They are few animals more bizarre than fishing rods, a species that has so much trouble finding a mate that when males and females mate underwater, males actually line their tissues with females for life. After the merger, the two share a single respiratory and digestive system.
Scientists have now discovered that fishing carries out this sexual parasitism because it has lost a key part of its immune system, which then allows two bodies to become one without rejecting the tissues. (Remember the symbiote Jadzia Dax from Deep Space Nine?)
All vertebrates, including humans, have two types of immune systems. The first is the innate system, which responds quickly to attacks by microscopic invaders with various chemicals such as mucous physical barriers such as hair and skin, and cells suffering from diseases called macrophages. The second line of defense is an adaptive system that produces both “killer”
But in a study published Thursday in the journal science, researchers at Germany’s Max Planck Institute and the University of Washington have found that many fish species (there are more than 300) have evolved over time to lose the genes that control their adaptive immune system, meaning they can’t produce antibodies and lack such T cells.
“Anglerfish have traded in their immune abilities, which we believe are essential for this reproductive behavior,” said Thomas Boehm, a professor in the Department of Immunobiology and Epigenetics at the Institute in Freiburg, Germany, and lead author of the paper.
To reach this conclusion, Boehm and his colleagues spent the last six years conducting genetic tests on tissue samples of marine fish taken from around the world. They tried to catch them with the help of deep-sea trawls that collect specimens 1,000 feet below the surface, but because fishing is rare and elusive, they were unable to collect any live specimens. To obtain enough tissue for their genetic analysis, the researchers instead looked at museum collections and other laboratories where marine fish have been stored in preservatives, some decades old.
There are several reproductive methods in the fishing family. Females of some species merge with one male; others protect themselves with multiple males; and yet another group have only a temporary merger. After grinding 31 tissue samples from 10 species, the team conducted genetic testing and found that species that temporarily merged with their friends lacked the genes responsible for antibody maturation. Species that form a permanent attachment to their partners have also lost a set of additional genes that are responsible for assembling T-cell receptors and antibody genes that underlie the innate immune system in all vertebrates.
“It was intuitive to think that there was some genetic predisposition to allow this to happen,” Bohem said of the unusual immune system of the species. “This is the first little evidence that these animals have this inability to reject a part of themselves and allow these purchases to take place.”