Crabs and crayfish are crustaceans, a subdivision of arthropods – the large group of animals without backbones (invertebrates) that includes insects, spiders, mites, scorpions and springtails. Arthropods have jointed legs and a hard outer shell that acts as a skeleton.
Crustaceans mainly occupy marine habitats, and it is in the world’s oceans that they show their greatest diversity. However, they are well represented on land, notably by woodlice or slaters and some sand hoppers, and in freshwater habitats. Between 50,000 and 67,000 species are known worldwide. However, scientists estimate the total number of crustaceans to be 10–100 times greater than this.
A crustacean has the following features:
- a segmented body with a hard exterior (known as an exoskeleton)
- jointed limbs, each often with two branches (termed biramous)
- two pairs of antennae
- seven or more pairs of appendages for feeding, swimming, walking, respiration and reproduction (clasping, sperm transfer, egg brooding and carrying young).
In order to grow, all crustaceans periodically cast off their old exoskeleton, to reveal a new one beneath. This process is called moulting and leaves the crustacean vulnerable to predation and cannibalism. In the common red crayfish (Jasus edwardsii) the first moult occurs soon after hatching, and moulting continues for the rest of its life. The markings on each crayfish are unique and are retained through each moult – the crustacean equivalent of fingerprints.
Crustaceans show a greater diversity of body form than any other animal group, and include worm-like slaters (isopods), short-bodied crabs and long-bodied shrimps and prawns. There is a great range in size, from less than a tenth of a millimetre (parasitic species and those that live between sand grains) to nearly half a metre (giant crabs, lobsters and slaters, which can weigh up to 20 kilograms).
The most recent classification of crustaceans comprises six classes. One of these (the cave-dwelling Remipedia) is not known in New Zealand, and another (the Cephalocarida) is known only from a single species. The Cephalocarida are believed to be close to the ancestral form from which other crustaceans evolved. The best-known crustaceans are edible species such as crabs, crayfish and shrimps. They belong to the Malacostraca class, along with slaters and sand hoppers.
New Zealand crustaceans
Most of the major crustacean groups are found in New Zealand waters, though many warm-water groups are absent or weakly represented. In 2005 the number of species known in New Zealand was 2,682, though the figure is always increasing. The actual number of species could easily be 10 times that figure because, with the exception of crayfish, crabs and shrimps, many of the groups have not been well studied. A number of species are endemic to New Zealand – unique to the country like kiwi and tuatara.