or Maja brachydactyla Balss, 1922
Spider Crab is well disguised by weed and hydroids.
SPIDER CRAB BEHAVIOUR
The Spider Crab, Maja squinado, is, without doubt, one of the most distinctive of the British Crustacea. The crab's size and orange spiky shell render it easily identifiable. The sexes can be separated by the huge size of the male's claws which reach a span of 1-5 metres or more.
When in my late teens I was canoeing off the Erme estuary, I noticed large mounds of Spider Crabs near the rocks of Battisborough Island. The mounds consisted of up to a dozen individuals, mainly female. Crabs could be caught by snorkelling down and picking them up. Even though our initial interest was culinary, we noticed that the crabs had obviously feed feeding on the mussel spat carpeting the rocks at that time. Local people informed us that the crabs were gaffed at low spring tide for later eating.
I later discovered the phenomenon was well known but relatively little studied. The crabs' behaviour has even been recorded on film 'Under the Keel' a film by Laurie Emberson on marine life off the Devon coast made a few years ago.
Over the last few years I have kept a note in my diving log when I have encountered the phenomenon and for those who are interested in seeing it for themselves.
The observations were made on the stretch of coast between Burton Bradstock and Seatown. Shingle beaches slope into about 7 metres of water which does not deepen significantly for several hundred metres. The seabed consists mainly of sand with mud patches and low reefs of slabby blue has rock
Crabs can be seen from early July on but their numbers may reflect the sea temperature. In early July 1987 there were many crawling over the Seatown reef. A few days later at Burton Bradstock many others were seen particular on the mud patches at the seaward side of the reef. These were mainly females. Male crabs were noted to be fighting and one was seen after seemingly fending off another male, to pick up a female. Another male was noticed carrying a female on its back using its hind claws.
It is worth noting that the
reef was covered in a carpet of tiny juvenile mussels.
In mid July 1990, many moulting male spider crabs were noticed off Seatown beach mainly near the shore at the junction of sand and reef.
In mid August 1991 at Burton Bradstock many very large freshly moulted male spider crabs were seen on the reef, which was again carpeted in mussel spat. Nine days later I dived at the same site and found the crabs gathered in mounds of 8-10, mainly males. This time I had my camera and was able to record the phenomenon for myself.
Finally an undated observation but which I suspect was late June early July, about 1982, was of hundreds of grapefruit sized female Spider Crabs off Seatown beach - all freshly moulted.
What are we to make of this phenomenon? The late moult last year may coincide with the rather bad summer we had. Is moulting encouraged by the presence of food, and do crabs gather at particular sites rather like frogs spawning? Do they breed and moult at the same time? Why, sometimes, are the crabs predominantly female and at others predominantly male?
Large rocky area (Lydstep Point) with large numbers of spider crabs (about 4 per sq metre ) and denser in gullies, small caves etc... I've often seen this kind of thing snorkelling.
However...... I came round
the point and across a small sandy bay where there were many swathes of
spider crab aggregations. They weren't piled up in a mound, just
a single layer (with the odd one on top of another - but not piled up high).
Was this the precursor/aftermath of a mating mound, or an alternative method?
The Spiny Spider Crab, Maja squinado, is an effective predator, able to open small mussels to feed on them, it feeds on tunicates and starfish, carrion, and is able to capture and eat small fish amongst the rocks at night. AH.
remains of the female Spider Crab. These remains are occasionally washed
up on Sussex shores.
of a Spider Crab washed up on the beach in October
Katherine Hamblett found the remains of a Spider Crab, Maja squinado on Lancing beach, Sussex, on 20 August 2000.
This dead specimen found by Katherine Hamblett is missing a few claws and legs.
Report by Jason Koen6 July 2004
Whilst walking along Gurnard Beach on the Isle of Wight (near Cowes) with my sons we found hundreds of dead Spider Crabs along the shore line washed up with all the seaweed. Originally we thought it was just the shell possibly from a mass moulting but we found whole carcasses with legs and eyes intact.
Report by Jane HillComment: this mass mortality was probably as a result of the recent gales coinciding with the breeding aggregations when the Spider Crabs come inshore, even into the intertidal zone.
Report by Peter Glanvill
Booth Museum of Natural History, 194 Dyke Road BRIGHTON BN1 5AA U.K.
John A. Cooper, Visitor Services Manager (Natural Sciences)
Gerald Legg, Keeper of Natural Sciences
The most impressive report I've heard about was that fishermen are now catching spiny spider crab (Maja squinado) off Whitby, on the east coast of England.. In the past I would have put this down to a mis-identification of Lithodes.
I sent the following message to the Crust-L list on Saturday and received the reply below.
I received an excited call
late last night (Friday night) from a fisherman who found eggs in a large
male spider crab, Maja squinado. The fisherman said the crab
had all the external morphological features of a mature male crab... large
chelipeds, narrow abdomen etc. He was eating his supper and had eaten the
meat from the chelipeds and walking legs when he pulled the thoracic sternum
away from the carapace to find eggs on the right side of the body. He has
saved the remains of the crab for me. I will look at it today.
Reply from: Cédric d'Udekem d'Acoz [firstname.lastname@example.org]
It is not the first record
of gynandromorphism in Maja. Several years ago, two gynandromorphic
were found in the area of Dinard, France, Western English Channel. I have
seen one of these specimens. It had not a full external male morphology.
It had a normal male morphology on one side and a normal female morphology
on the other side. His abdomen was therefore totally assymetrical. The
specimen was not ovigerous when I saw it.
I do not think it is particularly unusual as most fishermen (Sussex) have mentioned Spider Crabs which they have been unable to sex.
There are numerous other
rhizocephalan barnacles parasiticising other crustaceans, notably a species
called Clistosaccus paguri infecting the Common Hermit Crab, Pagurus
bernhardus, the genus Peltogaster infecting other hermit
crabs and Drepanorchis neglecta, that infects spider crabs.
A review of the species complex around M. squinado was able to differentiate between specimens from the Mediterranean Sea and those from the Atlantic, and concluded that the Atlantic specimens were a separate species, called Maja brachydactyla Balss, 1922  . The specific epithet squinado derives from the Provençal name for the species — "squinado", "esquinade", "esquinado" or "esquinadoun" — recorded by Rondelet as early as 1554  .
The Giant Spider Crab, Macrocheira kaempferi, from deep water in the Pacific Ocean near Japan, have been recorded with a 35 cm (14") wide body and a claw span of nearly 4 metres (12 ft.). It has got a small body with very long legs.