THEY CAME FROM BENEATH THE SEA

RUNSWICK BAY

Excellent day on the rocks at Runswick. I arrived at about an hour before low water. There was a persistent breeze rippling the surfaces of the pools, which always makes it tricky to photograph into the water, but the threatened rain never materialised.

Sheltering under stones there were several 'berried' crabs carrying their clutch of eggs against their abdomens. If you find one with a smooth, yellow-brown, soft lump under the body which doesn't look at all like berries, it is the parasitic barnacle Sacculina carcini or one of it's relatives. These barnacles do not have the hard plates that surround their rock dwelling cousins. They are just a soft lump of tissue which extends itself into the crab's tissues.

A female crab with her eggs held under her abdomen 

Sponges are among the simplest of animals. They do not have seperate tissues and organs and if forced through a tight mesh, the broken pieces will reform again after a short period of time into many small sponges. Oscarella lobularis is a beautiful encrusting sponge, and this one was found beneath a large stone in one of the rock gulleys. 

Oscarella lobularis alongside another sponge Hymeniacidon perleve

Oscarella lobularis (detail)

Under the same large stone as the sponges was this beautiful brittle-star Ophiothrix fragilis. The species can be identified by its large radial shields which are triangular in shape and extend up to ⅔ of the central disc's radius. Brittle-stars are Echinoderms, and like their starfish relatives exhibit five-fold symmetry.

They are often found alongside sponges and other sessile organisms. As their name suggests, they break very easily and it is best not to handle them. Far better to photograph them and then carefully return the stone back to its original position.

 The brittle-star Ophiothrix fragilis

These two fuzzy blobs are colonies of  the ascidian Botrylloides leachii. Known as sea squirts, these tiny creatures form colonies in which individuals are clothed within a common mass of tough, jelly-like material called a test.

The larval stage of sea squirts is tadpole-like and has a notochord (a flexible, rod-like structure) and a dorsal nerve cord. These characteristics are essentially the first stages of vertebrate evolution, so although sea squirts look nothing like fish, birds, mammals, or indeed us, they are in fact our very distant relatives.

Two colonies of Botrylloides leachii