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Sipuncula

Introduction

The Sipuncula, popularly called -pea-nut worms, are common inhabitants of shores of polar, temperate and tropical seas. They have also been collected from the floor of the ocean, at great depths.

Rondelet (1555) for the first time described and figured two sipunculans as micro and macrorhynchoterous worms. Later, Bohadsch (1761) described a specimen to which Linnaeus (1766) gave the name Sipunculus nudus. Sipunculans were placed under Gephyrea (Greek gephyros = a bridge) by Quantrefages (1847) along with echiurans and priapulans, under the assumption that they form a link between the annelids and the holothurians. This concept was in vague for over a century though Sedgwick (1898) promoted sipunculans to the rank of a Phylum, Sipunculoidea. Recently, Stephen (1964) emended the name of the phylum as Sipuncula replacing Sipunculoidea of Sedgwick (1898) and Sipunculida of Hyman (1959), with sipunculan as a common name.

Sipuncula comprises unsegmented, coelomate, bilaterally symmetrical and soft-bodied invertebrates. They are cylindroid in shape; with body regionated into a slender anterior introvert, and a plumper posterior trunk. The introvert is highly muscular, and retractile in nature and lies along the same axis of the trunk, but sometimes displaced ventrally. The mouth lies at the anterior extremity of the introvert and is usually surrounded by tentacles, generally followed by a zone of hooks. In some, the anterior end of the trunk is armed with a calcareous cap.

The anu~ is situated dorsally in the anterior region of the trunk. The posterior end is blunt, pointed, acorn like or provided with a cap. The trunk bears papillae of various shapes and sizes, being more dense at both ends than in the middle, or localised regionally and sometimes modified as holdfast or may even be totally absent. Phylum Sipuncula has been divided into two classes consisting of four orders and six families

In the class Sipunculidea, the tentacles enclose the centrally placed mouth on the oral disc; inlrovert hooks, if present, are simple, sharply pointed and usually scattered; spindle muscle not attached posteriorly.

In Class Phascolosomatidea, the tentacles are arranged in a single crescent or near circle dorsal to the mouth but enclosing the nuchal organ; peripheral tentacles absent introvert hooks recurved and arranged in rings; spindle muscle attached posteriorly. Badri Prasad Haldu, Zoological Survey of Indi.t Calcutta. Sipunculans inhabit all possible habitats right from the intertidal region (Mean High Water Spring Tide) up to the abyssal depths of about 9,600 m of the marine environment Their size also varies from a few millimeters to 60 centimeters in length.

They are benthic forms with crawling and burrowing mode of life, excluding one species which is a swimming form. These marine' invertebrates are detritus feeders and their feeding habit, specially of the larger forms, helps them to rebuild the substrate in a manner somewhat similar to earthworms and holothurians. By their feeding and burrowing habits and also their ecological association with other organisms, the sipunculans carved for themselves a special status in a marine biotic community.

The boring activity of sipunculans causes a good deal oferosion to the coral and rock substrate by mechanical abrasion or chemical action. Their association with other animals ranges from predator-prey relationship and commensalism to parasitism.

Historical Resume

The knowledge of sipunculans from the Indian coast is fragmentary. The earlier investigations were carried out in a rather haphazard manner from different coastal belts of India.

Ll Col. Hardwicke collected a specimen from India which was described by Gray in 1828, but unfortunately, no specific locality was mentioned. At present, the species is restricted to the mangrove zone of India, particuarly the Sundarban.

The pioneering work on the Indian sipunculans dates back to 1903 when sipunculans of the Minicoy Island in the Lakshadweep, collected during Gardiner's expedition to the Maldive and Laccadive Archipelagoes, were studied by Shipley (1903). Unfortunately, many of the species described ~y Shipley (Ioc. cit.) were synonymised. Moreover, this collection was not traceable in any.of the museums of Great Britain. This work followed by Annandale's (1907) report on an anonymous species from the brackish water ponds of Port Canning, Lower Bengal (= southern West Bengal), and then by Gravely's (1927) work on the fauna of the Krusadai Island in the Gulf of Mannar and Prashad's (1936) publication on a small collection of sipunculans from the Indian waters.

For nearly three decades after Prashad (1936), no publication appeared on this group, in India, though work on this group progressed rapidly in other parts of the globe. Johnson (1964, 1969, 1971) of the Pilani University, Rajasthan, described several new species from stray collections from Port Blair, (South Andaman Island), Port Okha (Gujarat), Minicoy Island (Lakshadweep) and Krusadai Isalnd in the Gulf of Mannar, as also some new records from the Indian waters. Subba Rao (1970, 1975) reported a brackish water form and studied chloride regulation in the same species. Rajulu (1969, 1975) studied asexual reproduction and biochemistry of the c~lomic fluid in some sipunculans. Reddiah (1975) dealt with the sipunculan habitat in peninsular India.

For the last one and a half decade (1975-1990) a good progress has been made in the Zoological Survey of India, in the taxonomic studies on sipunculans. Haldar studied the forms from the Indian seas in 1975, Andaman and Nicobar Islands in 1976 and 1978,_ estuarin~ forms in 1985 and 1989. and Lakshadweep (in press), in the collection of the Zoological Survey of India. Recent contributions on this group from the estuarine ecosystem of the Hooghly-Matla and Chilka Lagoon have been finalised. In an extensive study, HaIdar (in press) has dealt with the Indian coastal sipunculan fauna with view to up-dating our knowledge on this group.

Manavalaramanujam(1978-1982),studied histochemical nature ofthe cuticle and related structures, mechanism of calcification of shields, myohemerythrin and mucus in sipunculan worms.

Studies from Different Environs

As has already been stated, habitats of sipunculans include sandy, muddy and hard substrata. from intertidal to abyssal depths. They inhabit such varied niches as under surfaces, clefts and interstices of rocks, rubbles, boulders, corals, etc., sand pools, mangroves and even estuarine or brackish environments.

Most of the earlier studies on sipunculans were of taxonomic and faunistic nature, and contain only casual references to the nature of the habitat Later investigations contained some information on the general nature of the environment from where the specimens were collected.

There had been only very few studies pertaining to the group with emphasis on the habitat, and all were related to the shallow water fauna. Johnson (1964, 1969, 1971) studied the taxonomy, ecology and zoogeography of some forms from Port Okha, Port Blair and Minicoy Island. Shallow water forms collected from different parts of the Indian coast were being reported from time to time which had been consolidated by Haldar (1975-1990). In these publications, insular ecosystems of the Andaman and Nicobar Islands, and Lakshadweep had been thoroughly explored so far as the intertidal forms were concerned.

Our knowledge of the deep sea fonns of sipunculans is very poor. The intertidal and shallow water forms of the RIMS 'Investigator!-collections were studied by Haldar (1975). During the international Indian Ocean Expeditions (1963-1964), foreign vessels "Anton Bruun" and "Te Vega" explored the Arabian Sea and the Bay of Bengal, and the material was studied by Cutler (1977), and Cutler and Cutler (1979). Reddiah (1975) studied the sipunculan habitat in peninsular India.

Indian universities are not lagging behind in sipunculan research. Among these, Pilani University has been in the forefront. Johnson (1964-1971) studied the taxonomy and zoogeography of this group and described several species. Subba Rao (1970 &1975) of the Andhra University has thrown some light on the salinity tolerance and chloride regulation in a sipunculan worm. Rajulu (1969, 1975a, 1975b) noted asexual reproduction (1969), and made biochemical studies on the coelomic fluid of two species (1975a, 1975b). Manavalaramanujam (1978a, 1978b, 1980a, 1980b, 1980c, 1981a, 1981b, 1982) investigated histochemical studies of the cuticle and related structures of tWi> species, mechanism of calcification of shield, myohemerythrin and mucus in sipunculan works.

Sipunculans of the estuarine ecosystem have attracted still less attention. Earlier work of Annandale (1907) was on an unnamed sipunculan from Port Canning, southern West Bengal. Haldar (1985) reported faunal wealth of this group from the Indian estuaries. Recent contributions on this group from the estuaries ecosystem of the Hooghly-Matla, and Chilka Lagoon are being finalised by Haldar.

Estimation of Taxa

Sipuncula of the world comprises 202 species in 17 genera under six families. Of these, nearly half the number of species in genera have been recorded from the Indian Ocean. The sipunculan fauna of the Indian coast is fairly rich, represented by more than one-third of the total species known from the Indian Ocean.

It may be noted that the family Phascolionidae, though represented by a good number of species from different oceans, including the Indian Ocean, has not so far been. reported from the Indian region. The members of this family live in deeper waters which have not been explored properly in the Indian region.

Of the 38 species known from India (Haldar, in press), 18 species are rock ..borers and at least two species are mud ..burrows, while seven species are sand-burrows, 11 species occur under rocks, stones and coral boulders or on sand. Truly deep water forms are still unknown either from the Arabian Sea or from the Bay of Bengal. Though Hyman (1959) stressed on the exclusive occurrence of the sipunculans in the marine habitat, at least three species have been reported in from the estuarine habitat of some parts of the globe, subsequently. Recently, two species have been reported from the estuarine and brackish water zone of India also (Ganapati and Subba Rao, 1970; Haldar, 1989; in press). Animal Resources ofIndia Genera No. of species World. Ocean Indian Oce~ Indian Coast

Total 202 104 38 Distributional Pattern of sipunculan fauna in India Region No. of species Arabian Sea; Off-shore waters 4 Bay of Bengal: Off-shore waters 4 Lakshadweep 17 Andaman Islands 25 Nicobar Islands 14 Northwestern coast (Gujarat & Maharashtra) 15 Southwestern coast (Kerala) Southwestern coast (ramil Nadu, Islands in the Gulf of Mannar) 14 Northeastern coast (Andhra Pradesh, Orissa &West Bengal) •7

Classified Treatment

In addition to the taxonomic studies, a few other aspects of sipunculans have been studied. Ganapati and Subba Rao (1970) studied the salinity tolerance of Siphonosoma australe, while chloride regulation in the same species was studied by Subba Rao (1975). Asexual reproduction by budding in Sipunculus robusluS was reported by Rajulu (1975).

Manavalaramanujam studied several aspects of sipunculans such as histochemistry of the cuticles in Sipunculus robustus and Eloeosiphon Aspergillum (1980a, 1980b), biochemistry of the same species (1980c), histochemistry of mucus in Thmisle lage~iformis and S. robustus (1980d). The myohemerythrin concentration and mechanism of calcification of the anterior shield in Cloeosiphon aspergillum were reported by Manavalaramanujam (1981, 1982). 

Current Studies

In the Zoological Survey of India, mainly the systematics and geographical distribution of sipunculans are being studied•at present,• with emphasis on state-wise faunal accounts. The projects on sipunculan fauna of the Lakshadweep, Hooghly-Matla estuary and Chilka Lagoon have been completed. The current project is on the fauna of Gujarat coast. Outside the Zoological Survey of India no work on sipunculans is being carried out at present

Outside India, several scientists, particularly those from U.S.A., U.K., France, Spain, Australia and Russia, arc engaged in revisionary, ecobiological. and physiological and developmental studies as also on evolution, classification, etc.

Expertise India

In Z.S.I

B. P: Haldar. Zoological Survey of India, 27, J. L. Nehru Road, Calcutta 700 016

Abroad

S. J. Edmonds, Honorary Associate, South Australian Museum, North Terrace, Adelaide, South Australia, 5000.

E. B. Cutler, Department of Invertebrates, Museum of Comparative Anatomy, Harvard University, Cambridge, MA 02138, U.S.A.

N. J. Cutler, Biology Department, Hamilton College, Clinton, New York, 13502, U.S.A. Mary E. Rice, Smithsonian Marine Station at Link Port. Fort Pierce Florida, 34946, U.S.A. V. V. Murina, Institute of Biology of Southern Seas, Academy of Sciences of Ukranian SSR, Sevastopol, 335000, U.S.S.R.

P. E. Gibbs, Marine Biological Association, The Laboratory, Citadal Hill, Plymouth, Devon, PLI 2 PB England.

Selected References

Cutler, F. B. &Cutler, N. J. 1979. Madagascar and Indian Ocean Sipuncula. Bull. Mus. natn. hist. nat. Paris, (4) 1 (Al4) : 941-990; 21 figs.

Cutler, E. B., Cutler, N. 1. &Nishikawa, T. 1984. The sipuncula of Japan, their systematics and distribution Pubis. Seto mar. bioi. Lab., 29 (4/6) : 249-322, 2 pIs., 16 text-figs. Edmonds, S. J. 1980. A revision of the systematics of Australian sipunculans (Sipuncula). Rec. S. Aust. Mus., 18 (1), 74 pp., 133 text-figs.

Haldar, B. P. 1975. Sipuncula of the Indian Seas in the collection of the ZOological Survey of India. Proc. intn. Symp. bioi. Sipuncula and EchiuTa, Kotor, 1 : 51-92, 2 tabs., 6 maps. Haldar, B. P. (in press). Sipunculans of the Indian Coast. Rec. zool. Surv. India, (Ms. 234 pp., 202 text-figs.)

Hyman, L. H. 1959. Phylum Sipunculida. In : The Invertebrates. 5 : 610-696, 213-241 text-figs. McGraw-Hill Book Company, New York, Stephen, A. C. &Edmonds, S. J. 1972. The phyla Sipuncula and Echiura. Trustees of the British Museum (Natural History), London, 528 pp., 60 figs., 17 tabs.