<P> The methods by which the anglerfish locate mates are variable . Some species have minute eyes unfit for identifying females, while others have underdeveloped nostrils, making it unlikely that they effectively find females using olfaction . When a male finds a female, he bites into her skin, and releases an enzyme 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 now - 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 stays alive, and can take part in multiple spawnings . This extreme sexual dimorphism 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 . </P> <P> One explanation for the evolution of sexual parasitism is that the relative low density of females in deep - sea environments leaves little opportunity for mate choice among anglerfish . Females remain large to accommodate fecundity, 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 parasitic attachment, then it 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 facultative parasitism or simply use a more traditional temporary contact mating . </P> <P> Parthenogenesis is a form of asexual reproduction in which growth and development of embryos occur without fertilization . In animals, parthenogenesis means development of an embryo from an unfertilized egg cell . The first all - female (unisexual) reproduction in vertebrates was described in the Amazon molly in 1932 . Since then at least 50 species of unisexual vertebrate have been described, including at least 20 fish, 25 lizards, a single snake species, frogs, and salamanders . As with all types of asexual reproduction, there are both costs (low genetic diversity and therefore susceptibility to adverse mutations that might occur) and benefits (reproduction without the need for a male) associated with parthenogenesis . </P> <P> Parthenogenesis in sharks has been confirmed in the bonnethead and zebra shark . Other, usually sexual species, may occasionally reproduce parthenogenetically, and the hammerhead and blacktip sharks are recent additions to the known list of facultative parthenogenetic vertebrates . </P>

Where do fish lay their eggs and why