Internal fertilization in the salp Thalia democratica

Abstract

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 (Forsskål, 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.