Sexual reproduction of the solitary sunset cup coral Leptopsammia pruvoti (Scleractinia, Dendrophylliidae) in the Mediterranean. 1. Morphological aspects of gametogenesis and ontogenesis.

Information on the reproduction in scleractinian solitary corals and in those living in temperate zones is notably scant. Leptopsammia pruvoti is a solitary coral living in the Mediterranean Sea and along Atlantic’s coasts from Portugal to southern England. This coral lives in shaded habitats from the surface to 70 m in depth, reaching population densities of over 17,000 individuals per square meter. In this paper, we discuss the morphological aspects of sexual reproduction in this species. In a companion paper, we report quantitative data on the annual reproductive cycle and make an interspecific comparison of reproductive traits in dendrophyllidae aimed at defining different reproductive strategies. The present study on Leptopsammia pruvoti is the first in-depth investigation of the reproductive biology of a species of this genus. As expected for a member of the family Dendrophylliidae, Leptopsammia pruvoti is a gonochoric and brooding coral. The gastrodermal tissue of the gametogenetic mesenteries we examined was swollen and granular which led us to hypothesize that interstitial cells could have a trophic function favoring gametogenesis. Undifferentiated germ cells arose in the gastrodermis and subsequently migrated to the mesoglea where they completed gametogenesis. During spermary development, spermary diameter increased from a minimum of 14 m during the immature stages to a maximum of 410 m during the mature stages. As oogenesis progessed, we observed a gradual reduction in the nucleus to cytoplasm ratio due to the steady synthesis of yolk. During the final stages of oogenesis, after having migrated to the extreme periphery of the oocyte and firmly adhered to the oolemma, the nucleus became indented assuming a sickle- or dome-shape. We can hypothesize that the nucleus’ migration and change of shape could have to do with facilitating fertilization and determining future embryonic axis. During oogenesis, oocyte diameter icreased from a minimum of 20 m during the immature stage to a maximum of 680 m when mature. Embryogenesis took place in the ceolenteron. We did not see any evidence that even hinted at the formation of a blastocoele; embryonic development proceeded via stereoblastulae with superficial cleavage. Gastrulation took place by delamination. Early and late embryos had diameters of 204-724 m and 290-736 m, respectively. When released, the larvae had completed ontogenesis and swam by a ciliary movement with the aboral pole at the anterior, their shape varied from spherical to cylindrical (in the latter the oro-aboral axis measured 695-1595 m and the transversal one 267-633 m).