Marine zoologists use ‘worm’ as a catch-all term for animals from at least 12 major groups (phyla). All worms have tube-shaped bodies, but they differ in the complexity and arrangement of their internal organs.
Flatworms and tapeworms
The New Zealand writer Sheila Natusch has likened marine flatworms to steam-rollered sea slugs. Wafer thin, they range in length from a few millimetres to a few centimetres. Most flatworms dwell on the sea floor, under stones or on seaweed, and move across surfaces by undulating the sides of their body. They have a mouth, but no anus. Two species found under intertidal rocks at Kaikōura have the extraordinary distinction of possessing hundreds of eyes and multiple penises – 229 in the case of Anonymus multivirilis.
Parasitic flatworms are also known as flukes and tapeworms. A common fluke in New Zealand, Curtuteria australis, completes its life cycle in whelks, cockles and oystercatchers. The fluke’s eggs are eaten by whelks, who become hosts to the growing and multiplying larvae. The larvae leave to contaminate the foot muscle of cockles, reducing their ability to burrow, and making the shellfish easy prey for oystercatchers and other estuarine birds, who in turn become infected.
Ribbon or boot-lace worms are common around the coast, where they may be found crawling among seaweeds and rocks, or buried in sediment. Twenty-five species have been described in New Zealand waters. Ribbon worms are the longest animals in the world, with some species growing to 60 metres. However, the largest New Zealand species is only about 30 centimetres long. These are thin, unsegmented worms, often flattened and usually brightly coloured. They have a distinctive tongue-like proboscis at one end. Normally concealed in a cavity above the mouth, the proboscis will shoot out to spear, grab or suck prey. Ribbon worms eat bristle-worms and small crustaceans such as barnacles. When resting under rocks, they often roll up into a mess of knots.
Raw fish and roundworms
Humans can become accidental hosts to the marine roundworm Anisakis from eating raw or undercooked fish or squid that has been infected with its larvae. A few hours after consumption, vomiting and severe abdominal pain may develop. Anasakis larvae have been detected in New Zealand fish fillets and arrow squid. Although only one case of infection has been recorded, it is possible that more cases will occur as eating raw fish becomes more popular.
Roundworms are the most numerous animals in the world. These skinny little wrigglers dominate in deep-sea sediments, and there is hardly an animal in the sea or on land that is not infected with one. Most of the 163 species recorded from New Zealand’s marine environment are free-living in the top few centimetres of the sea floor, where they feed on decomposing organic matter and bacteria.
Members of this group are thread-like and resemble roundworms. Adult hair worms are free-living, but juveniles are parasitic in insects and crabs. The only marine species known from the South Pacific area is Nectonema zealandica, a parasite of the common marbled rock crab, Hemigrapsus sexdentatus.