An ultrastructural comparative study of the eggs of two species ofTethya(Porifera, Demospongiae)

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Abstract
The organization of the eggs of Tethya tenuisclera and Tethya seychellensis (Porifera, Demospongiae), collected inside the lagoons of two Maldive Islands, was studied by SEM and TEM. The cytoplasmic matrix is vacuolated and filled with stored material. Phagosome-like and rod-like inclusions represent the major components. Phagosome-like inclusions correspond to the egg yolk; rod-like inclusions undergo modifications resulting in filament bundles that accumulate underneath the cell membrane. The occurrence of undigested bacteria in membrane-bound vacuoles within the cytoplasmic matrix suggests a maternal transmission of symbionts. In T. seychellensis the layered appearance of the cell surface results from the accumulation of membranes associated with cytoplasmic vacuoles. This structure constitutes an envelope along the egg peripheral border and is interpreted as a protecting sheath improving egg survival after spawning.

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