Story: Crabs, crayfish and other crustaceans
Page 2 – Crabs
With their stalked eyes, wide bodies, large nippers and sideways scuttling movement, crabs are well represented in New Zealand. There is even a freshwater crab (Halicarcinus lacustris) in the streams and lakes of Auckland and the Waikato.
Crabs have wide, flat bodies with no obvious tail. The head and chest are fused and protected by a shield-like structure called a carapace.
The tail is tucked under its body. Males have a narrow tail; females have a broad, rounded one that supports their eggs.
True crabs have a pair of claws followed by four pairs of walking legs. Various other crustaceans (hermit crabs, porcelain crabs and king crabs) superficially resemble crabs but have only three pairs of walking legs.
Habitat and food
At least 30 species are known from the intertidal realm. Some, such as the purple rock crab or pāpaka nui (Leptograpsus variegatus), spend much of their time out of water, sheltering in rock crevices during the day and venturing onto the shore at night in search of food. The purple rock crab is not a fussy eater and consumes seaweeds as well as live or dead animals, including smaller members of its own species.
Giant spider crab
With a leg span of 80 centimetres, the giant spider crab (Jacquinotia edwardsii) is New Zealand’s largest crab. It is a cold-water species, found around the southern South Island and the subantarctic islands in shallow to relatively deep (600 metres) habitats.
There is a lot of muscle in its body and long legs, but with little demand for crab meat in New Zealand no fishery has developed around this species.
The only commercially harvested crab is the paddle crab (Ovalipes catharus), which is a coastal species that supports a small and relatively low-value fishery. Its body grows very quickly – from 3 centimetres to 10 centimetres in a year, and when fully grown it can be up to 15 centimetres wide.
Hermit crabs inhabit empty shells of sea snails, which they cart about with them as they scuttle over the sea floor. They do this to protect the soft, vulnerable abdomen, which is offset to one side of the body. As the hermit crab grows, it needs to find larger shells for protection. Sixty species are known in New Zealand.
King crabs are the giants of New Zealand’s crustaceans, and they live in deep water. Scientists group them with hermit crabs rather than true crabs, for like hermit crabs they only have three pairs of walking legs and an offset abdomen. Our largest king crab (Lithodes murrayi) has a body width of 20 centimetres and leg span of up to 1 metre.