CORAL REEF ASCIDIANS of NEW CALEDONIA Fabrication Et Coordination: Catherine Guedj, 8Evislon Du Texre Anglats Liz Ct Todd Newberrv
Total Page：16
File Type：pdf, Size：1020Kb
CORAL REEF ASCIDIANS OF NEW CALEDONIA Fabrication et coordination: Catherine Guedj, 8evislon du texre anglats Liz ct Todd Newberrv. La 10; du 11 mars 1957 n'outonsont. aux termes des ouneos 2 et 3 de I'article 41, d'une part, que les " copies ou reproductions strictement reservees Cl I'usage prlve du copiste et non destinees Cl une utilisation collective" et, d'outre part, que les analyses et les courtes cita tions dans un but d'exemple et d'illustration, " toute representation ou reproduction inte grale, ou partielle, faite sansle consentement de I'auteur ou de sesayants droit ou ayants cause, est illicite" (alinea 1er de I'article 40) Cette representation ou reproduction, par quelque precede que ce sort. constituerait done une conrrerocon sanction nee par les articles 425 et sulvonts du Code penal. ©ORSTOM ISSN : 0152-674-X ISBN: 2-7099-1050-0 CORAL REEF ASCIDIANS OF NEW CALEDONIA Claude Monniot Fran<;oise Monniot Pierre Laboute Editions de I'ORSTOM INSTITUT FRAN9AIS DE RECHERCHE SCIENTIFIQUE POUR LEDEVELOPPEMENT EN COOPERATION Collection Faune tropicale noXXX Paris 1991 We gratefully acknowledge Orstom facilities and the "Vauban" and "Dawa" 's crew for their help in collecting the animals. We are also grateful to G. Bargiban, J. L. Menou and A. J. Bruce who graciously gave us some pictures in addition to P. Laboute's illustration. Special thanks also go to A. Crosnier whose continuing interest in our book has largely contributed to its production. We are much indebted to Liz and Todd Newberry for their skillfull and creative revlsion of the engllsh. La collection Faune tropicale a pour vocation de diffuser les connais sances les plus recentes sur la svsternotlque des rwertebres et des verte bres des regions chaudes, connaissances indispensables aux recherches orientees vers le developpernent. Le contenu des ouvrages varie de la mise au point taxonomique sur un groupe zoologique au guide faunistique de terrain ou Cl la faune regio nale illustree. touchant ainsl un public plus ou moins large: svsternotl clens. ecoloqlstes. enseignants, etudiants ou simples amateurs d'histoire noturelle, Pierre Le Loeuff Directeur de collection Derniers volumes porus dons cette collection: XXIX, Les Coieootetes Scarabaeoidea de Nouveue-Cotedonle - Renaud Paulian, XXVIII. Faune des poissons d'eaux douces et sauraattes de I'Afrique de I'Quest (tome 1) - Christian Leveque, Didier Paugy, Guy G, Ieuqels, XXVII. Lesserpents de la Guyane ttancoise - Jean-Philippe Chippaux, XXVI. Ctues fibres de I'Afrique Intertropicale - Jean Dragesco, Armelle Dragesco-Kerneis, XXv. Guide des etones de mer, oursins et autres echinodermes du lagon de Nouveiie-Coieoorne - Aloin Guille, Pierre Laboute et Jean-Louis Menou INTRODUCTION The natural world. although often unfamiliar. is waiting for us to acquaint ourselves with it and to understand it through close. patient observations. While we expect to encounter the unknown when we venture on hikes and dives. we often come upon it in disconcertingly intimate ways. For example. we generally know what we eat. But seafoods have a way of surprising us - and even leave us puzzled after our meal has been "identified". You may on occasion have eaten a "violet". an "oya", or a "plure". but did you realize you were eating an asctdian. a so-called sea squirt? On scuba or snorkeling trips you surely have seen asci dlans. probably without knowing it and perhaps without realizing that you were looking at an animal at all. In fact. ascidians are our closest relatives among the invertebrates. Even so. except for a few specialists. naturalists rarely have more than a passing ac quaintance with these inconspicuous creatures. Asctdians lack the external skeletons or shells that draw our attention to animals like molluscs, crustaceans, and echinoderms. Nor do they move about. Instead. ascidians usually form small. soft. immobile mas ses. often quite drab in color. that resemble sponges or even sto nes. But what a remarkable variety of shapes. patterns. and colors they reveal to the practiced eye! Ascidlans have been recognized as a coherent group only since the end of the 19th century. Aristotle mentioned ascidians but did not try to classify them among other animals. Names like stony or soft corals. zoophytes. molluscs without shells. or molluscoids all suggest how difficult it has been for zoologists since antiquity to interpret the structures of these strange creatures. In the late 19th century. however, when embryology was transforming the ideas of systematists. the Russian biologist Kowalevsky found C. Monniot. F,Monniot P. Laboute 5 similarities between ascidian larvae and frog tadpoles. With this discovery, he theorized a link between invertebrates and verte brates. The group Protochordata was born, and ascidians beca me the topics of lively debates about evolution. During the 20th century, much of the earlier interest aroused by ascidians has waned. No one now doubts that they are among the precursors of vertebrates, but the difficulties inherent in stu dying "soft" organisms have often discouraged research on the biology of ascidians. This decline in attention has been unfortu nate, because we find so many extraordinary traits in these orga nisms. First of all, ascidian adults are in many ways simpler animals than their own larvae - a situation unique among ani mals. Moreover. the tunic that envelops ascidian bodies contains large amounts of a substance very much like plant cellulose again, an extremely rare occurrence in the animal kingdom. Third, the ascidian heart and the body'S entire circulatory system reverse the direction of their bloodflow every few minutes. And finally, many of these protochordates not only breed sexually but also replicate their bodies asexually by growing new ones from tissue fragments. The current revival of interest in ascidians, however, has little to do with such odd traits as these. Instead, it grows out of recent recognition of the asctdian's possible medicinal value. as phar macological research has turned to natural products of marine origin. General screenings of organisms for possible medicinal use have consistently singled out two groups, sponges and asci dians, both of which are sedentary filter-feeders. Ascidians may also play a role in another major concern of our times - the quality of our environment. Their coastal marine habi- Coral Reef Asclcnor» of New Ccledoruc 6 rats are particularly affected by pollution. Since ascidians concen trate a number of toxic elements. including heavy metals and hy drocarbons. they are turning out to be very good indlcators of water quality. So ascidians are once again in fashion. Their abundance and di versity in reef habitats are now arousing the attention of scien tists and divers alike, Of course, the current scarcity of specialists in ascldlan biology, and even of naturalists generally. limits our cornprehenston of ascldian biology. But international efforts now devoted to studying reef systems - their biota and ecology - will certainly lead to a better understanding of these still mysterious creatures. C, Monniot,F. Monniot P. Loboute 7 General organisation of an ascidian FROM THE EGG TO METAMORPHOSIS Ascidians are hermaphrodites: they have independent male and female gonads (even if these gonads are tightly conjoined) in the same body. They all release sperm into the sea. Mature eggs may also be re leased as gametes. to be fertilized in the open sea (oviparous species), or they may be held and fertilized inside the parental body. In the latter case. the embryos are brooded until they com plete their pre-larval development (ovoviviparous species). In either case, the eggs, which are almost always less than I mm in diameter. undergo total cleavage and develop into swimming larvae. The larvae are called tadpoles because they have ovoid trunks and motile tails, features that recall frog tadpoles. Whether they develop from freely spawned eggs or emerge as active tad poles from the parent's body, ascidian larvae do not feed. In ovo viviparous species. the tadpole's free-swimming stage is extremely short, with metamorphosis occurring just minutes or, at most. a few hours after its release. Tadpole Structures and their Protochordate Characteristics The larva is divided into a trunk and a tail, and its whole body is covered by a delicate tunic. A notochord, consisting of a column of about 40 large cells. extends from the posterior part of the trunk to the tip of the tail. Along each side of the tail are bundles of striated muscle fibers. C. Monniot. F.Monmot P,Lcboute 11 Ascidian tadpole: the tail, co ntaining the notochord, encirc les the trunk, Three anterior adhesive papillae can be seen inside a ring of epidermal papillae. The sensory vesicle con tains black granules, the otolith a nd oc ellus, a nd isloc ated ne ar the still-closed oral siphon. The branc hial sac and the gu t ha ve begun their developmen t . Th e tadpol e's nervous sys tem is most highly developed in the trun k, where it forms an an terior ganglion and a large sensory v( < lcle. It, too , runs the leng th of the tail. as a hollow, dor sal neu ral tub e above ih e no tochord . Both in formation and position , the not och ord and the dorsal nerve are homologous to those o f ver teb rates , The trun k's se nsor y vesicle, a sort of larv al "br ain", co m bine s a b a la n c e o rgan , the statocyst, with a li g ht-s en s it iv e s tru c ture, the ocellus . Th e tad ooi es d i gl~s t ive tract cons is ts of an enlarged and perfora ted p haryn x lendi ng in to a rather un iform tubular gu t. Since the gu t lacks an op en mouth and ends blindly.