On the development and structure of the escal light organ of some melanocetid deep sea anglerfishes (Pisces : Ceratioidei)

The escal light glands of some deep sea anglerfishes of the genus Melanocet us were examined by light and electron microscopy. Sections of a larval Mel anocetus sp. showed that the gland originates from a solid, branched ingrow th of epidermal cells from the distal surface of the bulb-shaped esca. The light gland of metamorphosed specimens of M. murrayi and M. johnsoni wa s 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 r adial tubules of the gland open into a central escal cavity, from which a d uct leads to an epithelium-lined space, the vestibule, lying above the glan d. 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 es cal cavity and the glandular tubules is of a uniform thickness and structur e, consisting of flattened basal cells, cells extending to the lumen, and g oblet 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 A I. murrayi the distal (terminal) portions of th e glandular tubules have tall epithelial cells, while their wide proximal p arts and the central escal cavity are lined with a flattened epithelium; go blet cells are absent. Many glandular cells have processes projecting into the lumina. All glandular lumina and the central escal cavity contain numer ous rod-shaped bacteria and apparently isolated anucleate cytoplasmic profi les. The reflector is thick and well-developed; each cell contains several staggered layers of crystals. Bacteria have not been found in escae prior to the development of the duct connecting the light gland with the exterior, which suggests that the bacte ria gain access through the duct.