More than just molluscs
In zoological terms, ‘shellfish’ refers to species belonging to the mollusc group. But some people use the term to include edible species that have shells but are not molluscs – such as crayfish, prawns and sea urchins (kina).
Shellfish belong to the large group of marine animals known as molluscs. With more than 80,000 species known worldwide, molluscs are the second largest group in the animal kingdom. They include familiar shellfish that are partially or entirely enclosed within a shell, such as mussels, oysters, snails and limpets. Others possess very small or internal shells –for example, sea hares, shipworms and squid. Sea slugs and octopuses are also molluscs, but lack any type of shell.
There is no standard shape or body plan for molluscs, although they share some common features:
- an unsegmented, soft body
- a mantle (a fold in the body wall, that may produce a shell)
- a muscular foot or tentacles.
Many molluscs also have:
- an external shell that completely or partially encloses the animal
- a radula – a ribbon of teeth for rasping food.
Molluscs in New Zealand
There are over 3,660 species of mollusc living in New Zealand waters. This makes them the country’s largest group of marine animals, far outnumbering the crustaceans (crayfish, crabs, krill) and vertebrates (fish and marine mammals). Scientists working on fossils know of nearly 7,000 other types that once lived around ancient New Zealand.
New Zealand’s named species are found in all seven classes of mollusc:
- Gastropods – sea snails, sea slugs and limpets (over 3,000 species)
- Bivalves (436 species)
- Octopus and squid (129 species)
- Chitons (56 species)
- Tusk shells (11 species)
- Monoplacophorans (2 species)
- Aplacophorans (2 species).
Many of these species are not found anywhere else. Some groups such as ostrich foot shells (Struthiolariidae) have probably been in New Zealand since the break-up of the ancient Gondwana continent 85 million years ago. However, a number of the New Zealand species are widely distributed throughout the South Pacific region, and a smaller group is found in south-east Australia.