NatureToday | Hosts shape tiny pea crabs

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Biologist Werner de Gier, PhD student at Naturalis and the University of Groningen, studies the evolution of crustaceans that have a separate lifestyle: the cohabitants. They have evolved from walking or swimming shrimps and crabs to living in or on a host. One of his study groups is the pea crab family (Pinnotheridae).

Round and thick

In the Netherlands we also have a few types of pea crabs, such as the tiny crabs that you sometimes encounter in the mussels on your plate (Pinnotheres pisum). Werner explains: “Especially the females never leave their host, so they have evolved into round, fat crabs that can no longer walk normally.” In hosts such as mussels and oysters, the crabs often eat with the shell, or collect the hosts’ protective mucus. To live unnoticed in the shells, pea crabs often have developed a soft carapace.

“’Our’ pea crabs are mainly round shaped by the choice of host, but much more is possible,” explains Werner. “A few examples: extremely broad-stretched crabs can be found in tubular habitats, such as the corridors of worms and lobsters. Angular, flattened crabs can be found among the spines of sea urchins; and elongated crabs with a sharp ‘nose’ make themselves at home in sea squirts.”

Same problem, same solution

Using coordinates placed on drawings and photographs from the scientific literature, Werner has shown the diversity in forms and hosts of the crabs. The data was accompanied by added photos and CT scans of indispensable collection materials from Naturalis. Adding a phylogenetic tree, a kind of family tree of the crabs, made it possible to see which species resemble each other, and how they have ‘grown towards each other’ in evolution.

In this case, pea crabs appear to evolve into a shape that suits their host. This also happens if their ancestral species looked very different. “Evolving the same form from different branches of the family tree is called convergent evolution,” explains Werner. “This can be within a family, like here with my crabs, but also between completely different animal groups. Think, for example, of deep-sea fish and squid that have grown gigantic eyes to see in the dark – the same problem, so the same solution.”

“This interaction between form and function, host and inhabitant, species and evolution collectively forms a symphony of nature that can make biologists very happy. But it is also simply relevant knowledge to have,” explains Werner. “The economic impact of pea crabs in mussel farming alone runs into the many millions. Other species are part of the ecology around coral reefs, which are both ecologically and economically indispensable. There really is a whole story behind such a crab in your mussel shell!”

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Text and photos: Werner de Gier, Naturalis Biodiversity Center

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