Environment – The slater puzzle

The next time you are at the beach at low tide have a look in some of the little rock pools and you are likely to see a unique NZ creature. Even though this animal is abundant and only occurs in New Zealand, it surprisingly does not have a common name, only a scientific/Latin one – Isocladus armatus. This common marine isopod (or slater) can be easily seen at Stanmore Bay and many other beaches on the Hibiscus Coast.

If I were asked to name this animal, I would call it ‘the variable slater’ because of the diversity of colour patterns it displays. You could be forgiven for thinking there were lots of different species, but in fact they are all the same. There are white ones, striped, spotted, black, green and reddish ones – and everything in between. In the classic 1968 book The New Zealand Sea Shore, Morton and Miller described this species as “hardly two individuals are alike”.

Such diversity within a single species begs the question: why are they coloured this way? Why has evolution by natural selection generated so much diversity in this species? We currently study these isopods at Massey University. Their colour patterns are all about camouflage. Some match the background, some use bold patterning to make their outlines hard to see, and some pretend to be plain objects like shell fragments.

But when we think of how evolution works, we typically expect those individuals best suited to survive will pass on their traits to the most offspring. Why hasn’t natural selection resulted in all isopods having the same, most effective camouflage pattern?

Although this is still an unsolved mystery, our best explanation is that the most advantageous colours and patterns are those which are the most different than the rest. If isopods that have unique patterns survive better than more common types, the result is an incredibly variable population. Why would being unique provide an advantage? Well, it’s all about how the minds of their predators, fish and birds, work.

The predators hunting Isocladus have a difficult task because the prey blends in so well with the background. But as an individual predator gets more practice catching them, the predator starts to specialise in the more common colour varieties. They form a ‘search image’ in their brain that they are looking for – just as you would have the image of a red bottle in mind if you went to the pantry for tomato sauce. So those isopods with the most different patterns survive best. They can hide in plain sight because the predators don’t recognise them as prey – yet.

Hibiscus Matters welcomes new Environment column contributor James Dale. James is a naturalist who lives on the Whangaparaoa Peninsula and is Professor of Zoology at Massey University’s Albany campus. He will use his expertise to reveal some of the amazing creatures that live on the Hibiscus Coast.