Background: This study compares the morphology and cytology of olfactory organs in moray eels (Muraenidae), particularly Siderea grisea and some species of the genera Echidna, Gymnothorax, and Lycodontis, fishes that are top predators in shallow-water marine habitats. Some of the species search visually for food while others search by olfaction.
Methods: The morays were collected in the Red Sea; the nasal olfactory organs were dissected and fixed in Bouin's solution for light-microscopy, and 3.5% glutaraldehyde for electron-microscopy studies.
Results: In each studied species the olfactory rosettes are elongated structures situated in closed olfactory chambers between anterior tubular inlet nares and slit-form posterior outlet openings. The double row of lamellae constituting these rosettes are round in Siderea and Echidna and elongated in the other species. They are attached at their base to a median raphe and range in number from 20 in the youngest observed Siderea to 168 in Gymnothorax of 1,500 mm total length. As in other teleosts, the lamellae are covered by a ciliated epithelium composed of three types of sensory cells: two of these, ciliated sensory neurons and ciliated supporting cells, differ in detail, length, and thickness of their cilia and intracellular rootlet system; the third type of sensory cells bears microvillae as well as cilia. Proximal, axonal extensions of the ciliated cells cross the basal lamina in bundles and combine to form fila olfactoria from which the two olfactory nerves extend towards the olfactory bulbs. Lateral extensions at the basal parts of these ciliated cells, the so-called spines, cross the membranes of neighboring cells as dendrites, possibly changing part of all of the ciliated epithelium into an olfactory field. The density and number of sensory cells on the lamellae, as well as observed differences in their foraging behavior in nature and captivity, enable the morays to be divided into two groups: one group, in which the lamellae are richly covered with stereocilia, includes species of the genera Siderea and Echidna, that search for food by olfaction, and the second group, which has a great deal less cells with stereocilia and includes the studies species of Gymnothorax and Lycodontis, locates its food visually.