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

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Abstract
Oogenesis, spermatogenesis, embryogenesis, and the structure of the larva of Oopsacas minuta (Hexactinellida, Hexasterophora) were studied in a population living in a Mediterranean cave and which reproduces year round. The different stages in oogenesis and the total and equal cleavage of the zygote are similar to the same events in the Calcarea and Demospongiae. During early development, the embryo is clearly cellular. The first stages of cleavage are spiral, and gastrulation occurs by primary delamination. These features are unique in the phylum Porifera where the occurrence of true gastrulation is controversial. The larva possesses multi-flagellated cells that are linked by parallel junctions, as well as larval spicules, choanochambers, and symbiotic bacteria. Part of larval tissue is syncytial. Spiculogenesis is intrasyncytial rather than extracellular as in the Calcarea, and the ultrastructure of the sclerocyte is similar to that of Demospongiae except that it is multinucleate. The developmental process and the structure of the larva in hexactinellids show characteristics that are found in some demosponges and calcareous sponges, but also have features that are not found in other Porifera. We consider that the latter are secondary acquisitions, which do not substantiate the classification of the Hexactinellida as a separate phylum.