Another Life: king of crabs has a long shelf life

A windowsill lined with the bones of the beachcombed dead may be the devil to keep dusted, but its ossuary oddities still beg to be picked up, however carefully.

Among them, the jawbone of a monkfish, lined with needle teeth, suggests a necklace for medieval torture or masochistic penance. A tiny curlew’s skull, with fragile, probing arc of beak, speaks of a bird that lived on its nerves. The vertebral discs of a dolphin, marvellously carved as if from ivory, could be counters from some ancient Chinese board game.

Dwarfing them all is a prickly carapace the size of a baby’s head, one eye still jutting on its stalk, together with a side leg of 32cm and a knobbly front pincer claw of 50cm – some spider crab, indeed.

Paramola cuvieri, the giant box crab, is the biggest to be found around these islands, with a total span of up to a metre. It is most at home on the slopes of the continental shelf, around 1,000m deep. However, a few have been caught in shallower fishing trawls and displayed, to dropping jaws, in west coast aquariums.

In 2012, one was netted from a mere 70m by fishermen trawling for turbot a mile off the stags of Inishbofin. Another, caught off Achill, lived for some days in Galway’s Atlantaquarium, then died in its tank, perhaps from sheer stress, or from missing a proper weight of water over its head.

Mass strandings

My windowsill remains of P cuvieri were washed up on the strand years ago. What took me back to study their formidable form was a letter from a reader, Paul Bray, with his photograph of scores of spider crabs washed up on Donaghmore Beach, just south of Courtown, Co Wexford. What, he wondered, could explain such a mass stranding?

These were not, of course, specimens of the giant box crab, but the more familiar, if morphologically related, spiny spider crab (Maja brachydactyla), its carapace and claw-span about half the size. Often prettily studded with barnacles, its prickly shells make tempting beachcomber's trophies. Several fine ones, indeed, have hung here on my walls.

The spider crab winters in deep water, at perhaps more than 100m, and migrates to shallower depths as sea temperature rises. Near the coast, the crabs often cluster together in a dense, prickly carpet or even pile up into heaps. Some online photos show amazing pyramids of the Australian species. In August 1997, a huge mound of spider crabs, estimated at some 50,000, was reported off Dorset in the English Channel.

Mating and moulting

What the crabs are actually doing, and in what sequence, is not always clear from the scientific studies. Even the composition of the heaps, between adults, juveniles and sexes, high-ranking or low-ranking, seems to vary from place to place. But the crabs are certainly both mating and moulting, if perhaps not at once, and doing either might be safer in a heap against predators (octopuses, seals, rays and more).

Like any other crab, the spiny spider goes through several moults as it grows bigger. A somewhat disturbing video filmed in a Californian aquarium, telescoping a 40-minute process, shows the crab withdrawing every last morsel of its body from its shell and claws. The empty shells can be washed ashore in numbers.

In other instances, like that at Courtown, the crabs are simply belly-up and dead, suggesting some local spot pollution or communal disease.

Patience to poke

The flesh of spiny spider crabs can be very good to eat (gourmet Hugh Fearnley-Whittingstall prefers it to lobster) – if one has the patience to poke it out of the long legs. This makes trapping them a major industry in Spain, Portugal and France, but there has also been in a busy export from Co Kerry, set up since 1981 on the initiative of Bord Iascaigh Mhara. In what's known as the Magherees fishery, a score of half-decker boats set thousands of pots in spring and early summer, up to 20m deep in Tralee and Brandon bays.

These are at the northerly edge of the spider crab’s range, with a lower rate of reproduction, but the species could benefit from climate change. One concern has been a sharp decline in big male crabs caught in tangle nets, but all spider crabs in the bays are likely, it seems, to be captured at least once a year. The smaller ones are thrown back to moult a few times more.

Dr Edward Fahy, who led research on the crabs for the Marine Institute, commented on the Bord Iascaigh Mhara initiative in his recent book, Overkill. Developing a fishery of such low tonnage and value, he concluded, "was a signal that the more lucrative possibilities of the territorial sea were close to exhaustion".