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Structure of the external retina of the oviparous, hermaphroditic fish Rivulus marmoratus Poey.

The external retina of the oviparous, hermaphroditic fish Rivulus marmoratus was examined ultrastructurally. The outer nuclear layer is about half as thick as the inner nuclear layer, and a single layer of ganglion cells is present. Single and double cones are present and these are arranged in a square mosaic. The pigment epithelium is not very well developed and extends only to the level of the cone ellipsoids in the light-adapted retina. However, the sparse pigment granules do migrate in response to light and are capable of isolating cone outer segments. Cone synaptic ribbons show extreme changes in length. The pigment epithelium contains myeloid bodies, which show variations with the photic state; and structures reminescent of phagosomes but without the limiting membrane. The structure of the external retina of R. marmoratus is discussed in relation to its behaviour and habitat.