Scientists Finally Know Why Male Anglerfish Don't Get Rejected When They Fuse With Females

'Plz reply, I fused my blood supply with yours' - Shriveled male anglerfish, 2020. Neil Bromhall/Shutterstock

Rachael Funnell 30 Jul 2020, 19:08

Clingy partners can be problematic for humans, but for anglerfish in the murky deep there can be no getting away as some males literally fuse with females to increase their chances of reproductive success. This extreme variety of copulation, which sees the male shriveled to essentially a pair of accessory gonads, is unusual in more ways than one. Scientists were perplexed as to why the female’s immune system didn’t launch an attack on their latched partner, but new research published in the journal Science has revealed that the males are protected thanks to an altered immune response. How romantic…

Finding a mate is difficult at the best of times but for deep-sea denizens the process is even more fraught. Some species of anglerfish have adapted a form of sexual parasitism to ensure that once they find somebody, they never let go. The males latch on to the females, fusing tissues and eventually even circulatory systems. The union ensures both partners are able to reliably reproduce and as time goes by the male shrinks down until only the important parts remain.

Usually, this kind of fusion would be expected to result in the male being rejected in the same way that donated organs often fail to take in their recipients. Understanding why this didn’t happen has been hard as obtaining specimens of deep-sea creatures for investigation, be they alive or dead, is never easy. Jeremy Swann and colleagues turned to preserved specimens to find the answer, and sequenced DNA from 31 anglerfish species including non-attaching, temporarily attaching, and permanently attaching varieties.

They found that the genes of these fish varied significantly when it came to those that code for different aspects of the immune system. Aicda genes are involved in the maturation of antibodies, a key element in the adaptive immune response, and these were missing in males, which temporarily attached to females. Rag genes assemble T-cell receptors as well as antibody genes and these were missing in species known to permanently attach. Both of these genes were present in the anglerfish species that didn’t exhibit sexual parasitism.

The researchers state that these genetic differences demonstrate how sexual parasitism has been supported by evolved changes in adaptive immunity, which prevent the females from rejecting the attaching males. The finding is also remarkable as it paints a far more flexible picture of immunological shift among vertebrates, as it was previously thought that going back on an established immune response would only end in disaster.

Understanding how these unusual animals alter their immune response to cater to their sex lives could prove very useful in formulating therapies to help patients who are immunodeficient. As a reproductive strategy, it seems to be working pretty well for anglerfish, who are the most species-rich vertebrate taxon in the deep sea. As a means of copulation, however, we’d recommend that it stays with the fish. When the Spice Girls sang 2 Become 1, I’m quite certain they didn’t mean it literally.

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