Rights: © Copyright 2013. University of Waikato. All rights reserved. Published 20 June 2013 Download

At mating time, male New Zealand pea crabs leave the safety of their host green-lipped mussel to search for a female. In this video, Oliver Trottier (Leigh Marine Laboratory) describes how he used infrared video cameras to document the behaviour of male crabs as they attempted to enter mussels containing female crabs. Oliver observed the crabs ‘tickling’ mussels, sometimes for hours on end. He thinks the tickling may stimulate a mussel to open wide enough to let the crab in.

Jargon alert:
The inhalant siphon is the structure through which water flows into the mussel.
The mantle edge is the frilly structure around the outer edge of the mussel’s flesh.

Focus questions:
What problem did Oliver face before he could begin observing pea crabs?
Oliver thinks that the male crab’s tickling might encourage the tickled mussel to open more widely. How could he test his hypothesis


Oliver Trottier (Leigh Marine Laboratory)
Male crabs have a bit of stage fright – they won’t come out if you leave the lights on – so after about a week of fiddling with that, I started to film in night vision. It turns out that, when the lights are out, the male crab would actually come out of his host for me.

Once the male pea crab gets onto the female pea crab’s host and he’s sitting there at the inhalant siphon, which is where he makes entry into the mussel, it was a really interesting behaviour that I started to observe. He might tickle the mussel’s mantle edge.

It can be a long time while he’s waiting there, and he might tickle for a while and wait. He’s got to wait for that mussel to open, it’s not really much he can do. I think the tickling does have an effect on the mussel, it might actually cause it to go “Ohhhh” and open even larger than it would normally but I haven’t sort of proven that yet, but he could be there for hours.

Oliver Trottier – Leigh Marine Laboratory, Auckland University.