Larval development, ultrastructure and metamorphosis inChondrilla australiensisCarter, 1873 (Demospongiae, Chondrosida, Chondrillidae)

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Abstract
This study investigated the cleavage, development and ultrastructure of larvae and settlers in the oviparous sponge Chondrilla australiensis, using electron and light microscopy. Fertilization is internal, and cleavage is total and equal. Swimming coeloblastula larvae are present after 24 h, and are 50–75μm in diameter. They are completely ciliated, spherical or ovoid, with a flattened posterior pole on which longer cilia occur. Larvae are composed only of a monostratified layer consisting of a relatively small number of columnar, monociliated cells. Intercellular junctions join the apical parts of the cells. The small central cavity contains dense populations of bacterial and cyanobacterial symbionts. After a free-swimming period of 24–36 h, larvae settle on the substratum by the anterior pole. Metamorphosis occurs by the dedifferentiation and rearrangement of ciliated cells, which are the source of all adult cells in C. australiensis. Results of our research confirm the monophyly of order Chondrosida.