Topic summary
Sexual parasitism

Extracted from the Wikipedia article Fish reproduction.
Sexual parasitism
Sexual parasitism is a mode of sexual reproduction, unique to deep-sea anglerfish, as they are faced with unique evolutionary pressures in the deep pelagic zones: individuals are very thinly distributed which makes encounters very rare, and unlike most pelagic fish anglerfish are weak swimmers. Therefore, finding a mate is problematic, and drives the development of this behavior.