Who hasn't picked up a crab on a beach or a river bank and been soundly nipped for presumption? Those beasts can inflict an excruciating pinch. Perhaps this partly explains why crabs are such a successful group, with close on 7 000 species known in 93 families. They range in size from tiny pea crabs, which parasitise oysters, clams and other bivalves, to enormous Japanese spider crabs, which may have a leg span of up to 4m. They have strong bodies and a single pair of claws. Which is probably just as well; the damage a double set of claws could do doesn't bear thinking about. They live their lives encased in a chitinous exoskeleton, which provides them with protection from predators and rigid attachment points for their muscles.
Life as an armoured warrior has its downside though. As crabs grow, they outgrow their exoskeletons and must generate a bigger suit of armour and get rid of the too-small version by moulting. The crabs will retreat to a (hopefully) safe place while they moult: the old exoskeleton weakens at predetermined points while the crab makes its body swell, splitting the old armour. The crab will then struggle out of its old suit in minutes, and continue to pump its body up to enlarge the new exoskeleton as much as possible. The crab is then very soft and can't move much, and it is at this point that crabs are most vulnerable. They may be eaten by a predator or fail to extract themselves from their old exoskeletons, becoming trapped and dying in them. It is estimated that up to 90% of crustacean mortality occurs as a result of moulting, quite a burden on animals which spend the bulk of their lives until adults in some stage of moulting.
In some species though, this is the only stage at which mating can happen, the crabs being impervious for the rest of their life cycles. Crabs generally mate belly to belly and have internal fertilisation. The female, which typically has a rounder abdomen than males of the same species, can store sperm for some time until her eggs are mature. Once fertilised, these are deposited onto her abdomen in a sticky matrix and brooded until the larvae are ready to be released into the plankton.
The larvae undergo several developmental stages before settling out on a suitable surface to grow into their adult form. Though there are some land and swimming crabs, the majority of species live on the sea floor and on reefs in a staggering variety of forms.
Some crabs, presumably dissatisfied with the protection provided by their own armour, have developed a specialised pair of legs on their backs. They are called porter crabs and they may first use their claws to nip off a suitable piece of sponge, colonial sea squirt, soft coral or seaweed and then transfer this cloak to their porter legs. The purpose is usually camouflage, but some crabs go further and carry toxic animals on their backs. In several cases, porter crabs are known to carry living sea urchins. The crab benefits from the protection of the urchin's fierce spines, while the urchin gets carried around to different food sources.
Other crabs are decorators. They will carefully attach hydroids, sponges, sea weeds or feather stars to their carapaces, so that they become almost unrecognisable as crabs. Experiments involving removing the crabs' decorations have resulted in the unfortunate crabs laboriously reattaching their decoration. When uncleaned crabs have been put in a different area of reef, they have changed their decorating overnight so as better to blend in with their surroundings.
The toothed decorator crab, known from Saldanha Bay to Richards Bay, is an alarming shade of bright magenta or sometimes vivid yellow when it first moults. Until they have managed to complete their decorating, they like to shelter in fields of striped anemones, and use them for protection. Once decorated though, they become practically invisible.
Protection is a watch-word for hermit crabs, which have soft abdomens and must find a shelter in the form of an empty shell, stone or piece of wood. Some species inhabit fixed homes such as old worm tubes attached to reefs, or holes in corals and sponges. Hermit crabs must find bigger shells when they grow too big for their homes and this can be tricky. They can be observed inspecting shells for fit and will wait for up to 8 hours by a too-big shell for another seeker to arrive, sometimes making 'vacancy chains'. Here the crabs arrange themselves in a line from the smallest to the biggest. If a crab arrives which fits the too-big shell, each crab in line will quickly move to the slightly bigger vacated shell up the line. Some hermit crabs have anemones attached to their shells. This is a symbiotic relationship, providing protection for the hermit and transport for the anemones. When the crabs have to change shells, they persuade the anemones to move with them so that their relationship can continue.
Hermit crabs are not that closely related to true crabs, but are closer kin to porcelain crabs, which live under rocks or commensally with anemones. Porcelain crabs don't usually get much bigger than 15mmm across, but they have relatively large claws for defending their territories. Their mouthparts have been adapted into sieves which they use to strain plankton out of the water.
They are called porcelain crabs because of their habit of dropping their legs to escape predators. This is one advantage of moulting, which porcelain crabs share with true crabs: they can regrow their lost limbs over successive moults.
It's a process which makes crabs rather more formidable to humans. Not only can crabs give a serious nip to the unwary, their weaponry can be regrown if it suffers an accident. The trick lies in picking them up, if this must be done, by the sides of the carapace. Or else leaving these armoured marvels strictly to their own devices.
text: Georgina Jones
photo: Jenny Strömvoll
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(This article was first published in the Sea Rescue Winter 2015 issue.