An ultrastructural study of gametogenesis and early development in the sea anemone Actinia fragecea (Cnidaria : Anthozoa)

Larkman, Alan Urquhart (1986). An ultrastructural study of gametogenesis and early development in the sea anemone Actinia fragecea (Cnidaria : Anthozoa). PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000de54

Abstract

Large Individuals of the sea anemone Actinia fragacea were collected at approximately monthly intervals over a two year period. Their gonads were examined by light and electron microscopy, in order to follow the gametogenic process. The sexes are separate, and both show an annual cycle of activity. Oocytes arise in the gonad epithelia, but soon migrate into the mesogloea. During vitellogenesis, the oocytes accumulate compound yolk granuleso fibrillar and cortical granules, lipid droplets and glycogen. The surface of large oocytes bears tufts of large microvilli or cytospines. The oocytes reach some 150 pm in size. A group of specialized gonad epithelial cells projects through the mesogloea and contacts the oocyte surface, forming the trophonema, which is involved with nutrient transfer. The gonad epithelium can take up nutrients from the external medium, and the trophonema is particularly active in the uptake and incorporation of some small molecules. Not all fully grown oocytes are always spawned; some break down in an orderly fashion and are resorbed. Oogenesis was also examined, in less detallo in four other species of anemone. Spermatogenesis takes place in spherical testicular cysts, which are also associated with trophonemata. Spawning occurred in the laboratory on three occasions. Spawned eggs do not possess a vitelline coat, and do not undergo a cortical reaction. Gastrulae may take up numerous supernumary sperm.

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