Story: Starfish, sea urchins and other echinoderms
Page 3 – Other echinoderms
Brittle stars look like thin starfish. They usually have five long skinny arms – but sometimes as many as eight – connected to a central disc. As their common name suggests, the arms are fragile and break off easily. Like starfish, brittle stars are able to regrow damaged or severed limbs; unlike starfish, they have no anus or intestine. Some of the oddest looking specimens have arms that repeatedly branch, perhaps up to 20 times. These animals, such as Astroba granulatus, are also known as basket stars.
New Zealand has at least 165 species of brittle star, but they are often overlooked – most live in deep waters, and some are quite small. Those in shallow waters usually retreat under rocks and fine sands, or weave themselves among seaweeds. The snake star Astrobachion constrictum hides in black coral.
Brittle stars use their flexible, spiny arms to move through the water, and to seek and capture food. The common mottled brittle star or weki huna (Ophionereis fasciata) secretes mucus along its arms, which trap food particles as they sweep over the sea floor.
Sea lilies and feather stars
Sea lilies are stalked echinoderms resembling feather dusters. They are of ancient lineage and more likely to be found as fossils than as living animals. The 13 New Zealand species inhabit deep waters offshore, living a sedentary life attached to the rocky sea floor or underwater mountains.
Feather stars are brightly coloured, mobile creatures that look like shuttlecocks. They walk on the tips of their arms and can swim. New Zealand has 36 different species.
Both groups are suspension feeders. They stretch out their branching arms, the sticky tube feet on the undersides trapping nutritious titbits. The food is then passed along to the mouth.
In 1983, nine little creatures were discovered living in sunken wood on the deep sea floor off Castlepoint in the North Island, and off Hokitika in the South Island. They had the distinction of being placed in a class of their own. In scientific terms this is significant – the equivalent of finding a new group of animals such as birds or mammals or fishes. Disc-shaped and edged with a ring of spines, they were clearly echinoderms, but differed from any that were known. They had no arms, mouth, gut, or anus. Scientists named them Xyloplax medusiformis. A second Xyloplax has subsequently been found in decomposing wood off the coast of Bermuda. Resembling flowers, these oddities are known as sea daisies.
Since 2000 their unique position has been questioned. DNA analysis of echinoderm groups suggests that sea daisies may be very modified starfish.
Warty, brainless animals that breathe through their rear end and lose their guts at the slightest disturbance do not sound attractive culinary prospects, but sea cucumbers are considered a delicacy by some. There are about 100 sea cucumber types in New Zealand waters and, in places, they may be the most conspicuous animal of the very deep sea floor. Some show parental care and hold babies under their bodies.
Masses of the beautiful red and white-spotted strawberry sea cucumber (Squamocnus brevidentis) gather on rocky walls around the southern coast of the South Island, reaching densities of 1,000 animals in a square metre. One site in Preservation Inlet, Fiordland, is known as the strawberry fields.
Stichopus mollis, or rori, is a grey, soft-bodied, sausage-shaped sea cucumber. It favours sheltered coastal waters to a depth of 40 metres all around New Zealand, growing to 20 centimetres in length. Using its three rows of tube feet, it moves slowly over the sea floor in search of food. Stichopus is a detritus feeder, sifting through mud and sand for organic matter to digest. This animal has been harvested on a small scale in recent years in New Zealand and is being investigated for its aquaculture potential. Sea cucumbers are boiled, gutted, reboiled, and then smoked before use.