Rare visitors washed up on Cornish beach – Columbus crabs
10/10/2008 20:24:49
Columbus crabs. ©Aimee Crossley/National Marine Aquarium.
October 2008. While wandering on Summerleaze beach, near Bude, North Cornwall District Council Beach Ranger Alan Coltart found a piece of aluminium wreckage covered in goose barnacles. The rangers keep a regular look-out for the marine life that washes up along the North Cornwall coast and so he went to have a closer look.
As is common with floating debris it was almost completely covered in large fleshy barnacles, but most of these were now dying. However, he spotted something moving around among the limp barnacles and on looking deeper found a few small crabs feeding on the tiny organisms living around the barnacles. Some of these were collected, photographed and taken to the rangers' station. Alan and his colleague, Senior Ranger Jolyon Sharpe, sent these photographs through to Douglas Herdson, the Information Officer at the National Marine Aquarium in Plymouth. Doug was able to identify them as the seldom seen Columbus crab, which is more usually found in the Western Atlantic. James Wright from the aquarium travelled to Bude where he discovered that some had died, but he was able to bring back sixteen of the crabs to the National Marine Aquarium, where it is hoped that most will survive and can shortly go on display to the public.
The Columbus Crab
The Columbus Crab, Planes minutus, is also known as the Gulfweed Crab and is reputed to have been first found by Christopher Columbus on his voyages of discovery. It is a small crab with rectangular carapace shell about half an inch across (1 to 2 cm); they have fairly large claws for their size, and hairy fringes to their legs to help them swim. They are variable in colour and pattern from blue grey to rich ruby brown, but often have a whitish patch towards the front of the shell. These crabs spend all their lives drifting in the surface waters, but are not strong swimmers; hanging around and clinging on to floating Sargassum weed, or drift wood, buoys and other articles covered in barnacles, sometimes even on turtles.
Loggerhead turles
Many young loggerhead turtles in the open ocean have been found to have Columbus crabs as hitch-hikers. They sneak in the cosy corner at the back of their shell between the edge of the carapace and the rear flipper. They feed on algae and small animals that try to grow on the turtle as well as waste from the turtle. When the older turtles move into coastal waters the crabs disappear, probably eaten by sharksuckers, remoras and other fish. Cleaner wrasse then usurp their role.
Columbus crabs are very poorly known in European waters, the main population appears to be in the western Atlantic, from the eastern United States to Uruguay, but especially in the Sargasso Sea. An area sometimes referred to as "The Bermuda Triangle".
Columbus crabs with 5p piece. ©Alan Coltart.
The earliest records from NW Europe seem to be of ones found up the west coasts of the British Isles amongst Sargassum weed that drifted to our waters. In the nineteenth and early twentieth centuries they were found from the Channel Islands to Galway, and as far north as Solway Bay. Since then ten were found in 1989 on a floating rope found between Fair Isle and the Shetlands; five on Belgian Coast in 2001; one in June 2002 near Sark and then from December 2006 to January 2007 about 45 were discovered cast up on the strandline in logs and on buoys from Dorset, Guernsey and Cornwall. This present find appears to be one of the largest groups ever reported in northern Europe.
Portugese Man-o-war
It would seem that this bit of flotsam has originated in mid or western central Atlantic and carried its passengers right across to our shores driven by the summer's exceptional and persistent westerly winds. Nearly all previous occurrences of this crab have been in December or January so this is rather early, but August and September saw over twenty Portuguese Men-of-war from the central Atlantic washed up along the southern and western coasts, these are more normally seen in October and November. Thus it appears that this summer's unusual conditions may have brought us more Atlantic wanders than we are used to and earlier in the year than before.
What else is to come? It looks like being an interesting winter for beachcombers, who are welcome to report their findings to the National Marine Aquarium. Already hundreds of tiny relatives of the Portuguese Man-of-war, the beautiful blue but harmless By-the-wind Sailors have been seen on beaches in the north west of Scotland. Little clusters of Buoy Barnacles are possible and even the rare Violet Sea Snail may be next.
