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Among tunicates, gamete morphology and sperm-egg interactions have been extensively investigated in ascidians, and to a lesser extent in appendicularians and thaliaceans. Sperm-egg interaction has been studied in only one salp, Pegea socia (Bosc, 1802). To determine if the pattern of internal fertilization of P. socia is generally applicable to salps, we performed an ultrastructural study on blastozooids of Thalia democratica (Forsskal, 1775). The ovary, located in the mantle near the gut, consists of a single oocyte connected to the atrial chamber wall by a 'fertilization duct', resembling a stack of single cells without a lumen. The flagellate sperm has a long corkscrew-like head with the single mitochondrion twisted around the nucleus. Fertilization is internal, and sperm actively penetrate the atrial wall and bore through the cells of the fertilization duct. During this process, the fertilization duct shortens as the cells move apart, one to one side and the next to the other, and rejoin to form a central lumen, which contains many sperm. At the same time a few sperm reach the periovular space for fertilizing the oocyte. Comparisons with P. socia indicate that this singular mode of internal fertilization with a complex corkscrew sperm actively penetrating the fertilization duct cells, probably evolved in the salp ancestor and has been modified to some extent in various genera. Chez les tuniciers, la morphologie des gametes et les interactions spermatozoides-oeufs ont ete bien etudiees chez les ascidies, mais moins bien chez les appendiculaires et les thaliaces. Les interactions entre les spermatozoides et les oeufs sont connues chez une seule salpe, Pegea socia (Bosc, 1802). Afin de determiner si le patron de fecondation interne de P. socia s'applique de facon generale aux salpes, nous avons etudie l'ultrastructure des blastozooides de Thalia democratica (Forsskal, 1775). L'ovaire, situe dans le manteau pres du tube digestif, consiste en un seul oocyte relie au mur de la chambre de l'atrium par un << conduit de fecondation >> ressemblant a un amoncellement de cellules individuelles sans lumiere. Le spermatozoide flagelle possede une longue tete en tire-bouchon avec une seule mitochondrie tournee autour du noyau. La fecondation est interne et les spermatozoides penetrent activement dans la paroi de l'atrium et percent les cellules du conduit de fecondation. Durant ce processus, le conduit de fecondation raccourcit, alors que les cellules se detachent, les unes d'un cote, les autres de l'autre, et se rejoignent pour former une lumiere centrale qui contient de nombreux spermatozoides. Au meme moment, quelques spermatozoides atteignent l'espace periovulaire pour feconder l'oocyte. Les comparaisons avec P. socia indiquent que ce mode particulier de fecondation interne, dans lequel un spermatozoide complexe en tire-bouchon penetre activement dans les cellules du conduit de fecondation, a probablement evolue chez l'ancetre des salpes et s'est modifie jusqu'a un certain degre dans les differents genres. [Traduit par la Redaction], Introduction Tunicates are marine animals and include benthic ascidians, planktonic thaliaceans (salps, doliolids and pyrosomes), and appendicularians. Most are hermaphroditic and have highly varied modes of sexual reproduction that ensure [...]

Abstract: The distribution of sensory cells and nerves was studied in the siphons of Corella inflata Huntsman, 1912 and Corella willmeriana Herdman, 1898 by immunohistology and electron microscopy. Each siphon [...]