Some of New Zealand’s most beautiful animals are the sea slugs. They have no shell, but scientists group them with the gastropod group of molluscs, which are snail-like shellfish. Two main types of sea slug frequent coastal waters:
- nudibranchs: colourful, patterned slugs
- sea hares: brownish slugs, resembling seaweed.
Nudibranch means ‘naked gill’ and refers to the feathery growths near the rear end of the animal, which function as breathing apparatus. Over 80 species are known from New Zealand. The warm waters around the Poor Knights Islands marine reserve, in the north, are one of the best places to see the spectacular blue and brown gem nudibranch (Dendrodoris denisoni), and Verco’s nudibranch (Tambja verconis), resplendent in gold and blue.
In contrast, New Zealand’s largest species, the Wellington nudibranch (Archidoris wellingtonensis), is an ugly creature. Capable of reaching 20 centimetres in length, its orange-brown body is covered in large wart-like growths.
Defensive strategies of nudibranchs
Nudibranchs are carnivores and feed specifically on sponges, sea firs (hydroids) and sea squirts. Because they lack the protection of a hard shell, it might be thought that their soft bodies would be vulnerable to hungry predators such as fish and starfish. But most predators avoid them, probably because they contain unpleasant chemicals in their bodies. However, the Roboastra luteolineata preys on fellow nudibranchs.
A remarkable defensive system is used by the pink and white Jason’s nudibranch (Jason mirabilis). They feed on hydroid polyps, and by some unknown means incorporate the hydroid’s stinging cells into growths (cerata) on their backs. Any predator biting into these growths will trigger the stinging cells and receive a dose of poison.
Test for toxins
Scientists think the bright colours of nudibranchs are an adaptive strategy that warn predators of poisons or nasty-tasting chemicals. But how can you test for them? Marine scientist John Morton suggested that you cautiously touch the animal with your tongue. This may not be a good idea, as in 2009 dogs died after eating the rather drab-coloured nudibranch species Pleurobranchaea maculata, washed up on Auckland beaches. Testing found that the nudibranchs contained the toxin tetrodotoxin, which if ingested would also be fatal in humans. Signs were put up warning people not to touch the nudibranchs and to watch their dogs and children.
New Zealand has eight species of sea hare. This group of fleshy slugs has prominent tentacles that could, with some imagination, be thought to resemble the ears of a hare. Some species have a small internal shell protecting the gills. All are herbivores, feeding on succulent seaweeds, which afford good camouflage. They shoot out a poisonous purple dye if a predator gets close.
Sea hares are hermaphrodites (each animal has both male and female sex organs). Mating often occurs in groups, with chains of animals lining up to fertilise the released eggs of the animal in front. Sea hares release their eggs in a colourful tangle resembling spaghetti or knitting wool.