Journal of the Marine Biological Association of India

Tuniciers pelagiques de l'ocean Indien.

Volume 14 Issue 1

J.Godeaux

Abstract

Tunicates are distributed between three classes, the first, benthonic, the Ascidians, both others holoplanktonic, say, the Appendicularians and Thaliaceans. Among Thaliacea are grouped the Doliolidae, Salpidae and Pyrosomatidae.

 

The biological cycle of the DolioUds is a very complex one with not less than six stages: tadpole, oozooid, nurse, trophozooid, phorozooid and gonozooid, with striking differences in their anatomy as well. As a matter of fact, a complete cycle has only been described in three species and we also know that it is simpler in a fourth one, propagation of which is assumed mainly to be by blastogenesis. Moreover some sixteen species have been created based on the morphology of both phorozooid and gonozooid, but only four can be determined with the help of the oozooids: we have therefore cryptic species and more investigations are urged in order to solve the problem. The stolo prolifer possesses a complicated structure with several layers of anlage the destiny of which is not yet completely understood.

 

Salps and pyrosomes are also metagenetic animals: the oozooid is budding, the blastozooids

are sexed, either able to bud (pyrosomes) or not (salps). Thanks to the budding capacity of their blastozooids, pyrosomes build up permanent colonies, sometimes of a large size (up to 4 m. long) where zooids are embedded in a common matrix. On the other hand, in the salps, the blastozooids are so loosely bound together that connections are easily broken down by the waves.

 

Thaliaceans can be studied like the other tunicates, on an academic basis (e.g. dealing with embryology, blastogenesis, physiology, a. s. f.), but we could tentatively use them for more practical purposes, e.g. as indicators, giving information on the qualities of the waters in which they are living and on the currents which are carrying them.

 

Doliolids are very small and easily, either damaged or lost during the sorting out from the vast collections. Nevertheless, examples of their use as indicator species are available in Atlantic Ocean and in the Mediterranean Sea. In the Indian Ocean, two species are widespread and common: D. gegenbauri and D. denticulatum; D. indicum appears to be characteristic of the area and D. krohnt is recorded from the Gulf of Aqaba. Several other species have been observed in some stations.

 

Pyrosomes are known from a few scattered spots of the open sea in the Indian Ocean by at least four species : P. atlanticum, P. aherniosum, P. verticillatum and P. spinosum and their varieties. P. aherniosum is circumglobal and also found in the tropical Atlantic Ocean ; P. atlanticum and P. spinosum are more eurythermic and caught in the depths of both Atlantic (Gulf of Biscay, Azores) and Indian (Arabian Sea) Oceans. P. atlanticum is living in and often collected from the occidental Mediterranean Sea ; no pyrosome has been yet recorded from the Red Sea. Working on .an important collection of P. aherniosum from the eastern tropical Atlantic Ocean, a stratification according to the size of the colonies, say, the older, the deeper, was detected. Unfortunately, Indian records are far too less numerous to afford similar conclusions which imply systematic investigations in selected areas.

 

Salps are ordinarily common, sometimes available in tremendous amounts and often easily determinable. It seems likely they could be used, for those reasons, more conveniently than the other forms.

 

The Indian Ocean is still far from well known despite the fact that a lot of species of Salps has been recorded by the different oceanographic expeditions which cruised the Ocean since almost a century. Among fifteen species referred to in the tropical part, four have been seldom found (wrong determinations ?), seven are locally more or less regularly caught, the remnant is, at least in some spots, very common (e.g. Thalia dentocratica, Salpa cylindrica, Cyclosalpa pinnata and Brooksia rostrata). Most of the investigations have been done with open nets and little information is still available about the bathymetric distribution and exigences of the animals: some are only met above abyssal depths, others are caught in neritic shallow waters. A few are able to withstand severe conditions as prevailing in the Gulf of Aqaba where salinity rises to 40%;, (e.g. Ritterielh amboinensis, Salpa maxima). The trend is not precisely the drawing of maps but the better understanding of the laws governing the distribution, both bathymetric and latitudinal, of the planktonic animals with, as a background, the hope of an efficient exploitation of the Oceans by and for mankind.

Date : 30-06-1972