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The Spider Crab -  Is it an Invasion, or just a Seasonal Bounty?


Recently the television crew from BBC Wales was in both New Quay and Aberaeron to report on a supposed 'invasion' of Spider Crabs into the shallow waters of Cardigan Bay.  They spoke to local fisherman David Winston Evans in New Quay and Fishmonger Will Willis in Aberaeron (see the Seafood of Cardigan Bay). 

Mr. Evans spoke of his annoyance of the fact that the Spider crabs can reach the bait in his Lobster pots with their long claws and it was noted that 'lobsters were being frightened away from the Lobster pots'.

Spider Crab - Maia squinado

 

 

Often the Spider crabs form large mounds containing hundreds or even thousands of crabs, with females on the inside. This is thought to be  a method of protection against predators especially after moulting when the shells are still soft. This crab's carapace and legs are colonised by Acorn Barnacles - Balanus balanoides. (photo Rod Attrill)

Tourists to the area however, are unlikely to ever see a Spider Crab except in the fishmongers or in a Restaurant. Although the crabs come into shallower waters in the summer months, they are not found in the intertidal zone or on the beaches.

The Common Spider Crab is know scientifically as Maia squinado. Its range extends from France and Spain where it is commonly harvested,  up as far as the west coast of Wales - close to its northern limit.

Spider Crabs live in deeper water in the winter - in depths of up to 120 metres, but come closer inshore in the early summer months as water temperatures increase. There is no doubt that such a migration would tend to result in a greater population density closer to the shore in the summer. Winston Evans, New Quay fisherman, says they are present in numbers from about the middle of May for some three months.

The succulent leg and claw meat of the Spider Crab is prized on the continent and is now served in local restaurants as the crabs have increased in numbers. 

Hugh Fearnley-Whittingstall writes in the 'River Cottage Cook Book':  'The meat is sweet and beautifully textured, which is why the Spanish, who appreciate such things, will pay more for a large Spider Crab than for a Lobster of the same weight. I am a total convert, and rate them as even better than Brown Crabs.'  There is a recipe for Spider or Brown Crab Linguine on Page 336 of his cook book. However, removal of the meat can be a tedious process!

Crab Legs

Like other crabs, the females carry the eggs  - up to 150,000 , around with them under their abdomen which is tucked underneath the carapace or shell. Most females are 'berried' or carrying eggs by May. The eggs take from 60 -75 days to mature at which time a tiny larva hatches out and swims up into the plankton. 

The reason that crabs and other marine animals with planktonic larvae lay so many eggs is that there is a greater chance of wider distribution of the species. This leads not only to improved territorial coverage but to greater mixing of the gene pool within the species. The down side of this is that the vast majority of planktonic larvae become food for other animals - both  planktonic predators and filter feeders on the sea floor. In the tropics, coral reefs feed mainly on plankton.
The larvae are firstly known as Zoea (see drawing above). they later become somewhat more crablike in the Megalopa form (see drawing on right). 

Altogether, they spend some three weeks floating about in the plankton at the mercy of the ocean currents until they change into small crabs and begin life on the sea floor. If they survive their first year, they will grow to a carapace or shell width of seven cms in that time.

There is much speculation as to why the crabs are being found in such large numbers this year. It is well known that Spider crab numbers can fluctuate wildly from year to year, and this could well be part of a cyclical population trend. However, Dr. John Fish of Aberystwyth University is quoted in the Times as saying: 'I have seen more Spider Crabs on the beaches of Cardiganshire this year than in other years'.  Keith Stone, North Wales Sea Fisheries Officer, said 'It is rare to find them so far north. It could be because of global warming, ocean currents or changing migration patterns or they could be adapting to colder water conditions'.

There is also the possibility that fishing practices have resulted in a change of population numbers in the other large crustaceans that might compete with the Spider Crab.

Whatever the reason, the Spider Crab represents a bountiful natural resource that is more likely to grace the plates of local restaurants in years to come.

 

İRod Attrill, 2003