Story: Marine animals without backbones
Page 3 – 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.
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 Their mouth is at the end of a narrow neck, which can be turned inside out and withdrawn into the body.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
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.
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.
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 (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.
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.