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Biology for the Global Citizen

Stuck on You: Finding Love in the Dark and Deep

by George Shiflet
Copyright © 2023 George Shiflet. All rights reserved.

One hundred years ago, an Icelandic fisheries biologist first observed what turned out to be sexual parasitism in fish. There was no ready explanation, but this phenomenon has now been observed in a number of deep-sea anglerfish species. From the example shown in Figure 1, you can see that such fish could be the subject of nightmares and horror films. The specimen shown is a female, while males of the species are much smaller. In the deep parts of the vast oceans, it is really dark, and possible mates may be few and far-between. (Given the physical appearance of some of these fish, darkness could be advantageous for mating!) It makes sense for males, once they encounter a female of the same species, to attach to the female for life. They may have trouble finding another.


Figure 1. Female Cryptopsaras couesii.

Figure 1. Female Cryptopsaras couesii. Note the fleshy modification of tip of the dorsal fin ray (spine), which acts as a lure for potential prey and mates.

Copyright © Masaki Miya et al. at https://commons.wikimedia.org/wiki/File:BMC_-_Ceratiidae-Cryptopsaras.couesii.jpg


Applying Science to Life 1 How do mates find each other? Form one or more possible explanations. The explanation doesn’t have to be correct, but how would you test your hypothesis? Think of any mechanisms that might promote successful mating.

Females often have luminescent bacteria to light the way, and when a male, with its enlarged eyes, finds a female anglerfish of the same species, it latches on (Figure 2). Alternatively, he could become a snack, because females always seem hungry.


Figure 2. Female Cryptopsaras couesii

Figure 2. Female Cryptopsaras couesii, showing luminescence of bacteria associated with her teeth and fin rays. Even the fleshy modification of tip of the dorsal fin ray possesses these symbionts.

Copyright © Etrusko25 at https://commons.wikimedia.org/wiki/File:Cryptopsaras_couesii.png


Applying Science to Life 2 How would secure attachment take place? What adaptations might be necessary?

After the initial attachment, the body surface of the male fuses with that of the female, eventually tapping into the circulatory system of the female. She essentially provides him with life support, much like a developing human fetus to its mother. His tissues become hers, and he becomes a parasite that produces sperm for her eggs. He is stuck on her for life.

Well, this relationship is not only interesting, but very unexpected. If you needed a liver, kidney, or heart transplant, the new organ would need to be matched carefully so that your immune system wouldn’t attack and reject it. Why? On the surface of almost all your cells, you have molecules that essentially tell your immune system that your cells belong to you, and the molecules should not treat them as aliens. These molecules, called Major Histocompatibility Molecules (MHM), essentially serve as your bar or QR code. The combinations of these molecules are determined by an individual’s particular gene mix. In the event a pathogen or foreign tissue is introduced to your body, the immune system will attack and destroy it. That is why tissue and organ transplants must be carefully typed, because no one else (except an identical twin) will match closely enough to escape rejection. Even sufficiently immunologically close fits require immunosuppressive drugs for success. Consequently, the fusion between the bodies of two fish, even of the same species, would be impossible unless something was different about the fishes’ immune system. So, how is such a phenomenon possible?

Recently, scientists from Germany and the United States have answered this question. In their analysis, they found that species exhibiting sexual parasitism also lack genes to direct the production of the identity molecules (MHMs) of these fishes’ cells’ surfaces. Furthermore, the genes that influence other fundamental components and activities of the vertebrate immune system are also missing—killer T-cells (eliminate infected cells and attack foreign tissues like those in transplants) do not form, and at least some anglerfish seem to lack the ability to produce antibodies. Human beings would not be able to survive without either of these fundamental immune functions. Consequently, scientists are pursuing how these animals can survive with this deficiency in their immune response.

Video of a fanfin anglerfish and her mate:  https://www.sciencemag.org/news/2018/03/exclusive-i-ve-never-seen-anything-it-video-mating-deep-sea-anglerfish-stuns-biologist



Swann, Jeremy B., Stephen J. Holland, Malte Petersen, Theodore W. Pietsch, and Thomas Boehm. "The immunogenetics of sexual parasitism." Science (2020).