Crabs?

[RobAdair]

Bought 10lb of mussels this afternoon and when I started to shell them I found a number of tiny crabs inside? I\'m no expert but this has never happened to me before and they all had a distinctive marking on their backs that I had never seen before? If it\'s any help they were from Ireland.

[img=640x480]http://nesa.cibs.org.uk/crab.jpg[/img]

[Edited on 27/10/2005 by RobAdair]

[jonny_mc]

Pinnotheres maculatus

[BriH]

If it\'s any help they were from Ireland.

Look like baby Irish crab to me Rob LOL

But on a serious note had this a few times , i reckon they get hoovered up by mussels ,but then the mussel gets farmed and the baby crab remain insde - as for the markings on the backs , it could be the inards of the crab at a very early stage of growth

[MikeR]

Colwyn Bay mussels also get quite a few small crabs inside them.

[Doc]

Jonny MC was almost right, they are Pinnotheres pisum (Linnaeus), commonly called the pea crab and regularly found inside bivalves, especially mussels.

Its not absolutely certain how they get or remain there, however, thoughts may be that they are drawn in as larvae as the mussel draws water into its body cavity (filter feeding), the larvae evades being digested and escapes into the mantle cavity (inside the mussel), it then feeds on the organic material either as a waste product or other larger material drawn in by the mollusc. It then grows to its normal size.

This may be an example of a synergistic relationship in that the pea crab plays a role of housekeeper, it also may protect the inner soft tissue against other predators such as parasitic copepods, however, in the long term the pea crab does have a deleterious effect on the bivalve by causing gill erosion. This has a knock on effect by reducing filtration (feeding capacity), leading to weight loss and poor conditioning and reduced spawning potential.

As a result the relationship between the mussel (bivalves) and the pea crab is as yet unresolved between parasitic or commensalism.

[SIXFOOTSTEVE]

Excellent answer Doc but couldn\'t the relationship be symbiotic ,it\'s a possibility .

[Doc]

Absolutely Steve, the term symbiotic would have been more apt, given the biological relationship.

[SIXFOOTSTEVE]

Yeas Doc they must be feeding in there and possibly ridding the mussels of something unecessary while causing no damage.
Good post Rob ,I\'ve not come across these crabs before now.