An extreme solution to deep-sea dating
In the aphotic "midnight zone" of the deep sea, more than 1,000 meters below the surface, the environment is defined by crushing pressure, near-freezing temperatures, and a complete absence of sunlight. In this vast, dark space, finding a mate is an evolutionary challenge. For a group of deep-sea anglerfish known as Ceratioids, the solution is an extreme reproductive strategies known to science: sexual parasitism.
This strategy evolved from extreme sexual dimorphism. A female anglerfish, like those in the genus Ceratias, can grow to over a meter long, while the male is a dwarf, sometimes only a few centimeters in length. The male's life is a singular mission: find a female. He is ill-equipped for long-term survival, often possessing a poor feeding apparatus, but has large nostrils to track the female's species-specific pheromones through the dark. Once he finds a partner, he uses specialized teeth to bite into her flank or belly and holds on for life.
A permanent biological bond
The attachment is physical. In species that practice obligate parasitism, the male's mouth and the female's skin begin to break down, and the two fish fuse together. Their circulatory systems merge, and the female begins to provide all the nutrients the male needs to survive. After mating, the male's body degenerates. He loses his eyes, fins, and most internal organs, eventually becoming little more than a pair of testes permanently attached to the female. This ensures the female has a supply of sperm whenever she is ready to spawn.
This fusion is unusual. In most vertebrates, the immune system would violently reject foreign tissue. Scientists have discovered that these anglerfish have evolved a unique solution. Many species that engage in permanent attachment have lost the genes associated with adaptive immunity—the system that produces antibodies and T-cells. This deep alteration prevents the female's body from rejecting the male, allowing their tissues to merge seamlessly. Some females are found with multiple males attached; a female of the species Cryptopsaras couesii has been documented carrying eight parasitic males at once.