Hermit Crabs in Torbay
Of the fifteen or so British species of hermit crabs, a number may be found on the shore or in shallow water. The best identification guide is the AIDGAP crab key, which lists eight species likely to be found by divers or rockpoolers: the others are either rare or live in deep water. So far, Janet and I have found four species in Torbay, and our observations of them are described in the notes below. Another two might reasonably be expected to be present here and elsewhere on the south coast, and are briefly noted.
The most widespread species is the Common Hermit Crab, Pagurus bernhardus. This is the largest British hermit crab, reaching a size of around 15 centimetres across the legs. It can be identified by its size, and by the large right claw, which is covered in tubercles and has a distinctive red and cream banded pattern. Small individuals sometimes also show bluish tints.
This is the hermit crab that
will be familiar to most readers, being the common find in rock pools and
on sandy shores. It occurs everywhere in Torbay, living at all depths from
pools on the rocky shore to deep water, and happily tolerating almost any
substrate. Specimens found on the shore are generally small. The big ones
normally live below low water mark on soft bottoms, but can sometimes be
found on sandy beaches on low spring tides. Small individuals are usually
quite shy, and retreat into their shell if danger threatens, but the big
ones prefer to run away. When escaping they run backwards with their claws
trailing, and they can show an impressive turn of speed. Stranded individuals
usually settle onto the sand and retract so that it is necessary to pick
up the shell to see whether there is a crab inside.
Pagurus bernhardus has a very rich associated fauna. Small individuals often have a cover of the hydroid Hydractinia echinata on the shell. This hydroid usually lives on hermit crab shells and seems to prefer the smaller ones, perhaps because larger crabs occupy their shells for long enough for a community of other sessile animals to develop, smothering the hydroid. The nudibranch Cuthona nana preys on Hydractinia: we have seen Cuthona once at Anstey's Cove. Larger crabs often carry a rich community of animals on their shells. Some, like the Saddle Oyster, Anomia ephippium, and barnacles, Balanus species, are animals which colonise any hard substrate. The calcareous tube worm Serpula vermicularis forms twisting masses of tubes on the shell occupied by the hermit crab. Serpula seems to prefer shells and also occurs on Queen Scallops, Aequipecten opercularis. Of most interest is the Parasitic Anemone, Calliactis parasitica, which is fairly common in Torbay, on shells occupied by large hermit crabs below low tide mark. This is a species which, like the hydroid Hydractinia, has become specifically adapted to live with hermit crabs. In Britain P. bernhardus appears to be the only hermit crab associated with this anemone. The ragworm Nereis fucata lives inside the shell with the crab. We have found this species once or twice in empty whelk shells cast up on the strand line: clearly the crab is at least sometimes able to change shells and leave the intruder behind.
Hermit Crab with Cloak Anemone
Pagurus prideauxi is a medium sized hermit crab, reaching a leg span of around 6 centimetres. The claws are a dull pinkish, and the legs are striped with orange, purple and white, as if it were wearing rugby socks. A diagnostic feature is the eyes, which are large and have a bright red spot just behind them. However the most useful field character is that P. prideauxi almost always occurs with the Cloak Anemone, Adamsia palliata. These two species appear to form an obligate symbiosis in which neither can survive for long without its partner. In all but small individuals the anemone has overgrown the shell and provides the main protection for the crab. The crab is unable to retract completely and must rely on running away, and the defence provided by the anemone. Adamsia produces acontia (stinging threads) very readily, which must enhance this defence. P. prideauxi is almost exclusively sublittoral, though it may occur in shallow water. It seems to live on a variety of bottoms, usually with stony ground nearby. In Torbay we have seen large numbers on an eel grass bed near Brixham. Pete Glanvill has also found it at Thatcher Rock, and it is sometimes washed up at Paignton.
South-claw Hermit Crab
is easily identified since it is the only common British hermit crab in
which the left claw is larger than the right. Other useful features are
the bluish pink colour, and the hairy antennae. It reaches about 4 centimetres
across the legs. Diogenes lives on sand flats at low water mark
or just below, and in such places it may be commoner than Pagurus bernhardus,
with which it usually occurs. On sandy beaches in Torbay, it can be quite
abundant at lower shore levels, although it is rarely seen. The trick is
that Diogenes burrows at low tide. The best way to find it is to
paddle as the tide is going out, when the crabs can be seen running about
in the shallow water. As they are exposed they dig straight down, disappearing
completely within a minute or so, and remaining hidden until the tide returns.
There is much suitable habitat for this species on the sandy beaches around
Torbay and it appears to be generally distributed: this could well be the
most abundant species after Pagurus bernhardus.
The Hermit Crab on the left is Diogenes pugilator with the major claw on the left.
Other Hermit Crabs
The fourth species, Anapagurus hyndmanni, is much smaller than the others: a large one only has about a 2.5 centimetres leg span. The right claw is large and bulbous, and white or sometimes a pale orange in colour. The legs are long and banded with white and dull brown, and the antennae are hairy. This species is only usually exposed on the lowest spring tides and appears to be ill adapted to shore conditions: unlike the other species, A. hyndmanni carries on crawling about when exposed, despite the risk of desiccation. It only retracts into its shell if it is picked up or otherwise disturbed. A. hyndmanni occurs on stony ground, often under boulders, but sometimes crawling about in full view among the weed. It is most common at sheltered sites, where it may be abundant, but occurs widely throughout Torbay. This species can also be found in shallow water in the kelp zone. The shore habitat suggests it probably normally lives under the kelp, but underwater it is more easily seen on areas of rocks and sand at the edge of the kelp beds.
more small species occur in shallow water in South Devon. Pagurus cuanensis
is densely hairy, with matted brown hair covering the right claw, and legs
banded with reddish brown and white. Anapagurus laevis has red and
white banded legs and a red right claw, and hairy antennae. Neither has
yet turned up in Torbay but it would be surprising if they are absent,
since both are common and generally distributed. Two more species are listed
in the AIDGAP guide but these are of restricted or northern distribution
and are less likely to occur in Southern England.
CROTHERS, J. and CROTHERS, M. (1983), A key to the crabs and crab-like animals of British inshore waters. Field Studies Council AIDGAP guide, reprinted from Field Studies 5, 753-806.
British Hermit Crabs (Link)
Hermit Crabs for the younger reader
Studies Council Publications
Use these links if your are familiar with the scientific classifications of marine life