Pea crabs: male vs female

Male and female New Zealand pea crabs differ in their size, shape and colour. In this video, Oliver Trottier (Leigh Marine Laboratory) uses live crabs to demonstrate the key differences, and explains how the male crab’s small size, flattened shape and mottled colouration are all important for his survival.

Jargon alert: Cryptic colouration is a pattern of colour that works to disguise an organism and protect it from predation.

Focus questions: What are the key differences between male and female pea crabs? What is the purpose of these differences?

Teaching point: Students could research the concept of sexual dimorphism (the different appearance of males and females of the same species

Transcript

Oliver Trottier (Leigh Marine Laboratory) Here I’ve got a male and a female. The male pea crab, he’s cryptic – he’s got all sorts of colourations that help him camouflage into the background, and the reason for that is he has to come out of his host and search for the female, so if he’s coloured the same as the background, it’s easier for him not to get picked off by predatory fish. He’s also dorsoventrally flattened, which means he’s sort of flattened like a bit of a pancake. The reason for that is, if he’s got to make his way into and out of host mussels, it just lessens the chances of him getting crushed.

This is the female here, and you can see she’s gigantic compared to the male. She’s a lot larger and rounder – the majority of her body representation comes from these eggs that you see down here. She has no colouration because they don’t actually leave the hosts so she doesn’t need to have that protection that the male does.

Acknowledgements: Oliver Trottier – Leigh Marine Laboratory, Auckland University.