Story: Marine animals without backbones
Page 1 – 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.
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, 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.