|THE YEAR IS 1976 AND ITS MY FIRST VISIT TO THE RED SEA. We are camping at Ras Mohammed and trudging towards the beach by torchlight in the hope of seeing flashlight fish in the shallows. |
As we climb the final dune, we become aware of a furious clicking sound coming from the beach. Shining our torches onto the sand reveals thousands of hermit crabs scuttling from the light. It also explains those little circular tracks we find all around the camp and our sleeping bags in the mornings.
Strangely, this was my first real encounter with a hermit crab - and it was on land, to boot.
I have since developed a fascination for this species and always make an effort to spot them wherever I dive.
Hermit crabs carry their mobile homes on their backs and can be found all over the world. While many people believe they all look the same, there are in fact 600 species.
They are, of course, a member of the Crustacea class, but where other crabs, lobsters and shrimps develop their own protection with a hard outer shell, the hermit crab appears to have managed to evolve only halfway through this process.
The front half of the animal, including its head, claws and back, is encased in a hard shell which it moults as it grows, just like other crabs. However, the abdomen is soft and never develops this armour, which is why the hermit needs the additional protection of a secondhand shell from a gastropod. The sides of the soft abdomen have a rough surface for gripping the inside of the shell while it moves about looking mostly for carrion on which to feed.
The problem with the shell is that it does not fit for life and the hermit will generally look for a new model just before a moult.
Making the move can be quite a challenge and there can be great competition for the ideal home. Not only must it be the right shape and offer sufficient growing space for several moults, it mustnt be too heavy to tote around the reef every day.
You will often encounter movement from a large shell on the seabed which, when investigated, will reveal a small hermit crab struggling to haul it along behind.
This is probably a sign of a very competitive housing market, although females often choose a larger shell for carrying eggs.
A number of species have one claw larger than the other, and use this as a front door to their home. The claw is moulded to the shape of the shell opening to provide a perfect seal, but this moulding can be achieved only after a moult, when the claw is soft and can be shaped to fit the entrance as it hardens.
There is one exception to this behaviour, and that is the specialised hermit crab Parugrita, which uses a crack in the reef or a discarded tubeworm cast as a home.
This species is therefore immobile and has a specially adapted pair of antennae which it uses to collect plankton borne by the current. It also uses the claw door trick for added security.
There are also instances of symbiotic relationships between hermit crabs and other species. The most common sight in British waters is that of the common Pagurus prideauxi hermit carrying one or more anemones on its shell. This relationship serves both parties well, as the anemones offer protection to the hermit crab with its stinging cells, while the hermit in return offers mobility to the anemones and the chance to share meals with the crab.
To persuade the anemones to adopt their new home, the hermit has to practise a bit of foreplay. It strokes the anemone to relax it and prevent discharge from the sting cells before peeling it off a rock and placing it strategically on its shell.
Some hermits prefer lighter-weight protection and will decorate their shells with stinging hydroids, which offer similar protection, but allow the crab to move about much faster.
A less common example of symbiosis occurs when a hermit crab lives on another species. You will often find species of shrimp living on urchins and starfish in the tropics and, if you look carefully, might also find very small hermit crabs adopting the same behaviour, particularly on starfish.
Obviously the hermit is after a free ride and will benefit from the starfishs leftovers at meal-times, but it is not clear if this is practised only by juveniles in search of protection or if this is peculiar to a particular species.
One of the oddest examples I have seen was a small hermit living on a large anglerfish in Norwegian waters. I encountered this as I was moving towards the angler for a close-up shot of its eye and became aware of some movement close to its mouth.
This movement turned out to be a hermit crab carefully picking bits from the skin of the angler and eating them. As the crab continued to feed, the anglerfish remained motionless, waiting for an easy meal to swim by.
Hermit crabs are active throughout the day on the reef, but they tend to reserve their hunting and feeding habits for night.
On a night dive you are almost guaranteed to find them sifting through a gravelly bottom or coral rubble, although they are also frequently found on the fine silt of river estuaries.
Finding them on the reef is not so unusual, but they are mainly carrion-feeders and will often finish off someone elses leftovers. The reef edge is the best place to spot them.
Some are quite shy and will pull back into their shell and freeze when torchlight strikes them.
Others will treat you with arrogance and just ignore you - no matter how close you come. If you suspect that a shell is occupied, a gentle nudge is normally enough to encourage the occupant to move off cautiously.
Although most hermit crabs are marine creatures, there are one or two species which spend some or all of their time on land.
The largest of these is the coconut or robber crab, which has even managed to evolve a skin tough enough to discard the need for a shell. This species is often found climbing coconut trees in search of its favourite food.
Once again, searching the sandy shores at night is the best time to spot these land-based cousins, although they rarely stay still for long in the light before scuttling back to their burrows.
I have found meeting them in their marine environment far more rewarding. It allows you to get up close and personal with these cheeky-looking happy campers of the reef.
|This hermit has a large claw door to keep out predators |
|this immobile species can make a discarded tubeworm cast home |
|a Mediterranean hermit |
|a cleaner hermit picks debris from the face of an anglerfish |
|Red Sea hermit crab |
|growing to use a larger shell, crabs can be tempted into over-protection - this one is struggling along with four stinging anemones |
|this one has tried to climb the housing ladder too quickly |
|a symbiotic hermit survives on the lunar landscape of a spiny starfish |