Gonochorism and planula brooding in the Mediterranean endemic orange coral Astroides calycularis (Scleractinia: Dendrophylliidae). Morphological aspects of gametogenesis and ontogenesis.

Information on reproduction in temperate scleractinian corals is notably scant. Astroides calycularis is an azooxanthellate coral that inhabits the South-Western Mediterranean Sea, in shaded habitats from 0 to 50 m depth. Recently, it has been observed along the coast of the Adriatic Sea. This study is the first in-depth investigation of A. calycularis reproductive biology. Observations from the nineteenth century described A. calycularis as hermaphroditic; in contrast, we demonstrated gonochorism (male and female colonies) and brooding (planula releasing) as the reproductive mode, consistent with other members of the family Dendrophylliidae. Undifferentiated germ cells arose in the gastrodermis and subsequently migrated to the mesoglea, where they completed gametogenesis. During spermatogenesis, spermary diameter increased from 20 to 940 mu m. During oogenesis, a conspicuous presence of lipid vesicles of exogenous origin (phagocytes) was observed in the ooplasma. As oogenesis progressed, the synthesis of yolk gradually reduced the nucleus to cytoplasm ratio. In the final stages of oogenesis, the nucleus migrated to the extreme periphery of the oocyte adhering to the oolemma, and became indented. Nuclear migration and shape change may facilitate fertilization and determine the future embryonic axis. During oogenesis, the oocyte diameter increased from 25 to 1590 mu m. Embryogenesis took place in the coelenteron. Formation of a blastocoel was not observed, and development proceeded via stereoblastulae with superficial cleavage. Gastrulation took place by delamination. Embryo diameter ranged from 550 to 1140 mu m. Released larvae (length 1700 to 2000 mu m) were observed in the field during summer, along the bentho