The persistence of the embryonic (optic) fissure into adulthood and development of the falciform process in the eye varied among three fish species: guppyPoecilia reticulata, Mozambique tilapiaTilapia mossambica, and brown trout Salmo trutta. The falciform process is a ridge of pigmented and vascular tissue associated with the embryonic fissure in teleosts. In guppies, the embryonic fissure closed during embryonic development, and no falciform process developed. In Mozambique tilapias, the embryonic fissure persisted into adulthood but was lined only with retinal pigment epithelium and lacked photoreceptive layers; it was associated with a partially formed falciform process. In brown trout, the embryonic fissure remained well defined with both retinal pigment epithelium and photoreceptor layers, and the falciform process was well developed. The temporal zone of the fissure in adult brown trout exhibited active growth and had a ventral area of high cone density. The well-developed fissure in brown trout and the oblique orientation of its ventrotemporal cones relative to incident light may be correlated with the species' ability to locate prey above and in front of its body. This system also may allow brown trout to detect polarized light, which could assist in navigation during migration.