Echiura

T

he Echiura commonly recognized as 'spoon-worm' are inhabitants of the shores of polar, temperate and tropical seas. These animals are a group of marine worms with the first echiurans described appear to be Thalassema (= Lumbricus) thalassemum (Pa lla s, 1 7 6 6 ) a n d Echiura n s (= Lumbricus) echiurus (Pallas, 1766) and the first described bonellid is Bonellia viridis Ronaldo, 1824. Most species dig deep tunnels or burrows in fine sediments. Echiurans are one of the valuable organisms for managing our ecosystem as it used as food for rays and flatfish.

wider end to the burrow wall, and at its narrower end to the circle of glands. To feed, it produces conical mucus net that lines the burrow as water is sucked in at a rate of about 18L per hour. Edible particles are caught on the net, and after some time the worm slowly eats the net and all the edible matter sticking to it.

Sexual dimorphism The sexes are separate in the families Echiuridae and Urechidae and they are indistinguishable externally, but in remarkable sexual dimorphism can be seen in Bonelliidae: the males are relatively small and carried on or in the females commonly inside the nephridia, occasionally in the coelom, oesophagus, pharynx, or on the proboscis. The size of females individual may reach a total length of 2 meters, but males are just a few millimeters long, are reduced in complexity, and live attached to the female.

Morphology

Reproduction

They are unsegmented, bilaterally symmetrical coelomate animals, composed of two parts trunk and proboscis. Trunk, is sub-cylindrical or saclike in shape and muscular in composition. A muscular, bifid, sensory proboscis is attached to the anterior end of the trunk and cannot be retracted within the body of the animal. The mouth is situated at the anterior position at the base of the proboscis, and the anus at the posterior end. The superficial covering of the trunk is cuticular epithelium with or without papillae, in many echiurans containing a special toxic green pigment (bonelliin). A pair of setae is usually present on the ventral surface of the body just posterior to the mouth. These setae can elongate significantly when muscles are contracted, helping the animal in locomotion and in burrowing into the bottom deposits. One or two rings of anal setae encircle the posterior region in some genera. The body wall consists of longitudinal, circular, and oblique muscles. Euchiurans are most often found with their trunks buried, anterior end upward; members of genus Urechis create U-shaped burrows. Burrowing and other locomotion is accomplished through peristaltic movements of the body wall.

Fertilization is external and mass spawning can be seen. After external fertilization, larvae may drift in the plankton form up to 3 months. The initial larval stage is a trochophore, after 30 days, prototroch and preprototroch develop into the adult proboscis while the postprototroch develops into an elongated truck. Larval development takes two to three months. Male bonelliids are considered to be parasitic on females. When a larva touches the skin of an adult female, the larvae will develop as males. When the larva is alone, the larvae will develop into as a female.

Feeding Most of the echiurans are detritus feeders. They used to bury their trunk and extend the proboscis over the substrate and trap the organic particles by mucus on the proboscis. The mucus and food substances are moved by ciliary action along the ventral proboscis gutter to the mouth. Deposit feeders, keep their trunk inside the burrow and extend their long proboscis up to 300 or more times the trunk length out of the burrow to collect surface sediment and organic particles near the burrow opening, captured particles transevered to mouth by dorsal cilia. The organic components are digested, and the remaining sediments are deposited as small fecal pellets near the burrow opening. Filter feeders have a relatively short proboscis poorly adapted for surface deposit feeding. Echiurans secrete mucus net from a circle of glands around the base of the proboscis and the funnel-shaped net is attached at its

Fossil record The echiurans are specialized for the formation of U-shaped burrows, known in the fossil record from Cambrian times. The construction of burrows was not only made echiurans but also made by other organism which was inhabited by the echiurans. Body fossils of echiurans are much rarer, since echiurans have no hard parts. The oldest plausible echiuran fossil is Pennsylvanian.

Global and Indian status Phylum Echiura is comprised of 176 species under three orders: Echiuroinea, by far the largest order, with two families (Bonelliidae and Echiuridae) and 170 species; Xenopnuesta, with one family (Urechidae) and 4 species; and Heteromyota, which are made up of one monotypic family (Ikedaidae), are containing two species. A total of 43 species of echiura under the 14 genera were reported from Indian Ocaen (Table 1). 12 species of echiurnas are endemic to India.

Habitat Sand, mud, detritus surface, rubble, sunken pieces of wood, roots of sea grasses, and shells of coconuts are the optimum place for the echiurans to live. It is also documented that the some species inhabit rock galleries excavated

LESSER KNOWN MARINE ANIMAL by other invertebrates. The distributional patterns of the echiurans are extensive and very wide from the intertidal down to 10,000 meters in depth. All echiurans are marine, except for a few species that occur in brackish water.

Spoon-worm

Threats Marine pollution and dumping of solid waste in the coastal waters may affect the survival of ehiurans. Table 1: Species Diversity of Echiura Species World India 1.Acanthobonellia Fisher, 1948 3 4 2.Acanthohamingia Ikeda, 1910 3 3.Achaetobonellia Fisher, 1953 3 2 4.Alomasoma Zenkevitch, 1958 6 5.Archibonellia Fisher, 1919 2 6.Binophorus Salvini-Plaven, 1972 1 7.Bonellia Rolando, 1821 12 2 8.Bonelliopsis Fisher, 1946 3 9.Bruunellia Zenkevitch, 1966 1 10.Charcotus Datta-Gupta, 1981 2 11.Choanostomellia Zenkevitch, 1964 4 1 12.Eubonellia Fisher, 1946 4 1 13.Ikedella Monro, 1927 4 1 14.Jakobia Zenkevitch, 1958 3 15.Kurchatovus Datta-Gupta, 1977 2 16.Maxmuelleria Bock, 1942 5 17.Metabonellia Stephen & Edmonds, 1972 2 2 18.Nellobia Fisher, 1946 6 19.Protobonellia Ikeda, 1908 3 1 20.Pseudobonellia Johnston & Tiegs, 1919 1 21.Pseudoikedella Murina, 1978 6 1 22.Sluiterina Monro, 1927 1 23.Torbenwolffia Zenkevitch, 1966 4 24.Vitjazema Zenkevitch, 1958 1 25.Zenkevitchiola Murina, 1978 6 1 26.Echiurus Guérin-Méneville, 1831 17 27.Anelassorhynchus 7 28.Arhynchite Sato, 1937 2 10 29.Lissomyema Fisher, 1946 9 1 30.Listriolobus Spengel, 1912 35 16 31.Ochetostoma Leuckart & Ruppell, 1828 9 2 32.Thalassema Lamarck, 1801 2 33.Ikeda Wharton, 1913 4 34.Urechis Seitz, 1907 1 35.Sactosoma Genus

Total

176

LEKMAI-2015 ZOOLOGICAL SURVEY OF INDIA Government of India Ministry of Environment and Forests and Climate Change Port Blair - 744102, Andaman and Nicobar Islands

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Released on the occasion of “National Workshop on Lesser Known Marine Animals of India” during11th to 13th June, 2015 held at ZSI, Port Blair

Prepared by: K. Venkataraman, C. Raghunathan, Tamal Mondal and R. Raghuraman