Morphology and fine structure of the swimming larvae of Ircinia oros (Porifera, Demospongiae, Dictyoceratida)

The fine structure of the free-swimming larvae of the sponge Ircinia oros (Dictyoceratida, Demospongiae) from the northern Mediterranean was studied using light and electron microscopy. Larvae are of the parenchymella type, oval. The larva is extensively ciliated. A band of long cilia and pigment-filled protrusions separates the posterior region from the antero-lateral region. The parenchymella is three-layered, with a ciliated epithelium, an intermediate layer and an internal zone. The surface of the larva is made of pear-shaped monociliated cells, which form a pseudo-stratified columnar epithelium. Zonula adhaerens, which join the apical parts of the ciliated cells, can be observed. The single basal ciliary rootlet is fibrillar and not associated with nucleus. A network of microtubules extends from the side of the basal body and forms electron-dense bundles. These bundles are opposite to the basal foot, run parallel to apical part of the plasmalemma and are oriented towards the posterior pole of the larva. A nucleus without a nucleolus is situated in the basal part of the cell. Above the nucleus there is a complex of large lipid droplets. The basal part of the cells are anchored by thick bundles of collagen fibrils with a regular transverse banding pattern of 25 nm periodicity. A prominent spindle-shaped sheath of subepidermal cells separates the epithelium from the central region of the larva. These cells are situated perpendicular to the epithelium and synthesize abundant fibrils of collagen. A part of these cells degrades. In the internal part of the larva there are multiple intercellular endosymbiotic bacteria and amoeboid nucleolated cells. Our data show that the study of sponge larvae is a source of diagnostic characters at the order level.