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Marine animals without backbones

by  Dennis Gordon and Maggy Wassilieff

About 90% of the animals in New Zealand waters are marine invertebrates. These creatures without a backbone come in a huge variety of body shapes. Millions of sponges, jellies and worms live, burrow and wriggle in sea, sediment, sand, and inside other animals.

Sponges and jellies

Plentiful and diverse

Invertebrates, or animals without backbones, make up the bulk of animals within the oceans. Of about 12,700 animal species known in the waters around New Zealand, 90% are invertebrates. They are an incredibly diverse assemblage of organisms, exhibiting a range of body forms and lifestyles. The best represented groups are molluscs (shellfish, octopus and squid) and crustaceans (crabs, crayfish, and barnacles).

New Zealand’s marine zone is 15 times its land area and only a tiny proportion has been systematically sampled for animal life. On average, animals new to science are discovered in New Zealand waters every two to three weeks. Most are small marine invertebrates. During a 2003 survey of seamounts north-west of New Zealand, 600 new species of tiny shellfish were discovered.


Sponges are an ancient group of animals with a simple body that functions as a living sieve. Their surface is perforated with tiny pores, which let in water. The water enters internal cavities lined with special feeding cells. Food particles in the water are trapped by the cells and digested. Sponges have a sort of skeleton, made up of glass-like splinters and collagen fibres.

There are about 700 known sponge species in New Zealand, but the real number may be twice this. Most (around 95%) are endemic – found only in New Zealand waters. The sea is their domain; only a few are adapted to live in fresh water. Brightly coloured and often of indeterminate shape, about 300 are known from coastal areas where they spend their life attached to the shaded sides of coastal rocks or the seafloor. Others are deep-water specialists and have been found growing 7 kilometres down in the Kermadec Trench.

Living fossils

New Zealand is home to a species of ancient sponge that still exists. Thirty-five million years ago the rock sponge Pleroma aotea flourished in shallow waters off the Ōamaru coast, and today it grows on deep-water mounts around northern New Zealand.

Although sponges are simple in form, they can contain complex surprises. Some defend themselves with poisonous chemicals, some of which may be of value to humans – New Zealand scientists are researching Mycale hentscheli, a common coastal sponge that produces compounds with anti-cancer properties.

Deep-water sponges known as glass sponges grow long fibres that can carry light in the same manner as fibre-optic cables. They produce the fibres at low temperatures – a feat that is of great interest to glass scientists, who have yet to accomplish it. Glass sponges have been found at quite shallow depths around the Fiordland coast.

Comb jellies

Comb jellies, or sea gooseberries, are a small group of exclusively marine animals. Their common name describes the eight comb-like rows of hair that line their surface. Of 100 species worldwide, 19 are known from New Zealand waters. Their small transparent, jelly-filled bodies make them difficult to see. Cast ashore, they sparkle along the tide line.

Comb jellies are major predators of fish and crustacean larvae. Some troll for plankton in surface waters, trapping them with their sticky tentacles. Beroe comb jellies, lacking tentacles, use their large mouths to bite or engulf prey. Large (40 centimetres) iridescent pink Beroe have been recorded in the bays around Wellington.

Simple worms

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 worms

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.

Hair worms

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.

Advanced worms


Bristle-worms, or polychaetes, are a major group of worms in the sea, occupying every type of marine habitat. There are enormous numbers in estuarine mud and deep-sea sediments. Nearly 500 species have been described from around New Zealand, and most are distinct to the southern hemisphere. One well-known species is the dark-green ragworm, Perinereis, which lives on rocky coasts and looks like a giant centipede.

Bristle-worms have segmented bodies and are closely related to earthworms. Some roam freely over the sea floor or tunnel through sediments, devouring mud for its nutrients. Most are sedentary, living in burrows or in tubes that they secrete around their soft bodies. They wait for their food to come to them and have specialised tentacles for trapping particles or sweeping them up from the sea floor.

The spiny tube worm Spirobranchus cariniferus cements grand constructions of lime around itself on tidal rocks. At high tide it spreads its plume of deep-blue feeding gills into the water; when the tide goes out, it withdraws them into its tube.

Peanut worms

New Zealand has 26 species of these exclusively marine animals. They are burrowing sediment feeders, up to 20 centimetres long. Their mouth is at the end of a narrow neck, which can be turned inside out and withdrawn into the body. Somebody in New Zealand must love them, but not the country’s bristle-worm expert, who has written: ‘these warty little hole-dwellers are as uninteresting as their appearance suggests.’ 1

Spoon worms

With only six species known from New Zealand waters and most of them restricted to deep-water sediments, few New Zealanders will have encountered spoon worms. The country’s largest species, Urechis novaezelandiae, resembles a raw sausage and lives in a U-shaped tunnel half a metre down in coastal mud. They trap food particles in a mucus net secreted from their snouts. Snapper love them.

Three of New Zealand’s deep-water species belong to a family that exhibits pronounced sexual dimorphism – males and females are very different sizes. The males are miniature and live inside the female’s reproductive system.

Penis worms

There are 16 known species worldwide, and three of these tunnel in the sea floor around New Zealand. Some feed on surface deposits; others are carnivores and venture out of their burrows to hunt for bristle-worms and other soft-bodied creatures. Like insects, crabs, and crayfish, penis worms moult.

Horseshoe worms

Only three species of this insignificant group occur in New Zealand – a quarter of all known horseshoe worm species. They live in marine sediments or inside tubes they have made. Some bore into shells. New Zealand’s Phoronis ovalis burrows into empty mussel shells. Superficially, horseshoe worms resemble bristle-worms, but they lack bristles and their internal structure has more in common with bryozoans – a fellow group of animals with specialised filter-feeding and breathing structures (lophophores), and a U-shaped gut.

Goblet worms

Goblet worms (entoprocts) have a rounded body carrying a ring of tentacles on a stalk. Most live in the sea and are tiny (1–5 millimetres). They are found attached to coastal rocks or to other animals such as shellfish, sponges and bryozoans. Twelve species have been found in New Zealand’s coastal waters.

Arrow worms

Fifteen species of arrow worms are known from the upper layers of New Zealand waters. Ranging in length from 1 to 10 centimetres, they are torpedo-shaped with lateral and tail fins, and resemble transparent juvenile fish. They are predators of tiny fish and animal plankton. By flexing their bodies, arrow worms make swift darting movements towards their prey. They grasp their victims with mouth hooks and kill them with poison.

Acorn and wing-gill worms

New Zealand has six species of these worm-like animals, which are usually found in shallow waters. They are of interest to scientists because they have gill slits, a feature otherwise found only in animals with backbones. Acorn worms have three-zoned bodies consisting of acorn-shaped probosces, cylindrical collars and worm-like trunks. The species most likely to be encountered around the coast are Balanoglossus, a 20-centimetre burrower in mud, and the much smaller scarlet Saccoglossus, which lives among seaweed. Tiny wing-gill worms live in tubes and form colonies.

  1. Geoffrey B. Read, book review of Polychaetes & allies. New Zealand Journal of Marine & Freshwater Research 34, no. 3 (September 2000): 571. › Back

Lace corals and lamp shells

Lace corals

The scientific name for lace corals is Bryozoa, from the Latin meaning moss animals – these delicate individuals form colonies in thick, moss-like crusts. New Zealand is a centre for bryozoan diversity. Almost 1,000 species are found in the region’s waters, and about 600 of these occur nowhere else.

They are an ancient group with a well-known fossil record: their calcareous skeletons have been preserved well in compressed marine sediments. Waitomo and Ōamaru limestones are composed mainly of bryozoan remains.

Outwardly, some bryozoans resemble corals or sponges; however, bryozoans are much more complex animals. Their evolutionary relationship is closer to lamp shells (brachiopods), shellfish, and worms. Although tiny (0.5–1 millimetre), individuals form large colonies that spread over stones, shells and the sea floor. A number of species foul the hulls of ships. Others form extensive structures on the sea floor that can be considered the temperate-water equivalents of tropical coral reefs. Bryozoan beds formed by the Tasman Bay ‘coral’ Celleporaria agglutinans around the northern South Island, and Cinctipora elegans in Foveaux Strait, provide important habitats for juvenile fish and shellfish. Some of these beds have been damaged by trawling.

A cure for cancer?

Like sponges, bryozoans contain complex chemicals that may be beneficial to humans. Bryostatin-1 is one anti-cancer drug that has been used in human trials. It was isolated from Bugula neritina, a marine-fouling bryozoan of Mediterranean origin. This animal first turned up in Wellington Harbour in 1949, where it was noticed forming burgundy-coloured growths on wharf pilings and seaweeds.

Lamp shells

Lamp shells, or brachiopods, superficially resemble clams or scallops (molluscs), but internally they are quite different. Inside lamp shells are large filamentous structures known as lophophores, which are used for filter-feeding and breathing. New Zealand species range in size from 2 millimetres to 5 centimetres across. Most attach themselves to the sea bed or underwater rock walls with a short stalk, although New Zealand’s largest species, Neothyris lenticularis, is free-living as an adult. Lamp shells once dominated ancient seas, but declined when molluscs gained ascendancy.

New Zealand has 525 fossil species of lamp shell, but only 35 living species. Fiordland is home to one of the richest assemblages of lamp shells in the world. In the dim waters of the fiords, seaweed growth is limited and sedentary animals dominate the steep rock walls. Lamp shells grow at densities of 500 individuals per square metre, alongside bryozoans, soft corals, and sponges.

Tiny sea animals

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.

Tunicates: sea squirts, salps and appendicularians

The filter-feeding marine animals, sea squirts, salps and appendicularians are collectively known as tunicates. The scientific name refers to their protective cellulose covering or ‘tunic’.

Tunicates are distant relatives of vertebrates – animals with a backbone – and zoologists have traditionally grouped them together. At some time in their life cycle, both tunicates and vertebrates have gill slits, a nerve cord, and a supporting rod running down their backs. Tunicate larvae look like microscopic tadpoles and possess these vertebrate features. However, as adults they show little resemblance to fish or other mobile backboned animals. Many settle permanently and grow anchored to solid surfaces.

Sea squirts

Sea squirts are the best known of the tunicates. New Zealand has at least 166 species, some of which are solitary, while others form colonies. Individuals are flask shaped, with a siphon that takes in sea water. The water is passed through gills to extract oxygen and filter out food particles. Water and waste products are squirted out of the body through a discharge siphon. One conspicuous species around the South Island is the stalked sea tulip (Boltenia pachydermatina or kāeo), which is common on rocks and wharf pilings at low-tide level.

About 12 species have recently arrived in New Zealand waters as hitchhikers on ships’ hulls or stowaways in ballast water. Two have created problems because of their rapid spread into marine farming areas. Ciona intestinalis outgrows the mussels cultivated on rope lines, preventing them from feeding. Didemnum vexillum, which arrived attached to a barge in the Marlborough Sounds in 2001, forms colonies on artificial surfaces – it is only a matter of time before they spread to mussel lines.

Early fieldwork

Salps were collected by Joseph Banks in the first piece of fieldwork conducted in New Zealand. On board the Endeavour, on 7 October 1769, he dipped his net into the waters of Poverty Bay and reported retrieving salps, bryozoans and seaweeds.

Salps and appendicularians

There are two groups of tunicate living as plankton in the upper ocean. Salps are transparent barrel-shaped animals that may live singly or in colonies. The colonial species join together in chains which are usually a few centimetres long, although 20-metre colonies of fire salps have been observed in summer in waters around northern New Zealand. Salps reproduce rapidly when the plant plankton they eat is abundant. The salps, in turn, become food for fish, marine mammals and seabirds. There are 19 known salp species in New Zealand waters.

Less well known are appendicularians, of which five species have been found in New Zealand. These creatures are very small (3–5 millimetres long), and drift through the sea in a mucus house. They construct these houses around them by secreting mucus from glands in their heads. The houses are inflated with water and serve to catch food.

Lancelets, lampreys and hagfish


Lancelets belong to a subgroup of vertebrates. Technically, they are invertebrates because they lack bony structures; however, their bodies are supported by a gelatinous rod of tissue – a precursor to a backbone.

Lancelets, or puhi (Epigonichthys hectori) are small, transparent animals that spend most of their life partially buried in sandy burrows just below the low-tide zone. They filter-feed, using the whiskery growths around their mouths to trap food. New Zealand has just one species.


Lampreys are slime-covered animals that resemble eels but lack bones. And like eels, they spend part of their lives in rivers and part in the sea. For their first four years lampreys are blind filter-feeders, then they develop eyes and migrate to sea. It is not established how long they remain there, but during this time they become blood suckers, preying on large fish and marine mammals. Lampreys attach their circular sucker mouths to their host’s body, rasping away the flesh and sucking at the tissues. They return to freshwater rivers to spawn before dying. The adult form of New Zealand’s sole species, Geotria australis, was a favoured food of inland Māori.

Trapping lampreys

Whanganui Māori used an extensive system of weirs to trap lampreys, or piharau, as the creatures made their way upriver in winter to spawn. The projections of mānuka stakes were built from the riverbank towards the middle of the river, directing the lampreys into a net trap. Māori in Southland used to catch lampreys by hand on rock walls as they ascended the Mataura Falls.


New Zealand has three species of hagfish, an entirely marine group. Hagfish are generally scavengers of dead fish and marine-mammal corpses, although they will attack living fish, worms and crabs. They exude copious quantities of blue slime from their skin if disturbed, which is why they are also known as snot eels. For some people the common coastal hagfish, or tūere (Eptatretus cirrhatus), is good eating, once the slime is removed. At nearly 1 metre in length, it is one of the larger hagfish known.

Hononga, rauemi nō waho

More suggestions and sources

How to cite this page: Dennis Gordon and Maggy Wassilieff, 'Marine animals without backbones', Te Ara - the Encyclopedia of New Zealand, (accessed 24 June 2024)

He kōrero nā Dennis Gordon and Maggy Wassilieff, i tāngia i te 12 o Hune 2006