The escal light glands of some deep sea anglerfishes of the genus Melanocetus were examined by light and electron microscopy. Sections of a larval Melanocetus sp. showed that the gland originates from a solid, branched ingrowth of epidermal cells from the distal surface of the bulb-shaped esca.
The light gland of metamorphosed specimens of M. murrayi and M. johnsoni was found to be constructed in the same way as that of most other ceratioids, i.e. as a branched tubular gland enclosed by a cup-shaped reflector; the radial tubules of the gland open into a central escal cavity, from which a duct leads to an epithelium-lined space, the vestibule, lying above the gland. A duct from the vestibule opens on the upper-caudal surface of the esca.
In the smaller of two specimens of M. murrayi, the epithelium lining the escal cavity and the glandular tubules is of a uniform thickness and structure, consisting of flattened basal cells, cells extending to the lumen, and goblet cells. No bacteria were found anywhere within the esca. The reflector enclosing the gland contains only a few scattered crystals.
In the larger specimen of M. murrayi the distal (terminal) portions of the glandular tubules have tall epithelial cells, while their wide proximal parts and the central escal cavity are lined with a flattened epithelium; goblet cells are absent. Many glandular cells have processes projecting into the lumina. All glandular lumina and the central escal cavity contain numerous rod-shaped bacteria and apparently isolated anucleate cytoplasmic profiles. The reflector is thick and well-developed; each cell contains several staggered layers of crystals.