Wednesday, 19 April 2017

How to avoid predators in a rock pool

I was snorkelling at Murrays Beach (Booderee National Park, Jervis Bay, New South Wales) last weekend and it was fabulous with warm clear water and a blue sky. The rocky shore dips into the bay creating ribs of reefs where a multitude of sea-life live. The highlights on this trip were cuttlefish hiding under ledges and huge schools of small fish of various shapes and colours.

But I don't bother taking photographs when snorkelling, I like to just watch the life go by. It was only when up top-side and walking along the shore afterwards that I took a few shots of life in the rock pools.

At first, things looked quiet and as though there were no animals living in the pools, until I saw a little movement in the green seaweed. A Sea Hare Aplysia dactylomela was slowly munching its way through the algae. It was only when it crossed a relatively open patch that I noticed it. If it had stayed in the thicker fronds, I would not have seen it.

So, I sat quietly and sure enough other creatures began to move.

A small shrimp twitched. The sea hare depended on coloured skin as camouflage in the weed, the shrimp had evolved a mostly transparent body for hiding from predators. That method allows it hide on any colour of substrate.

Then several shells began to walk across the bottom, but not with molluscs inside them, hermit crabs. They have adapted to use shells for protection.

And lastly, I noticed a school of small fish. As they were swimming up near the surface they should have been the first things to detect. Their bodies too, were quiet transparent and it was their shadows on the pool bottom that attracted my predator's eye. But, their schooling and safety in numbers appears to be their main defence tactic, which is fine as long as there are others in the school slower than oneself. There are twelve in the photograph, how many were there the next day, or the day after. There were White-faced Herons about.

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