The limpet of light virtue

 Bank in the southern coast of Hayling Island (near Portsmouth, UK), composed of millions of slipper shells.

Bank on the southern coast of Hayling Island (UK), composed of millions of slipper shells (Crepidula fornicata).
Image by Rob Farrow, used under a CC-BY-2.0 licence from Geograph.

Recently, while on fieldwork in Norfolk, I came across several shells of the Slipper limpet (Crepidula fornicata), on the beach. Hardly worthy of comment, you might think, but the Slipper Limpet shouldn’t be here. Looking at its known distribution range, it was reported in Norfolk for the first time just a few years ago. Yet, there it was, now during my marine field trip, and in considerable numbers

First introduced into the UK with imported oysters (Crassostrea virginica) towards the end of the 19th century1, the slipper limpet has now spread to 20 countries. How did such a tiny, snail-pace species, native to the Atlantic Coast of North America make its way to the Pacific coast, Uruguay, Japan, entering the Mediterranean all the way to Greece and northern Europe? The limpet’s peculiar adaptations might provide some clues towards solving this mystery.

Veliger – the advantages of looking like dumbo

In its larval state, the tiny limpet is called a veliger because of its large, ciliated lobes called velum – the Latin word for veil. In limpets, however, this veil is not used to cover the animals face, but is instead used for swimming, feeding, absorbing oxygen and releasing carbon dioxide. But the lobes are only useful in the juvenile stage, and are lost when the limpet metamorphoses into an adult2.

Microscope photo of a veliger of slipper limpet, with standard fluorescent imaging.

Microscope photo of a veliger of slipper limpet, with standard fluorescent imaging.
Image by Andreas Hejnol (2007)

This structure not only makes them look a bit like Dumbo, but it probably also plays an important part in making the limpets highly mobile2, allowing them to spread fairly rapidly, for a snail. Mobility is a lethal feature for any invading army to have, especially if it bears little energy costs to the invader. Perhaps the “ears” that make this animal look so harmless and almost silly in its larval state are actually its strongest weapon for invading foreign coastlines.

Game of shells – between a rock and a hard place

In its native range the juvenile slipper limpet’s main enemies are crustaceans and drilling snails3, but in Europe it has very few predators, which has probably allowed it to thrive, and warmer waters brought by climate change are also expected to help it expand Northwards.

But this safe haven might be short lived, since one the limpet’s predators is itself an invasive species – the Japanese shore crab (Hemigrapsus sanguineus) from Asia began to invade the British shores last year4. Maybe the ongoing expansion of this new invader will curb the limpet’s own expansion, although this might not actually be a good thing.

Two Aliens Doesn’t Make a Right

The Japanese shore crab is itself quite a damaging species – scientists have documented troubling impacts on native crab species, and worse still, once stablish the crabs become so abundant that they’re considered almost impossible to eradicate5, and can continue to spread at a rate of 12 km2 each year7. Our slipper limpet on the other hand, although it has been found to reduce survival and growth of the native European blue mussel (Mytilus edulis), can also reduce the chances of the mussel being eaten by star fish (Asterias rubens)8.

Despite fears that the limpet would compete with the cultivated oyster (Crassostrea gigas), a study found that the limpet had no effect on the oyster’s growth or mortality11. But this doesn’t mean the limpet is a benevolent or a harmless invader, it still disturbs the water flow and competes with native species for rocky habitats, placing native fauna literally between a rock and a hard place.

Etymology – what’s in a name?

The slipper limpet’s latin name, Crepidula fornicata actually means ‘a small arched boot’ – Crepidula means ‘small boot’ in Latin, while fornicata comes from the Latin word fornix, meaning arch.

Roman ladies of light virtue often plied their trade under arches; in fact this is where the word fornication originates. The limpet might sound a bit dull in comparison, but just wait until I introduce you to this creature’s sex life.

A slipper limpet begins life as a male, and later turns into a female as it matures.

When the dumbo-like veliger metamorphoses into a small limpet, it begins looking for a place to settle. Adult slipper limpets often live in stacks of up to 20 individuals, and juvenile limpets are attracted to chemicals released by the adults9. This chemical signalling system guides most settling juveniles to land on, or very near a stack of limpets.

The stacks consist of a large female at the bottom of the pile, with one or more males attached to each other in a size gradient – the largest male attaching directly to the female, and smaller males attaching themselves to a larger male bellow, forming an almost pyramid shape stack. So when small juveniles arrive at the stack, they make their way to the top, where they can mature into young males.

Crépidules_groupeCrepidula fornicata

Mating stack of slipper limpets (Crepidula fornicata) with several young males at the top, some of which are trying to get close from the female at base by attaching themselves sideways. This species competes with native fauna for every hard habitat available, including the minute rock to which this stack is attached, which is considerably smaller than they are (Southsea, UK)
Images by (left) F Lamiot and (right) JoJan, used under creative commons licences from Wikimedia commons.

The larger males, which can sometimes reach the same size as the enormous female at the bottom, father most of her offspring, although younger males further up the stack do have some success9.

Eventually, the larger males may choose to change gender and become a female. Males have the choice of when to change gender, meaning these tiny limpets have some serious mental arithmetic to do – they must calculate when is the best time to switch. Large dominant males may be better off as females, rather than trying to compete with many other males. In contrast, small males that fail to attach to a stack for some reason may benefit from becoming a female earlier in life – by developing into a small female these individuals hope that new juvenile males will settle on top, and a stack will form around them. The limpets show quite a plastic behaviour during this process, and social interaction appears to play a major role in determining when a limpet undergoes its sex change.

With their unusual mating strategy and global conquest, the remarkable slipper limpet proves that the link between arched structures and sex long pre-dates the Romans.

References

1. de Montaudouin, Xavier, Corinne Audemard, and Pierre-Jean Labourg. “Does the slipper limpet (Crepidula fornicata, L.) impair oyster growth and zoobenthos biodiversity? A revisited hypothesis.” Journal of Experimental Marine Biology and Ecology 235, no. 1 (1999): 105-124.
2. Chan, Kit Yu Karen, Houshuo Jiang, and Dianna K. Padilla. “Swimming speed of larval snail does not correlate with size and ciliary beat frequency.” PloS one 8, no. 12 (2013): e82764.
3. Pechenik, Jan A., Olivia V. Ambrogio, and Steven Untersee. “Predation on juveniles of Crepidula fornicata by two crustaceans and two gastropods.” Journal of Experimental Marine Biology and Ecology 384, no. 1 (2010): 91-98.
4. Seeley, Becky, Jack Sewell, and Paul F. Clark. “First GB records of the invasive Asian shore crab, Hemigrapsus sanguineus from Glamorgan, Wales and Kent, England.” Marine Biodiversity Records 8 (2015): e102.
5. Dauvin, Jean-Claude, and Fabien Dufossé. “Hemigrapsus sanguineus (De Haan, 1835)(Crustacea: Brachyura: Grapsoidea) a new invasive species in European waters: the case of the French English Channel coast(2008-2010).” Aquatic Invasions 6, no. 3 (2011): 329-338.
6. Hejnol, Andreas, Mark Q. Martindale, and Jonathan Q. Henry. “High-resolution fate map of the snail Crepidula fornicata: the origins of ciliary bands, nervous system, and muscular elements.” Developmental biology305, no. 1 (2007): 63-76.
7. Leppäkoski, Erkki, and Sergej Olenin. “Non-native species and rates of spread: lessons from the brackish Baltic Sea.” Biological invasions 2, no. 2 (2000): 151-163.
8. Thieltges, David W. “Benefit from an invader: American slipper limpet Crepidula fornicata reduces star fish predation on basibiont European mussels.” Hydrobiologia 541, no. 1 (2005): 241-244.
9. Proestou, Dina A., Marian R. Goldsmith, and Saran Twombly. “Patterns of male reproductive success in Crepidula fornicata provide new insight for sex allocation and optimal sex change.” The Biological Bulletin 214, no. 2 (2008): 194-202.

I reported the sighting to the Marine Sightings Network, as this is the best way for scientists to monitor changes in the distribution of invasive species, and involve local communities in understanding and protecting the wildlife around them
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