Reproduction in certain deep-sea anglerfishes involves the permanent attachment of dwarf males to much larger females and fusion of their tissues leading to the establishment of a shared circulatory system. This unusual phenomenon of sexual parasitism enables anglerfishes to maximize reproductive success in the vast and deep oceans, where females and males otherwise rarely meet. An even more surprising phenomenon relates to the observation that joining of genetically disparate male and female anglerfishes does not evoke a strong anti-graft immune rejection response, which occurs in vertebrates following allogeneic parabiosis. Recent studies demonstrated that the evolutionary processes that led to the unique mating strategy of anglerfishes coevolved with genetic changes that resulted in loss of functional genes encoding critical components of the adaptive immune system. These genetic alterations enabled anglerfishes to tolerate the histoincompatible tissue antigens of their mate and prevent the occurrence of reciprocal graft rejection responses. While the exact mechanisms by which anglerfishes defend themselves against pathogens have not yet been deciphered, it is speculated that during evolution, anglerfishes adopted new immune strategies that compensate for the loss of B and T lymphocyte functions and enable them to resist infection by pathogens.
The morphology and the feeding strategies of the eerie-looking deep-sea anglerfishes (Ceratioidei) have been shaped during millions of years of evolution in the eternal darkness of the deep oceans [1,2]. Most are characterized by having a globular body with a huge head and enormous crescent-shaped mouth that is filled with long, fang-like teeth. Their retractable jaw, lack of ribs, and overall pliable bodies enable them to feed on prey that is almost twice their own size . In addition, they have perfected their feeding strategy by developing a specialized luminescent lure organ that enables them to attract prey in the hostile environment of the dark ocean . This fishing rod-like apparatus, termed illicium (from the Latin word “Illicio”, which means “to entice”), is derived from a modified dorsal-fin spine which extends from the dorsal region of their head and is tipped with a sphere-shaped luminescent organ, called ēsca (which means “bait” in Latin) [4,5].