Castrator pea crabs live up to their name. They live inside the sex organs of marine molluscs and prevent them from reproducing. But it turns out the pea crabs’ parasitic ways also make it terribly tricky for them to find a mate.
Castrator pea crabs () are tiny parasitic crustaceans found off the east coast of South America, from southern Brazil down to Argentina’s Valdez Peninsula. They spend most of their adult lives in the sex organs of various .
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“They castrate, which means that they halt or stop the reproduction of the snail they use as host,” says at the National Council of Scientific Research in Mar del Plata, Argentina.
Pea crabs only seem to parasitise female limpets. These molluscs normally store their eggs in their sex organs, but won’t lay eggs if they have a resident pea crab. It may be that the weight of the pea crab misleads the limpet into believing it already has eggs.
However, the pea crabs don’t eat the limpet eggs. Their diet consists entirely of a green mucus, which the limpets make out of phytoplankton that they filter from the water to feed themselves.
There is just one problem with the pea crabs’ lifestyle: it is hard to find a mate when everyone is hiding inside a limpet’s gonads.
Juvenile pea crabs live in the open ocean and only settle down in limpets when they are older. Since females then stay put inside the limpets, researchers previously thought that they must be mating as juveniles and storing sperm for later.
To confirm this, Ocampo and his colleagues examined females using a scanning electron microscope. But they found that the sex organs of juvenile female pea crabs cannot store sperm.
There have also been observations of males leaving their hosts and wandering about. Ocampo says it’s likely that males abandon their own hosts to look for females.
“The female lives inside the host and waits for the male pea crab to copulate with them,” Ocampo says.
The finding implies that limpets hosting a male pea crab may be better off than those with a female, as a male is much smaller and may ultimately leave.
It would be interesting to see whether the females are “picky about accepting sperm from males that come knocking on her host,” says at the University of New England in Australia. He says females may not entertain males in their choice abode, or may discard their sperm after the deed is done.
Journal of Morphology
