NEW CALEDONIA

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CORAL REEF ASCIDIANS

OF NEW CALEDONIA

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Fabrication et coordination: Catherine Guedj, 8evislondu texre anglats Liz ct Todd Newberrv.

La 10; du 11 mars1957n'outonsont. aux termes des ouneos 2 et 3 de I'article41,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 sans le consentement de I'auteur ou de sesayants droit ou ayants cause, est illicite" (alinea 1er de I'article40)

Cette representation ou reproduction, par quelque precede que ce sort.constituerait done une conrrerocon sanction nee par les articles425et sulvonts du Code penal.

©ORSTOM ISSN : 0152-674-X ISBN: 2-7099- 1050-0

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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 no XXX

Paris 1991

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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 andA.J.Bruce who graciously gave us some pictures in addition toP.Laboute's illustration.

Special thanks also go toA.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 Scarabaeoideade 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. Lesserpentsde la Guyane ttancoise - Jean-Philippe Chippaux, XXVI. Ctuesfibresde I'Afrique Intertropicale - Jean Dragesco, Armelle Dragesco-Kerneis,

XXv. Guide des etonesde mer, oursins et autres echinodermes du lagon de Nouveiie-Coieoorne - Aloin Guille, Pierre Laboute et Jean-Louis Menou

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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 ascidlans. 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 masses. often quite drab in col or. that resemble sponges or even stones. 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

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Coral Reef Asclcnor»

of NewCcledoruc

6

similarities between ascidian larvae and frog tadpoles. With this discovery, he theorized a link between invertebrates and vertebrates. 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 studying "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 organisms. First of all, ascidian adults are in many ways simpler animals than their own larvae - a situation unique among animals. 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 ascidians, 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-

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rats are particularly affected by pollution. Since ascidians concentrate 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 diversity 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

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General organisation

of an ascidian

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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 released 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 complete 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 tadpoles from the parent's body, ascidian larvae do not feed. In ovoviviparous 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

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Ct;"!fOIf(? et Asc1dlOr_ otNewCol6<:JOl".IQ

12

Ascidiantadpole:

the tail,conta ining the notocho rd, encirclesthetrunk, Three anterior ad hesive papillae canbe seeninside aringofep idermal papillae.

Thesensoryvesicle containsblackgra nules, the otolithand oc ellus, andisloca ted near the still-close doralsiphon.

Thebranchialsacandthe gut havebegun theirdevelopmen t.

The tadpol e's nervous system is most highl y dev elo p ed in the trun k, where it forms an an terior ganglion an d a large sensory v(<lcle. It,to o,runs the leng th of the tail. as a hollow, dor sal neu- raltube above ihenoto chord . Both in formation an d position,the not ocho rd and the dorsal nerve are homologous to tho se ofver- tebrates,The trun k's sensor y vesicle, a sort of larv al "brain", co m- bines a balanc e organ, the statocyst, with a lig ht -sen s it ive structure, the ocellus .

The tadooiesdigl~stive tract cons is ts of an enlarged and per fora- ted pharyn x lending into a rather un ifo rm tubular gut. Sinc e the gutlacks an op en mouth an d ends blindly. it is not yetfunctional in the tadpole. t-ven so, the ou tlines of th e adult's incu rren tan d excu rren t water siphons arc already visible on the do rs alpart of the trun k. Ad hesiv e pap illae (either two or three. depending on the spec ies)proj ectfrom the anterio r part of the trunk.These papillae will attach the tadpole to the subsrrare when itsettles and m(:l<:unor pho scs.At th is stage . the trun k also con tains the rudi- men:ofaheart and masses of undifferentiatedcells.

The tad p ole swims by beating its tail sporadically bu tvig o rous ly trom sideto side.It tends to swim upw ar d and to w ard ligh t.

Metamorphosis

Suddenly,within a sp an of jus: a few minutes , the tad p ole turns awa y from the lightand seeks the substrare .When itenco un te rs a sui table surfa ce. it affix e s itself with its adhe sive papi llae. Immedi ately, the tail co llaps es and is res o rb e d into the trunk,as larv al tis sues break up and disappear. The an imal then rota tes

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90° wlthi n the tunic , so that the ora l siphon now points aw ay from the subsrrate: it then opens. The sensory vesicl e an d the rest of the larv al nervous system disappear,while a new neural gang lio n forms. The animal begins to feed. 11 grows rapidl y,its body wall develops an arra y of smooth muscles, and the tuni c thickens The young asci d ian has no w lost all trac e of its larval notochord and ne ural tube .11 will not sho w anysigns of ceph ali- zati on and, wit h rare exceptions . will rem ain fixe d in place, im m o bil e.

ORGANS OF THE ADULT ASCIDIAN

Solitary or colon ial, all ascidians have the gene ral body plan de- pict ed in the figure.Since the larva's nerv o us system is dorsal, the neural ganglion,situa ted between the siphons, is now used to define the dorsal surface oftheadult.

The oral sip hon is taken to mark the body'Santer ior end.The posterior end is mo re arbitraril y designa ted but usually is consi- dered to lie bey on d thegut-lo op, as faras possib le fro m the oral siphon.Thus the clo acal sip hon is roughly "mid-d or sal", and the endostyleofthebran ch ialsac marks the body'Smid-ventrallin e.

Solitary tndivtduals. the largest of which may reach 30 cm fro m bas e to apex. are general ly much larg er than the comp onent

C.~lIot.F. lot P. loboole

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bodies of colonies, whose zooids are usual1y measured in mil1i- meters. Some colonies, however, may spread over an enormous surface, as large as a square meter or more.

The adult body is hollow, a complex sac open to the outside through two tubular projections, the siphons. One of these openings allows water to enter the body and be filtered, and the other conducts filtered water out, along with digestive wastes and sexual products.

The Tunic, an Animal Cellulose

The adult body is entirely embedded within a tunic, which forms a somewhat flexible exoskeleton, The tunic's composition is very unusual, since it consists largely of polysacchartdes much like the constituents of plant cellulose. Various proteins also occur in the tunic. as do some blood cel1s. The tunic is secreted in part by the cells it contains and in part by the external epithelium of the body wal1.

The consistency of the tunic varies enormously. In some didemnids it can be very soft, even slimy. Some pyurids and styeltds, on the other hand, have such hard, thick tunics that they feel like leather and look like small stones. Between these extremes, the tunic ranges from soft and yielding to tough and cartllaginous.

The tunic's appearance varies so Widely, in fact, that it may be of little use in identifying species in the field. Transparent as glass or densely pigmented, drab or brilliant in color, the tunic can easily

Coral ReefAscidlons ofNew Caledonia

---

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ADiplosomacolony'sslimy tunic, is so tran sparent

that thezooidsare easily seen.

lead one 10 confuse asci di ans with othe r organism s . espec iall y sponges or alg ae.

The tunichas seve ra l funct io ns. First, itanc hor s the animal to the substrate. Insome species,it produces long filam ents that help the ani ma l anchor itse lf on soft sed im ents.The Tunic can also retainallsorts ofanima l. mineral. andplan telemen tson its su rfa- ce or within its matrix These inclus ions tend to increase the tunic's firmness . to isolate the anim al from environm ental extremes . and toprov idesomepro tec tio nagains t predato rs.Some tunics produce papillae or spi nes in vario us arrangemen ts. scales, and othe r short ourgro w ihs. an d some con tain calca reo us spi- cules withshapes ch arac teristic of the partic ular speci es.

A firm tunic.the mostcom mon type.also maintains the animal's shape .preve n ting the collapse of its intern al cav ities.In tact.the tunic acts antagonistically 10 the musc ul ature of the body wall. When the animal is disturbed, these muscles cont ract, closin g the siphons and reducing the body'S internal cavities: the bod y regains its full volume and its open apertures on ly when the muscl es relax and the elasti c support of the tunic can reasse rt itself

The tunic is by no mean s an inertstruct ur e.Itsactivity is impl ied by its many blo o d sinuses, and especi all yby the vartety of blo od cells disp ersed with in it. It is als o a rout e by which to eliminate

C.Monrsot.F.Mon nlOl p Lobo ute

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PhalluSlO julinea(Ascld iida e) has0cartllaglnous and clearlunic onoofte ngrows,as here, among mad rep ororloncorals

Polycarpa pigmentata is a solitary ascidian ofthe family Styelidae.

I1 has a Ihick, very hard, lealhery tunic

Only its siphons reveal the animal here,

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some metabolic wastes and to accumulate others, perhaps for later reuse.

1\ variety of epibionts - sponges, algae, bryozoans, barnacles, polychaetes - may settle and live on some hard tunics. Some bivalved molluscs and amphipods may even dig pits in the tunic and live there.

The Mantle

The body wall, or mantle, lines the enveloping tunic. It compri- ses two epithelial sheets, an external one lying against the tunic and an internal one delimiting the peribranchial cavity. Between these two layers lie mesenchymatous tissues, the dorsai neural complex, muscuiar flbers, and extensive circulatory sinuses. The mantle's musculature consists of smooth muscle and shows two arrangements: on the one hand, a fine, felt-like coating of short fibers, variously distributed within the mantle, around the gut, and elsewhere; and, on the other hand, juxtaposed circular and longitudinal bundles of long fibers.

Strong sphincters encircle the siphons: elsewhere, circular muscles may wrap the entire body. The bundles of longitudinal fibers generally lie beneath the circular muscle-bands, at sharp angles to them, and radiate from the siphons or the intersiphonal dorsal area. They may in some cases reach all the way to the posterior part of the body.

These long bundles of muscle fibers enable either siphon to close and can contract the body itself, either partially or completely. Such contractions can be very sudden, producing the squirt

C.Monniot,F.Monniot P.Loboute

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TI I1I1,. -;"

\IJ: lIlo..

\tLH'01ISstrtll KI

(:i1i.IIt', i,:-.1IgJI1,i1,l----::''-'-'-'- ' "

I' 'ntJr(.U\illial - - - -(·~l\'I:\

SIII>-c·l ldnslylar

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Diagrammatic tra nsverse section of an asc idian

show ingthe filtrationprocess throug hthebranc hial tissue.

Ascid ia n generalstructure.

OtlJr~i!'e' A~CJdlmu of~..CaIe<:X)r.!r.

18

of "sea squirts". 13lH these con tra ctio ns exh a us t the as c id lan's rep ert ory of mov ements;asci dransare truly seden tary cre a tu res.

T h e BranchialSac

within the ascidlan body is sus pen ded a pharynx . or branchial sac, su rroun d ed by Clpertbrancbia l cavn y The pharynx is attached anterlorly to thebo d y wall(or marutc) <It the baseof tile oral apertur e, slightly beiow Clcircle of rhrcad-Itke tentacles, and it is also attache d along the body'Smid-ventral line . I\t the bottom of the phary n x .a narrow open in g marks the entrance to the cso-

pll; I~US.

Thebranchralsac is held open wtrrur: thepertoranchratcavity by v. iscular stru ts, wh ic h connect the outer epithelium ofthe bran- ch ialsac to tile in ne r epuheuurn of the mantle. Blo od flows back

<I11dforth thro ughthese struts ,betweenthesinusesofthe mant le and those of th« pharynu cal wall. The branchial wall is perfora- ted by many slits, called stigm a ta , which are usually arrangedin very regu lar patte rns . Th e stigmata are bord er ed by cilia. Their strong beat draws a current of water into tile oral apertu re arid torc es it th ro ugh the stigmata, into the peribranch ial cav ity, an d then out the dOnCCII aperture. SC<:l water thus bathes the entire branchialsac and assures gns-exct 1angeatthe pharyngealwall. The branchia l

sa r

is also attac he d to the mantle alo ng the mid- ve-n tral. aruero-p osreno r line .whe re tile wall of the sue carries Cl complex gro ove,theendosryle.The wallsgroov e of the endostyle hav e alrcrn anng longitudinal bands of glandular and ciliated cells: thes e glcm d u lar strips sec re te mucus .m d enzym es . Along

Ora l ten tac l f'~---...,~:r.

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Ciliatedlongitud inalperforations (stigmata)seen from within

thebra nchial sac. Tra nsverse and longitud inalbars delimit'meshes' of thepharyngea l wall (sc anningelectronrnicrosc opv aftercritical-point drying) .

;;:,..,,_ - - - - - - N'eur·a l comp l e x

Cl oa c a l

~1It--- sLp ho n

Per-L br-anchi a I cavi ty

- - - - -Rectum

- - - - -E'sophagu s

_ - - - -Rh izoid

me bottom of the end osry le runs a lin e ofcells bearing long tla- gella. Tile endost yl e presen ts us with still ano the r asc ldian trait thathas intriguingvertebrateparallels. Bothin its structure and its phys io logtc al acuv un·s. such as iodine-Ilxauon and thyroxin-production.tile ascid ian en dostyle closely resembles mat ofCep ha- lo chord ata (lancetets) an d ev en of the arnrn oco ete larva of the cyclo stom eushes (lam p reys).Opposit e the endosryle.alo ng the bran c hialsac's dorsal line. eith er a row of tentacular dorsal 121£1- gueis or else a co n tinuous memb rane (called the dorsal lamina) proj ec tsinto the lumen of thesac.

In tilesimp lestascid ians.thebranchial sac has aflat sur face.per- ror ared bya largenumber ofstigmataarranged intransverse row s In more com plex forms . the branchial surface deve lops internal proj ecting elemen ts: pap illae and internal longitudinal and trans- verse bars I\t its most co mplex. the wall of the phary nx greatly mcreus--s its surface by for ming longitudinal pleats. As we sha ll se-c . these levels of branchial complexity are im p ort ant in in ter- pretin g major taxono mic relati onshipsamong theasc idians.

Filtration

Each branchial perfo ration . or stigma. is bordered by a ring 01 seven paralle lrows of ciliated cells.All the cilia of astigm a beat syn chro nous ly. for cing water from the branchial sac to the pert- brancnial cavuy This curre n t can be brisk: a SOli tary asc idi an 8 cm long can mo ve threeor four liters of water pe r hourac ro ss itspharyngealwCl11.

C.Monrno1,FMonn;ol P LOOOlte

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Mucous web

covering a fewstigma ta.

Note thetiny size of the web's openings comparedtothe size of the stigmatacilia

Cc-c-Q , Ai.;.II1icrl$

d NewCdooonto

- - - -

20

Tile cilia drive wate-rbut the y do nOI filter fo o d-p art icles from it.

111;\1 tClsk is <Jc curllp lislled by very fine muc o us webs. that are se-creted along the en tire length of the endosryle and that glide transv ersel yup on lIlt'phar yngc,t1wall .toward tilebrancntalS;\c'S mid -dorsal line.Tlicsc:webs "Ire carried along bycilia located on the pharyn geal wall oron ihe sac'Sin ternalbranch ial papillae or bars .Alo ng me dor sal line.these webs are roll ed into a rop e-like strand rarucressuspended in the curren t ofsea waterare caught and held on the sticky muco us webs .whose pores range in size from O.I to 03 urn.Th is partlcu late fo od gels mixedinto the dorsal muc ous strand.whic h move-s alo ng the dorsa l languers or lamina 10 tile CSOpl1;lg US. wh ere strand and food together al-e ingested.

Filtratio n by asridians is not selective :part icl es are not activ ely sor ted. Of course . the oral tentacl es. mside the oral sipho n. do block very large objects.An d whe n un destrabtc-parttclcs are en- gu lfed.or to o much se di ment en ters the bran ch ialsac. the ascidian Gin cont ract sud denl y and vi ole n tly . ej e c ting the sac 's co n tent s back through the oral siph on. This behav ior prev ents cloggingof tile,1l 1irll<il's respira toryand food filtra tio n ap p aratus.

The Digestive Tract

Th e rather simpl e ascidian dlge stiv e tract form s a loop that brings Hi e rectu m back close to the csophagus.Starting at til e bottom of the bra nch ial sac. wc find. in tile following order:a cytmd nca l csophagus.a more or less globular sto mach. and an in testine thatoc casionally isdiv ided byco nstrictions into sep ara te

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Det ailsoftwo differentkinds ofmucousad hesivesecretions respon sibleforbranchi alfilter-feeding.

regions.Thedigestiveepimeuurnis ciliated alongitswhole length.

I1 also includes mucus-se creting cells ,gland ular cells,en docrin e cells,and ofcourse food- absorbing cells,There are no elab orate diges tive diverticula or accessory structures . In some famili es, near where it joins the intestine, the sto m ach does con tain a

"caec um", but this small. ringer-like sac has no sp ec ial histologi- cal characte ristic s, its function is unknown.The stomach wall may be creased by lo ngit u d inal folds or orn amen ted by tuber- cles ,which corres pond 10 cellular differentiation of the stom ach wall. In the families Pyur idae and Mol gulld ae.bunchesof glandular cells within dive rticula of the stom ach wall for m a so-called liver.

Theposterior intestine is surrounded by Cl ne two rk of anas to m o- sing tubules whose tips lie against the intern al digestive epithe- liu m . These tubules con v erge into thin ducts tha t run alo ng the gut-loop.Tile sm all ducts joi n in turn to form a co m mon pyloric duct. sometimes dilated into anampu lla.that penetra tes the wall of the stomach. Vario us acuvtues. excretory or dig estiv e, hav e been attribu ted to this "pyloric gland",which is fo und in all ascidians. butits actualroleremains unkno w n,

Food movescon tinuousl y through thegut. Feces form etrhersau- sage-nke shapes or, mor eonen,ov alpellets.They areejected in to the peribro nch ial cavity,which functions as a cloacal cavity,an d pass ou tof the bod y with the water current pourin g through the clo acal siph o n

Depending on the family,th(' gut-loop may be lo cated below the branchi alsac or besi de il, gen erally on the lert (in one family, the Cor ellidae . it lies on the righ t).The gut-tract is alw ays separated from the peribrarutua tcavity bythemantle'sinner eplrneliurn.

C.~.1of1r'ot.F.M<>n.-.cl prcoccto

21

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Cornl~fA.:;.o:,,·JiQr4

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~~ut projec ts very far into the portbranchl al cav ity,it is con - nected to Ihe body wall on ly by mre rmluem trabecu lae ,In many kind s 01" ascrdia ns. tile dis ta l part of th e rectum, and its some- limes rattle r elabo r.uely frilled anus, lie completely free of the clo ac atcavity's wall,ne ar the base of thecioac atsiphon ,

The " Kidney"

Thereis no specializedexc retory organ in ascrdrans.Excretion is carried ou t instead tJy specia lized blood cells, tile nep hrocyr es l':v en so , in Ihe Molg u lidae. an d only in this family. a clo s ed.

bean-shaped vesicle called a "k idney" or "renalsac", lies on tile righ tside of the bo dy.Th is vesic le contains a liquid is oton ic wuh sea water. nitrogen ous concretions (especially uric acid). an d symbio tic bacu.r!a and fungi. The actu a l role of this org an remains unclear:itmay be a nitrogen-storage org an rather than an exc retory one.

The Nervous System

The nerv ous system comp rises nerv es.a single ganglion. and a neura lgland that is tightl y associatedwiththis ganglion Thegan- glion liesdorsally insi de tile mantle, betwee n th e t\>\IOsiphons .ItS

cells arearrang c.d in an externa lcortexaround a fib rous medulla. The ganglion IS elongated, and its ante rior and po sterior corne rs extend into nerves that quickly ramify through the man tle tow ard the siphons and ov er the flanks of the body. t\ vis c eral nerve

Tub ules ofthe pyloricgland covering the externalwall of the intestine (scanningelectronmicroscopy afte rcritic a l-point drying). The internalepithelium of themantlehasbeen removed toshowthe outside oftheintestineitself.

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:"eural ganglion pcripharyngeClI band

Nervous system,

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'::::,:::', ::.:-",' f ' ; -

\.

...

/

.\, (

.)0\,

-

- - - --,---,./ ,

^{. :}

^'\,\ ^'\

:"eural gland

xerts

DuetOrIlleneural gland ' . , . . . , - - - - norsaruoercule

- . ; - - - \ I a m l e

...-;:''--',,''"'""''JJL.-''---TUnic Oralteruarles

::....

SAGITTi\L SECTIO:" INTEHNAL SIDE

penetrates ventrally, as well, toward tile dorsal part of tile pl la- rynx. It is important to note that neither the ascidian's heartbeat nor the movement of its branchial cilia is neurogenic at all. let alone controlled by the neural ganglion.

We still know very little about the sensory organs of these creatures. Sudden changes in light, vibrations, and chemical and ther- mal stimuli may cause ascidians to contract. Sensory cells have been described in tile internal epithelium of tile mantle, near the siphonal apertures, but no one has yet studied the behavioral traits and sensory physiology of ascidians very thoroughly. The little one can say is that stimuli provoke both direct and crossed reflexes. Some experiments indicate that the neural ganglion is involved only in crossed reflexes and that it thereby provides a certain amount of muscular coordination.

The neural ganglion is always associated with a round neural gland, made up of clumps of cells forming an intricately convolu- ted surface that surrounds a central lumen. A ciliated duct leads from this gland to a dorsal opening at the base of the oral siphon, just ahead of the branchial sac, between tile oral tentacles and the beginning of the dorsal lamina. This opening is variously called the ciliated funnel, the ciliated slit, or the vibratile organ: its site is called the dorsal tubercle. The neural gland shows cyclic cellular activity to which different roles have been attributed:

excretory, neurosecretory, and especially endocrine. Some inves- tigators have shown that tile neural gland secretes hormones

C.Monniot, F. Monniot P. Laboute

23

(26)

Coral ReefAsctdions of New Caledonia

24

resembling those of the posterior lobe of the vertebrate hypophy- sis. Others believe that the neural gland induces spawning of eggs or sperm when an individual detects certain chemical stimuli. especially other individuals' sexual products, in the sea water around it.

Blood and

the Circulatory System

Ascidian blood is a colorless plasma, isotonic with sea water, that carries a large array of circulatory cells. Again we know too little about ascidians to assign roles confidently to these structu- ral elements. The ascidian circulatory system is not closed; such vessels as exist are not lined by an endothelial vascular wall, except near the heart. Rather, blood circulates among lacunae in the loose mesenchyme ("connective tissue") of the mantle, the pharyngeal wall. and around the gut and gonads. These spaces sometimes form veritable channels or sinuses that resemble real vessels. They do so alongside the endostyle and the branchial sac's dorsal line, and in the branchial sac's longitudinal and trans- verse bars. Indeed, these branchial sinuses are often called vessels. Inmany ascidians, still other 'vessels" penetrate Widely into tile tunic.

The ascidian heart is extraordinary. It entirely reverses its blood flow every few minutes. Lined by a simple epithelium, it forms a v-shaped tube of striated muscle fibers arranged spirally around its long axis. Waves of contractions. spaced several seconds apart, move blood through the heart in one direction for a few

(27)

minutes. Then the heart pauses before waves of contractions begin again. driving blood through in the opposite direction.

Among tile blood cells, undifferentiated hemobJasts are probably the source of all the other types of blood cells: Iymphocytes, amoebocytes. nephrocytes, phagocyres. pigment cells, and vacuolated cells (which include granulocytes. signet-ring cells. com- partment cells, and morula cells). Some types of vacuolated cells can store metals - in some species large amounts of vanadium, an ability unique to ascidians. The role of these metal-storing cells.

called vanadocytes or (if they concentrate iron) ferrocytes. is stilI poorly understood. Recent studies indicate that the metals are associated with "tunlchrome". an animal polyphenol unique to ascidians. Some vacuolar cells are well known, too. for their strong acidity. Still other ascidian blood cells promote coagula- tion by secreting lectin.

Blood seems to play a minor role in ascidian respiration. It carries no respiratory pigments, either in its plasma or its cells. It serves primarily to store and transport metabolic products. In many kinds of ascidians. too, the circulatory system plays an important role in processes essential to the growth of colonies.

Sexual Organs

Asrtdian ovaries and testes are independent of each other even though they are simultaneously present in the same individual.

Tlley are often juxtaposed to form a single gonad. but their gono- ducts remain separate. There may be one or many gonads in an ascidian body, depending on the species. and they may mature

C, Mormiot. F.Monniot p,Laboute

25

(28)

Corn!~f~dk)l1s of NowC.OltKJQO JO

26

eitherslrn u haneo usly or succe ssively oreven in such away that ma u-and k-rn .ilc phases alte-rnate during the body's adulthood.

Both mature sperm an d eggs are exp elled fro m the gon o d ucrs into the penbranchral av ity -sperm to be sweptout the: cloac al siphon. eggs to UC spawned or retain ed.Cru ss-Ie rtiliza tio n is the rule, IJl1tscn-reruliznnonmayals ooccur.

The lo cauon ofgonad s in the body varies among the different major gro u p s 01il~ ci u ii1I1 S. In the cll ti re ly colo nial aplo u s o- branchs. whose hooics (Ire divided into tw o or th re e part s. tile g()n Clds me rou nd ill the abdom en or post-abdomen,below the thor acic bran chialsac.In phlebobrancns.rhev adhen:to the gut, usually with in th e intestinal lo op. Among stoltdobra nc hs . the gonads may be distribu ted an yw nen~ in rh« pcrtb ranchi al wull of themantle ,dependingon tilespecies.

Ascidian s lack access o ry sexu a l orgi:ttIS. but ovo vivipa ro u s forms may hav e structu res to shelter their ottspriru; thro ug ll em- bryog en esis. TI1 U S. so me ovoviv ip ar o u s species brood the ir yo u ng in the peribranchial cavity, but others ha v e evolv ed an incu b ato ry potnh formedby adilat ionofthedistalpartof the ovi- duct. Tl iis pouch may hol d on o or se-v eral emb ry os <It v.m ous stag es of developm en t Some colon ial ascid tans - the Iiidc-n\- ntda«.for e-xample -hol d their embry o s in cha m b e rs inside the colony's comm on tunic until they arc released ^,IS activ e tadp oles.

(29)

Example of a stolonial colony, In this species,

Ecteinascidia ndouae, the zooids are far from each other, linked by their stolons.

so the colony occupies a large surface of the substrate, which is made here mostly of calcareous algae,

BUDDING AND REGENERATION:

ASEXUAL REPRODUCTION

All ascidians reproduce sexually. Many also replicate their bodies (reproduce asexually) by strobilation or budding to form colonies.

The first individual of a colony. an oozooid, begins to bud very soon after it settles, Depending on the order and family it be- longs to. an ascidian may bud one or more individuals. cal1ed blastozooids. The blastozooids will bud more individuals, al1 of them genetically identical to the parent zooid. The oozooid generally has the same anatomy as its budded offspring. It may per- sist in the colony or disappear once colony growth is underway Among the three orders of ascidians. aplousobranchs are always colonial. Phlebobranchs and stolidobranchs have both solitary and colonial genera. At least twelve modes of asexual reproduction have been described so far. varying from family to family and especially from order to order. We have greatly condensed them into the following groups.

Stolonic BUdding

Stolonic budding appears to be the simplest mode of asexual multiplication. The family Perophoridae shows stolonic budding very clearly. /\ stolen protrudes from the basal or posterior part of

CMonniot.F.Monniot P. Loboute

27

(30)

CC'f;:,!RoetAJcldans

0' l,-~cceocoo

28

the zooid . It is <Ill evagin ation of the mantle . containing mes en- chymato uscellsand bloo d.that elongates,all the while secre ting tunic and ad herin g to the sub strate. Her ni as develop either at its tip oralo l li-( its len gth. Some of them may torm branches of the stol en , bu t most of these swellingsenlarge. their tissues beco me denser. and organog enesis produces complete and functio n ing new zooids . All theseblasto zooidsrem ain connected by sto lons. Dep ending on the species. pero p horld srolo n s may be long an d thread-like. with the individ uals well separated. or they may be short.in which case thecolony resemb les ;1bunchofgrapes.

Bu d ding by Stroblotlon

Strobilation. universal in the Pol yclinidae andwidespread among the Polycitoridae.inv olve-s a segmentatio n of theelong ated body int o two or more parts. Each part un dergoes parti al or compl ete disorgan iza tion. then regrow s and reorgaruzes to reconstitu te an en tirezoo id.InthePolycllnk lae.whose-zooids are quitelon g.the process is very clear.Zooids have three parrs:a thor ax with the branchial sac.an abdo men with the gu t-loo p. and a post-abd o- menwith thegonadsandheart.The zooid first shows aseriesof constrictions along its whol e length.Next tJHC' body div id es at thesecons trtcnons .Tiletissuesof theresul tant ves icles(strobilae) dedttteren ttate.The n their reorgani zation begi ns. EllCh section of the paren tal body retain s its original polarity as the new individuals grow. In oth e r words. the an te rio r part of each stro bila buds a thor ax. and the posterior pan bud s an abdomen and a post-abdome n.E;ICh bud'sgro wthbrings it to the colony's surface , whereit (JJ!CflSand begins to feed.In thosePol ycitoridaethatcan strobuate. only the ab do mendoes so.an donly in a few place s.

In thisspec ies of Aplidium onecolonyismassive. while the othe r'sbuds haveorga nized into circularsyste ms, mostlyonsepara telobes,

(31)

Schematic structure of a polyclinid colony, showing, on the left

the splitting of zooids into strobilae following dedifferentiation, On the right, each strobila

reorganizes to constitute a new zooid, and groups of zooids

together reconstitute "systems".

Esophageal-rectal Budding in the Didemnidae

In this family. zooids are tiny. about I mm long on average. They have a thorax containing the branchial sac. and an abdomen that is connected to the thorax by a thin waist. As the figure shows.

two different buds appear at this waist; one protrudes a new abdomen near the parental thorax; the other. a new thorax near the parental abdomen. For a little while. the result is a bizarre double-body with two thoraxes and two abdomens. But as they grow and adjust the proportions of their new parts, the two newly forming zooids eventually separate. In fact, thoracic and abdominal protrusions are not always simultaneous; the new thorax generally appears first. So vigorously budding didemnid colonies may well contain scattered zooids with two thoraxes and only one abdomen. This type of budding is esophageai-rectal, because the esophagus and rectum are the sites where the new body parts actually appear, as the figure shows.

Budding by strobilation interrupts sexual reproduction. since

C. Monnlot. F.Monniot P.toboute

29

(32)

Esophageal-rectal budding in the Didemnidae:

2- Both buds organize,

1-A thoracic bud has already differentiated while the abdominal bud remains rudirnentorv.

each strobila's tissues must be reorganized, But budding in didemnids can go on during the maturation of the gonads, which are not disturbed in the parental abdomen, Most often, however, periods of colony growth and bouts of sexual reproduction alter- nate with each other, probably as the colony'S energy budget is directed toward one activity or the other.

Peribronchial, or Pallial, Budding

Among stolidobanchs, the family Styelidae is tile only one with colonial representatives, Several colonial styeltd genera show sroloruc budding much like that of tile Perophoridae or Cionidae among the phlebobranchs. But in Botryllus and "polystyellds", budding proceeds in still another way. The mantle on each side of the body swells into the tunic as a hernia. In Botryllu5, this her- nia becomes a bud, which itself forms new buds before it com- pletes its own development. When the first buds reach the colony's surface. the parent zooid collapses and disintegrates.

Thus. the zooids of a botryllid colony proliferate extremely rapid- ly in the spaces between their parents. various polystyelids have a comparable mode of budding, although their zooids do not show Botryllus's hlghlv organized schedule of zooidal develop- ment and disintegration.

There are, as well. other modes of budding that appear to be de- rived from the major types we have described. This brief survey merely suggests how extensively ascidians have exploited budding and colony formation as pan of their growth strategies.

Each of the four main modes of budding gives rise to colonies of very different shapes. Derivative modes increase the array of colony forms still further.

Coral Reef Ascidions of New Caledonia

30

3- The old thorax and newly formed abdomen join.

while the new thorax isconnected to the old abdomen,

4- The two individuals progressively separate,

(33)

Pallealbud ding ofBottvtiu».

The colonydevelopsnumerousvascular ampullae inthe commontunic on itsexterna ledge.

Eachzooid budsa newindivid ua l on eachsid e.

and these inturn bud asthe parent zooid disorganizesand disappears.

Thedidemnid budd ing sta ge-2 seen throug h a micro scope.

with thetunicrem oved

(34)

Differentkinds of budd ing

prod ucedifferentshopes ofcolonies.

TheWhiteandredBotryllusteptus hasall Itszooidsdeeplyembed d ed in(Jcommo n mass oftunic. ou :tneyellowindividualsof Euhe rdmoniaclaviformisshore only acommonbasalsnee rortunic to crea te acolony

wilnthe appearanceof abouquet.

(35)

FROM THE INDIVIDUAL TO THE COLONY:

ASCIDIAN DIVERSITY

Solitary Ascidians

During a dive, an observer may have a hard time singling out these solitary animals. attached as they are to the subsrrate and often colored like the rock itself or the other animals and algae that grow on it. Furthermore. ascidians may be hidden in crevices. under stones. or among the branches of coral. Most solitary ascidians are small. on the order of several centimeters or less. and even this size is reached only by the oldest animals.

Growth is continuous but always slows down after the settled animal's first few months. The life span of a solitary ascidian has been reckoned only from animals living in aquariums. It seems to range from several months to two or three years.

The shape of a solitary ascidian depends heavily on the consistency of its tunic. If its tunic is thick and hard. the animal will often have a globular, erect shape and may carry a fairly dense coating of epibiotic organisms. This appearance is especially common among stolidobranch ascidians, such as pyurlds and styellds, some of which are even stalked. In New Caledonia, Po!ycorpa clouota is a striking example of this body form. As the animal grows, the body grows away from the substrate by secreting a flexible stalk of especially firm tunic. The animal sways back and forth on its stalk.

C.Monnlot.F.Monniot P.Laboute

33

(36)

With itslargesize(30 cm) andbright calor,

Polycarpaclavata isoneofthe most spectacularoscknons of New Ca led onia.

The animalswa ys atopitsstalk;

theoraland clooco lape rtures arealmostop p ositeeachothe r.

andtheir gapingallowswater topa sseasily throug hthebod y,

(37)

The tunic of small solitary phlebobranch ascidians is generally clean and often transparent and soft, butit may become opaque and rather carulagtnous in large species. These animals often lie broadly attached to the subsrrate, sometimes adhering to it by almost their entire left sides. Perhaps because their tunics are relatively fragile. many of these animals prefer less exposed habitats - crevices and the gaps among coral branches, for example - where they find some protection from predators, consequently, except in quiet waters, phlebobranchs can rarely be found just by swimming above the bottom; instead, they have to be sear- ched out patiently in recessed, half-hidden, often dark places. In some solitary ascidians, the tunic is encrusted with sediment, and the animals are ovoid, with their two siphons close together.

These forms live in silty habitats, where they can hide in the sediment. Their tunics may hold onto sand or mud with adhesive papillae or with hair-like processes, or sediment may even be incorporated during growth into the tunic's matrix.

Most solitary ascidians live as isolated individuals, although their distribution and density can vary greatly. Their camouflage can be so perfect that an experienced diver senses the presence of an ascidian more than he actually sees it. It is only by touching the animal that he can be sure it is really there: Large ones may betray themselves by their gaping siphons, which may be pale or brightly colored internally and which shut abruptly at the sligh- test disturbance.

In areas rich in organic matter or in quiet places like ports. some kinds of solitary ascldlans settle next to or even on each other.

The individuals, which may be of various ages, often adhere so tightly to each other that the resultant mass resembles a colony.

C. Monnlot. F.Monnlot P.tcboute

35

(38)

PO/W ( l/jJCI Ilioric(I/)S, '-ill w/n C(// )OPUS, and p!]Uf(l cOIl {rooosa

ag-

grc g illcill tll b WilY TIIC::;(' cl!-\.\~ r t 'g;lI i o ns gcn crally shclrer an ep i- 1;1\111, 1thar li\'('~ ill llll' 'sIll,t11cdvit ies I)('I W(' ('II the as cidians lJod i('s. Tllis entire li\'ing su ostratc may its('lf now be come a Iavorahlt-p!;](' (' rorothe rI,illdsofascicJi; 1I1Sto settle. Thus ,aggre-

~;lIi()l1s rlILl~' be' rn' J!lns rwcific or be lIlodc lip ofseve ralsp ecies .

III rnos: (,ilS':s, 011(' Sp ('('ics domi n ates numcireally or by she er IIld SS. Tllis ",llgll'g,lli\'(' br-liavio r eases cro ss-fertilization among I1 C' glDupedP,II ('111S.I\&jlc g il live Ix-havioris pro b ably enhanced

!>~' ,II(' s,xwtin ll nl SllbSlill l<T S 111<11attract la ter larv ae la se ttle ClllCIjoit)lilt' (rowct.

.'\sNrqylliul ls ,11(' notlimited to solnar y sp ecies .The larv a e ofdit-

k'wI I1species 01,rplol lsobr211lcils IllLly sel l I!' I1C;1I each other The-ir su/)scqllt'rll lolollil's pack tllg(' l lwran d may even part ially

i"lJ\'!'I' \)!\C ;111011\('1', Tilb suuanon is co mmon whe re solid substra rcs'lI " rw\ ' dillI<\150densel y p<'I JU!clled-on narborfloats and piling s,tor('X,lI11plt',

Co n»Pool~cion5 c .~·UCOl-

Po lycorpocryp toc orpo hererevealsonly itsWhiteapertures.

Thetunicis covered with sediment.

(39)

Thepresenc e of one individ ual ofthesolitaryPalycorponigricons attractsother lorvoeofthesome species.which settleclose to it.

Theaggregatedbodies

mayeven fusetheirtunic stoget her.

toresemb lemembersofacolony.

Polyc OrpoQUI/to5 hardfunicIS covered

with many pib io nts.

Wtl entnisanimo's ye llowsip honsare clo sed.

itcon hardly e een,

(40)

Ea c h element

ofthis Eusynsfyelocolony islinkedto neighbors bythin transp arentstolons which are almostinvisible on the irregularsubstrate.

(41)

By exte nding bud din gsto lons.

Symp legma a/lemaco nform

ocolony tha texploits0 thin, linear substratesuchasasmall, branc hing seaweed.

Colonial Ascidians

STOLONIC COLONIES

Colon ies of thi s typ e occu r amo n g the Perophoridae and tile Sty e lidae. They result from budd ing by sto lonlc ouigrowths of the mantle. as we hav e already described.The settled larva me- tamorphoses in to the col ony's foundin g zooid. or 00200id.It puts outstolons in se veraldirections; then blasi ozooids dev elopfro m sid e-o r en d-po c kets of these sto lo ns. The.se zoo ids in turn put out new sto lo ns ,which produce still more buds. tf the grow ing colony enco unters tavorable conditio ns , zoo ids devel op quic kly and bud ,so the whol e colo ny gro ws in this direction. if. on the other hand, a sto len gro ws into a less fav orable area,us zo oids differen tiate mor e slowly or not at all.And ifco nditions deteriora- te generally,partofthe colony may dege nerate or die.Thus.SI O-

Io nic budding allows a colony a certain choice of hab itat. since the effect ofth ese patterns of grow th and regress ion is to shifl thewhol ecolo n y'S pos itionon the substra te.

All co lonies that can putou t srolo ns in a partic ular directio n are potentially mobile as lo ng as the substrare can accommo d ate the ir displacem en t.tnsuchco lonies,reg ression occurs on the side away from activebudding.and material fro m thosedegenera- ting zoo ids is broken dow n and reused by the growing parts of the co lony. In spec ies wheresto lons are long and individualswi- del yspaced ,co lonialmo v em en tover the sub straie may be quite rapid, ev en though eac h ind ividu al zoo id remains immobile du- ringitslifeti me.

C.MonniOl.F.Monnlol P.loboute

39

(42)

CoralReef Ascrdions Of New Caledonia

40

Very short stolons, by contrast, produce colonies with individuals Joined closely together (the stolidobranch genera Amphicarpa and El1synsryela are examples), Zooidal contact can be so tight that the tunic around zooids may actually fuse. The colony then forms a virtually continuous crust, and the boundaries between individual zooids appear to be mere grooves.

BOUQUET-LIKE COLONIES A colony of thiS sort has a shallow but usually sheet-like tunic that unites the colony only at the substrate. Either the zooids produce short basal stolons (e. g., Perophora) or they strobllate (as in El1- herdmanjo). After strobilation. each piece regenerates separately, grows within a newly secreted tunic, and is joined to the other zooids only by the thin basal sheet of tunic. In the clavelinids (Po!ycitoridae), too, zooids are embedded in a common tunic only along their posterior parts, while almost all the rest of each zooid remains anatomically isolated from the others. The attach- ment surface of the colony may be very small in relation to the entire surface and volume of the clustered zooids that make up the colony. Colonies of this design have several advantages:

diverse orientation of the zooids, fixation onto restricted surfaces, and the projection of zooid siphons away from the substrate.

ThiS arrangement minimizes potential damage to the colony by predators, because one destroyed zooid need not endanger the health of the rest of the colony. In fact, damaged parts of the colony heal quickly. Colonies whose zooids form clustered "bou- quets" adapt well to frequent and abrupt changes in bottom cur- rents: since zooids' orientanons vary, some zooids can close their siphons while others keep theirs open.

(43)

Cla velinaflava(Polyc itorid ae) makesbouqu et-like colonies inwhic h the zooids ore united onlybytheirba saltunic.

Theirthora xesremain independent ofeach other.

In almo stall colonialascid ians . the vary ing ages that result from co n tin uo us budding spread outgameto ge nes isoverlime.Conse- quently. thes e spe cies enj oy exte n d e d breeding seasons an d thusenha nced cha nces for a broad distributlon.Butwhe n

, I

^co^{lo -}

ny is large. or whenoc olo gicalcond itions be c o meless favor able. budding slow s down throug hout the co lony.an d the formation of gametes begins.Sexua l maruruy th us may occur simultan eou sl y over the whole colony, shortening an otherwi se exten ded bre e- ding peri od.

COLONIESWITHZOO IDS PARTIALLYEMBEDDED IN THECOMMO N TUNIC

Thereare many enc rusting or pe du ncul ate colonies in which the ab d o mens of the indivi dual zo o ids are enclo sed with in a dens e mass of tunic, itself lightly auacn ed to the substra te. while the zo o ids' thorax es project more or le s s independently from this common mass. OX~Jco r!)nio ioscicutons is a dra rn auc example This arran g em e nt is intermediate between colon ies whose zoo id s arc woupe d in "bouq ue ts". on ly basall y CUIlJ1CCle d. an d ones whose zo o ids are fully em bed ded in a com m o n tunic. In these uuermediatety thick colonies. zooids show no orderly arran ge-rncn tin relation to eac h other.They simply are hel d close together,The zooids'exp o sed thoraxes are les s pro tected from predati on th an rheir mor e embedded abdo mens: it is in their abdomens th atacti ve reproducti on,whether se xual or asexual, occurs.

C:k....mloLJ=.".-\T.ruot P LOC)(X.;le

41

(44)

Coral Reef .Ascidians of New Caledonia

42

COLOI\JIES WITH ZOOIDS ENTIRELY EMBEDDED IN A COMMON TUNIC These colonies have diverse shapes. Some encrust the substrare in thick or thin sheets. Others form small cushions or even balls that are attached to the substrate by only a small part of the tunic. There are, as well, massive colonies that grow away from the substrate on stout peduncles or stalks

rclavate'

forms) and thereby project their ZOOids well out into the water. The surfaces of these colonies, whatever their forms, may be smooth and even, or contorted into folds. lobes, or finger-like projections that greatly increase the colony's surface for water intake. Depending on the environment in which it grows, even a single species (of Polyclinum, for example) may either grow erect, pedunculate lobes or spread out as an encrusting sheet.

A colony's zooids are arranged quite independently of the colony's overall shape. In all cases, every zooid's oral siphon opens at the surface of the colony's tunic. In the most simply organized type of colony, each zooid opens independently to the outside through its cloacal siphon, as well. occasionally, all the zooids orient their siphons in the same way. giving the colony an orderly appearance. This regular arrangement of zooids is especially visible in Citorclinum laboutei, where each siphon is encircled by a plgmented white ring. In other species (some species of Eu- distoma, for example), the zooids form "systems" in which they are arranged spoke-like in circles. Their separate cloacal siphons lie close together toward the center of the circle, and their oral siphons are on the periphery.

(45)

In the polyci toridOxycorynlotoscicuons, theabd omensofthe zooids are packed tog ether in a densecommontunic and formthe stalksof thecolonies.

In the colonies's "head s·, thethoraxesare partia llyisolate d,

each inits own thintunic.

and are easilyseen.

thanksto theirpigmenta tion.

At a mor e co m plex level of organi zat ion. the zooids for m even more thoroughly integ rated systems. in which the common tunic has on e or more cav ities in to whic h the cloacal apertures of a few or many lnd lv id ualzooids empty:eac h cavity itselfopens to theouts ideby a common cloacaJaperture. This ty p e of organi za- tio n.simple when the common cloaca l cav ities are themselv es simpl e.becom es cornpllcar ed in avarie ty of waysas these cav i- ties ramify. A mere po uch-lik e inv ag in ati on may pene tra te the tunic below the colo ny'ssurface an d spread outat the le v el of the zooids' thoraxes to crea te channels . Theseduc ts may branc h

C.Mor-r tot,F.Monnlot P.LabOula

43