No ordinary worms
Science Centres: Aquatic Biodiversity and Biosecurity
An international symposium Species 2000: New Zealand was held in Wellington in 2000 to review and inventory New Zealand’s total biodiversity as a contribution towards global species databases. The next stage is the publication of a review volume, due in late 2004, comprising kingdom-by-kingdom or phylum-by-phylum reviews of bacteria, algal, fungal, and plant kingdoms, and all the phyla of animals. Future issues of Aquatic Biodiversity & Biosecurity will feature updates on some of the organisms to be covered in the volume. Here we begin with one of the smallest, the little-known phylum of spoon-worms – Echiura.
Scientist collecting Urechis novaezelandiae.
Urechis novaezelandiae.
Never heard of spoon-worms? Not to worry, neither have most people. Spoon-worms are rarely encountered, difficult to collect, and most New Zealanders have never seen one. There are only about 150 species worldwide (at least these are the ones we know about), including some six species in New Zealand waters. Soon New Zealand’s largest species Urechis novaezelandiae (the one you are most likely to see if you are lucky) could get a name change.
Japanese scientist Teruaki Nishikawa visited New Zealand in 2002 and, aided by NIWA biologist Geoff Read, captured a good spoon-worm specimen in Wellington Harbour for a molecular study involving two pairs of species of Urechis – a Californian and Chilean species, and a Japanese and New Zealand species. Re-examination of the external characters and internal anatomy of all four species indicates that the Californian and Chilean species are identical. The same appears true for the Japanese and New Zealand species. If so, then U. novaezelandiae may have to be called U. unicinctus, as the name applied to the Japanese species predates the name novaezelandiae introduced by Arthur Dendy in 1897. We are now waiting on the results of the molecular study to confirm what the other information points to.
Spoon-worms are plump-looking marine worms with a flexible proboscis at the mouth end that is shaped like a small scoop in Urechis and some other genera. Spoon-worms inhabit mud tunnels of their own making or live in rock crevices or empty shells. Urechis worms, which squirt water when handled, make a U-shaped tunnel up to half a metre down and between 15 and 100 cm across the bottom of the U, so it takes a lot of digging to capture one.
The smallest echiurans are a few millimetres long and the largest is about 40 cm. They can be important food for fish (for example, snapper in the Hauraki Gulf include some U. novaezelandiae in their diet) and may be used as bait by some fishers. They have been around for a long time judging by overseas evidence of fossil feeding traces and burrows in Silurian sandstone from 408–438 million years ago. The traces are made by the proboscis during feeding, and in some living species the proboscis can extend up to 1.5 m, greatly exceeding body length. The appearance of the proboscis misleadingly accounts for the phylum name (echis, snake; -ura, tail).
Unlike segmented worms, echiurans have no internal body partitions. Most echiurans have separate sexes, but members of one family, Bonelliidae, show marked sexual dimorphism. The dwarf males (1–5 mm long) have juvenile characteristics like surface ciliation and no proboscis. They live with the female, either inside its genital ducts, on the surface near the genital openings, or in other nearby specialised tubes. New Zealand has three bonelliids.
The relationship of echiurans to other worms has long been linked with segmented worms of the phylum Annelida, which includes marine bristle worms, earthworms, and leeches. Recent studies around the world support this idea, although the jury is still out. The way the larva develops, the fine structure of echiuran and annelid bristles, the type of sperm, and some gene sequences suggest that echiurans may have evolved from bristleworms.
The biology of the New Zealand species U. novaezelandiae is almost unknown. We do not even know, for example, if it resembles its Californian counterpart U. caupo ecologically. The latter is known as the ‘fat innkeeper’ because it shares its burrow with a fish, a polychaete worm, several crabs, and a clam, which all benefit from the spoon-worm’s active pumping of fresh, food-laden water through the burrow. New Zealand’s other species are all found in deeper water; they include the bonelliids Pseudoikedella achaeta, Sluiterina kaikourae, and Torbenwolffia galatheae, the echiurid Echiurus echiurus? (probably E. antarcticus), and the thalassematid Thalassema malakhovi (possibly T. sydniense). Emeritus Professor Ed Cutler of Harvard University reviewed the New Zealand fauna for the Species 2000 project and said that even with the few species we have, taxonomic problems abound because there are only a handful of experts worldwide.