Deep-sea anglerfishes employ an incredible reproductive strategy. Tiny dwarfed males become permanently attached to relatively gigantic females, fuse their tissues, and then establish a common blood circulation. In this way, the male becomes entirely dependent on the female for nutrient supply, like a developing fetus in the womb of her mother or a donor organ in a transplant patient. In anglerfishes, this unusual phenomenon is referred to as sexual parasitism and contributes to the reproductive success for these animals living in the vast space of the deep sea, where females and males otherwise rarely meet. The permanent attachment of males to females represents a form of anatomical joining, which is otherwise unknown in nature except for the rare occurrence in genetically identical twins. The immune system represents an extraordinary obstacle here. It attacks foreign tissue as it would destroy cells infected by pathogens. Just witness the difficulties surrounding organ transplantation in humans, which requires the careful cross-matching of donor and recipient tissue characters, together with immunosuppressive drugs, to ensure the long-term survival of the organ graft. But how is it possible then that, in case of anglerfishes, that individuals of the same species accept each other so readily when tissue-rejection is the usual and expected result of any such union? The phenomenon of sexual parasitism has posed an enigma that has existed for 100 years, ever since the first attached couple was discovered by an Icelandic fisheries biologist in 1920. Now, scientists from Germany and the USA have solved this century-old conundrum and report their findings online on July 30, 2020 in Science.
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