Cryptopsaras couesii is one of the most commonly collected ceratioid anglerfishes. The ceratioid anglerfishes are a group of 11 families with representatives distributed around the world below a depth of about 300 meters (they may sometimes be found at much shallower depths as well, e.g. see Cryptopsaras couesii in Stewart and Pietsch 1998). They are most strikingly characterized by having an extreme sexual dimorphism in which males are dwarfed and, in some species, become parasitically attached to the body of a relatively gigantic female. The males of most species (but not C. couesii or its sister taxon, Ceratias) are equipped with large nostrils, apparently for homing in on a female-emitted, species-specific pheromone (Pietsch 2005). Normal jaw teeth are lost during metamorphosis, but are replaced by a set of pincher-like denticles at the anterior tips of the jaws for grasping and holding fast to a prospective mate. In some forms (including Cryptopsaras couesii) attachment is followed by fusion of epidermal tissues and, eventually, by a uniting of the circulatory systems so that the male becomes dependent on the female for blood-transported nutriment, while the female becomes a sort of self-fertiliizing hermaphroditic host (Pietsch 1976; Pietsch and Orr 2007 and references therein). A Cryptopsaras couesii female may have as many as eight males attached to various parts of her body (Pietsch 2005). Attached C. couesii males are almost invariably found facing anteriorly, as if they approached their mates from behind (Pietsch 2005).
Most ceratioid anglerfishes, including Cryptopsaras couesii, have a bacterial bioluminescent bait or lure (known as an "esca"), the structure of which is extremely useful (at least to humans) in distinguishing species (Pietsch and Orr 2007). Remarkably, at least in C. couesii luminescence is not limited to the caruncle and esca. In shipboard experiments, Young and Roper (1977) found that this species is luminescent over most of its body, with the glow apparently emanating from its skin (except where damaged). In their experiments the fish varied its luminosity to match varying levels of overhead illumination, achieving both lateral and ventral countershading that made it nearly invisible.
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