(Echiurans)
Phylum: Echiura
Number of families: 5
Thumbnail description
Worm-like invertebrates with non-segmented, bilaterally symmetrical bodies
Evolution and systematics
As the echiurans' body has no large hard parts, fossils of these animals are rare. There are two fossils for this group: a fossil from Illinois, United States, dated from the Late Carboniferous and another fossil from Namibia, dated from the Late Cambrian.
A free-swimming trochophore larva is present in echiurans, sipunculans, mollusks, and annelidans, which suggests a phyllogenetic relationship. Echiurans and annelids have many features in common. The most important difference between them is the absence of segmentation in Echiura. Some authors consider echiurans to be properly placed within the phylum Annelida, though other studies have shown that echiurans and pogonophorans have a close affinity and both may be closer to mollusks than to annelidans or sipunculans.
The phylum Echiura encompasses about 160 species and 40 genera, divided into two classes: Echiuridea, with three orders and four families (Echiuridae, Bonellidae, Ikedidae, and Urechidae), and Sactosomatidea, with a single family, Sactosomatidae, with one species, S. vitreum.
Physical characteristics
Echiurans, also known as spoon worms, have a body divided in two distinct regions: a sausage-shaped saccular, nonsegmented trunk and a ribbon-like proboscis at the anterior end. The length of the trunk may range from 0.39 in (1 cm) up to >19.6 in (>50 cm) and may be gray, dark green, reddish brown, rose, or red. It may be thick or thin, smoothed or roughened by glandular or sensory papillae. Internally, layers of muscles are responsible for peristaltic movements of the trunk. A pair of chitinous golden-brown chaetae usually occurs ventrally on the anterior part of the trunk. Some echiurans have one or two rings of chaetae around the anus.
The proboscis may be short or long, scoop- or ribbon-like, and flattened or fleshy and spatulate. It is generally white, rose, green, or brown. The distal end may be truncate or bifid. It is muscular, mobile, and highly extensible and contractile. It is able to extend 10 times its body length and can reach 3.2–6.5 ft (1–2 m). The ventral surface of the proboscis is ciliated, which helps in the feeding process. The mouth is located ventrally at the base of the proboscis and the anus is at the posterior extremity of the trunk.
Distribution
Echiurans are mainly marine, but some species live in brackish waters. The majority of spoon-worms are found in intertidal and shallow waters, but there are also species living at depths of 32,800 ft (10,000 m).
Habitat
Echiurans usually live in a U-shaped burrow with both ends of the burrow open. They are found mainly in soft benthic substrata such as sand, mud, or rubble, occupying burrows excavated by themselves or by other animals. Some species live in rock galleries excavated by boring invertebrates, whereas others live in empty shells, sand-dollar tests, coral or rock crevices, inside dead corals, or under stones. In general, some commensals are present inside the burrow, including polychaetes, crabs, mollusks, and fishes. The burrow provides a protected, ventilated home, and remains of food discarded by the spoon-worm may be eaten by the commensals.
Behavior
Echiurans are slow but not sedentary, and animals without a proboscis can swim. One of the most important movements is the peristalsis of the trunk, which allows the animal to move slowly over the surface and construct burrows in the sand or mud. The movements by the peristalsis forces water through the tube, permitting the animal to obtain a supply of oxygen. In general, the burrow is kept clean and free from debris and fecal matter.
Feeding ecology and diet
The food of echiurans consists of dead organic matter and microorganisms that live on the substratum. Echiurans may be detritus feeders. They extend the proboscis out of the burrow onto the surface of the sediment. The tip of the ventrally ciliated surface collects particles, and glands produce mucus to adhere to these particles. Movements of the cilia conduct particles and mucus to the mouth. The proboscis is then extended in a new direction, and the procedure is repeated. A few species are filter feeders. The innkeeper worm, Urechis caupo, constructs a mucous net placed near the opening of the burrow. Peristaltic movements of the trunk draw water through the burrow, and particles and small organisms are trapped in the net. Ultimately, the worm eats both the net and the food.
Reproductive biology
Echiurans reproduce strictly by sexual means. Sexual di-morphism is pronounced only in the family Bonellidae, in which the male is much smaller than the female. Sexes are separate, and sperm and eggs are usually liberated at the same time in seawater where fertilization occurs, except in Bonellidae, in which individuals undergo internal fertilization.
Free-swimming and feeding trochophore larvae develop 22 hours to four days following fertilization. The larvae may drift in the plankton for up to three months, and during metamorphosis, it increases in length. The larva settles on the sub-stratum and begins life as an adult.
Conservation status
No species of Echiura are listed by the IUCN.
Significance to humans
Some species of echiurans are commonly used as laboratory animals for physiological, embryological, and biochemical studies. The substance bonellin has been studied because of its antibiotic properties.
Species accounts
Green bonelliaInnkeeper worm
Resources
Books:Beesley, P. L., G. J. B. Ross, and C. J. Glasby, eds. Polychaetes and Allies: The Southern Synthesis. Fauna of Australia. Vol. 4A, Polychaeta, Myzostomida, Pogonophora, Echiura, Sipuncula. Melbourne, Australia: CSIRO Publishing, 2000.
Grassé, P. P. Traitè de Zoologie. Vol. 5. Paris: Masson et Cie, 1959.
Stephen, A. C., and S. J. Edmonds. The Phyla Sipuncula and Echiura. London: Trustees of the British Museum (Natural History), 1972.
Periodicals:Agius, L. "Larval Settlement in the Echiuran Worm Bonellia viridis: Settlement on Both the Adult Proboscis and Body Trunk." Marine Biology 53 (2002): 125–129.
Fisher, W. K., and G. E. MacGinitie. "The Natural History of an Echiuroid Worm." Annals and Magazine of Natural History 10 (1928): 204–213.
Jaccarini, V., L. Agius, P. J. Schembri, and M. Rizzo. "Sex Determination and Larval Sexual Interaction in Bonellia viridis Rolando (Echiura)." Journal of Experimental Marine Biology and Ecology 66 (1983): 25–40.
Nishikawa, T. "Comments on the Taxonomic Status of Ikeda taenioides (Ikeda, 1904) with Some Amendments in the Classification of the Phylum Echiura." Zoological Science 19 (2002): 1175–1180.
Other:"Introduction to the Echiura." [July 24, 2003].
Murina, V. V. "Phylum Echiura Stephen, 1965." 1998 [July 24, 2003].
[Article by: Tatiana Menchini Steiner, PhD]
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