The comparative ultrastructure of the egg membrane and associated pore structures in the starry flounder, Platichthys stellatus (Pallas), and pink salmon, Oncorhynchus gorbuscha (Walbaum)

Abstract

Eggs of the starry flounder (Platichthys stellatus) and pink salmon (Oncorhynchus gorbuscha) were examined by scanning and transmission electron microscopy to determine differences in egg membrane structure with reference to contrasting ecological conditions in which the eggs normally develop. The egg membrane of the starry flounder constitutes 0.22–0.50 % of the egg's diameter. The zona radiata is composed of 6 continuous horizontal lamellae, covered by a thin triple layered border, and pierced by numerous regularly spaced pore canals. The micropyle canal measures 8 μm at the opening and tapers to 3.6 μm as it penetrates the membrane. In contrast, the thicker membrane of the pink salmon egg forms 0.80–1.0% of the egg's diameter, is composed of numerous short discontinuous lamellae which are pierced by pore canals, and is covered by a coating of irregular thickness. The 15–16 μm micropyle opening is surrounded by an area of protrusions, and the funnelshaped canal tapers to 2 μm at its terminal aperture. Contrasting environmental conditions during embryogenesis of these two species may be reflected by the thin membrane and simple lamellar structure in the pelagic egg of the starry flounder, and the thick membrane and complex lamellar structure in the demersal egg of the pink salmon.