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The bodies of spoon worms are generally cylindrical with two wider regions separated by a narrower region. There is a large extendible, scoop-shaped proboscis in front of the mouth which gives the animals their common name. This proboscis resembles that of peanut worms but it cannot be retracted into the body. It houses a brain and may be homologous to the prostomium of an annelid. The proboscis is used for feeding and has rolled-in margins and a groove on the ventral surface. The distal end is sometimes forked. The proboscis can be very long; in the case of the Japanese species Ikeda taenioides, the proboscis can be 150 centimetres (59 in) long while the body is only 40 centimetres (16 in). Even smaller species like Bonellia can have a proboscis a metre (yard) long. Respiration also takes place through the proboscis although some larger species also use cloacal irrigation. In this process, water is pumped into and out of the rear end of the gut through the anus. The proboscis additionally has a sensory function.
There are a pair of large chitinous setae (bristles) on the front end of the body and in some species, one or two rings of setae near the hind end of the animal. The body wall is muscular and surrounds a large coelom which leads to a long looped intestine with an anus at the rear tip of the body. Most echiuroids are a dull grey or brown but a few species are more brightly coloured, the translucent green Listriolobus pelodes being an example.
Distribution and habitat
Echiuroids are exclusively marine and the majority of species live in the Atlantic Ocean. They are mostly infaunal, occupying burrows in the seabed, either in the lower intertidal zone or the shallow subtidal. A few are found in deep waters including at abyssal depths. They often accumulate in sediments with high concentrations of organic matter. In the 1970s, the spoon worm Listriolobus pelodes was found on the continental shelf off Los Angeles in numbers of up to 1,500 per square metre (11 square feet) near sewage outlets. The burrowing and feeding activities of these worms churned up and aerated the sediment and promoted a balanced ecosystem with a more diverse fauna than would otherwise have existed in this heavily polluted area.
A spoon worm can move from one location to another by extending its proboscis and grasping some object before pulling the body forward. Echiurus can also swim by use of the proboscis and by contractions of the body wall.
Some spoon worms live in U-shaped tunnels in sand, mud or other soft substrate. Echiurus for example is a detritivore and extends its proboscis from the rim of its burrow with the ventral side on the substrate. The surface of the proboscis is well equipped with mucus glands to which food particles adhere. The mucus is bundled into boluses by cilia and these are passed along the feeding groove by cilia to the mouth. The proboscis is periodically withdrawn into the burrow and later extended in another direction.
Urechis has a different method of feeding on detritus. It has a short proboscis and a ring of mucus glands at the front of its body. It expands its muscular body wall to deposit a ring of mucus on the burrow wall then retreats backwards, exuding mucus as it goes and spinning a mucus net. It then draws water through the burrow by peristaltic contractions and food particles stick to the net. When this is sufficiently clogged up, the spoon worm moves forward along its burrow devouring the net and the trapped particles. This process is then repeated and in a detritus-rich area may take only a few minutes to complete. Large particles are squeezed out of the net and are eaten by other invertebrates living commensally in the burrow. These typically include a small crab, a scale worm and often a fish lurking just inside the back entrance.
Ochetostoma erythrogrammon obtains its food by another method. it has two vertical burrows connected by a horizontal one. Stretching out its proboscis across the substrate it shovels material into its mouth before separating the edible particles. Other spoon worms conceal themselves in rock crevices, empty gastropod shells, sand dollar tests and similar places. Some are scavengers or detritivores, while others are interface grazers and some are suspension feeders.
While the proboscis of a spoon worm is on the surface it is at risk of predation by bottom-feeding fish. In some species the proboscis will autotomise (break off) if attacked and the worm will regenerate a proboscis over the course of a few weeks.
The sexes are separate in echiuroids. There are paired gonads in the body cavity and the gametes mature here before being released into the sea. In most species fertilisation is external and the eggs hatch into planktonic trochophore larvae. In some species, some of the developmental stages are segmented and this is believed to show the distant annelid ancestry of these worms. After developing through several stages, the larvae settle on the substrate and undergo metamorphosis into juvenile spoon worms. Bonnelia is an exception to this course of development. In this genus, the male is much smaller than the female. If a larva comes into contact with an adult female, it will enter her body and undergo metamorphosis there into a male spoon worm which will live permanently in her uterus. Other larvae develop into female spoon worms.
- Order Bonelliida
- Order Echiurida
- See references at Echiura
- Toonen, Rob (2012). "Part 6: Phylum Sipuncula and Phylum Annelida". Reefkeeper's Guide to Invertebrate Zoology. Retrieved 2012-11-10.
- Felty Light, Sol. Intertidal Invertebrates of the Central California Coast. Google Books. p. 108. Retrieved 2011-11-30.
- Walls, Jerry G. (1982). Encyclopedia of Marine Invertebrates. TFH Publications. pp. 262–267. ISBN 0-86622-141-7.
- Stull, Janet K.; Haydock, C.Irwin; Montagne, David E. (1986). "Effects of Listriolobus pelodes (Echiura) on coastal shelf benthic communities and sediments modified by a major California wastewater discharge". Estuarine, Coastal and Shelf Science 22 (1): 1–17. doi:10.1016/0272-7714(86)90020-X.
- "Echiuroidea". Encyclopedia Britannica 1911. Retrieved 2012-11-11.
- Chuang, S. H. (1962). "Feeding Mechanism of the Echiuroid, Ochetostoma erythrogrammon Leuckart & Rueppell, 1828". Biological Bulletin 123 (1): 80–85. JSTOR 1539504.
- Echiuroidea World Register of Marine Species. Retrieved 2011-11-30.
- Barnes, Robert D. (1982). Invertebrate Zoology. Holt-Saunders International. pp. 870–873. ISBN 0-03-056747-5.