Hermit Crabs

Hermit Crabs in Torbay

by Chris Proctor

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.

Pagurus bernhardus

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.

Commensals

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 prideauxiis 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

Diogenes pugilator 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.

Two 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.

12 March 2016
A tiny and brightly colourful hermit crab was discovered by volunteers on a Cornwall Wildlife Trust ‘Shoresearch’ survey at a Castle Beach, Falmouth. The tiny hermit crab measuring only six millimetres in length hasn’t been recorded in Cornish waters since 1985. The species which doesn’t have a common name is only known as Clibanarius erythropus (from the Latin meaning soldier, clad in mail with red legs!) This is a southern species which is common in Channel Islands and along the French coast.
Cornwall Wildlife Trust Report