To mate, a male pea crab must leave the relative safety of hismussel to seek out a female. In doing so, he is at risk of death by crushing (between the two shells of a mussel as it closes) and by predation (usually by fish). In this video, Oliver Trottier (Leigh Marine Laboratory) describes the risks taken by male pea crabs in search of a female.
Oliver Trottier (
Every time that crab comes out of the , there’s a likelihood that he’s going to get killed. One of the things that’s been observed is that the ratio of male to female crabs being born is actually equal, but the population of crabs within a farm or a wild population of the New Zealand pea crab is 90% female and only 10% male. And where we think that large population skew is coming from is the males are either getting killed coming in and out of their hosts or they’re getting preyed upon in search of a female, so that’s how I know it’s risky behaviour.
When a male will approach a female, the main risk is going to be getting crushed by the female’s hosts when he’s making his way in. So a mussel will close probably anywhere to about that fast, and the male crab, of course, he has to avoid that as best he can, so he has to make a decision whether he’s going to go for it and go in or sort of abandon ship and come out. And inside the mussel, there’s a bit of a fringe for the mantle edge, and that’s not something that he could probably just push his way through easily, so more often than not he probably bails out than bailing in.
Oliver Trottier – Leigh Marine Laboratory, Auckland University.