Deep-sea anglerfish fuse bodies to mate thanks to an odd immune system

By Donna Lu

Anglerfish have an unusual way of mating and an unusual immune system

Edith A. Widder

Some species of anglerfish – the deep-sea predator that uses a luminous lure to attract prey – have a bizarre way of reproducing: they fuse with their mates. We now know how the fish can fuse tissues without triggering a potent immune response. They have a strange immune system.

There are 168 known species of anglerfish, which are found at ocean depths beneath about 300 metres. Some species mate through a process known as sexual parasitism. Males, which are often less than 10 millimetres in length, attach to the body of the larger female.

For some species of anglerfish, this attachment is temporary. In others, it is permanent: the skin tissues of the two fish fuse and eventually their circulatory systems connect, and the male becomes dependent on its mate for nutrients.

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In all other vertebrate species, the fusion of tissues would trigger a significant immune response, because an animal’s immune system attacks cells it recognises as foreign – the reason why people have to take immunosuppressive drugs after receiving an organ transplant.

By analysing the DNA of 31 anglerfish specimens from 10 species, Thomas Boehm at the Max Planck Institute of Immunobiology and Epigenetics in Freiburg, Germany, and his colleagues found that fusing anglerfish species are missing key immune system genes.

All other vertebrates have some form of adaptive immunity, in which white blood cells known as T-cells and B-cells protect the body by recognising foreign pathogens and producing specific antibodies against them.

“Patients with defects in adaptive immunity are very poorly,” says Boehm. But the anglerfish seem to have traded adaptive immunity for reproductive success without severe consequences.

Species with temporarily attaching males didn’t have functional aicda genes, which are needed for antibodies to mature. Permanently attaching species also had non-functioning rag genes, which are needed to assemble T-cell receptors.

The deep sea isn’t entirely devoid of pathogens, so how the anglerfish are able to defend themselves from infection remains a mystery, says Boehm.

Journal reference: Science, DOI: 10.1126/science.aaz9445

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