Spider crabs at Donegal Point: the animals that garden themselves

You will swim over a dozen of them at Donegal Point before you see one. That is the point of a spider crab — it does not hide under a rock, it makes itself into part of the reef and then stands there in plain sight.

What it is

The one you meet here is Maja brachydactyla, the European spider crab — for years lumped in with the Mediterranean Maja squinado until the two were separated. Reddish-brown, a domed spiny shell up to roughly 20 centimetres across on a big one, and a leg span far greater than the body would suggest.

Compared to an edible crab it looks fragile, and it is slower and far less inclined to argue. A brown crab squares up. A spider crab mostly just stops and hopes you have not noticed.

The gardening

Spider crabs are decorators. The shell carries small hooked bristles that work like the hooked half of velcro, and the crab actively picks up bits of seaweed, sponge and hydroid, trims them, and presses them onto itself until they catch and keep growing. It is deliberate, it is done with the claws, and the crab reworks the arrangement as it goes.

The result is a living animal wearing a garden that matches the ground it happens to be standing on. It is one of the better camouflage systems in the sea and it is the main reason you keep missing them.

Look for the legs, not the crab. The shell is disguised; the long jointed legs against the rock are what give it away.

Walking out of your own body

A crab cannot grow inside a rigid shell, so it has to leave it. The old shell splits at the back and the crab reverses out — and it takes everything with it, not just the body armour: the legs, the claws, the eye coverings, even the lining of the gills and the front of the gut. What is left behind is a complete hollow crab, which is why people find perfect empty spider crabs on the shore and assume something died.

The new crab comes out soft and defenceless and takes days to harden. It is at that point that everything on the reef would happily eat it — which explains the next part.

The heaps

Spider crabs gather in numbers to moult and to mate, piling into mounds — sometimes hundreds of animals deep in a heap. The bodies on the outside take the risk and the soft ones in the middle are protected while they harden. If you drop onto one of these it is genuinely startling: a mound of moving legs where there was bare seabed a week earlier.

They also move seasonally, coming into shallower water in the warmer months and heading deeper for winter, which is why they seem to appear on a site and then vanish.

A note on taking them

Spider crab can only by fished commercially in Ireland and most of it goes for export. Scuba divers are not permitted to take Crabs, Lobsters or Crawfish.

Donegal Point is one of the sites we dive regularly from Kilkee. Conditions are live on our home page, and if you want to see this ground properly, come and talk to us on the East Pier.