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Effects of an alien invasive gastropod on native benthic assemblages in coastal lakes of the iSimangaliso Wetland Park, South Africa
Abstract
Tarebia granifera (Lamarck, 1822) is one of the most widespread alien invasive molluscs around the world and was accidentally introduced into South Africa via the aquarium trade during the 1990s. It has invaded and spread in the estuaries and lakes of the iSimangaliso Wetland Park, where it is often present in extremely high densities. There are no studies directly addressing the impact of T. granifera on benthic fauna of invaded habitats in South Africa. This study aimed to compile useful historic and current benthic community and environmental data for a comparison between pre- and post-invasion scenarios involving T. granifera in the shallow water habitats (depth < 2.6 metres) of different lakes in the iSimangaliso Wetland Park. Results provide evidence that T. granifera can dominate and thus significantly affect native invertebrate assemblage composition. Under certain environmental conditions, some native gastropod species can be displaced by T. granifera. However, T. granifera does not appear to markedly affect the biodiversity of aquatic coastal ecosystems. This study serves as a baseline for future research. There is a need for empirical approaches, addressing the extent and specific mechanisms of the ecological impact of T. granifera.
... During the 1990s aquarium trade, T. granifera may have been introduced into KwaZulu-Natal but was discovered in the St. Lucia estuary in 2005 for the first time [33]. However, this has changed as T. granifera has invaded different water bodies in the northeastern areas of South Africa [33]. ...
... During the 1990s aquarium trade, T. granifera may have been introduced into KwaZulu-Natal but was discovered in the St. Lucia estuary in 2005 for the first time [33]. However, this has changed as T. granifera has invaded different water bodies in the northeastern areas of South Africa [33]. In our study, T. granifera was found to be the snail with the highest abundance (n = 2201). ...
Disease distribution is correlated to the distribution of the freshwater snails which in turn is influenced by the physicochemical status of the habitats. This study aimed to evaluate freshwater snail species diversity, abundance, and distribution in KwaZulu-Natal (KZN) province, South Africa, between December 2020 to February 2021. A total of 4576 freshwater snails consisting of 8 species were collected from 127 sites in 11 districts. Tarebia granifera snails were the most abundant (n = 2201), while bivalves (n = 95) were the least abundant. The highest and least Shannon–Weiner Simpson’s diversity indices were recorded in Ugu and iLembe districts, respectively. A negative relationship was observed between rainfall, Bulinus tropicus, Lymnaea natalensis, bivalves, and Physa acuta, while temperature had a positive relationship with B. globosus, B. pfeifferi, and T. granifera. A positive relationship was observed between B. globosus and B. pfeifferi (r = 0.713, p < 0.05). Snail presence constitutes potential health and economic risks to humans and animals in contact with the waterbody. Hence, our study described the current distribution, abundance, and species diversity of freshwater snails in the KZN province with insights into the possibilities of snail-based biological control for schistosomes intermediate host snails.
... The former was accidentally introduced into South Africa through the aquarium trade and has since invaded several rivers, lakes, wetlands and estuaries in the eastern and northern parts of the country (Appleton et al. 2009). It can reach high population densities, and has been implicated in displacing native snails and becoming the dominant component of invertebrate assemblages in invaded areas (Miranda and Perissinotto 2014). Theba pisana can reach high densities on the west and south coast of South Africa, with the potential to impact on native fauna and flora (Odendaal et al. 2008). ...
Studies of the impact of alien species on the environment are increasingly being carried out, and there has been ongoing debate about how to standardise the description of these impacts. This chapter evaluates the state of knowledge on the impacts of alien species on biodiversity in South Africa based on different assessment methods. Despite South Africa being one of the most biologically diverse countries in the world, there have been very few studies that formally document the impacts of alien species on biodiversity. Most of what is known is based on expert opinion, and consequently the level of confidence in the estimates of the magnitude of these impacts is low. However, it is clear that a significant number of alien species cause major negative impacts, and that there is cause for serious concern. There is a growing global effort to assess all alien species with standardised protocols to alleviate the problem of comparing impacts measured using different approaches. Formal assessments have been done for a few alien species in South Africa, but most naturalised and invasive species have not been evaluated, and, we suspect, for most alien species there has been no attempt, as yet, to document their impacts. However, red-listing processes found that alien species were frequently included as a significant extinction risk for several native species of fish, amphibians, and plants. There are very few studies that cover the combined impacts of co-occurring alien species in particular areas, and these studies could provide the rationale for regulation and management, which is often absent. While reductions due to alien species in the value of ecosystem services, the productivity of rangelands, and biodiversity intactness are relatively low at present these impacts are expected to grow rapidly as more invasive species enter a stage of exponential growth.
... This is potentially due to the uniqueness and vulnerability of island habitats (Mueller-Dombois & Loope, 1990). One gastropod, Tarebia granifera, was implicated in causing local extinctions (MR) to native snails in wetlands in Puerto Rico (Giboda, Malek, & Correa, 1997), Venezuela (Pointier & Giboda, 1999), South Africa (De Kock & Wolmarans, 2008;Miranda & Perissinotto, 2014), and ...
Aim
Alien gastropods have caused extensive harm to biodiversity and socioeconomic systems like agriculture and horticulture worldwide. For conservation and management purposes, information on impacts needs to be easily interpretable and comparable, and the factors that determine impacts understood. This study aimed to assess gastropods alien to South Africa to compare impact severity between species and understand how they vary between habitats and mechanisms. Furthermore, we explore the relationship between environmental and socioeconomic impacts, and both impact measures with life‐history traits.
Location
Global.
Methods
We used the Environmental Impact Classification for Alien Taxa (EICAT) and Socio‐Economic Impact Classification for Alien Taxa (SEICAT) to assess impacts of 34 gastropods alien to South Africa including evidence of impact from their entire alien range. We tested for correlations between environmental and socioeconomic impacts per species, and with fecundity and native latitude range using Kendall's tau tests. Kruskal–Wallis tests were used to compare impact magnitude among mechanisms and habitats, respectively.
Results
This study presents the first application of EICAT and SEICAT for invertebrates. There was no correlation between environmental impacts and socioeconomic impacts. Habitats did not differ regarding the severity of impacts recorded, but impacts via disease transmission were lower than other mechanisms. Neither fecundity nor native range latitude was correlated with impact magnitude.
Main conclusions
Despite gastropods being agricultural and horticultural pests globally, resilience of socioeconomic systems makes high impacts uncommon. Environmental systems may be vulnerable to gastropod impacts across habitats, having experienced multiple local extinctions of wetland island snail fauna. South Africa stands out as the only continental country that follows this trend. The knowledge gained on severity and nature of gastropod impacts is useful in risk assessment, which can aid conservation management. To make impact assessments more realistic, we suggest alternative ways of reporting impacts classified under EICAT and SEICAT.
... River system, it is expected that it will also become an invasive species in this system (Miranda & Perissinotto, 2014). ...
In this study macroinvertebrates, fish, amphibians and birds were selected as focal groups to represent the biodiversity spectrum from invertebrates, lover chordates to higher chordates. The single most important aspect that was identified by each of the four taxonomic groups was the integrity of the Phongolo floodplain and the Ndumo Nature Reserve as a whole. Key corner stones that will determine this integrity of the study area includes the flood regime, sedimentation and habitat heterogeneity.
... The abundance data demonstrate that T. granifera is firmly established throughout the Nseleni River system, with an overall preference for shallow waters and an apparent seasonal pattern of abundance. Distributions of T. granifera were patchy, a common phenomenon with freshwater benthic macroinvertebrates (Covich et al. 1999), which was consistent with T. granifera distribution patterns in other South African localities (Appleton et al. 2009;Miranda et al. 2010Miranda et al. , 2011bMiranda and Perissinotto 2014b). The seasonal abundance pattern and sporadic pulses in abundance are likely explained in part by the continuous boom and bust population dynamics reported for T. granifera (Perissinotto et al. 2013) and its capacity for reproduction, embryo development and live births throughout the year (Kun-Jun 2004; Miranda et al. 2011b). ...
The invasive freshwater snail Tarebia granifera (Lamarck, 1822) was first reported in South Africa in 1999 and it has become widespread across the country, with some evidence to suggest that it reduces benthic macroinvertebrate biodiversity. The current study aimed to identify the primary abiotic drivers behind abundance patterns of T. granifera, by comparing the current abundance of the snail in three different regions, and at three depths, of the highly modified Nseleni River in KwaZulu-Natal, South Africa. Tarebia granifera was well established throughout the Nseleni River system, with an overall preference for shallow waters and seasonal temporal patterns of abundance. Although it is uncertain what the ecological impacts of the snail in this system are, its high abundances suggest that it should be controlled where possible and prevented from invading other systems in the region.
... However, the competitive interactions between M. tuberculata and T. granifera remain unclear and are yet to be fully investigated. The introduction and establishment of T. granifera within coastal lakes of the iSimangaliso Wetland Park has been associated with the decline of historic M. tuberculata populations , with the exception of those from Lake Nhlange (Miranda and Perissinotto, 2014). Differences in behaviour have also been recorded between M. tuberculata from Lake Nhlange and those from St Lucia Estuary Mouth (Raw et al., 2015). ...
... T. granifera can dominate, and thus significantly affect native invertebrate assemblage composition. 13 Therefore, the snail fauna and the bilharzia status of water bodies may change after invasion by introduced snails. For example, there is evidence that P. acuta has replaced B. globosus on Inhaca Island, Mozambique, and that as a result, bilharzia transmission there has been reduced, if not stopped. ...
Bilharzia surveillance at local authority level in South Africa relies on public health personnel being able to locate transmission sites. Although several methods have been tried, the method of choice is collecting snails, identifying those that serve as intermediate hosts for bilharzia, and examining them for evidence of infection. Unfortunately, public health personnel are seldom trained to recognise these snails or the parasites? larval stages, and few laboratories are able to assist. Therefore, the intention is that these guidelines will help to equip personnel with the basic information needed to collect and identify the bilharzia-carrying snails, and then examine them for infection. The intramolluscan or larval stages of trematode parasites likely to be encountered in dissected snails are described. However, emphasis is placed on the furcocercariae, a group which includes the infective stage of bilharzia flukes. A family key to furcocercariae is presented.
... The species was introduced in the late 1990s (Appleton and Nadasan 2002) and has since been recorded from estuarine lakes (sensu Whitfield 1992Whitfield , 2000 on the KwaZulu-Natal coast at least since 2006 (Pillay and Perissinotto 2008). In the coastal lakes of northern KwaZulu-Natal, T. granifera has been shown to displace native gastropods under certain environmental conditions (Miranda and Perissinotto 2014a). However, the specific mechanisms which drive the success of this species are yet to be elucidated. ...
Chemical cues released by organisms have the potential to impact the behaviour of species, particularly in aquatic environments. These interactions affect the dynamics of resource utilisation at a small scale. Chemical cues may also be significantly involved in heterospecific competition and in reducing biotic resistance thus facilitating invasion success. This occurs if one species causes another to vacate optimal niches. This study aimed to assess (1) the behavioural responses of native gastropods to chemical cues released by other native gastropod species with which they did not co-occur, and (2) whether native gastropods from different localities exhibit similar behavioural responses to chemical cues released by the alien invasive Tarebia granifera, irrespective of their prior exposure to the alien. Behavioural responses were measured using an experimental approach both in situ and in the laboratory. Principal Components Analyses were used to visualize the variation in the behavioural responses of gastropods which were measured in terms of displacement, number of steps and turning angles. Gastropod behaviour was similar for populations from different sites, irrespective of their prior exposure to T. granifera. The behaviour of native gastropods was only different when exposed to chemical cues naturally released by T. granifera. Additionally, native gastropods did not behave differently when exposed to the chemical cues of an alien physid, Aplexa marmorata. This suggests that a change in behaviour when exposed to chemical cues of T. granifera may not be the result of having different evolutionary histories, but rather that T. granifera has evolved a unique metabolite which acts a deterrent to potential competitive snails. The role of chemical cues in mediating interactions between native and invasive species needs further research, as they may be important factors influencing biotic resistance and facilitating invasion success.
The spread of freshwater invasive species through aquarium trade poses a threat to the ecosystem, economy and human health. The availability of the exotic freshwater gastropod mollusc, Planorbarius corneus (Linnaeus, 1758) (Gastropoda: Planorbidae), in pet shops in Kolkata, West Bengal, India, raises concern for its potential establishment as an invasive species. Assuming the role of the native predators as a potential biotic barrier for invasion, observations on the predation pattern of the malacophagous leech, Glossiphonia weberi (Blanchard, 1897) (Clitellata: Glossiphoniidae) against P. corneus was made in single as well as in combination with the native snail Indoplanorbis exustus (Deshayes, 1834) (Gastropoda: Planorbidae). Using varying size classes and densities of P. corneus, the predation potential of G. weberi was estimated after a 24 hour period. In another set of experiments, the predation of G. weberi against the snail P. corneus was observed in the presence of I. exustus under four different habitat conditions. The purpose was to justify - (1) the effect of conspecific and heterospecific conditions of the prey availability and (2) the effect of habitat complexity on the predatory efficacy of G. weberi. The results indicated that G. weberi consumed varied numbers of P. corneus, depending on the size and the densities of the prey and predator. Although the prey heterogeneity and complex habitats caused significant reduction of predation on both of the prey species, G. weberi significantly preferred P. corneus over I. exustus in open (P < 0.0001), macrophyte (P = 0.002) and pebbles and macrophyte containing habitats (P < 0.0001). Apparently, G. weberi preferred the exotic snail P. corneus in presence of alternative prey snail I. exustus, under different habitat conditions. Therefore, G. weberi may act as a significant biotic resistance against the colonization and establishment of P. corneus in the Indian context. However, further studies including the multiple prey and predators are required to ascertain the food web level impact of the exotic snail P. corneus in invaded freshwater ecosystems.
All primary (name-bearing) types of Patelloidea, Lottioidea, Fissurelloidea and Scissurelloidea deposited in the KwaZulu-Natal Museum are presented. The reference to the original publication, including the original generic position, the type locality, the collector and the cited dimensions of the type specimen(s), is provided for each species, followed by information from the label for each type in the NMSA collection (type locality, collector and catalogue number), size of the type specimen, brief remarks and colour photographs.
Historical gaps and biases in the literature may have influenced the current knowledge of the impacts of invaders on global biodiversity. We performed a systematic review and compiled the main gaps and biases in the literature and the reported negative, neutral and positive effects of exotic species on local invertebrates worldwide. We analysed the relation of these reported effects to the biogeographical origin of the exotic species, the environmental characteristics of the invaded area, the trophic level of the exotic species and of the invaded local fauna, and the elapsed time after first introduction. We analysed 1276 publications comprising 2984 study cases. From these, 1786 cases included “control” situations (without exotics) and provided quantitative supporting evidence of the effects of exotic species on local invertebrates. The main gaps in the literature included tropical and arid climates, estuaries and marine ecosystems, as well as exotic species coming from Neotropical, Australian, Oriental, Ethiopian and Antarctic regions. Carnivorous and herbivorous species were underreported as exotic species and as impacted invertebrates. The considered variables were mostly unrelated to the reported effects, suggesting that the effects of exotic species on local invertebrates are heterogeneous and not unidirectional. Many impacted invertebrates were assemblages of undefined composition in terms of the native or exotic nature of the invaded organisms. Further avenues to reduce the identified biases in the current knowledge about the effects of exotic species on local invertebrates are also indicated.
Mangroves occur in South African estuaries at their poleward distribution limits, extending into temperate habitats. In 1963, William Macnae published the first comprehensive assessment of mangrove swamps in South Africa and made firsthand observations of these mangrove ecosystems. This article reassesses South African mangrove habitats, highlighting changes since Macnae’s assessment, through a literature review of research done in the past 50 years and using the results of a dedicated mangrove survey spanning 2012–2017. Until now, changes have been recorded mostly for mangrove vegetation, including a change in mangrove cover and a poleward shift of mangrove species. While some mangrove-associated fauna have disappeared from most sites (e.g. the gastropod Terebralia palustris), others, such as fiddler crabs, have spread farther south. The effects of decreasing diversity with an increase in latitude were not observed along the South African coast. Instead, habitat quality and estuarine mouth state seem to exert greater influence on species diversity in the mangroves, and a poleward shift in species distribution is now evident not just for the mangrove flora but for the fauna as well. South African mangrove research needs to include a continuous monitoring plan, especially if we are to contribute to global knowledge on blue carbon, the effects of sea-level rise, and the resilience of the mangrove ecosystem.
The successful invasion of freshwater and coastal lakes of South Africa by the recently introduced thiarid snail Tarebia granifera may be due in part to release from predatory pressure. This study aimed to determine the comparative vulnerability of T. granifera and the widespread native aquatic thiarid Melanoides tuberculata to predation. These species also account for many thiarid invasions in the Americas, Europe and parts of Africa. We quantified the shell crushing resistance of these snails, as well as the maximal shell crushing capability of native freshwater crab predators, Potamonautes sidneyi and P. perlatus. Using an Instron isometric transducer, we showed that Tarebia granifera shells were significantly stronger than Melanoides shells, and exceeded the crushing strength we documented for both potential predatory crabs. The greater shell strength of Tarebia granifera was due to shape, sculpture and thickness characteristics. Shell strength of Melanoides, however, remained within the range of crushing strength of their potential predators. Assuming crushing to be the main form of crab predation on snails, we inferred T. granifera to be less vulnerable to durophagous attack and that their population growth is thus not limited by predation pressure.
Two Asian freshwater gastropods, Radix rubiginosa (Michelin, 1831) (Lymnaeidae) and Gyraulus chinensis (Dunker, 1848) (Planorbidae) are reported from a facility supplying fish and plants to the aquarium trade in KwaZulu-Natal, South Africa. Neither species has been found in South Africa previously. Both are considered potentially invasive. Updated keys for Lymnaeidae and Gyraulus spp. in South Africa are presented. An analysis of data on the dates of first collection and regions of origin for all 14 freshwater snails known to have been introduced into the country, regardless of whether or not they have survived, shows three phases of introduction. These are (i) from North America between the 1940s and 1960s, (ii) from South America during the 1970s and 1980s and (iii) from Asia between the 1990s and the present. Further research may show two species long regarded as indigenous to also be introductions.
A new species of the genus Parachiton, P. hodgsoni n.sp., which inhabits shallow water off the South African coast, is described. The radula is unusual for the genus Parachiton. It consists of about 160 transverse rows of very small mature teeth of which the major lateral teeth have very narrow tridentate cusps. The intermediate valves are granulose with the granules arranged in widely spaced, longitudinal rows on the central area of the valves. There are 20-31 longitudinal rows on the central areas of the intermediate valves. Variability in the morphology of the tail valve is shown.
South African temporarily open/closed estuaries (TOCEs) and similar systems along the coastlines of other regions of the world, especially Australia, are amongst the most productive aquatic ecosystems. They shift seasonally from mostly open mouth states during rainy seasons to mostly closed mouth states during the dry part of the year. This allows a whole range of juvenile forms of estuarine-dependent and estuarine-associated marine species to be recruited inside their sheltered and productive reaches, where they complete their growth to maturity. This new book covers topics such as the structure and function of open/closed estuaries in South Africa, as well as outlining the future management decisions that need to be made in order to ensure the longevity of these productive ecosystems.
The recent dry phase experienced by the St Lucia estuarine system has led to unprecedented desiccation and hypersaline conditions through most of its surface area. This has changed only recently, at the end of 2011, with the onset of a new wet phase that has already caused a major shift to oligo-and mesohaline conditions. The estuary mouth, however, remains closed to the ocean, making the weak connection re-cently established between the St Lucia and the Mfolozi estuaries the only conveyance for marine recruit-ment. As a result, only 10 indigenous and two alien aquatic gastropod species are currently found living in the St Lucia estuarine lake. This is out of a total of 37 species recorded within the system since the earliest survey undertaken in 1924, half of which have not been reported in the literature before. The tick shell, Nassarius kraussianus, which was consistently found in large abundance prior to the recent dry phase, ap-pears to have temporarily disappeared from the system, probably as a result of the extinction of Zostera marine grasses inside the lake. Population explosions of the bubble shell Haminoea natalensis, with its dis-tinct egg masses, were recorded seasonally until 2009, but the species has subsequently not been observed again. A molecular DNA analysis of the various populations previously reported as belonging to the same assimineid species, variably referred to as Assiminea capensis, A. ovata, or A. bifasciata, has revealed that the St Lucia assemblage actually comprises two very distinct taxa, A. cf. capensis and a species provisionally referred to here as "A." aff. capensis or simply Assimineidae sp. In the mangroves, the climbing whelk Ce-rithidea decollata is still found in numbers, while ellobiids such as Cassidula labrella, Melampus semiaratus and M. parvulus are present in low abundances and all previously recorded littorinids have disappeared. A number of alien freshwater species have colonized areas of the system that have remained under low salin-ity. These include the invasive thiarid Tarebia granifera, which can be found in concentrations exceeding 5000 ind.m -2 , the lymnaeid Pseudosuccinea columella and the physid Aplexa marmorata. ZooKeys 440: 1–43 (2014) doi: 10.3897/zookeys.440.7803 www.zookeys.org Copyright Renzo Perissinotto et al. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. A peer-reviewed open-access journal
The Assimineidae are a family of amphibious microgastropods that can be mostly found in estuaries and mangroves in South Africa. These snails often occur in great numbers and are ecologically important to the St Lucia Estuary, which forms a crucial part of the iSimangaliso Wetland Park, a UNESCO World Heritage Site. Genetic and shell morphometric analyses were conducted on individuals collected from nine localities distributed from the northern lake regions to the southern lake and the mouth of the St Lucia estuarine lake. Mitochondrial (COI) and nuclear (28S) DNA was used to construct Bayesian Inference, Neighbour-joining, Maximum Parsimony and Maximum Likelihood trees. Principal Component Analysis and Cluster Analysis were performed on standard shell parameter data. Results indicate that two different taxa are present in St Lucia. The taxon comprising individuals from the South Lake and St Lucia Estuary Mouth is identified as Assiminea cf. capensis Bartsch, in accordance with the latest taxonomic consensus. The taxon comprising assimineid individuals from False Bay, North Lake and South Lake, is here tentatively named “A.” aff. capensis (Sowerby). These two taxa exhibit patterns of spatial overlap that appear to vary depending on environmental parameters, particularly salinity. The need to resolve the complex taxonomy of assimineids is highlighted.
Ten species of freshwater gastropod have been introduced into South Africa, mostly through the aquarium trade. Two of these, Lymnaea columella (Lymnaeidae) and Physa acuta (Physidae), have been invasive in river systems across the country for many years, probably since the 1940s or 1950s, and two recent arrivals, Tarebia granifera (Thiaridae) and Aplexa marmorata (Physidae), are spreading. Research on one of the well-established invaders, P. acuta, is reviewed and updated and a suite of attributes is identified that may have enabled it to become invasive.
The Lake St Lucia estuarine system is the most important nursery ground for juvenile marine fish and prawns along the KwaZulu-Natal coast. The estuary mouth closed in June 2002 because of drought and remained so for 4 years and 9 months. A study to determine the impacts of extended mouth closure, hypersalinity and low lake levels on the mesozooplankton, macrobenthic invertebrates and fish fauna was initiated in 2004. Zooplankton and benthic invertebrate diversity declined, benthic invertebrate community composition changed and the diversity and abundance of fish decreased between 2004 and 2007. In the case of fish, the declines were related to die-offs in the lake and the failed recruitment of post-larvae and juveniles from the marine environment as a result of the mouth having been closed. Options for management intervention under closed-mouth conditions are limited at this time, particularly in the short term, to breaching the mouth and facilitating the inflow of sea water. In the medium term, as was the historical situation, the reconnection of the Mfolozi system to St Lucia should be viewed as a major priority.
* Corresponding author Abstract The Tuxpam and Tecolutla rivers in the Gulf of Mexico, are located in the Mesoamerican biodiversity hotspot and support different human activities: crude oil extraction, agriculture and livestock, that provoke environmental disturbances. Aquatic mollusks, physicochemical water characteristics and substrate type were examined along the main watercourse of these rivers during three periods of one year: the wet season, dry season, and during the northern winds season, when hurricanes are more frequent. In both rivers, physicochemical water characteristics, types of substratum and mollusk fauna demonstrated environmental gradients and differentiate two river zones: freshwater and estuarine. Differences were also found between the dry season, with higher inorganic salts content, and the wet and northern winds and hurricane seasons, when inorganic and organic nutrient inputs occurred. The mollusk fauna is composed of nine and eleven taxa in the Tecolutla and Tuxpam rivers, respectively. In both rivers, the introduced gastropod Tarebia granifera is the dominant species in the freshwater zone with regard to population density and the area it covers within the ecosystem, followed by the introduced bivalve Corbicula fluminea. Native mollusk species were confined to point-locations and attained very low densities. The gastropod Neritina virginea and the bivalve Brachidontes exustus were dominant in the estuarine zone of both rivers. Mollusk population densities declined during the wet, northern winds and hurricane seasons, while in the dry season both alien species reached higher densities, which could indicate that alien mollusks are removed by effect of climatic events. T. granifera and C. fluminea exhibit traits characteristic of invasive species and pose a risk to native mollusk biodiversity in these rivers.
Since the establishment of the alien mussel Mytilus galloprovincialis in South Africa, several authors have studied its interactions with individual indigenous species. However, the broader implications of this invasion on the intertidal zone remain undocumented. This paper analyses the impacts of this mussel on the rocky-shore invertebrate community structure at Marcus Island on the west coast of South Africa. The effects of the invasion were linked to 3 key elements and were not consistently spread across the intertidal zone, but were focused within the mid-to-low shore. Firstly, physical stress in the mid-intertidal zones was ameliorated by the presence of M.galloprovincialis beds. Secondly, habitat complexity was increased where M. galloprovincialis replaced bare rock or less complex secondary habitat. Thirdly, habitat became less patchy as mussel beds blanketed the shore. Consequently, invertebrate density and species richness increased substantially, and community composition changed significantly in the mid-shore. Lower on the shore, significant changes in invertebrate community structure were driven by a switch from mono-layered beds of the small indigenous mussel Aulacomya ater to multilayered beds of M galloprovincialis, despite no change in total species richness.
Invasive aquatic species, such as the gastropod Tarebia granifera, can cause ecological disturbances and potentially reduce biodiversity by displacing indigenous invertebrates. In South Africa, T. granifera was first recorded in an estuarine environment in the St Lucia Estuary. Its tolerance to salinity and temperature was investigated through the experimental manipulation of these factors. T. granifera can tolerate temperatures between 0 ºC and 47.5 ºC, allowing it to survive high temperature extremes. The species may also survive cold snaps and invade higher altitude areas. More remarkably, this snail survives high salinity for a relatively long time, as LS50 (lethal salinity for 50% of the population) was reached at 30 psu over 65-75 days. However, higher salinity adversely affected the T. granifera population. Snails acclimated to freshwater conditions and suddenly transferred to 30 psu experienced 100% mortality within 48 h. Snail activity also declined with increasing salinity. T. granifera's environmental tolerance and parthenogenetic characteristics are the keys to successful introduction and establishment. Therefore, the management of T. granifera may prove difficult in the short to medium term. The present findings constitute a contribution to the knowledge of biological invasions in Africa and to the understanding of estuarine invasions by T. granifera.
Chemical cues provide aquatic organisms with sensory information that guides behavioural responses and thus interactions among themselves, each other and the environment. Chemical cues are considered important for predator avoidance, foraging, larval settlement and broadcast spawning in aquatic environments. However, the significance of their role as drivers of direct interactions between heterospecifics has been largely overlooked.
A video camera and a demarcated arena were used in situ to record behavioural responses of three native gastropod species, Assiminea cf. capensis, Melanoides tuberculata and Coriandria durbanensis, exposed to treatments representing chemical cues released by a non-native invasive gastropod, Tarebia granifera. The responses were measured quantitatively as displacement and orientation of movement at locations in St Lucia Estuary, within the iSimangaliso Wetland Park, a UNESCO World Heritage Site on the east coast of South Africa. All native gastropods exhibited a negative taxis response to chemical cues released by T. granifera, while T. granifera individuals responded randomly to conspecifics. Displacement was measured relative to the source of the extract, the number of steps taken were determined with path analysis and orientation was determined from the mean (±95% CIs) turning angles, with significant negative turning angles representing negative taxis. Responses to treatments corresponding to the environment and conspecifics were random and undirected, indicating kinesis.
This study presents evidence for interactions driven by chemical cues between a non-native invasive gastropod and several gastropods native to South Africa. The results indicate that chemical cues can facilitate invasion success as the behavioural response of native gastropods is to move away allowing additional food and space resources to become available to T. granifera.
Four primary habitats have been identified in the St Lucia estuarine system, each having a suite of characteristic vegetation types. These can be described as follows: (1) open water; (2) intertidal shoreline; (3) 'dry' shoreline and island (i.e. those habitats without any freshwater source other than rainfall); and (4) groundwater-dependent shoreline. The responses of these habitats to changes in water level, salinity and other determinants have been described. An understanding of these responses facilitates the management of the estuary mouth. Past records indicate that the mouth closed naturally at the start of a drought, but the management policy was to keep it open by dredging the berm which developed at the mouth. While the mouth was open, seawater replaced estuary water lost to evaporation and the system eventually became hypersaline. An assessment of the range of physical conditions that occur in St Lucia Estuary and the response in each habitat indicated that the open mouth hypersaline condition is the most detrimental, resulting in die-back of shoreline vegetation. This understanding supported the management decision that resulted in the mouth being kept closed since 2002. Since then, the estuary has been characterised by high salinity but low water levels, and therefore large-scale die-back of shoreline vegetation has not occurred.
This paper is the first comprehensive and illustrated account of living Polyplacophora from the Maldive Islands. The only important previous contribution is that of E.A. Smith (1903), describing eight species. We determined 15 species sourced from 24 sampling sites, mainly at a depth of 20–40 meters, represented by 19 living specimens and 1112 valves. Only nine species (Parachiton hylkiae mauricejayi, Ischnochiton albinus, Ischnochiton sp., Rhyssoplax maldivensis, Tegulaplax hululensis, Lucilina sp., Acanthochitona penicillata, Craspedochiton cf. laqueatus, Cryptoplax burrowi) are represented by living specimens, the others by separate valves (Callochiton clausadeae, Lucilina perligera, L. carnosa, Acanthochitona sp., Craspedochiton sp., Cryptoplax sykesi), three of them (Lucilina perligera, Acanthochitona sp., Craspedochiton sp.) by only one or two valves. We did not find four of the species previously reported for the Maldives, i.e. Ischnochiton feliduensis, Callochiton platessa, Squamopleura miles and Schizochiton incisus. The specimen collected by J.S. Gardiner, and reported in Smith (1903) as Callochiton platessa Gould, has been studied, and it is not possible to attribute to any of the other Callochiton species from the Indian Ocean. It thus remains undetermined as Callochiton sp., and Callochiton platessa Gould must be deleted from the list of chiton species living in the Maldive Islands. The holotype of Ischnochiton feliduensis (the only specimen known) is illustrated to provide a complete and exhaustive survey of all the 19 chiton species now known from the Maldive Islands. Two species from other localities, Squamopleura miles and Schizochiton incisus, also are illustrated. The type material of Craspedochiton laqueatus and Cryptoplax burrowi has been examined. Eleven species are reported for the first time for the Maldives: Parachiton hylkiae mauricejayi, Ischnochiton albinus, Ischnochiton sp., Callochiton clausadeae, Lucilina perligera, L. carnosa, Lucilina sp., Acanthochitona penicillata, Acanthochitona sp., Craspedochiton sp. and Cryptoplax sykesi.
Base-line limnological and biological data are given for Lake Bhangazi South, a small lake on the coastal plain of Kwazulu-Natal, South Africa. This shallow (Zmax < 6 m), subtropical lake is a warm and seemingly continuously polymictic system, but experiences severe (continuous?) deoxygenation of its deeper muddy sediments. Nutrient status (of N in particular), light attenuation, phytoplankton productivity and zooplankton biomass are high relative to other regionally comparable coastal lakes investigated. While the ichthyofauna is quite rich, zooplankton and especially zoobenthos communities are species-poor, and lack the relict estuarine components which often dominate these latter assemblages in comparable coastal lakes. The zooplankton is typically freshwater in composition, and contains a new species of copepod (Tropodiaptomus bhangazii Rayner); the lake may be older, or seen longer evolutionary divergence than its proximate counterparts. Benthic species diversity in the lake is especially low amongst the Crustacea and Mollusca, possibly reflecting the effect of relatively acid waters (pH ca 6.5) on the calcium budgets of these groups. Scanty observational data indicative of ecosystem changes over the past 35 years are reported and briefly evaluated from a conservation/management perspective. Along with the endemic copepod, several rare fish species give the lake added conservation status.
Taxonomic comments are given on some South African members of the polyplacophoran genus Chaetopleura. The species C. pustulata (Krauss, 1848) is here given species rank, based on examination of the type material. Chaetopleura natalensis Kaas & Van Belle, 1990 is placed in the synonymy of C. pustulata (Krauss, 1848). Chaetopleura destituta Sykes, 1902 formerly interpreted as synonym of Chaetopleura pertusa (Reeve, 1847) is shown to be conspecific with Chaetopleura papilio (Spengler, 1797). The transfer of Lepidozona debruini Strack, 1996 to Chaetopleura is indicated by a distinct perinotum coverage, radula characters and the position of the nephridiopore in relation to the ctenidia.
The Asian prosobranch snail Tarebia granifera was reported from South Africa (and Africa) for the fi rst time in 1999 in northern KwaZulu-Natal though it was probably introduced sometime prior to 1996. In the 10 years since its discovery it has spread rapidly, particularly northwards, into Mpumalanga prov-ince, the Kruger National Park and Swaziland. The snail has colonized diff erent types of habitat, from rivers, lakes and irrigation canals to concrete lined reservoirs and ornamental ponds. It reaches very high densities, up to 21 000 m -2 , and is likely to impact on the entire indigenous benthos of the natural waterbodies of the region – more so than any other invasive freshwater invertebrate known from the country. The indigenous thiarids Thiara amarula, Melanoides tuberculata, and Cleopatra ferruginea are con-sidered particularly vulnerable. Preliminary accounts of the reproductive biology and population fl uc-tuations of T. granifera in KwaZulu-Natal are also presented.
Non-native species and habitat degradation are two major catalysts of environmental change and often occur simultaneously. In freshwater systems, degradation of adjacent terrestrial vegetation may facilitate introduced species by altering resource availability. Here we examine how the presence of intact riparian cover influences the impact of an invasive herbivorous snail, Tarebia granifera, on nitrogen (N) cycling in aquatic systems on the island of Trinidad. We quantified snail biomass, growth, and N excretion in locations where riparian vegetation was present or removed to determine how snail demographics and excretion were related to the condition of the riparian zone. In three Neotropical streams, we measured snail biomass and N excretion in open and closed canopy habitats to generate estimates of mass- and area-specific N excretion rates. Snail biomass was 2 to 8 times greater and areal N excretion rates ranged from 3 to 9 times greater in open canopy habitats. Snails foraging in open canopy habitat also had access to more abundant food resources and exhibited greater growth and mass-specific N excretion rates. Estimates of ecosystem N demand indicated that snail N excretion in fully closed, partially closed, and open canopy habitats supplied 2%, 11%, and 16% of integrated ecosystem N demand, respectively. We conclude that human-mediated riparian canopy loss can generate hotspots of snail biomass, growth, and N excretion along tropical stream networks, altering the impacts of an invasive snail on the biogeochemical cycling of N.
We followed the invasion dynamics of the Oriental thiarid snail Thiara granifera on the Martinique island, French Antilles. This freshwater species was first discovered in 1991 in the Charpentier River, and its spread has since been analysed based on a yearly survey of the malacological fauna at more than 100 sites covering the whole island and representing 50 river systems and three pools. Four river systems were sampled at many sites. Thirteen river systems were colonized by 1997. Colonization within river systems occurred at a speed greater than 1km per year, probably resulting from both active and passive dispersal. Our results can, on the whole, be explained by a simple diffusion process. However, stratified diffusion has to be invoked in at least one river. Moreover, colonization was faster downstream than upstream, suggesting that current velocity plays a significant role in dispersal. Dispersal also occurred between river systems at a mean distance of almost 10km, though with a large variance, in accordance with the scattered colony model of stratified diffusion. The relative frequencies of T. granifera and Melanoides tuberculata, another recent invader of Martinique, were followed at three sites on the Lzarde River. The first species quickly outnumbered the second, though never wiped it out. The data therefore do not support any exclusion phenomena between these two parthenogenetic invaders. Our analysis does not indicate any obvious influence of the rise of T. granifera on the local freshwater fauna.
We examined the patterns of distribution, vectors of introduction, and potential ecological impacts of freshwater exotic species
in Texas over the last 45years. Currently, five species of exotic gastropods are established: channeled-type applesnail (Pomacea insularum), red-rim melania (Melanoides tuberculatus), quilted melania (Tarebia granifera), giant rams-horn snail (Marisa cornuarietis), and Chinese mysterysnail (Cipangopaludina
chinensis). In contrast to the northern part of the US, where shipping appears to be the most important vector for the introduction
of aquatic invasive species, aquarium and ornamental trade dominated among unintentional vectors of introduction of all freshwater
exotics in Texas, resulting in different patterns of distribution, spread, and ecological impacts. The rate of spread of exotic
gastropods in Texas varied from 39 waterbodies colonized over 18years for P. insularum to only three waterbodies during last 45years for C.
chinensis. Four of five exotic gastropods were found in highly vulnerable aquifer-fed springs and rivers, which contain numerous endemic
and endangered species. The fifth species, Pomacea insularum, is an agricultural pest. Potential negative ecological effects of exotic gastropods include impacts on wetlands and wetland
restoration, competitive exclusion of native snails, and the introduction of exotic parasites, trematodes, which could infect
fish and waterfowl, including federally protected species. Aquifer springs with stable temperature regimes are refuges for
both cold and warm intolerant species.
Individual animal species can impact ecosystem processes, but few exotic invaders have demonstrated ecosystem-scale impacts, even when population sizes are large. We combined whole-stream measures of carbon and nitrogen fluxes with rates of consumption and ammonium excretion to show that an exotic freshwater snail, Potamopyrgus antipodarum, dominated these fluxes in a highly productive stream. The snails consumed 75% of gross primary productivity, and their excretion accounted for two-thirds of ammonium demand. Such large fluxes were due to high snail biomass rather than high rates of excretion or consumption. This exotic species may dramatically alter ecosystem function in rivers, with potential consequences for food web structure and element transport.
All primary types of Scaphopoda and Cephalopoda deposited in the KwaZulu-Natal Museum are presented. The reference to the original publication, including the original generic position, the type locality, the collector and the size of the type specimen(s), is provided for each type, followed by information on the type in the NMSA collection (catalogue number, size of the type specimen, type locality and collector), other types mentioned in most recent publication(s) with the reference(s) to such publication(s), brief remarks and colour photographs. A lectotype is designated for Sepia (Rhombosepion) acuminata Smith, 1916.
We use a three-year study of sheltered low shore assemblages colonised by the non-indigenous Asian kelp Undaria
pinnatifida to explore survey design issues for assessing the ecological impacts of invasive species. The weight of evidence overall suggested little impact from Undaria on low shore assemblages, with control–impact contrasts that could plausibly be interpreted as impacts probably reflecting natural causes. We demonstrate that the potential for reaching incorrect conclusions regarding the impacts of invasive species using control–impact designs is greater than when such designs are used to assess traditional forms of anthropogenic impact. We suggest that a before–after control–impact framework is essential, but recognise that such an approach has a number of limitations. In particular, there is no assurance that the before–after impact site will be invaded at all, or to the extent that provides worst-case impact information for coastal managers. We discuss possible ways of assessing invasive species impacts, but suggest that the uncertainty inherent in extrapolating impact information to other places and times means that the precautionary principle should be applied, and ‘worst-case’ impacts assumed, until the level of scientific uncertainty is reduced. Such an approach should only be applied, however, after an evaluation of the feasibility, costs and benefits of managing a particular pest in relation to other priorities for invasive species.
An invasive operculate gastropod, the freshwater snail Tarebia granifera, is reported for the first time from Israel. In a survey of 52 freshwater sites populations of T. granifera were discovered at four sites, in abundance at two of them, one of which included individuals infected with the trematode Philophthalmus sp. In both sites T. granifera outnumbered its close native relative and potential competitor, the thiarid Melanoides tuberculata, in abundance and mean embryo counts, and in one of the habitats almost excluded it. This invasion may have important public health implications, due to T. granifera s role as an intermediate host of the human eye flukes. Furthermore, evidence of long-distance dispersal or multiple invasion sites may facilitate the host's and parasite's spread to other water bodies used for agriculture and fish ponds.
Tarebia granifera is a freshwater/estuarine gastropod invading many tropical and sub-tropical areas around the world. This snail is native to southeast Asia and was accidentally introduced into South Africa during the last decade. The current study investigated shallow-water benthic assemblages of different invaded and uninvaded localities across locations spanning a large range of environmental conditions in the iSimangaliso Wetland Park. Using a correlation-based approach, we found that native benthic assemblages were more closely associated with environmental conditions than with densities of T. granifera. However, there were significant negative correlations between T. granifera abundance and Shannon Diversity at two of the invaded locations. This alien species has successfully invaded, and become dominant in, different types of water bodies with different assemblage compositions and physico-chemical characteristics, ranging from freshwater ponds to saline estuaries and lakes. The current data set
Four new species of chitons (Mollusca, Polyplacophora) are described, viz Leptochiton (L.) dispersus and L. (L.) permodestus from Transkei, L. (L.) meiringae from the eastern Cape Province, S Africa (note 11) and Ischnochiton (Stenosemus) vanbellei from the Mediterranean Sea (note 14). New records are given for little known species, such as Leptochiton (L.) chariessa (Barnard, 1963) (note 11), Ischnochiton (I.) obtusus Carpenter in Pilsbry, 1893 (note 12), Leptochiton (L.) lineatus (Nierstrasz, 1905), L. (L.) rissoi (Nierstrasz, 1905), Loricella oviformis (Nierstrasz, 1905), and Placiphorella albitestae Is. Taki, 1954 (note 13). New synonyms: Squamophora Nierstrasz, 1905, and Componochiton Milne, 1963 = Loricella Pilsbry, 1893; Paricoplax profundior Dell, 1956, and Componochiton raceki Milne, 1963, are junior objective synonyms of Loricella oviformis (Nierstrasz, 1905) (note 13).
The St Lucia estuarine lake in South Africa forms part of a World Heritage Site and is an important local source of biodiversity. Like many estuarine systems worldwide, St Lucia has experienced significant anthropogenic impacts over the past century. Abstractions have decreased fresh water inflows from the lake catchments by about 20%. Furthermore the Mfolozi river, which previously shared a common inlet with St Lucia and contributed additional fresh water during droughts, was diverted from the system in 1952 because of its high silt loads. The separated St Lucia mouth was subsequently kept artificially open until the onset of a dry period in 2002 when the mouth was left to close naturally. These changes and the current drought have placed the system under severe stress with unprecedented hypersaline conditions coupled with desiccation of large portions of the lake. Long-term simulations of the water and salt balance were used to estimate the occurrence and persistence of water levels and salinities for different management scenarios. The risks of desiccation and hyper-salinity were assessed for each case. The results show that the configuration of the Mfolozi/St Lucia inlets plays a key role in the physicochemical environment of the system. Without the Mfolozi link desiccation (of about 50% of the lake area) would occur for 32% of the time for an average duration of 15 months. Artificially maintaining an open mouth would decrease the chance of desiccation but salinities would exceed 65 about 17% of the time. Restoring the Mfolozi link would reduce the occurrence of both desiccation and hypersaline conditions and a mostly open mouth state would occur naturally. Integrating these modeled scenarios with observed biological responses due to changes in salinity and water depth suggests that large long-term changes in the biological structure can be expected in the different management scenarios.
Individual animal species can impact ecosystem processes, but few exotic invaders have demonstrated ecosystem-scale impacts, even when population sizes are large. We combined whole-stream measures of carbon and nitrogen fluxes with rates of consumption and ammonium excretion to show that an exotic fresh-water snail, Potamopyrgus antipodarum, dominated these fluxes in a highly productive stream. The snails consumed 75% of gross primary productivity, and their excretion accounted for two-thirds of ammonium demand. Such large fluxes were due to high snail biomass rather than high rates of excretion or consumption. This exotic species may dramatically alter ecosystem function in rivers, with potential consequences for food web structure and element transport.
A criterion for comparing diversity is offered based on dominance patterns involving all the proportional species abundances. The method is applied by plotting percentage cumulative abundance curves. This can reveal that some assemblages cannot be compared in terms of diversity or equitability and that intrinsic diversity indices cannot under these circumstances be relied upon. The behaviour and interpretation of these dominance curves under different circumstances is explored using examples from macrobenthic and marine nematode studies. Dominance curves are also compared with Sanders' rarefaction curves and the relative advantages discussed. It is recommended that these graphical methods be routinely applied to marine biological data before calculating more complex diversity or equitability indices.
The bottom substrates of the freshwater Lake Sibayi are mainly fine sands. Silt substrates occur mainly in the valleys leading from the western and northern arms of the lake underwater in the main basin of the lake.
The fauna included an estuarine crustacean component and a polychaete. The freshwater component consisted principally of chironomid larvae, oligochaetes and molluscs.
Differences in density of animals were shown for three species in depths up to 20 metres and depths below 20 metres. Other animals are more evenly distributed with regard to depth.
The infauna of the bottom sediments is mainly confined to sandy substrates. The epifauna is more generally distributed. The fauna of weed beds far from the shore of the lake in 5 metres of water is like that of the marginal vegetation.
Some of the fauna showed seasonal changes in density.
The value of different methods that were used in the survey are discussed.
Drought conditions have prevailed in many areas of NSW since 2002. On the mid-north coast, below-average rainfall resulted in reduced riverine flows and the extended closure of intermittent estuaries within the Solitary Islands Marine Park. Patterns of structure of benthic infaunal communities were evaluated at the height of the drought to determine if they differed between closed, intermittent estuaries and permanently open estuaries within the region. Replicate van Veen grab samples were taken in the upper, mid- and lower reaches of six intermittent and three permanently open estuaries and sieved to retain the macrofauna. A range of physico-chemical measures was also taken at each sampling time. Multivariate analyses of assemblage data revealed a significant difference between the structure of the two estuary types and also among estuaries within each type. Differences between estuary types were attributable to small differences in the abundance of a number of taxa but also to the absence of the amphipod Urohaustorius metungi from most of the intermittent estuaries. In contrast, these small amphipods dominated communities in the lower reaches of the permanently open estuaries. Physico-chemical variables were highly variable among estuaries and were not strongly correlated with assemblage patterns. Correlations with catchment size were the strongest and, as most of the intermittent estuaries in the region are smaller than the permanently open estuaries, this confounds the interpretation of assemblage patterns in this preliminary study. In order to differentiate between the effects of catchment size and entrance status, the same estuaries need to be resurveyed during periods when at least some of the intermittent estuaries are open.
The St. Lucia Estuary is the largest estuarine system in Africa. The estuary is part of the Greater St. Lucia Wetland Park, which has been declared a World Heritage Site. This ecosystem has been subjected to severe drought conditions over the last four to five years, resulting in its mouth being closed off from the ocean in June 2002 for a period of over four years. The main aim of this study was to document the effects of the prevailing drought on the macrofauna of the system, since the last work on this benthic component had been undertaken over a decade ago, during a normal-to-wet phase. Macrofauna samples together with physico-chemical data were collected at representative sites in the Narrows, and the South and North lakes in February, April, August and October 2005. The drought exerted a strong influence on the system, leading to hypersaline conditions developing in its northern regions (maximum of 126 at Hell's Gate), and to the loss of aquatic habitat. Ordinations and clustering indicated that the macrofauna of the system could generally be separated into three clusters viz. (1) the Narrows and the southern portion of South Lake, (2) the northern half of South Lake, and (3) the North Lake–False Bay complex. Multivariate correlations indicated weak relationships between macrofaunal community structure and physico-chemical parameters. The distinction in macrofaunal assemblages between these clusters was probably caused by these habitats being physically separated at the peak of the drought, with no water flow between them, thereby preventing exchange of planktonic larvae and retarding colonisation of habitats. There was a northward decline in taxonomic richness and diversity of macrofauna in the system, which correlated positively with water depth and negatively with the biomass of microphytobenthos. It is evident that the drought structured macrofauna communities primarily through its effects on water depth and habitat fragmentation. The results of this investigation provide valuable information regarding the effects of droughts on estuarine–lake systems and the possible mechanisms by which they occur.
St Lucia Lakes (Zululand, South Africa) were sampled with a Van Veen grab in January 1972, July 1972, and January 1973. The system had high salinities at the commencement of the programme (70%0-80%0 False Bay, 55%0-60%0 North Lake, 45%0-58%0 South Lake) which quickly dropped to less than sea-water values in the first six months. The January 1972 samples showed that only a few Polypedilum (Chironomidae) survived in False Bay, while a very reduced faunal list was shown for North Lake (9 sp.). Nevertheless South Lake showed a fairly complete faunal list (21 sp.). By July 1972 nine species were recovered in False Bay, 18 in North Lake and 23 in South Lake, indicating reinvasion consequent upon dilution of the salts in the water. No great change was shown by the result of the January 1973 samples.Evidence suggests that most forms would tolerate salinities up to about 55%0 salinity. Beyond this level only Chironomid larvae and Ostracods survived. Most rapid reinvasion of the denuded parts of the system was by rapidly maturing forms with planktonic larvae. There is evidence of exchanges of water between the various areas consequent upon wind induced seiches which would carry the planktonic larvae to these denuded areas.Numerical and gravimetric analysis of standing stocks showed that sandy substrates are dominated by the small Assiminea bifasciata (Mollusca; Gastropoda) which is probably the most important food item in the benthos, apart from the larger bivalves which were not sampled. Muddy samples were similar to sandy samples when Assiminea was excluded from consideration. Estimates of standing stocks for the St Lucia system were: for sandy substrates between 1,08 and 3,26 g/m and muddy substrates 0,013 and 0,063 g/m.
Lake Nhlange (KwaZulu, South Africa) is connected to the sea through the Kosi estuary and has a salinity between 36%0 and 5%0. The benthos of the lake was sampled at approximately six-monthly intervals from July 1965 to January 1969. The substrates are mainly sandy in shallow water and also on steeply sloping profiles even in the deepest water of 30 m. Flat areas are mainly silty. In December 1965, following the closure of the estuary mouth and severe rains, the lake level rose to about 2 m above its normal level. Although the salinity changed little (5%0, to 3%0), quantities of peat-stained water and detritus were brought into the lake. A waning pattern of low oxygen conditions below about 12 m in the hot season, followed by reasonably well-oxygenated conditions in the cold season, was found. Particularly severe oxygen depletions were discovered at the mud water interface, even where well-oxygenated conditions pertained in the open water column at equivalent depths.
Evidence that the fauna was driven into shallower waters by the influx of detritus, with its high oxygen demand, is given and a slow recolonization of the deeper water is shown. Large fluctuations in standing stocks were noted, together with changes in dominant forms. The question as to whether this was entirely due to the effects of the flood or whether fluctuations in fairly stable conditions are to be expected is examined with reference to some Northern hemisphere lakes. It is concluded that fluctuations are to be expected under fairly stable conditions, but that those shown in the present study were in the main due to the effects of the flood.
This book provides a comprehensive introduction to all aspects of biological invasion by non-native species. Highlighting important research findings associated with each stage of invasion, Invasion Ecology provides an overview of the invasion process from transportation patterns and causes of establishment success to ecological impacts, invader management, and post-invasion evolution.--Jacket.
1. Grazing by invasive species can affect many aspects of an aquatic system, but most studies have focused on the direct effects on plants. We conducted mesocosm and laboratory experiments to examine the impact of the invasive apple snail Pomacea canaliculata on macrophytes, filamentous algae, nutrients and phytoplankton.
2. In a freshwater pond, we confined 500 g of Myriophyllum aquaticum or Eichhornia crassipes with 0, 2, 4 or 8 apple snails in 1 m × 1 m × 1 m enclosures for approximately 1 month. Apple snails grazed heavily on both species of macrophytes, with higher overall weight losses at higher snail densities. The damage patterns differed between the two macrophytes. In M. aquaticum, both leaves and stems suffered from substantial herbivory, whereas in E. crassipes, only the roots suffered significant weight reduction.
3. In addition to grazing on macrophytes, apple snails appeared to have controlled the growth of filamentous algae, as these did not develop in the snail treatments. The ability of P. canaliculata to control filamentous algae was supported by a laboratory experiment where the consumption was as high as 0.25 g g−1 snail DW d−1. Because of a lack of native herbivorous snails in the pond, the growth of filamentous algae (mainly Spirogyra sp.) reached 80.3 g m−2, forming a spongy pond scum in the no-apple snail control. Together with previous reports that apple snails could eat the juveniles and eggs of other freshwater snails, our results indicated that P. canaliculata could have out-competed native herbivorous snails from the pond by predation on their juveniles or eggs. Alternatively, P. canaliculata might have out-competed them by monopolisation of food resources.
4. Nitrogen and phosphorous concentrations remained low throughout both experiments and were not correlated with apple snail density. The treatment effects on chlorophyll a (Chl a) and phytoplankton composition varied in the two experiments. In the M. aquaticum experiment, with increasing snail density, Chl a increased, and the phytoplankton community became dominated by Cryptophyceae. In the E. crassipes experiment, Chl a level was independent of snail density, but with increasing snail density, the phytoplankton community became co-dominated by Cryptophyceae, Chlorophyceae and Bacillariophyceae.
5. Given the multiple effects of P. canaliculata on wetland biodiversity and function, management strategies should be developed to prevent its further spread. In invaded wetlands, strategies should be developed to eradicate the apple snail and re-introduce native snails which can control the development of filamentous algae.
Abstract Procedures used to detect environmental impacts that occur as a result of planned disturbances are often inadequate. Widely used designs for univariate measures, such as the abundance of a population, lack proper spatial replication and have unjustified patterns of temporal sampling. Asymmetrical analyses of variance derived from repeated measures models can be used to detect many types of impact that are not identifiable using widely recommended BACI (Before/After, Control/Impact) sampling. These asymmetrical, beyond BACI designs are also more logical because of spatial replication. The mechanics of these procedures are discussed, including worked examples of calculations, considerations of their power to detect impacts of a specified magnitude and the integration of various temporal and spatial scales into the design. Related issues are briefly discussed concerning optimization of sampling and how to proceed when no data are available before a disturbance.
Gut fluorescence and carbon budget techniques were applied to Tarebia granifera (shell height 10–12 mm) at the iSimangaliso Wetland Park, a UNESCO World Heritage Site. This snail has recently invaded a number of estuaries in northern KwaZulu-Natal, where it reaches densities of over 1000 ind. m−2 and becomes a dominant component of the benthic community. Its rapid establishment and spread have raised concerns about potential top-down impacts on the ecosystem. This study shows that T. granifera can utilize large amounts of microphytobenthos (MPB) in addition to detritus. In situ total available MPB pigment concentrations ranged from 11.6 to 110.5 mg pigm. m−2. T. granifera’s gut pigment content ranged from 54 to 1672 μg pigm. ind−1. Gut evacuation rates (k) ranged from 0.36 to 0.62 h−1 (R2 range: 16.2–35.2, P < 0.05). Individual ingestion rates ranged from 6.6 to 30.4 μg pigm. ind.−1 d−1. T. granifera was estimated to consume from 0.5 to 35% of the total available MPB biomass per day, or 1.2–68% of the daily primary benthic production. The carbon component estimated from the gut fluorescence technique contributed 8.7–40.9% of the total gut organic carbon content. The average carbon daily ration contributed by microalgal biomass was ≈16% body carbon per day. Variability in the data was attributed to the complex feeding history of snails. Further studies are needed to validate these results and provide more information on the ecological impact of T. granifera on this wetland and other similar invaded ecosystems, both estuarine and freshwater.Research highlights► Gut fluorescence and carbon budget techniques were applied to a benthic invertebrate. ► This in situ approach provided critical insight into the diet of an invasive species. ► T. granifera's feeding rate is comparable with other invasive gastropods in the world. ► T. granifera has significant top-down effects on freshwater and estuarine ecosystems.
The Olympia oyster (Ostrea lurida) was historically abundant in Willapa Bay, WA, but populations were decimated by overexploitation in the mid to late-1800s and have failed to recover. We investigated the potential role of two introduced predatory gastropods, the Japanese drill (Ocinebrina inornata) and the eastern drill (Urosalpinx cinerea), in limiting Olympia oyster recovery. We quantified the bay-wide distribution, local abundance, and per capita effects of drills, and asked how each of these three components of total invasion impact might be influenced by another dominant introduced species, the Pacific oyster (Crassostrea gigas). Bay-wide sampling revealed differences in spatial distribution of the two drill species, with U. cinerea more abundant toward the head of the estuary and O. inornata more abundant toward the mouth. Individual feeding trials indicated that both drill species preferred Pacific oysters to Olympia oysters of similar size, and preferentially attacked smaller oysters. We used field enclosures to quantify the direct effects of Japanese drill predation on Olympia and Pacific oysters, intra and interspecific competition, and indirect effects mediated by the shared predator. Predation reduced the survival of both oyster species, but the per capita impact of Japanese drills declined with increasing density of either Olympia or Pacific oysters, consistent with a type II functional response. This positive indirect effect of Pacific oysters on Olympia oysters was offset by asymmetric competition, in which Pacific oysters reduced Olympia oyster growth and survival but not vice versa. Despite the large drill impacts seen in these experiments, Olympia oysters transplanted to intertidal sites throughout the bay experienced low and variable rates of drill predation compared with other mortality sources. Introduced drills may be only one of a suite of factors that prevent rebuilding of Olympia oysters in the intertidal zone in Willapa Bay.
Abstract Several recent methods have been developed for detecting anthropogenic perturbations. Most have been analyses of data collected before and after some anthropogenic disturbance. It may, however, be more common that data can be collected only after a disturbance has occurred. In such situations, the only appropriate sampling design to use will often be an asymmetrical design because it will avoid problems of spatial confounding. Here, I describe in detail the steps involved in constructing asymmetrical analyses of variance using a case study of subtidal epibiota around marinas as an example. Differences between the marina and control locations were detected for a number of taxa, but this was often only possible after post-hoc pooling of non-significant terms. Marina and control locations varied greatly from estuary to estuary and consequently it was not possible to identify suites of species that were typical of either type of location. This result highlighted the need for multiple control locations near each marina to allow a reliable estimate of the variability among controls. Large variability among controls would mean that if differences existed between disturbed and control locations they would rarely be detected. These and other problems associated with analysing ‘after data’ are discussed in addition to the precautions to take when designing environmental sampling regimes.
In marine macrobenthos studies the identification of organisms at species level is the best entry to ecological and biological information about the animals. An accurate identification requires excellent conservation of the organisms, reliable fauna description, experts and lengthy work in the laboratory. The aim of this work is to test taxonomic sufficiency (TS) in two deliberately selected different case studies to understand whether and how the taxonomic complexity of a benthic assemblage influences the results of TS and where it works better. The first benthic settlement was collected in an area characterized by homogeneous depth and grain size composition (case study A) around an off-shore gas platform, while the second one was collected along a coast-wide transect in an area with human pressure limited to fishing activities (case study B). Univariate and multivariate statistical analysis was used to assess differences in the taxonomic structure of benthic assemblages and to test TS on the two different datasets. TS seems to work in both sites, from species to higher taxonomic levels, and the family taxonomic level appears the best compromise for taxonomic resolution when an accurate identification is not achievable. The application of TS does not indicate a significant difference between the two datasets and appears therefore to be a valid instrument to analyse and describe the structure of benthic settlements in the case of taxonomically complex communities.