Sagitta nairi, Casanova & Nair, 2002

Casanova, J. - P. & Nair, V. R., 2002, A new species of Sagitta (Chaetognatha) from a Laccadive lagoon (Indian Ocean) having fan-shaped anterior teeth: phylogenetical implications, Journal of Natural History 36 (2), pp. 149-156 : 149-156

Sagitta nairi n. sp.

(®gures 2, 3, 4A±D)

Diagnosis. Maximal length 5.2 mm after preservation in formalin (4%); tail segment 28.5±30.2% of total body length without tail ®n; anterior teeth fan-shaped, stouter and more numerous (11±14) that the posterior ones (11, 12).

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Materials. Agatti Atoll is one of the numerous atolls of the Laccadive Archipelago. It is situated at about 11ssN, 72ssE in the Arabian Sea , oOE the southwest coast of India (®gure 1). On the west side of the atoll lies the lagoon which is orientated along the north±south axis. In the channel, the depth is about 6 m , but inside the lagoon it is only between 0.3 and 3.5 m, the mean being 1.5 m.

Surface plankton tows were made for 10 min, with a square net (mouth area 5 0.25 m 2, mesh size 5200 m m) on 9 and 10 December 1988. The chaetognaths were preserved in 4% buOEered formalin. They were very rare or absent in daytime collections, but during the night, they ranged from 48 to 99 specimens per m3 (samples included the present species and Sagitta madhupratapi which was not then separate d).

A total of 42 specimens was used for systematic studies.

Description. Sagitta nairi is small in size, the maximum length observed being 5.2 mm. The tail constitutes 28.5±30.2% of the total body length without tail ®n (®gure 2). The body is slender, slightly rigid and transparent (®gure 3a). In dorsal view, the head is peculiar owing to the bases of each set of anterior teeth, right and left, that are well seen and aligned in regular rows, so that its anterior side has two more or less prominent swellings (®gure 3b, c). These teeth, 11±14, are large and strong, a characteristic feature that is original in Sagitta . Their shape also is curious (®gure 4a±c): in ventral view, they look like ¯attened arrows, the outermost overlapping the innermost; in lateral view, they are S-shaped. Their narrow base is ornamented with ridges constituted of rows of small spines. Posterior teeth, 11 and 12, are less numerous than the anterior ones, which is not common; they are long, thin, bent at their bases and in close contact with each other (®gure 4c). Hooks are seven to eight; the base of the anterior ones are enlarged and thus their inner and outer sides have no regular curve (®gure 4a, d). The vestibular organs are smooth. Two symmetrical plates enclosing the inner bases of the hooks correspond probably to the ventral plates (®gure 4a). Such a disposition seems never to have been seen since the plates are covered by the cuticle lying on the ventral side of the head ( Kapp, 1991).

The eyes are large and rounded; the pigmented area is also roughly rounded 14

at of by (®gure 3f), but a careful observation of some specimens allows recognition of the pigment cell looking like a stumpy star. The collarette is thin and restricted to the neck region (®gures 2, 3c). None of the specimens has a complete corona ciliata, but the observation of its remnants on the dorsal anterior part of the body of diOEerent specimens indicates that it begins behind the eyes and reaches posteriorly at least a distance equal to twice the length of the head. Two well-marked intestinal diverticula are present at the beginning of the gut, in the neck area (®gures 2, 3b). The ventral ganglion is large, representing about 11% of the body length (®gure 2). The anterior ®ns begin at the level of the posterior end of the ventral ganglion. The posterior ®ns do not reach the seminal vesicles. They lie much more on the tail than on the trunk, so that the ratio T/C (part on trunk/part on caudal segment) 5 0.40±0.53.

The ovaries are empty rather than immature in all the specimens observed since in a few of them the seminal receptacles are still more or less ®lled with sperm; but 14

at of by whatever it may be, their characteristics (length, number and size of ova) remain unknown; they can reach at least the level of the ventral ganglion. In the largest specimens, the tail coelomic cavities are ®lled with clusters of sperm. The seminal vesicles are small and elongated (®gures 2, 3d, e); they open laterally at the anterior part that protrudes into a slight swelling. They are apart from the posterior ®ns, and separated from the tail ®n by a distance equal to less than their mid-length when mature.

Types. The holotype, a mature individual, 5.0 mm long, is deposited in the National Institute of Oceanography , Regional Centre, Cochin, India (IOBC- 14

at of by 0509-14-66-200 0). Five paratypes are presented to the MuseÂum national d’Histoire naturelle, Paris (Reg. no. UE 937).

Etymology. The species is named in honour of Mr G. R. Nair, husband of the second author (V. R. Nair) in recognition of his invaluable support throughout her career.

Comparisons with other species. The anterior teeth being more numerous than the posterior ones immediately distinguish S. nairi from most other species of Sagitta . 14

at of by Indeed, only three of them are known to exhibit this characteristic feature: S. helenae Ritter-Zahony, 1910 , S. americana ( Tokioka, 1959) and S. johorensis Pathansali and Tokioka, 1963 . The ®rst is localized on the warm American side of the Atlantic. Most of its morphological characters indicate that it is very diOEerent from the new species, e.g. its large size, 11±13 mm, and its seminal vesicles in contact with both the lateral and tail ®ns.

Sagitta americana , described from the Paci®c oOE California to Peru, belongs to the robusta group whose characteristics have been recently described at length (Casanova and Goto, 1997); it also diOEers considerably from S. nairi , e.g. its anterior teeth are slender while the posterior ones are much thicker ( Tokioka, 1959).

Sagitta johorensis , described from a few specimens in Malay waters, is undoubtedly the species most related to S. nairi . Indeed, it is also a small species, attaining a maximum length of 5.9 mm, and the number of teeth and hooks are comparable. However, diOEerences in the disposition and shape of these components of the head armature are noticeable. Pathansali and Tokioka (1963) described the piriform head of S. johorensis , and opined that`the two rows of anterior teeth meet each other at an obtuse angle’ and that these teeth were not stout. Later, Panthansali (1974) redescribed the piriform head and stated that the teeth are short, with their tip directed forward, while the posterior ones are larger and stouter. The hooks also are diOEerent since their two edges are regularly curved. In the description of S. johorensis, Pathansali and Tokioka (1963) mentioned that they dissected the heads of two specimens; so one can be sure that S. nairi is a diOEerent species since Tokioka, a renowned expert of chaetognaths, would have noticed the peculiar shape of the anterior teeth and hooks. The seminal vesicles also are diOEerent: ovoid when mature in S. johorensis , in contact with the posterior ®ns and well separated from the tail ®n by a length equal to their largest diameter, while elongated in S. nairi and separated from both the posterior and tail ®ns. Finally, the absence of collarette in the former is not convincing since the development of this tissue in a single species may be variable; indeed, for instance, in S. crassa Tokioka, 1938 its importance varies from season to season ( Tokioka, 1974) and in S. lyra Krohn, 1853 , it is only present in specimens living in the coldest waters of its area (Casanova and Ducret, 1970).

The geographical distribution may also be support for the existence of the two species. Indeed, S. johorensis was described in near shore waters of the strait of Malacca, far away from the Laccadives Sea. Also the lagoons of atolls, and more particularly those constituting this archipelago, are known to house endemics ( Emery, 1968; Madhupratap et al., 1991). So, it is not surprising to ®nd a new species in the Agatti Lagoon, as the con®ned shallow waters promote speciation, since another endemic chaetognath, S. madhupratapi , has already been discovered from the same atoll ( Casanova and Nair, 1999).

Phylogenetical importance. In his authoritative taxonomical revision of Chaetognatha, Tokioka (1965a) emphasized that two characteristics diOEerentiating the genera are`of an essential importance’. One is the presence of a ventral transverse musculature in the primitive Order Phragmophora , this`basic feature’ being absent in the`advanced’ Aphragmophora . The other is the peculiar appearance of hooks and teeth, that he considered to be`a high grade of specialization’ allowing division of the Aphragmophora into two suborders, Ctenodontina (genera Sagitta and Pterosagitta ) and Flabellodontina (genus Krohnitta ). In the former, hooks are gently curved, teeth-rows comb-shaped and teeth slender; in the latter, hooks are curved rather abruptly, and teeth stouter than in the former and arranged as a fan (®gure 4e). This classi®cation is admitted among specialists even though some minor arrangements have been proposed for the Phragmophora (Casanova, 1985) .

When they described S. johorensis, Pathansali and Tokioka (1963) noticed that the anterior teeth are`closely set that they overlap and may appear fan-shaped’. This trend is more marked in its related species S. nairi by the ¯attening of these teeth and increasing of their size with regard to the posterior ones. Moreover, the hooks, similar in S. johorensis to those of the other Sagitta species , are diOEerent in S. nairi : when large, there is an abrupt change of curve at a certain distance from their base. These features recall the two characteristics of the Flabellodontina stated above.

Certainly, the two Krohnitta species have other characters that distinguish them from the genus Sagitta , i.e. no posterior teeth, one pair of lateral ®ns, tail ®n oval, and for these reasons S. nairi belongs undoubtedl y to the genus Sagitta . Nevertheless, this species ®lls a part of the gap between the head armature of the genera Sagitta and Krohnitta .

In a second paper, Tokioka (1965b) again focused attention on the structure of hooks and especially of teeth of Krohnitta , which are unique among the chaetognaths. 14 His conclusion was that since the Phragmophora ( Heterokrohnia , Spadella , Eukrohnia ) and the Ctenodontina ( Sagitta , Pterosagitta ) have a comparable armature structure, the Flabellodontina (Krohnitta) must rather constitute a particular strain, diOEerent from that comprising the Phragmophora and the Ctenodontina, than a at

branch derived from the Ctenodontina. The discovery of S. nairi pleads in favour of this second possibility.

of There are now only two ways to clear up this problem. One is to expect the discovery of other species having original characters comparable to that of S. nairi . Further investigations in the Indo-Paci®c, considered to be the natal area of chaetognaths ( Tokioka, 1965b), would certainly be interesting. A second ®eld of research is to undertake biomolecular investigations on the two Krohnitta species and S. nairi , by for comparing these data with those reported by Telford and Holland (1997) for 18 species of the genera Sagitta , Eukrohnia and Spadella . The depth of branching of these taxa on the phylogenetical tree, combined with morphological results, would certainly give clues to the real situation.

Acknowledgement

The authors express their sincere thanks to Dr M. Madhupratap (National Institute of Oceanography, Dona Paula, Goa, India) for providing the zooplankton samples from Agatti Lagoon.

References

CASANOVA, J.-P., 1985, Description de l’appareil geÂnital primitif du genre Heterokrohnia et nouvelle classi®cation des chaetognathes, Compte-rendus hebdomadaire s des SeÂances de l’ AcadeÂmie des Sciences, Paris, 301, III(8), 397±402.

CASANOVA, J.-P. and DUCRET, F., 1970, Contribution aÁ l’ eÂtude morphologique du chaetognathe Sagitta lyra (Krohn 1853) , Rapport de la Commission Internationale de la Mer MeÂditerraneÂe, 20 (3), 259±361.

CASANOVA, J.-P. and GOTO, T., 1997, Sagitta siamensis, a new benthoplanktonic Chaetognatha living in marine meadows of the Andaman Sea, Thailand, Cahiers de Biologie Marine, 38, 51± 58.

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CASANOVA, J.-P. and NAIR, V. R., 1999, A new species of Sagitta (Phylum Chaetognatha) from the Agatti lagoon (Laccadive Archipelago, Indian Ocean), Indian Journal of Marine Sciences, 29, 169 ±172.

EMERY, A. R., 1968, Preliminary observations on coral reef plankton, Limnology and Oceanography, 13, 293 ±303.

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MADHUPRATAP, M., ACHUTHANKUTTY, C. T. and NAIR, S. R. S., 1991, Zooplankton of the lagoons of the Laccadives: diel patterns and emergence, Journal of Plankton Research, 13 (5), 947 ±958.

PATHANSALI, D., 1974, Chaetognatha in the coastal waters of Peninsular Malaysia with descriptions of two new species, Ministry of Agriculture and Rural Development, Malaysia, Fisheries Bulletin, 2, 1 ±30.

PATHANSALI, D. and TOKIOKA, T., 1963, A new chaetognath, Sagitta johorensis n. sp., from Malay waters, Publication of the Seto Marine Biological Laboratory, 11 (1), 335 ±357.

TELFORD, M. J. and HOLLAND, P. W. H., 1997, Evolution of 28S ribosomal DNA in chaetognaths: duplicate genes and molecular phylogeny, Journal of Molecular Evolution, 44, 135 ±144.

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