Steve Ludwig, of St Pauls School, asks :-

I found a small white crab inside a wild green lip mussel. Is this a parasite or does it have a symbiotic relation with the mussel?

Kevin Heasman, an aquaculture scientist at Nelson's Cawthron Institute, responded.

The Crab you found is the Pea Crab (Pinnotheres novaezelandiae) and it is essentially a parasite on the mussel. Some people however considered the relationship to be commensal (living together for mutual benefit) with the mussel.

The crab lives in the mussel cavity but does not eat the mussel. It actually feeds on the food that the mussel is gathering. So it does not harm the mussel and lives with it (commensal) but the mussel gains no benefit and actually experiences a net loss of food (parasitic). The mussels that are infected with Pea Crabs do not show any noticeable reduction in weight or any ill effects.

This species of Pea Crab is found only in New Zealand including the Chatham Islands. Although it is most often seen living in the Green Lipped mussel it is also found occasionally in Oysters (Crassostrea gigas), Ribbed mussels (Aulacomya ater) and Blue mussels (Mytilus edulis). Some researchers have found infection rates of mussels can range from 0% to as high as 70.3%. The occurrence of the crab in farmed mussels appears to be less than of wild mussels and it is not of any great concern to mussel farmers.

The crab can grow up to 20mm in size and is generally proportionate in size to the mussel, (i.e. a small mussel has a small crab).

The crabs are solitary therefore it is very rare to find more than one crab in a mussel. This obviously generates challenges for the crabs to breed. If the truth be known, researchers are still not absolutely sure where breeding takes place. Some suggest that the crabs may leave the mussel host and get together in the water column then return to the host. Others suggest that the male will track a ripe female to a particular mussel, enter it and breed with the female in the mussel. Future research will tell which method the crabs use or possibly show it to be a combination of both methods.

The crabs appear to breed all year round but mainly from August to March. They can produce up to 10,000 eggs at a time and may produce eggs more than once a year. Egg incubation takes 2 months or more depending on temperature and there are several plankton (the larvae live in the water column) stages before they take on the shape and lifestyle of an adult. The adult also moult throughout their lives and can reach maturity in less than a year.

The crabs are quite edible. In fact some people hope to find them in their mussels and eat them raw or from a cooked mussel.