The male pea crab leaves the safety of his green-lipped mussel host when it’s time to mate. He’s looking for a mussel that contains a female crab who’s also ready to mate – but how does he know which mussels contain females? And how does he avoid getting killed as he searches?

Oliver Trottier at the Leigh Marine Laboratory (University of Auckland) has been investigating these questions. His research so far suggests that male pea crabs can ‘smell’ which mussels contain females and that they use an unusual tickling technique to encourage those mussels to open and let them in.

Getting in and out: a dangerous time

The ocean is a dangerous place for a male pea crab. Each time he leaves his mussel host to find a mate, he risks getting crushed by his host mussel or the host mussel of a female crab. His crush risk is about 15% for every journey he makes into or out of a mussel. On top of that, he’s at risk of being eaten by fish such as snapper and spotties. These and other reef fish species are very common around mussel beds as they are hot spots for diverse marine life.

A long wait to get into the mussel shell

The male pea crab’s life is made even more dangerous by his method of gaining entry to the female’s host mussel. Using an infrared camera to film crabs at night, Oliver showed that the male crab can spend hours on the shell of a mussel containing a female, apparently ‘tickling’ it on and off to gently encourage it to open wide enough for him to climb in! The average ‘tickle time’ observed by Oliver was about 3 hours, and throughout that period, the male crab is at risk of being eaten by predators.

Finding a mate through smell?

Because leaving his mussel is so dangerous, it’s likely that the male pea crab has a mechanism for detecting which mussels contain a female crab. Without this, he’d enter too many shells that had no female inside and be crushed or eaten before finding a mate. Oliver’s observations from the ‘tickle’ experiment seemed to suggest this too. He noticed that the crabs spent far more time on mussel shells that had females inside.

Oliver hypothesised that female crabs were releasing a pheromone (a chemical ‘scent’) into the surrounding seawater when they were ready to mate. This would let males know which mussel shells contained receptive females.

Testing the pheromone idea

Oliver set up an experiment to look at whether male pea crabs could detect a chemical released by female pea crabs. He used a plastic tube with seawater flowing through it. Within the tube, he placed two mussels – one containing a male crab and the other a female. The male crab’s mussel was placed downstream to make sure the male crab could detect any scent being released by the female.

Crucially, Oliver included a trap in the tube between the two mussels – so if the male crab detected a pheromone from the female and left his mussel to find her, he would fall into the trap and be stuck there. Male crabs are more likely to leave their shells under cover of darkness, so Oliver set up his first experiment one afternoon and left it overnight. He says he was “pretty stoked” in the morning when he opened the trap to find a male crab inside.

Next steps

Oliver’s next task is to nail down exactly what pheromone the female pea crab is releasing. If the identity of the pheromone can be determined, the pheromone might be used to lower the infection level of pea crabs on mussel farms.

Find out more in the article, Biocontrol of the New Zealand pea crab.

    Published 14 June 2013