IN DEFENCE OF CRABS

Forest and Bird, Volume 19, Issue 1, 1 February 1988, Page 38

iN DEFENCE OF CRABS

eaches and rocky shores are wonderful places to visit or live near, but for some animals, like our native crabs, survival on the seacoast can be quite difficult. A crab’s life is full of danger. Fish, seabirds, kingfishers and people all eat them. ) Crabs protect themselves in lots of different ways. Their thick, heavy shell-like cover — called a carapace — works like a suit of armour and many use their strong pincer claws to frighten or attack their enemies. Anyone who has been nipped by one will tell you that a crab‘s pincers are very powerful for their size. . But not all crabs are fierce. Some such as the little native tunnelling mud crab dart quickly sideways into their holes at the slightest hint of danger. New Zealand's two other common mud crabs, the hairy-handed crab and the stalk-eyed crab, are also timid despite their ferocious sounding names. Both are quick to take advantage of the burrowing skills of their cousin, and often take over the tunnelling crab’s hole. When threatened some crabs such as the swimming crabs burrow backwards into the sand. They use their rear paddlelike legs as shovels to rapidly sink themselves into the sand, leaving only their eyes and feelers protruding. Some crabs hide under stones, especially at low tide. If you carefully lift up damp rocks you can sometimes uncover the cancer crab which has a piecrust-edged carapace. Other crabs like the large shore crab creep into narrow rock crevices and are nearly impossible to dislodge. Several small crabs, like the secretive hairy crab, will stop moving when disturbed.

Most crabs blend in fairly well with their surroundings. Some are even able to change the colour of their shells to match rocks or sand nearby. The clever hairy seaweed crab is well known for dressing up and attaching bits of seaweed to its shell as camouflage . . . but it often betrays itself by zigzagging across the sandy sea floor — quite unlike a clump of growing seaweed! Hermit crabs have one of the most unusual ways of protecting themselves from predators. They take refuge in empty sea-snail shells and only emerge if they feel safe. When frightened the hermit crab shoots back into its shell, using its main nipper as a front door. Like all crabs, a hermit moults or sheds its skin regularly as it grows. This is usually carried out inside its shell, but sooner or later, it becomes too big and needs to find a new home. When hermit crabs look around for a larger shell to move into they explore it carefully with their feelers and pincers, before deciding to move in. Hermit crabs occasionally use sea anemones for extra protection. The anemones with their stinging tentacles live on their shell homes. Even when the crab swaps shells the anemone usually goes with it, either moving itself or sometimes being stroked and coaxed by the crab. Crabs have another very unusual way of avoiding capture. If one of their legs is caught by a bird or a fish they are able to shed the leg — just like a lizard sheds its tail. In time a complete new leg grows to replace the lost one. Crabs of course have to breathe and use a pair of gills to extract oxygen from water. If their gills are full of seawater they can survive for quite long periods out of the sea and sometimes use this water-holding ability to ‘‘blow bubbles" when disturbed. Crab eggs hatch into tiny larvae and gradually change by moulting into small versions of a mature crab. They are very vulnerable when small, especially at the moulting stages and very few of the many thousands of eggs or larvae survive to become adult crabs. New Zealand's ‘Giant Crab’ is enormous and may reach a width of 20cm across the back, and its clawed arms are known to grow up to 40cm. Yet this so-called giant is a mere dwarf when compared to the massive Japanese spider crab which has legs that can spread up to 3.5 metres! g&