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Archiannelids comprise a heterogeneous group of small segmented marine worms, usually without parapodia or setae, by which they are readily differentiated from their closest relatives, the polychaetes. These worms are aberrant annelids well adapted for interestitiallife. Their length is highly variable between 0.25 mm and 80 mm, with the somatic segments ranging from 5 to 250. The body is dorso-ventrally flattened and pale-white or transparent, with the cuticle often supporting hypodermal glands. Head is distinct and bears two tentacles and-in some cases statocysts, nuchal organs and ciliary bands. Eyes are absent in the majority. Hind end or pygidium bears anal cirri or lobes or papillae, which are mostly adhesive in nature. Anal segment and its lobes are richly supplied with adhesive hypodermal glands. Sexes are separate. Gonads are paired and produced distinct ova and sperm.
All the archiannelids are free-living, harmless and occur in marine sediments, with the exception of the sole freshwater genus Troglochaetus. These worms generally prefer coarse sands with sufficient interstitial spaces near low water level, although some occur in sublittoral and subsoil habitats. Few archiannelids also live in littoral muds or algae. The smaller specimens are interstitial in their habit, while the larger ones are burrowing in the substratum. They.are highly thigmotactic and adhere to substratum firmly during any commotion in the habitat. Their mode of locomotion is mostly by creeping, although the larger worms burrow in sand with remarkable ease. The archiannelids are largely omnivorous feeding on fine organic detritus, bacteria, diatoms and other smaller metazoans.
They are gregarious, the phenomenon being closely related to tidal rhythm in some cases. Being intertidal animals, they are largely eurythermic and euryhaline, tolerating a wide range of temperature and salinity in the habitat. Higher temperature and salinity of coastal waters during summer months act as a stimulus for these worms to breed and reproduce more during March to August. Quantitatively, their densities in marine sediments are known to range from a few individuals to a few hundreds of specimens per 10cm2 of the sediment. Besides fonning food for larger animals in the littoral ecosystem, archiannelids also proved quite sensitive to ecological stress resulting from organic pollution in the habitat, thereby serving as indicators of pollutiop. They also form suitable material for experimental studies on ecology.
Interest in the study of archiannelids inhabiting the intertidal marine sediments and littoral algae dates back to the middle of the 19th century when the earliest worms were discovered and described on the European coasts, as Nerilla antannata Schmidt (1948), Dinophilus gyrociliatus Schmidt (1857), Polygoridus lacteus Schneider (1868), Saccocirrus papillocerus Bobretzky (1872) and Protodrilus leuckarti hatschek (1882). Since then, a good number of species, genera and families of the archiannelids from different parts of the world were described by several authors as new to science. However, nothing was known of this. group of worms from Indian coasts until the pioneering contribution on archiannelids was made by Aiyar and Alikunhi (1944) and Alikunhi (1964, 1948) from intertidal sands on the Coromandel.coast. These authors described with much anatomical detail two species of Polygordius (P~ madrasensis, P. uroviridis), two species of Protodrilus (P. pierantonii, P. indieus) and four species of Saccoeirrus (S. minor, S. eirratus" S. orientalis, S. krusadensis) as new to science. Subsequently, while studying the interstitial meiofauna from the beach sands on Waltair coast, R~o &Ganapati (1968) reported the occurrence of 12 species of archiannelids, including those known from the European coasts (Nerilla antennata Schmidt, Nerillidium mediterraneum Remane, Diurodrilus minimus Remane, D. benazzii Gerlach, Trilobodrilus nipponicus Uchida and Okuda, Dinophilus gyrociliatus Schmidt) as well as those from the Coromandel coast. Further ~nvestigations of the Waltair beach sands have led to the description of a new species, Trilobodrilus indicus (Rao, 1973).
A good number of archiannelids were also reported from intertidal sediments on the Orissa coast (Rao, 1969, 1989). Outside the Indian mainland, several known and unknown species of these worms were commonly reported in littoral sediments on the coasts of Andaman and Nicobar Islands (Rao, 1975, 1980, 1987, 1988) in the Bay of Bengal and Lakshadweep (Rao, 1983, 1990) in the Arabian Sea. More Recently, Jouin and Rao (1987) made detailed morphological investigations employing SEM on archiannelids of the Indian Ocean including those from the Indian subcontinent, Lakshadweep and Andamans, resulting in the description of a new subspecies Polygordius eschaturus brevipapillosus.
All the above taxonomic studies threw considerable light on the wide geographical distribution of these interesting worms in the seas around India. Yet, many areas of Indian coast, particularly on the west, remain unexplored or underexplored for this group of annelids. Hence, detailed exploration of the littoral habitats are like to reveal the existence of more species in this region.
Estimation of Taxa
Taxonomic features generally utilised in the identification of genera and species of Archiannelida are : their body size, number of somatic segments, shape of pygidium and its adhesive papillae, structure of tentacles, palps, eyes,.nuchal organs, ciliary bands, etc. on head, disposition of salivary glands, pharyngeal bulb, structure and position of circulatory apparatus, excretory and reproductive organs, sperms and the number and structure of parapodial setae when present
The archiannelids were earlier considered as a class of primitive annelids, but according to the recent opinion of some zoologists, they arc polychaetcs secondarily adap.ted for interstitial mode of existence by the loss of lateral parapodia and hence are to be considered as an order of the Polychaets (Hermans, 1969). However, the archiannelids hitherto known comprise six families representing 21 genera and over 120 species as given below: Polygordiidae (Polygordius -15 species), Protodrilidae (Protodrilus -32 species, Astomus -1 species), Protodriloidae (Protodriloides -2 species), Saccocirridae (Saccocirrus -18 species), Nerillidae (Nerilla -10 species, Nerillidium -8 species, Mesonerilla -10 species, Meganerilla -2 species,. Nerillidopsis ¬1 specie,S, Paranerilla -1 species, Bathynerilla 1 spccies, Afronerilla -1 species, Troglochaetus'-1 species, Bathychaetus -1 species, Thallassochaetus -1 species, Psammoriedlia -1 species), Dinophilidae (Dinophilus -8 species, Trilobodrilus -4 species, Apharyngtus -1 species) and Diurodrilidae (Diurodrilus -6 species). But, the archiannelids known within the Indian region represent all the 5 families (except Protodriloidae), 8 genera and 21 species.
Out 'of the 21 species known in this region, 16 species are eurytopic occurring in wide]y distributed areas of the Atlantic, Pacific and Indian Oceans, while about 5 (25%) are endemic. The eurytopic species, such as Prolodrilus indicus Aiyar and Alikunhi, Nerilla antenneta Schmidt and Dinophilus gyrocilialus Schlnidt are most widely distributed, while the endemic species Saccocirrus cirratus Aiyar and Alikunhj, S. orientalis Alikunhi and Trilobodrilus indicus Rao are restricted to a few pockets in this region.
CurrenLly, studies on the systematics and distribution of Archiannelida along the Indian coast are being pursued by G. C. Rao in the Zoological Survey of India. Material of these worms was being collected during general faunistic surveys of the coastal areas. No attempts are being made outside ZSI to carry out any kind of study on these annelids. Outside India, Mme Claude louin is an active worker and authority on this group, having made a remarkable contribution to the knowledge of these worms from different parts of the world. However, the natural habitats of these animals are under constant and serious threat due to effects of pollution, disturbance, removal of beach sands for construction purposes, etc., resulting in the elimination of many of these sensitive species on our coasts.
O. C. Rao, Zoological Survey of India, Andaman &Nicobar Regional Station, Port Blair ¬744 101.
C. Jouin, Laboratoire de Biologie et Physiologie Marines, Universite Paris VI9 4 Place Jussieu, F 75252 Paris Ccdex 05, France.
W. Westheide, Universitat Osnabruk, FB 5/Spezielle Zoolgie, Postfach 4469, D-4500 Osnabruck, Germany.
P. Schmidt, Zoologischcs Institute der Universitat Gottingen, Gottingen, Germany.
S. Sasaki, Department of Zoology, University of Hokkaido, Hokkaido, Japan.
J. S. Gray, Institut Marin Biologi Limnologi, Universitat Oslo, Norway.
P. J. S. Boaden, Marine Biological Station, Portaferry, Northern Ireland, U.K.
R. Brown, Macleay Museuln, University of Sydney, Sydney, Australia.
E. Kirsteur, Department of Living Invertebrates, American Museum of Natural History, New York -10024, U.S.A. Selected References Aiyar, R. G. and Alikunhi, K. H. 1944. On some archiannelids from the sandy beach. Madras. Proc. Nat. Inst. Sci. India, 10: 113-140. Hartmann-Schroder, G. 1986. Polychacta (including Archiannelida). In: Stygofauna Mundi, (Editor: L. Botosaneanu), 1: 210-233. Hermans, C. O. 1969. The systclnatic position of the Archiannelida. 5yst. Zool., 18: 85-102. Jouin, C. 1971. Status of the knowledge of the systematics and ecology of Archiannelida. Smithson. Conlr. Zool.,76: 47-56.
Jouin, C. &Rao, G. C. 1987. Morphological studies on some Polygordiidae and Saccocirridae (polychaela), from the Indian Ocean. Cah. Bioi. Mar., 28: 389-402. Rao, G. C. &Ganapati,~. N. 1968. On some archiannelids from the beach sands ofWalta.ir coast. Proc. Ind. Acad. Sci., 67: 24-30.
Remane, A. 1932. Archiannelida. Tierwelt der Nord-und Ostsee. 6: 1-36. Schmidt, P. &Westheide, W. 1977. Intcrstitielle fauna von Galapagos: Polyrdiidae, Saccocirridae, Protodrilidae, NerilHdae, Dinophilidae (Polychaeta). Mikrofauna Meeresbondens 62: 1-38.