Amphibalanus improvisus - Bay barnacle
There remains uncertainty about the origin and non-native status of the bay barnacle. It used to be difficult to tell white barnacles apart until Darwin described numerous species in 1845. Based on very early observations, it can be said that this species is either a native species of Europe or a very early immigrant [2]. For example, a species list of the Dutch coast does not mention the exotic origin of the bay barnacle [3]. Others describe this barnacle species as originating from warmer seas [4] or the North(west) Atlantic [5]. Due to the lack of clarity in its origin, this species of barnacle is therefore characterised by many scientists as being cryptogenic [6].
The bay barnacle is a typical species of cold to temperate regions and occurs mainly in intertidal areas and estuaries, but can also be found far from the coast [7]. This species can be found up to a depth of 90 metres [8]. It attaches itself to hard substrates, such as coastal defences, ship hulls and harbour infrastructure, but can also be found on the shells of molluscs (e.g. oysters), crustaceans (e.g. crabs) and even on algae
First observation in Belgium
On the Belgian coast, the bay barnacle was reported with certainty for the first time in 1895. The species was then found not only in the sea but also in brackish water and the Sea Scheldt [10]. It is noteworthy that Charles Darwin first described the bay barnacle in 1854 and thereby noted that this species probably occurred in Belgium [11]. Given the doubtfulness of this sighting, it is not considered to be the first sighting in Belgium. However, recent archaeological research has shown that the bay barnacle lived in our regions as early as the 17th century [7].
Spreading in Belgium
In the early 1960s, the bay barnacle was one of the four most common barnacles found off the Belgian coast [12]. It is the most common barnacle in the coastal harbours and the sluice-dock of Ostend [13, 14]. The species is also found in Belgian estuaries [8] and the almost freshwater of the Baudouin Canal [15]. Furthermore, this species can be found in seawater. In the late 1990s, scientists found the species on buoys in the Belgian part of the North Sea up to 25 kilometres from the coastline [16]. Today, the bay barnacle is still regularly spotted on buoys
Spreading in neighbouring countries
The very first reported sighting of the bay barnacle in Europe comes from the Netherlands. There, the species was reported for the first time in 1827, under the name Balanus ovularis, a name that is no longer used for the bay barnacle Amphibalanus improvisus [17]. In 1844, reports from the Baltic Sea [18] and the Black Sea [19] followed. However, these reports all predate Charles Darwin’s official description of the bay barnacle in 1854 [11]. Therefore, it is unclear whether these introductions have always involved the bay barnacle. In this time, species such as the wrinkled barnacle Balanus crenatus and the acorn barnacle Semibalanus balanoides already occurred here. These species show strong similarities to the bay barnacle; white colour, similar size and they also occur in the intertidal zone. The timing of introduction in Europe should therefore be regarded as uncertain [7].
In his description of 1854, Charles Darwin mentions that this barnacle species already existed in Europe off the coast of England and Scotland [11]. Not long after, in 1858, the bay barnacle was spotted in Germany on buoys in the Elbe estuary [4]. In France, the species was first recorded in 1872 in the southern part of the Bay of Biscay [20]. This was followed in 1952 by the introduction in the Caspian Sea [21].
Currently, the bay barnacle is a common species in the marine and brackish coastal waters along the Atlantic coast from northern Spain to Norway, including Great Britain, Ireland, the Baltic Sea, the Mediterranean Sea, the Black Sea and the Caspian Sea, and it is still expanding its range [9].
Initially, the spread of the bay barnacle occurred by attachment to the hulls of ships [11]. Some assume that the bay barnacle was introduced to our regions from the east coast of North America [18], but this hasn’t been proved [8].
More recently, the transport of larvae in ships’ ballast water has also been responsible for secondary dispersal in Europe. Another dispersal option is the attachment of adult individuals to imported oysters [7].
Few indigenous barnacles tolerate the brackish water environment of estuaries and port areas. This largely eliminates competition in less saline environments and explains the success of this species [22].
The bay barnacle is hermaphroditic and can also fertilise itself, with several broods per year possible. For example, bay barnacles in the Baltic Sea produce two broods per year. The larvae stay free in the water column for two to five weeks and – with the help of the water current – can reach new areas before settling permanently [21].
Its tolerance to a wide gradient of temperatures and salinities positively contributes to the spread of the bay barnacle. The optimum temperature gradient varies between 0 and 30°C [7]. Optimal salinities range from 6 PSU (brackish water) to 30 PSU, but its occurrence in the North Sea (35 PSU) indicates that it can tolerate higher salinities. It can also survive in freshwater conditions, although this species cannot reproduce there. The Dutch name ‘brakwaterpok’ (literally translated as ‘brackish water barnacle’) is somewhat confusing, as the species also occurs in seawater.
The bay barnacle has little competition with native barnacle species but does compete with other introduced barnacles such as the striped barnacle Amphibalanus amphitrite, the New Zealand barnacle Austrominius modestus and the wrinkled barnacle Balanus crenatus [9].
The bay barnacle can dominate the present biological community and compete with other species for both space and food [7]. A remarkable example of this is the settlement of the bay barnacle on juveniles of the softshell clam Mya arenaria. This can cause a shortage of food and oxygen for the softshell clam [23].
Their fouling on hulls, buoys and many other artificial hard materials – such as fishing gear – may cause economic problems. For example, ships experience a less efficient use of fuel and biofouling also entails increased maintenance costs. Attachment of barnacles can be solved by cleaning the hull or can be prevented by treatment with antifouling paint, which is, however, an expensive undertaking [24]. Moreover, these paints often damage the ecosystem. Some toxic substances from antifouling paints cause long-term environmental damage long after they have been withdrawn from being used, such as tributyltin (TBT), the use of which has been banned since 2003 [25].
In the colder and brackish waters of the Baltic Sea, relatively few filter feeders are naturally present. In this region, the introduced bay barnacle ensures a good flow of nutrients from the water column (by filtration) to the bottom (by excretion). In this way, the species ensures that many large algae can grow well and are maintained. One of the species that noticeably benefitted from this is the gutweed Ulva intestinalis, whose population in the Baltic Sea has increased significantly since the introduction of the bay barnacle [26].
The bay barnacle has a white conical shell that can grow to almost 2 cm in diameter and 1 cm high. The central opening is pentagonal and slightly toothed, although two corners may be rounded, giving the opening a more teardrop-like appearance [22].
The bay barnacle is almost like the twin brother to the wrinkled barnacle Balanus crenatus. To identify these two species, you need a magnifying glass and detach the barnacle to look at its calcified base, with which they attach themselves. The bottom of the bay barnacle is perforated and you can see channels running towards the centre. The wrinkled barnacle, on the other hand, does not have a pierced bottom. The calcareous plates of the operculum are also different and used for identification [3]. Two other white barnacles living in our region, the New Zealand barnacle Austrominius modestus and the acorn barnacle Semibalanus balanoides, do not have a calcified base.
Barnacles only feed when they are submerged. Then the central opening is opened and they stick out their six pairs of long and hairy feather-like appendages, called cirri. With these protrusions, they filter small food particles from the water column [22].
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VLIZ Alien Species Consortium (2020). Amphibalanus improvisus – Bay barnacle. Non-native species of the Belgian part of the North Sea and bordering estuaries anno 2020. Flemish Institute for the Sea (VLIZ). 7 pp.