Distribution of Aplysiidae in Australian waters

Australian distribution records were sourced from the literature, books, websites and personal records. The geographic extent includes the waters of continental Australia plus its oceanic islands and reefs (e.g. Lord Howe Island, Middleton and Elizabeth Reefs, Cartier and Ashmore Reefs) as well as external territories of Norfolk, Christmas and the Cocos (Keeling) islands.

Whilst many distribution records for the mainland can be pinpointed to individual locations, some give only broad regional descriptors. To accommodate this wide spatial range, presence/ absence data for each taxon were consolidated into currently recognised marine bioregions - the IMCRA 4.0 shelf provinces (COA 2006) (Tables 1 and 2). A comparison of species richness among IMCRA provinces and between the east and west coasts, for the purposes of identifying latitudinal change, was facilitated by calculating the mid-point for each province as the summary independent variable.

Distribution of Aplysiidae in Australian waters

The western South Pacific Ocean has long been known to support the greatest diversity of aplysiids in the world (Eales 1960; Medina et al. 2004). In Australian waters there are the 25 described and undescribed species (23 listed in Nimbs et al. (2017)). Of these 25 taxa, 10 are in the genus Aplysia but, as these may include several synonymous taxa, the actual number is currently uncertain.

In a revision of the world species of Aplysia, Eales (1960: 380) listed 12 species from Australia (A. cronullae Eales, 1960, A. dactylomela Rang, 1828 (syn. A. argus Rüppell & Leuckart, 1830), A. denisoni E. A. Smith, 1884, A. extraordinaria (Allan, 1932), A. gigantea G. B. Sowerby I, 1869, A. juliana Quoy & Gaimard, 1832, A. nigra d’Orbigny, 1837, A. oculifera A. Adams & Reeve, 1850, A. parvula Mӧrch, 1863, A. reticulata Eales, 1960, A. sowerbyi Pilsbry, 1895, A. sydneyensis G. B. Sowerby I, 1869). Later, Carefoot (1987: 170) listed 14 species, adding A. keraudreni Rang, 1828 on the east coast and A. pulmonica Gould, 1852 on the west. However, the latter was recently synonymised with A. argus (Alexander and Valdés 2013).

Eales (1960: 322) described a preserved individual of the east coast species A. keraudreni as “A small specimen, much contracted, but probably belonging to this species, [which] came from Sydney, Australia.” However, later in the work, as part of a global analysis of distribution (p. 380), it was omitted from a discussion of Australian animals and mentioned as occurring only in New Zealand. Furthermore, in a tabulated summary of species found in the South West Pacific (encompassing all Australian coastal waters), A. oculifera was omitted from the regional list (p. 381) despite being mentioned in the text (p. 380) as occurring in Western Australia (WA).

Whilst Carefoot (1987) and Eales (1960) both report A. nigra as an eastern Australian species, these records are most likely to be for A. juliana. Indeed, Eales discussed the numerous similarities between them. Later, Rudman also highlighted the probability that A. nigra brunnea Hutton, 1875 from NZ was most likely to be A. juliana (Rudman 2003a). Another Aplysia, A. reticulata, has not been mentioned in the literature or observed in Australian waters since Wells and Bryce (1993) recorded it and is also likely to be synonymous with another taxon.

In his treatise on the sea slugs from the Abrolhos Islands, WA, O’Donoghue (1924) described a new species, Dolabrifera pelsaertensis, using two specimens collected from Pelsaert Island. As “They do not appear to be referable to any described forms...” (1924: 535), he considered them to be a new taxon. Nevertheless, species of Dolabrifera can be highly variable in both colour and morphology (Rudman 2003b; pers. obs.), depending on diet and habitat. As with many of the Australian taxa described using preserved specimens, D. pelsaertensis is likely to be synonymous with another Dolabrifera. Few data are available regarding this species and a thorough search of the literature failed to nd any contemporary reference to, or observations of, D. pelsaertensis.

Aplysia argus and A. parvula are the most widespread aplysiids in Australian coastal waters, being found in 16 and 15 shelf bioregions respectively (Figs. 1A and 2A), and they are the only taxa recorded from the Great Australian Bight. Dolabella auricularia (Fig. 2G) and Bursatella leachii (Fig. 2E) also have wide distributions around much of Australia (Rudman 1998b) with both occurring in 12 bioregions. Aplysia sowerbyi (Fig. 2C), currently the only Aplysia recorded from the Northern Shelf Province, is probably circum-continental, however, some gaps remain, particularly in northern Western Australia. With just the single record, Aplysia keraudreni (Fig. 1G) has a highly restricted distribution and A. cronullae (Fig. 1B), A. denisoni (Fig. 1C) and A. reticulata (Fig. 2B), also found in only a few bioregions, are all likely to be synonymous with more widely observed species (Nimbs et al. 2017).

Petalifera petalifera (Fig. 3D), Phyllaplysia sp. (Fig. 3F) and the undescribed Bursatella sp. (Fig. 2F) from WA have comparatively short ranges, whereas Dolabrifera brazieri (Fig. 2H), Petalifera ramosa (Fig. 3E), A. sydneyensis (Fig. 2D), A. juliana (Fig. 1F) and A. extraordinaria (Fig. 1D) are more widespread, but restricted to the east coast. Dolabrifera dolabrifera (Fig. 3A), A. oculifera (Fig. 1H) and Syphonota geographica (Fig. 3I) are found on both the east and west coasts, but not in between, and Notarchus indicus (Fig. 3B) exhibits an apparently disjunct distribution: Central Eastern Province in NSW, Spencer Gulf Province in SA and three bioregions in WA.

Both Stylocheilus striatus (Fig. 3H) and S. longicauda (Fig. 3G) have been found in several bioregions; however, records are patchy and there are broad spatial gaps. Patchy distribution is also evident for the undescribed Petalifera sp. (Fig. 3C). The only Aplysia found exclusively on the west coast is A. gigantea (Fig. 1E), which occurs in both the Southwest Province and Southeast Transition bioregions.

All taxa mentioned above have been illustrated in Nimbs et al. (2017), except Aplysia gigantea, Bursatella sp. and Phyllaplysia sp. (presented here in Fig. 4). No images of Aplysia cronullae, A. reticulata or A. keraudreni were available.

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Fig. 1.

Fig. 1. Distribution of the Australian Aplysiidae. (A) Aplysia argus; (B) Aplysia cronullae; (C) Aplysia denisoni; (D) Aplysia extraordinaria; (E) Aplysia gigantea; (F) Aplysia juliana; (G) Aplysia keraudreni; and (H) Aplysia oculifer

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Fig. 2.

Fig. 2. Distribution of the Australian Aplysiidae. (A) Aplysia parvula; (B) Aplysia reticulata; (C) Aplysia sowerbyi; (D) Aplysia sydneyensis; (E) Bursatella leachii; (F) Bursatella sp.; (G) Dolabella auricularia; and (H) Dolabrifera brazieri.

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Fig. 3.

Fig. 3. Distribution of the Australian Aplysiidae. (A) Dolabrifera dolabrifera; (B) Notarchus indicus; (C) Petalifera petalifera; (D) Petalifera ramosa; (E) Petalifera sp.; (F) Phyllaplysia sp.; (G) Stylocheilus longicauda; (H) Stylocheilus striatus; and (I) Syphonota geographica.

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Fig. 4.

Fig. 4. Top left: Bursatella sp., Woodman Point, WA, 21 March 2017, photo: Matt Nimbs; top right: Phyllaplysia sp. Sydney Harbour, NSW, 4 February 2002, photo: Andrew Trevor-Jones (identified by W. B. Rudman (Rudman 2002), photo used with permission); bottom: Aplysia gigantea, Woodman Point, WA, 23 March 2017, photo: Matt Nimbs.

Distribution of Aplysiidae in Australian waters

Aggregating data into IMCRA 4.0 shelf provinces revealed latitudinal patterns of distribution on both the east and west coasts that are consistent with those mentioned for NSW alone (Nimbs et al. 2017), where aplysiid diversity increases with latitude, peaks in subtropical waters at 31-32°S and decreases in temperate waters. This pattern is stronger on the east coast (Fig. 5). Species of Aplysia are more diverse on the east coast, whilst other genera are more evenly distributed on both the east and west coasts.

In keeping with the our hypothesis that the central NSW coast is possibly the most speciose region for aplysiids globally, Lord Howe Island and the nearby Middleton and Elizabeth Reefs, both with 8 recorded species, support the highest diversity among the offshore Australian islands and territories. This is followed by Norfolk Island, Christmas Island and the Cocos (Keeling) Islands with 7, 3, and one species records respectively. Aplysia argus, A. parvula and Petalifera sp. have been recorded from Ashmore and Cartier Reefs, two of the emergent shoals in the northeastern Timor Sea (Willan 2005).

As with most other sea slugs, local distribution of sea hares is predominantly determined by the availability of their food (Edmunds 1977). However, with a tropical affinity (Rudman and Willan 1998), their broader geographic distribution is likely to be mediated by water temperature (Sunday et al. 2015). Whether on the east or west coast, conditions in the subtropics and warm-temperate regions appear optimal for the development and survival of sea hares, and this is reflected in their higher diversity. At these latitudes, warm water is supplied by southward-flowing boundary currents, the East Australian Current in the east and the Leeuwin Current in the west (Malcolm et al. 2011; Wernberg et al. 2013), which also carry a supply of larvae as potential recruits (Booth et al. 2007; Smith 2011). Further south, periods of coastal upwelling can provide nutrients (Gersbach et al. 1999; Roughan and Middleton 2002) and conditions conducive to the development and proliferation of algal food sources, whilst periodically adding considerable thermal stress to any tropical herbivores present (Pörtner 2010).

The vagaries of abiotic conditions can result in situations where, at any one time, a given sea hare species, potentially undetected for some time, can become the dominant heterobranch taxon in terms of abundance (pers. obs.). This phenomenon can result in community shifts in which one taxon becomes supplanted by another as populations wax and wane. Over longer temporal scales, these patterns can be visualised as a ‘stochastic corkscrew’ as described in Smith and Nimbs (2017).

Environmental conditions in large embayments are generally more conducive to settlement and development of sea hares - e.g. Port Stephens, NSW (Nimbs et al. 2017). In contrast to the large embayments on the coasts of southern QLD, NSW and south-western WA, aplysiid diversity in similar, calm-water habitats in Victoria, Tasmania and South Australia is comparatively low, despite high algal diversity and abundance. In fact, algal diversity in the Great Australian Bight and SA gulfs is amongst the highest in the world (Kerswell 2006) and yet, according to Shepherd and Thomas (1982), observations of aplysiids in South Australia are very rare. Indeed: “Out of all the thousands of dives I have done in southern Australia... I have only seen sea hares once, although I have often dived in their habitat.” (S. Shepherd pers. comm.). Low aplysiid diversity in these areas is unlikely to be driven by food availability but rather the effects of water temperatures which, particularly in winter, probably truncate or interrupt the aplysiid life-cycle.

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Fig. 5.

Fig. 5. Latitudinal diversity of the Aplysiidae on the eastern and western Australian coasts. Data represent aplysiid species richness for the middle latitude of each IMCRA 4.0 provincial shelf bioregion.