Where did you see your last velvet swimming crab ? It was probably on your most recent dive and it could have been anywhere ! They are usually described as rocky habitat animals and they certainly like reef crevices and hidey holes but two of my recent dives were over a muddy seabed and a rich seagrass bed ; there were velvet swimming crabs in both places .
Something I have noticed more and more recently is how often these crabs can be seen among sea anemones , especially snakelocks anemones and , to a lesser extent , plumose anemones . In some habitats where crevices are scarce , virtually every velvet swimming crab is tucked in next to a large snakelocks , seemingly using them as shelter . The stinging cells carried by the anemone tentacles could well help to ward off the approach of the crab ’ s predators – such as cuttlefish and large fish – and it would be good to know if you have noticed this association in a variety of locations .
This crab ’ s versatility also extends to their diet . They often eat seaweed but , contrary to some descriptions , are most certainly not vegetarians . I have seen them eating barnacles , other crabs ( including a spot of same-species cannibalism ), worms and assorted different molluscs . The hard mollusc shells are a challenge for their relatively light claws , as velvet swimmers are built more for speed than strength , but can be opened with their patience , persistence and dexterity . Look out for them holding the point of a shell with one claw , like an ice-cream cone , while gradually chipping away at the entrance with the other claw . Like so many of their crustacean relatives , velvet swimming crabs are also enthusiastic scavengers .
Even their breeding arrangements are remarkably flexible . Especially in the south of the UK , velvet swimming crabs can breed at almost any time throughout the year – I see couples getting ready to mate , or a female with eggs , on virtually every dive around our local Devon reefs . They are more likely to take an autumnwinter break in cooler northern waters .
As is typical for crabs , the eggs carried dutifully underneath a female ’ s abdomen hatch into larvae that spend several weeks drifting in the open sea . I sometimes see velvet swimmer females using their claws to gently prod and scrape at their mature egg mass , as if to clean it or perhaps even help the offspring on their way . Following development in the plankton , those youngsters develop into miniature crabs ( a few mm in size ) that settle back on the seabed .
Velvet swimming crab pair getting ready to mate
In a sea of snakelocks anemones … a female carrying eggs shelters
This crab has broken open a flame shell
Despite their reputation for ferocity and the alternative name of ‘ devil crab ’, velvet swimming crabs are not always ‘ top dog ’ among their neighbours . Shore crabs of the same size seem able to intimidate them , and they are absolutely no match for a quick-thinking tompot blenny that wants them out of his territory . With an angry blenny attacking their eyes or rear , a velvet swimming crab will soon shrink away apologetically . There is also evidence where they are exploited commercially , that populations are very vulnerable to over-fishing .
Incidentally , this species is living proof that scientific ( Latin ) names aren ’ t the unambiguous ‘ handle ’ we are sometimes led to believe . Since I first became fascinated by crabs , admittedly a very long time ago , it has had four different scientific names ( Portunus , Macropipus and Liocarcinus before Necora if you ’ re asking ) while always being known as the ‘ velvet swimming crab ’! �
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