One explanation for the of sexual parasitism is that the relatively low density of females in deep-sea environments leaves little opportunity for mate choice among anglerfish. Females remain large to accommodate , as is evidenced by their large ovaries and eggs. Males would be expected to shrink to reduce metabolic costs in resource-poor environments and would develop highly specialized female-finding abilities. If a male manages to find a female, then parasitic attachment is ultimately more likely to improve lifetime fitness relative to free living, particularly when the prospect of finding future mates is poor. An additional advantage to parasitism is that the male’s sperm can be used in multiple fertilizations, as he stays always available to the female for mating. Higher densities of male-female encounters might correlate with species that demonstrate parasitism or simply use a more traditional temporary contact mating.
Finally, sexual parasitism may be an optional strategy in some species of anglerfishes. In the , parasitized females have been reported in and —and others that were not still developed fully functional gonads. One theory suggests the males attach to females regardless of their own reproductive development if the female is not sexually mature, but when both male and female are mature, they spawn then separately.
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Parasitism is not the only method of reproduction in anglerfish. In fact, many families, including the , , , and , show no evidence of male parasitism. Females in some of these species contain large, developed ovaries and free-living males have large testes, suggesting these sexually mature individuals may spawn during a temporary sexual attachment that does not involve fusion of tissue. Males in these species also have well-toothed jaws that are far more effective in hunting than those seen in parasitic species.
However, the methods anglerfish use to locate mates vary. Some species have minute eyes that are unfit for identifying females, while others have underdeveloped nostrils, making them unlikely to effectively find females by smell. When a male finds a female, he bites into her skin, and releases an that digests the skin of his mouth and her body, fusing the pair down to the blood-vessel level. The male becomes dependent on the female host for survival by receiving nutrients via their shared circulatory system, and provides sperm to the female in return. After fusing, males increase in volume and become much larger relative to free-living males of the species. They live and remain reproductively functional as long as the female lives, and can take part in multiple spawnings. This extreme ensures that when the female is ready to spawn, she has a mate immediately available. Multiple males can be incorporated into a single individual female with up to eight males in some species, though some taxa appear to have a "one male per female" rule.