Crepidula fornicata - Slipper shell
SCIENTIFIC NAME
Crepidula fornicata (Linnaeus, 1758)
1901 - 1950
NW Atlantic
Aquaculture - importing live animals
The slipper limpet is native to the east coast of North America, from Nova Scotia (Canada) to the Gulf of Mexico [2].
First observation in Belgium
The first individual got observed on the 28th of September 1911 attached to an oyster in Ostend. This specimen is preserved at the Royal Belgian Institute of Natural Sciences (RBINS) [3].
Spreading in Belgium
The slipper limpet has been an established species in Belgium since the 1930s. In the 1960s, research on the presence of this species along our coast led to further observations in De Panne, Koksijde, Blankenberge and the Sluice Dock of Ostend [4]. Recently, the population near the Belgian coast significantly increased, and anno 2011, the species is now common along our coast. Slipper limpets are found on hard substrates along the Belgian coast, including buoys and wind turbine banks [5], and on soft sediment, where they attach to empty shells [6].
Spreading in neighbouring countries
The first European observation of the slipper limpet was in Liverpool Bay (England) in 1872. There, the species was unable to establish itself permanently. Between 1887 and 1890, the slipper limpet was repeatedly introduced in Essex (southeast of England) via the import of American oysters Crassostrea virginica. These oysters were deposited on existing English oyster beds and carried the slipper limpet as a stowaway [7].
From Essex, the slipper limpet spread along the European coast [7]. The North Sea has a north-eastward tidal residual current. The larvae of the slipper limpet – which live freely in the water column (planktonic) – can ‘hitch a ride’ on this current and can, therefore, quickly migrate in a northerly direction to other coastal areas along the North Sea [8].
The species was first reported from the Netherlands in 1924 [9]. These observations were of empty shells attached to washed-up seaweed. The first living individual got noticed only two years later, in October 1926 [10], attached to washed-up wreckage. Soon afterwards, the species became common in Zeeland [11].
Nowadays, the slipper limpet occurs in all European countries bordering the Atlantic Ocean (including the North Sea), the Baltic Sea, and some Mediterranean countries (Greece, Italy, Malta) [12-14].
The slipper limpet was transported to Europe with the American oyster Crassostrea virginica, which got released for aquaculture purposes [7]. In addition, it cannot be ruled out that this sea snail ended up in our regions as part of the biofouling community on ship hulls or as planktonic larvae in ballast water [7, 15].
On a smaller scale, the species may spread because of fishing activities. Fishing vessels that fish for shells use fishing gear that gets dragged across the sea bottom for a certain distance. When they haul up their catch, everything gets sorted. What they cannot use is thrown overboard. If slipper limpets are part of the bycatch, they can easily disperse [16].
The success of the slipper limpet can be explained by some specific characteristics of this species, such as its unusual way of reproduction. Slipper limpets sit on top of each other, with the youngest and smallest individuals on top of the tower (see also ‘Specific features’). The close presence of conspecifics stimulates their reproduction [16, 17].
The slipper limpet has few to no predators in our regions [7] and thrives on different types of hard substrate and shells. Occasionally, they colonise muddy bottoms [16].
The slipper limpet has a planktonic larval stage of about two to three weeks [4], which contributes to the rapid expansion of its distribution range. The planktonic larvae passively ‘hitch a ride’ on the prevailing sea currents. This allows them to bridge large distances. Local currents also play an important role in the local reproductive success of the species[18]. Large numbers of slipper limpets are observed in Mont Saint-Michel Bay because the planktonic larvae are held inside the bay by an annular sea current [19]. The opposite is happening in the Bay of Morlaix (Brittany, France). Here, the slipper limpet population cannot sustain itself because the larvae get carried away from the bay [20].
A more northern expansion is likely hindered by lower winter temperatures, which may slow down or prevent development [17]. The larvae and juvenile individuals generally grow faster at higher water temperatures. They tolerate temperatures of 30 and 32°C, respectively [21-24]. Additionally, higher water temperatures result in greater reproduction and, hence, higher abundances of slipper limpets [25]. Global warming is expected to promote further northward migration and increase their occurrence at greater depths [21, 26]. The species would also disappear from tidal areas, where it currently lives at high temperatures [27]. Hard offshore substrates, such as buoys or wind turbines at sea, can serve as ‘stepping stones’ towards new areas.
Juvenile individuals and larvae can tolerate a low salinity of 15 PSU [22, 28], although growth and development will be slower. In the case of a low salinity combined with a low food supply, a juvenile slipper limpet will invest more energy in creating more body tissue to enlarge the organ with which it feeds. In this way, it can absorb more food. Nevertheless, the rate at which juvenile slipper limpets feed remains slow. Due to their slower development and growth, individuals remain a smaller size for a longer time, making them more vulnerable to predators [29, 30]. The slow-growing larvae get transported over long distances in the plankton. They can end up in suitable or unsuitable (e.g. low salinity) habitats. If the slipper limpet lives for a long time in low salinity water, it appears to be better able to adapt to future changes in salinity [21].
The acidification of the oceans, due to the high anthropogenic CO2 emissions, has negative effects on the larvae of the slipper limpet. Not only do they stay smaller, but they also have difficulty producing their shells. This makes them more fragile. They also remain in the larval stage longer, with a higher risk of predation and disease
According to recent studies, successful dispersal and establishment of the species get determined by the conditions during and after the larval settlement. The choice of a good microhabitat (e.g. a mussel) on which the larva settles is, therefore, essential for its further survival [36]. Particularly in tidal areas, the species seems unable to choose a good microhabitat. The metamorphosis from the larva to the adult stage requires a hard substrate [37]. However, as their density increases, the sediment may become muddier and more anoxic due to their biogenic deposition and the suspended material that gets retained between the organisms. This may explain why higher densities are observed in muddy deposits [36].
The slipper limpet competes for food and space with other filter feeders, such as mussels and oysters [38, 39]. However, studies suggest that mussels and slipper limpets depend on different food sources [40]. Competition between slipper limpets and oysters occurs when the food supply is limited, and the oysters need a lot of energy to produce male or female reproductive cells [41]. Mussels with attached slipper limpets grow less efficiently because they need to invest more energy to adhere to the substrate. This effect does not occur in oysters [40]. The growth of slipper limpets on mussels offers two advantages for the affected mussels, (1) the snails protect the mussels from predation by sea stars [42] and (2) the snails attract parasites, which would reduce the number of parasites that infect the mussels [43]. In addition, the slipper limpet promotes the deposition of mud (pseudofaeces) when the water is rich in dissolved material. However, this kind of sedimentation is a real plague for oyster beds and makes the environment unsuitable for the establishment of juvenile oysters [38].
Slipper limpets can be a good host for the yellow boring sponge Cliona celata. The slipper limpets are not affected by the sponge. So, the sea snail forms an ideal substrate for this sponge. The latter is detrimental to those species with which the slipper limpet lives. The reason for this is the higher risk of parasitism because of the presence of the boring sponge [44, 45].
In southern England (near Weymouth), the slipper limpet is the dominant species among the macrofauna (> 1 mm). Densities of up to 1,750 individuals per m² have been observed [46]. If slipper limpers are massively present on a soft sea bed consisting of sand or silt, the soft substrate gets eventually transformed into a ‘harder’ substrate. This shift in substrate type brings about a change in the present faunal community [16]. This change in habitat may lead to a reduction in the reproductive success of the common sole (Solea solea) [47] and may result in the movement of flatfish to other suitable habitats. The massive presence of slipper limpets also results in a reduced food supply and increased competition between flatfish in areas where there is still food available. These snails consume the primary production suspensions, which are also eaten by the prey of flatfish (i.e. benthic invertebrates) [48-50].
The slipper limpet can also have positive effects. For example, the species makes silicon more readily available in the water column, allowing diatoms to grow continuously. Continuous diatom production makes the formation of toxic algal blooms more difficult [40]. On the other hand, the consumption of the primary production, produced by slipper limpets, may control a red tide of Aureococcus anophagefferens in shallow estuaries [51].
Slipper limpets are rich in lipids. The lipids found in these sea snails are of great value in medicine. The lipids can reduce the risk of colon cancer and treat psoriasis [52-54]. Slipper limpets are very nutritious and were used as a source of protein during the war [55].
The slipper limpet changes gender during its life cycle. Fertilisation takes place internally. Slipper limpets are born as free-living larvae. After the larval stage, they transform into a crawling male and look for a female to attach to. The male stage lasts about two years, after which a gradual transformation takes place to the female stage. This transformation takes about 60 days. During this period, other slipper limpets can attach themselves to the transforming shell until a chain of about 12 individuals is formed. On average, one additional snail attaches itself per year, and the oldest individuals are always at the bottom of the chain.
Slipper limpets are mainly found on breakwaters and groynes. In the English Channel, they are found at great depths (up to 60 metres deep) [26].
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