Chaetognatha
Introduction

Chaetognaths are a small phylum of about one hundred species living in various marine habitats, in the open sea as well as on and near the bottom, from polar to tropical regions, and at all depths. They are considered one of the most taxonomically isolated animal groups, with obscure phyletic origin and relationships with other phyla. In spite of their small size, between 2 and 120 mm, these carnivorous animals play an important role in food chains. Reeve (1970) concluded that their biomass in the pelagic world ocean is 30 % of that of copepods.

The ecosystem role of benthic species is unknown, but judged minor by Feigenbaum (1991), owing to their small size. Indeed, most of the neritic Spadella and Paraspadella species do not exceed 3-4 mm in length, but deeper species are larger. Spadella birostrata, recorded in most benthic hauls between 90 and 555 m off the southern European coast (Casanova, 1988), may reach 11 mm. Hemispadella dauvini, the most recently described spadellid, reaches up to 18.5 mm in length (Casanova, 1996). Moreover, many newly discovered deep bentho-planktonic species (Casanova, 1986a) are larger (up to 38 mm for Heterokrohnia murina), and sometimes numerous. So the importance of these taxa on, or a few metres above, the bottom has very probably been overlooked.

Although chaetognaths are eaten by numerous larger carnivorous organisms (Reeve, 1966), they are themselves active predators. Prey are grasped with the two sets of rigid hooks. The first evidence of venom injection into prey was shown by feeding experiments (Parry, 1944). The venom is a tetrodotoxin produced by a bacterium, Vibrio alginolyticus, whose exact location in the head (teeth, vestibular organs, mouth, or gut) is still unknown (Thuesen et al., 1988).

Secretory cells in the pharynx probably serve to coat prey with mucus before they are swallowed entire (Arnaud et al., 1996), so the size of prey is limited by maximal mouth opening. The diet of pelagic species includes a wide range of organisms, reflecting the composition of the zooplanktonic community. Thus it varies seasonally, but consists mainly of copepods, usually the dominant component of plankton. Their impact on these crustaceans is important: Stuart and Verheye (1991) indicate that, off the west coast of South Africa, Sagitta friderici alone may eat up to 14% of the summer daily production of copepods. On the other hand, Feigenbaum (1991) reports that the impact of chaetognath predation on fish larvae could be exaggerated because of the scarcity of larvae in the plankton. Nevertheless, chaetognaths must contribute to reduction of larval abundance during periods of fish reproduction.

Prey selection has been reported, large species feeding preferentially on larger prey than smaller species. Cannibalism is known, particularly in certain species. It is generally accepted that movements of prey provoke attacks by planktonic and benthic chaetognaths. Species living in dark submarine caves are attracted by the odour of bait in traps (Casanova, 1986b). Deep-living bentho-planktonic species may feed on organic matter and bacteria adhering to mineral particles in the sediment (Casanova, 1986a).

Curiously, the gut in many surface-dwelling species is often compressed into a lamina when empty, whereas it always remains cylindrical in deep species. In some Sagitta species, vacuolized cells that are more or less numerous on the lateral parts of the gut provide buoyancy (Bone et al., 1987), and are of phylogenetic significance (Dallot, 1970).

Growth and reproduction of chaetognaths have been reviewed recently by Pearre (1991). Chaetognaths are protandrous hermaphrodites. Testes and ovaries are connected in the heterokrohniid family (Casanova, 1985), but separated in other members of the phylum. Self-fertilization has been demonstrated by laboratory experiments in Sagitta setosa (Dallot, 1968), and subsequently in other species. Cross-fertilization would be the most common mode in the sea (Alvariño, 1965), although reports on mating are rare. Eggs are generally laid freely in the water. Those of Pterosagitta draco are embedded in a floating gelatinous mass. Benthic species (spadellids), and Sagitta hispida, which sometimes lives in seagrass meadows, attach their eggs to the substratum. Eukrohnia species keep their eggs and newly hatched "larvae" in two gelatinous brood sacs, or marsupia, protected by the recurved posterior part of the lateral fins. According to Terazaki and Miller (1982), the sacs are ovoid, and markedly different in Eukrohnia fowleri and Eukrohnia bathypelagica. Inasmuch as there is no metamorphosis, the term "larva" is not appropriate, but changes during early life are so significant that many authors use this term. The duration of ovulation is variable; Sagitta setosa, for example, deposits its eggs in clusters at intervals of about 24 hours, in total up to 6 such clusters are laid (Dallot, 1968). Eggs generally hatch after 2-3 days, depending on species and environmental variables (mainly temperature). Growth is characterized by allometric changes: the tail and the ventral nervous ganglion are proportionally larger in juveniles than in adults. The number of teeth and hooks increases, but in Sagitta lyra the number of hooks decreases in fully mature animals, and their shape changes.

Chaetognaths are subject to parasitism and diseases, but their impact on the populations is still very poorly known (Nagasawa, 1991). Epibionts have been reported frequently, but they are not always interpreted as parasites. For example, bacteria sometimes observed on the ventral surface of the head of Xenokrohnia sorbei might be simply food remnants in specimens caught just after feeding (Casanova, 1993a). Bacterial infections are observed in freshly sampled chaetognaths, as well as those kept alive in laboratory experiments. Infestations by various parasite groups have been reported, mainly protozoans, larvae of trematodes, nematodes and cestodes, and more rarely copepods. According to Alvariño (1965), the incidence of parasitism is higher in neritic chaetognaths. Mazzoni (1990) provided a complete review of this subject. In Argentine waters he reported larvae of the parasitic nematode genus Centracoecum in Eukrohnia hamata, Sagitta friderici, Sagitta tasmanica and Sagitta gazellae, including interesting data on percentages of infested individuals at all seasons of the year. He also emphasized the role of chaetognaths as initial hosts in the biological cycle of this parasite, since adult nematodes infest the Argentine hake, Merluccius hubbsi, a very important commercial species. Although evidences of impact of the parasite on hake development and growth have not been reported, the presence of nematodes in processed fish meat degrades its visual aspect, thus interfering with marketing.