Reproduction of a hexactinellid sponge: first description of gastrulation by cellular determination in the Porifera

Oogenesis, spermatogenesis, embryogenesis, and the structure of the larva o f Oopsacas minuta (Hexactinellida, Hexasterophora) were studied in a popula tion living in a Mediterranean cave and which reproduces year round. The di fferent stages in oogenesis and the total and equal cleavage of the zygote are similar to the same events in the Calcarea and Demospongiae. During ear ly development, the embryo is clearly cellular. The first stages of cleavag e are spiral, and gastrulation occurs by primary delamination. These featur es are unique in the phylum Porifera where the occurrence of true gastrulat ion is controversial. The larva possesses multi-flagellated cells that are linked by parallel junctions, as well as larval spicules, choanochambers, a nd symbiotic bacteria. Part of larval tissue is syncytial. Spiculogenesis i s intrasyncytial rather than extracellular as in the Calcarea, and the ultr astructure of the sclerocyte is similar to that of Demospongiae except that it is multinucleate. The developmental process and the structure of the la rva in hexactinellids show characteristics that are found in some demospong es and calcareous sponges, but also have features that are not found in oth er Porifera. We consider that the latter are secondary acquisitions, which do not substantiate the classification of the Hexactinellida as a separate phylum.