Abstract
This study reports on the transfer of heterotrophic bacteria from parental tissue to oocytes in the Mediterranean bacteriosponge Corticium candelabrum (Homosclerophorida) and the description of the successive locations of the microsymbionts during embryo development through transmission and scanning electron microscopy. Eight different types of symbiotic bacteria are described morphologically. These eight bacteria morphotypes are found in both adult individuals and larvae. Symbiotic bacteria are transferred to oocytes, but not to spermatocytes. Bacteria are first located at the oocyte periphery below a thick collagen layer and then they migrate to the oocyte cytoplasm, forming spherical clusters. After cleavage, the bacteria can be found in the free space between blastomeres but mainly accumulate at the embryo periphery below the follicular cells that surround the embryo. Once the blastocoel is formed, the symbiotic bacteria move to this central cavity where they actively divide by bipartition, increasing their number considerably. Many examples of phagocytosed bacteria in the proximal zone of the larval cells are observed at this stage. Consequently, bacteria may represent a complementary source of energy for free larvae and settlers before they are able to capture food from the surrounding water.
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