A female specimen of the deep-sea anglerfish species Melanocetus johnsonii of about 3 inches (75mm) in size with a one inch (23.5mm) male fused on her belly
The Anglerfish that are found in deep-sea, 'fuse' bodies with their partners when they mate. The smaller male anglerfish becomes permanently attached to the relatively gigantic female. This results in their tissue fusing together and the two animals establishing a common blood circulation system.
According to a report by Daily Mail, the male anglerfish becomes entirely dependent on the female for nutrients. This process is called 'sexual parasitism.'
In this process, the male supplies sperm to his mate while simultaneously receiving these nutrients.
In order to support the mating-fusion process, female anglerfish have evolved a new type of immune system that does not regard the male as 'foreign tissue', allowing them to host up to eight mates simultaneously.
In a shot from footage of live anglerfish mating, a fist-sized female completely dwarfs the male fused to her underside (seen as the tiny object just beneath her belly)
Quick Recap
The Anglerfish that are found in deep-sea, 'fuse' bodies with their partners when they mate.
This process is called 'sexual parasitism.'
Anglerfish that permanently attach are largely lacking the genes that encode these MHC molecules, the team discovered.
According to scientists, this permanent attachment of males to females is a form of 'anatomical joining'.
In other vertebrate species, sexual parasitism would automatically trigger an immune response and attack foreign tissue, just as it would to destroy cells infected by pathogens.
But for anglerfish, protective immunity would actually stand in the way of reproductive success.
These findings hinted at the possibility that the immune system of anglerfishes was very unusual among the tens of thousands of vertebrate species,' said study author Jeremy Swann at the Max Planck Institute of Immunobiology and Epigenetics (MPI-IE) in Freiburg, Germany.
With scientists from the US, the MPI-IE team studied the genomes of different anglerfish species, including the structure of molecules called major histocompatibility (MHC) antigens.
These molecules are found at the surface of the body's cells and signal an alarm to the immune system, when the cells are infected by a virus or bacteria. MHC molecules are extremely varied and it is hard to find identical or near-identical forms in any two individuals of a single species.
This helps explain the problem of tissue-matching that makes human organ and bone marrow transplantation rife with complications. Anglerfish that permanently attach are largely lacking the genes that encode these MHC molecules, the team discovered.
The scientists say that it's as if the permanently attached males 'had done away with immune recognition' in favour of tissue fusion. This inhibited the function of T cells, a type of white blood cell of key importance to the immune system.
The study shows that vertebrates can survive without the adaptive immune facilities previously considered to be irreplaceable. 'We assume that as yet unknown evolutionary forces first drive changes in the immune system, which are then exploited for the evolution of sexual parasitism,' said Boehm.
The findings have been published in the journal Science. In 2018, scientists revealed footage of live anglerfish mating for the first time, according to a report in Science Magazine.
Before then, anglerfish mating pairs had reportedly only been observed in dead specimens caught in nets. The footage was captured at a depth of 2,600 feet off Portugal’s São Jorge Island by husband and wife deep sea explorers Kirsten and Joachim Jakobsen using a remotely operated vehicle.
