The escal photophore of ceratioids (Pisces; Ceratioidei) — a review of structure and function

The article deals with the structure and presumed functions of the escal photophore found in the bulbous tip of the cephalic fin ray or illicium, situated on the upper part of the head in metamorphosed females in most species of deep‐sea anglerfishes (ceratioids). The escal photophore consists of the light gland proper and certain accessory structures. An accessory structure common to all species is a lightproof cup enclosing the light gland and provided with a distal opening, while the escae in a number of ceratioids in addition posses reflecting structures, tubular modifications of which form light guides in some species. The light gland proper is a roughly oval or spherical body, typically consisting of radiating branched glandular tubules arranged around a central escal cavity which communicates with the exterior via the vestibule, a usually slit‐like epithelium‐lined space lying above the distal part of the light gland. All lumina within the light gland contain bioluminescent symbiotic bacteria. The esca is generally thought to function as a lure, prey being attracted by the light emitted from the photophore and movements of the illicium. A possible additional function may be the ejection of a luminous material to confuse predators.