Reproduction is central to the survival of all living organisms. Over millions of years, animal species on Earth have evolved a remarkable variety of mating strategies, ranging from physical fights with competitors to mesmerizing color changes, songs that attract mates and rhythmic movements that resemble dancing.
A particularly intriguing mating behavior has been observed in some anglerfish, predatory fish living in the deep sea. Tiny dwarf males belonging to specific anglerfish species can permanently attach themselves to larger females, fusing their tissues with those of the females and sharing the same blood supply.
This unusual behavior, known as sexual parasitism, results in males depending entirely on a specific female for both survival and reproduction. While many past studies have explored the biological underpinnings of sexual parasitism, they did not conclusively explain why only some anglerfish species rely on this mating strategy.
Researchers at the University of Tokyo recently tried to shed more light on the evolution of sexual parasitism in anglerfish, using a newly developed mathematical model.
Their paper, published in Proceedings of the Royal Society B, suggests that this mating strategy emerged in species for which securing a reproductive partner in deep marine environments outweighed the costs of being permanently attached to a single mate.
"I have long been interested in how animals evolve strategies to avoid reproductive failure when mates are difficult to find," Takeaki Satow, the paper's first author, told Phys.org. "Anglerfishes with parasitic males represent one of the most extraordinary examples of this idea. Once a male finds a female, he permanently attaches to her body and receives nutrients from her, ensuring that he will always be available when she is ready to reproduce."
As anglerfish live hundreds or thousands of meters below the sea's surface, observing their behavior and tracing their history is extremely difficult. Satow and his colleague Gaku Takimoto thus decided to create a new mathematical model that could shed light on why some anglerfish species may have evolved such a remarkable mating strategy.
"Our main objective was to test whether reproductive assurance alone could explain the evolution of parasitic males and to identify the conditions under which this strategy is expected to evolve," Satow explained. "To do this, we developed a mathematical model describing the life cycle of male and female anglerfish."
Modeling the costs and benefits of parasitic mating
The mathematical model created by the researchers simulates the evolution of different anglerfish mating strategies under varying ecological conditions. Specifically, it simulates encounters between males and females in the deep sea, predicting the probability that they will form a permanent attachment, successfully reproduce and pass on their traits to future generations.
Notably, the team's model also accounts for the costs of a permanent, parasitic attachment, such as limiting the offspring that females can produce over their lifespan and preventing males from having multiple mates. It also considers the possible benefits of permanent attachments, such as guaranteed access to a mate and an increased rate of reproductive success.
"We assumed that females invest resources in maintaining attached males, which reduces the number of offspring they can produce and may also reduce their survival," Satow said. "For males, attachment means giving up the opportunity to mate with any other female. By considering these trade-offs, we could identify the conditions under which sexual parasitism becomes an evolutionarily favorable strategy."
The simulations run by Satow and Takimoto suggest that sexual parasitism evolved to allow anglerfish to secure a reproductive partner in the deep sea, where mates are difficult to find. However, it may have evolved only in species for which these reproductive benefits outweighed the costs of being permanently tied to a single mate.
"This perspective may also help explain why permanent sexual parasitism is so rare, even among deep-sea fishes," Satow explained.
"Many deep-sea species probably experience similar difficulties in finding mates, yet almost none have evolved this strategy. Our results suggest that the balance between benefits and costs is the key, rather than low encounter rates alone."
Revisiting the evolution of deep-sea fish
This study could deepen the understanding of anglerfish mating strategies, offering a plausible explanation for why only some species engage in sexual parasitism. The insight gathered by the researchers indicates that anglerfish may possess specific biological characteristics that make them particularly well-suited for this unique reproductive strategy. Future studies could help identify these unique characteristics.
"More broadly, I hope this study highlights reproductive assurance as an important evolutionary force that may help explain many different reproductive strategies across the animal kingdom," Satow said.
"One important next step will be to understand why anglerfishes evolved permanent sexual parasitism while other deep-sea fishes did not. Our model suggests that the balance between benefits and costs is important, but we still need to identify which biological traits make anglerfishes particularly suited to this strategy."
Satow is also interested in further exploring how reproductive assurance contributed to the evolution of specific species. This could be particularly relevant for species inhabiting environments where potential mates are scarce or where there are too many same-sex competitors.
"Much of evolutionary research on reproduction focuses on what happens after a mate has already been found—for example, which reproductive strategy produces the highest reproductive success," Satow added.
"I am interested in an even more fundamental question: how important is simply succeeding in reproduction at all? I hope to develop a broader theoretical framework that explains how the need to avoid reproductive failure has shaped the remarkable diversity of reproductive strategies observed across different organisms."
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Publication details
Takeaki Satow et al, Reproductive assurance drives the evolution of parasitic males in anglerfishes, Proceedings of the Royal Society B: Biological Sciences (2026). DOI: 10.1098/rspb.2026.1268.
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Citation: Why some male anglerfish parasitically fuse with females (2026, August 10) retrieved 10 August 2026 from https://phys.org/news/2026-08-male-anglerfish-parasitically-fuse-females.html
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