Formation of the gut in the first two naupliar stages of Acartia clausi and Hemidiaptomus roubaui (Copepoda, Calanoida): comparative structural and ultrastructural aspects

In this study, the structure and ultrastructure of the digestive system are compared in the early larval stages (nauplii I and II) of two copepod calanoid species, Acartia clausi Giesbrecht, 1889 and Hemidiaptomus roubaui Richard, 1888. The nauplii I of both species have no functional digestive tract, which is represented initially only by a blind esophageal slit and yolky endodermal cells, which fill the most part of the naupliar body, whereas at the nauplius II stage the differentiated digestive tract becomes functional. The resorption cavity corresponding to the future midgut is progressively formed in the endodermal mass during the premolt phase; it is surrounded by differentiating epithelial cells. In the ecdysial phase the foregut has associated labral glands, the midgut young R-, B-, and R′-cells of epithelium, and there is a short open hindgut. Résumé : La structure et l’ultrastructure du tube digestif ont été étudiées comparativement sur des stades larvaires (nauplii I et II) de deux espèces de copépodes calanoïdes, Acartia clausi Giesbrecht, 1889 et Hemidiaptomus roubaui Richard, 1888. Chez les nauplii I, l’appareil digestif n’est pas développé. Seule une fente œsophagienne aveugle est présente, la plus grande partie du corps étant occupée par une masse de cellules endodermiques, riches en vitellus. Les nauplii II possèdent un tube digestif fonctionnel. L’intestin moyen s’édifie progressivement, pendant la phase de prémue, à partir d’une cavité de résorption formée au sein de la masse endodermique et entourée d’une enveloppe de cellules épithéliales en cours de différenciation. Au moment de la mue, le nauplius est pourvu d’un œsophage avec des glandes labrales associées, d’un intestin moyen dont l’épithélium révèle trois types de cellules (R, B et R′) et d’un court intestin postérieur ouvert. 244 Baud et al. Introduction Studies of copepod crustaceans focusing on anatomy and the cellular structures involved in the digestive function are scarce. These works, done mostly during the last three decades, are restricted in the case of calanoid copepods to adult forms (Lowe 1935; Arnaud et al. 1978, 1980; Muskó 1988) and copepodids (Hallberg and Hirche 1980; Baldacci et al. 1985; Nott et al. 1985); some deal with cyclopoids (Defaye et al. 1985; Muskó 1986), harpacticoids (Sullivan and Bisalputra 1980; Yoshikoshi 1988a, 1988b), or the digestive tract of some parasitic species (John and Nair 1975; Briggs 1977; Gresty 1992). Regarding the anatomy of larval stages, only scanty information is available on a free-living harpacticoid (Fanta 1972), cyclopoid (Dudley 1966), and on some parasitic species (Sabatini et al. 1987; Benedetti et al. 1992), but none of these works have used an ultrastructural approach. The only study of free-living calanoid nauplii dates from the end of the 19th century (Grobben 1881) and the data need to be reconsidered. So the purpose of this paper is to compare the digestive system during early ontogenesis in nauplius stages I and II in calanoids to elucidate its morphogenesis and cellular structure. Material and methods Collection of nauplii I and II Acartia clausi This neritic species was collected with a net towed horizontally 10 m deep at the bottom of the Bay of Villefranche (Alpes-Maritimes, France). The plankton was poured into insulated containers and brought to the laboratory within 1 h after catch. Adults were then sorted under a dissection microscope. At least 120 females, to which about 50 males were added to insure fertilization, were put in a 5-L beaker in filtered (0.45 μm) seawater supplemented with Hymenomonas elongata (Haptophyceae, about 12 μm diameter) as food at an excess concentration of 15000 cells/mL. While in the beaker the copepods were kept in a plastic cylinder closed at the bottom with a 200 μm mesh net. This device separated the adults from their eggs, which fell through the net to the bottom of the beaker. The water in the beaker was kept at a temperature close to that of the seawater, about 23°C at the time of collection. The diel rhythm was 16 h light : 8 h dark. To obtain enough eggs to raise a cohort, females were Can. J. Zool. 80: 232–244 (2002) DOI: 10.1139/Z01-219 © 2002 NRC Canada 232 Received 28 February 2001. Accepted 27 November 2001. Published on the NRC Research Press Web site at http://cjz.nrc.ca on 1 March 2002. A. Baud,1 R.-M. Barthélémy, and M. Brunet. Equipe d’acceuil Biodiversité, Laboratoire de Biologie Animale (Plancton), Université de Provence, Place Victor Hugo, 3, 13331, Marseille, France. S. Nival. Station Zoologique, O6234, Villefranche-sur-Mer, CEDEX, France. 1Corresponding author (e-mail: [email protected]). J:\cjz\cjz80\cjz-02\Z01-219a.vp Wednesday, February 27, 2002 8:12:12 AM Color profile: Generic CMYK printer profile Composite Default screen