Fine Structure of Photophores in Gonostoma elongatum: Detail of a Dual Gland Complex

Gonostoma elongatum Gunther 1878 is a mesopelagic fish that has numerous light-bearing organs (the photophores). Some of the photophores are unusual in that they incorporate two different glands: glands D and V. These dual gland photophores are found: (1) in the upper serial row on the body, (2) among the suborbital photophores, and (3) in the caudal photophores. In a ventral serial row of photophores, each organ possesses one gland only: gland D. Glands D and V in the upper serial photophores have secretory ducts that fuse to form a common duct that empties to the surface. Gland D, which produces a bluish light, has dioptric material that could provide both spectral reflection and filter transmission. Gland V also has dioptric material associated with it. The entire glandular complex is covered by a shield of alternately layered collagen, which has a lens-like bulge over gland D. Immediately beneath the shield are slanted, overlapping rows of dioptric material. The detailed morphology of the dual gland photophore in the upper serial row is presented and briefly compared to some of the other photophores on the fish. Noteworthy are the various arrangements of materials that can efficiently guide the light from gland D and, possibly, from gland V. The result would be a diffuse glow suitable for counter illumination.