Story: Marine animals without backbones

Page 5. Tiny sea animals

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New Zealand’s marine environment teems with a multitude of microscopic invertebrate animals.


The microscopic parasites grouped as myxozoans (literally, slime animals) were once classified as protozoans – single-celled creatures. Now they are known to be highly reduced multicellular animals, perhaps related to roundworms or to jellyfish. There are at least 57 species living as parasites in New Zealand marine fish. Some myxozoans are relatively harmless, but one causes whirling disease in trout and salmon.


Tardigrades, or water bears, are microscopic animals commonly found wherever there are drops of water. New Zealand has five marine species. Some inhabit sea-floor sediments, while others appear to spend their lives on the surface of sea cucumbers. Tardigrades crawl along on four pairs of stumpy legs, sucking in scraps of food.


New Zealand’s marine zone is home to 44 known species from the gnathiferan group, including lesser jaw worms, rotifers, and thorny-headed worms.

Transparent lesser jaw worms are less than 1 millimetre long and reside in oxygen-depleted sediments around the coast. Rotifers are microscopic planktonic animals characterised by a crown of hairs at their head end. Thorny-headed worms are gut parasites of vertebrates. Their larval stages infect arthropods such as insects and crabs. Researchers have discovered that almost all adult crabs in the intertidal zone around the Otago coast are host to larvae of the thorny-headed worm Profilicollis. When the crabs are eaten by black-backed or red-billed gulls, the larvae attach themselves with their spiny probosces to the bird’s gut wall, where they complete their life cycle.

Other miniature beasts

Orthonectids are the tiny parasites of other marine invertebrates. One orthonectid has been discovered in a New Zealand bryozoan.

Dicyemids are a microscopic group of parasites that infect the kidneys of octopus and squid. They have a simple body, which consists of a single large cell surrounded by 30–40 smaller ones. They have a complicated life cycle. Six species occur in New Zealand.

Gastrotrichs are poorly researched microscopic worms. Four species have been recorded in New Zealand seas, but these have not been formally described.

Kinorhynchs, or mud dragons, are microscopic sediment dwellers. Seventeen species are known in New Zealand. They are worm-like and creep about in sea-floor sediments using spines on their head to anchor them while they contract their body. Their tail spines re-anchor the body as they extend their head forward.

Loriciferans were not discovered until 1983, and only one is known from New Zealand. They are flask-shaped animals with spiny heads and well-developed brains. They live between grains of sand.

How to cite this page:

Dennis Gordon and Maggy Wassilieff, 'Marine animals without backbones - Tiny sea animals', Te Ara - the Encyclopedia of New Zealand, (accessed 14 April 2024)

Story by Dennis Gordon and Maggy Wassilieff, published 12 Jun 2006