In the sparse population of the dark levels of the sea, finding a mate must present a problem. Possession of photophores or the ability to communicate by sound or scent must greatly increase the chances of success. Light signals can provide a means of communication in social and courtship displays and in warning and territorial behaviour. An additional adaptation for this purpose has been the evolution of dwarf parasitic males. For example, in some ceratioid angler fish the male is much smaller than the female, and relatively undeveloped except that his eyes and olfactory organs are large and his teeth sharp and specialized for gripping the female. Once she has been found, perhaps partly by scent, the male apparently bites into her skin and remains permanently attached. His eyes and olfactory organs then degenerate and a partial fusion of tissues occurs between the two individuals, the male drawing nutriment from the female by intimate, placentalike association of the two blood systems. The testes develop and ripen so that sperms can be shed in the immediate vicinity of the eggs.
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