Looking rather like a miniature armadillo encircled by a snake, chitons can usually be found on coastal rocks along with limpets, where they graze on rock-hugging growths of seaweed.
A chiton is easily recognised by the eight overlapping shell plates on its back, which have a leathery rim. They cling to rocks with a muscular grip and move slowly over surfaces at night when the tide is in. They have relatively flexible bodies and can squeeze into narrow crevices – unlike limpets, whose rigid shell prevents entry into such spaces. If detached from rocks, they roll into a ball just as a slater (woodlouse) does, their soft body protected by the shelly plates.
Despite minimal development of a head, and no sensory tentacles, some chitons know exactly where they are on the shore, and return to a home site after feeding. They are sensitive to pressure and detect light with numerous shell eyes. These are tiny holes in their shell plates that connect to structures containing a lens, a retina and pigment cells.
New Zealand chitons
Most of New Zealand’s 56 species of chiton are coastal animals, but a few are known in deep water. New Zealand’s most common species, the snakeskin chiton (Sypharochiton pelliserpentis), grows to 4 centimetres and is found on all rocky shores. Less commonly seen because it favours subtidal, shaded locations is the noble chiton (Eudoxochiton nobilis). At 10 centimetres, it is New Zealand’s largest species.
Tusk shells, named for their resemblance to elephant’s tusks, are carnivorous snails with a tubular, tapering shell. They are burrowing animals, living partially buried in the sea floor. Both ends of their shells are open: the broad end is buried and the narrow end sticks out into the water.
Tusk shell snails are simple creatures: they lack eyes, gills and a heart. They have a tiny head that consists of a proboscis with a mouth and threadlike tentacles that sweep the sediments and trap micro-organisms for food. The best-known tusk shell in New Zealand, Anatalis ana, inhabits coastal muds as well as deeper sediments. In pre-European times Māori collected tusk shells that washed ashore, and threaded them together for necklaces and anklets.
Two rare classes of shellfish, the Monoplacophorans and Aplacophorans, are uncommon in New Zealand waters. Monoplacophorans are deep-water shellfish that superficially resemble limpets. An eagle-eyed scientist must have been at work the day the first New Zealand monoplacophoran was discovered in sediments trawled up from a depth of 1 kilometre. Micropilina tangaroa is just 1.5 millimetres long, and only one specimen was found. Since then a second related monoplacophoran has been discovered off the Southland coast.
Aplacophorans are small worm-like molluscs that live and feed on deep-water corals and hydroids. They have no shell, eyes or tentacles. Two species are known from New Zealand waters. One (Dorymenia quincarinata) has been found at shallow depths in Lyall Bay, Wellington, and at depths of 240 metres near the Chatham Islands. The other species (Neomenia naevata) was collected from the Taiaroa Canyon, off the Otago coast, at a depth of 380 metres.