A small but ferocious predator, the male dragonfish, will apparently do anything for love. Or at least to find a partner.
A new study by researchers at Boston College has found that the male dragonfish’s eyes enlarge when searching for a mate, making the dragonfish an anomaly in vertebrate evolution, the team reported today in the journal Royal Society. Biological letters.
Like many creatures that live in the dark depths of the sea, dragonfish survive thanks to numerous adaptations, including glowing bioluminescence. But females are not as smart as males, according to Boston College biologist Christopher P. Kenaley, lead author of the report “Sexually Dimorphic Eye-size in Dragonfishes, a Response to a Bioluminescent Signaling Gap.”
“We found that these male dragonfish have evolved larger eyes to find females that produce less light,” says Kenaley. “It’s quite stunning and a very important insight into how these poorly known species exist and thrive in the deep sea.”
Sexual dimorphism in the visual systems of vertebrates is extremely rare, Kenaley added. “This may be the second known case in fish, the largest group of vertebrates.”
The deep sea, the largest biome on Earth, is a dark, arid place and the distance between individuals living there can be enormous, Kenaley said. Most animals in this kingdom produce their own light, bioluminescence, and use this to attract prey and often find mates. Many deep-sea fish are sexually dimorphic in this respect: males have larger photophores, the organs that produce light, than females.
That can pose a problem when it comes to finding a partner, Kenaley said.
“This creates a paradox: if bioluminescence is important in sexual signaling and finding each other, this dimorphism results in a scenario where females can detect males at shorter distances than the other way around,” he said. “In short, there is a detection gap in bioluminescence. We hypothesized that, to close this gap, men would have evolved larger eyes to capture more light.”
Kenaley and his team of Boston College students, working with Stockholm University biologist Valentina Di Santo, studied two species: Malacosteus niger And Photostomias guernei – of deep-sea dragonfishes that are sexually dimorphic in the size of their photophores. The team modeled the distance at which males and females of each species could see members of the opposite sex. This was based on eye size and how much light a member of the opposite sex could produce.
The researchers found that there is a significant detection difference in both species, ranging from just a few meters to more than 100 meters, they report in the journal. They also found that males of both species have larger eyes and that this dimorphism closes the opening by about five meters. It’s a modest improvement, they noted, but important for the difficult task of finding a rare mate in such an open system.
In previous work, some deep-sea biologists have argued that a bioluminescence detection gap exists. The Boston College-led team is the first to assess the extent of this gap and, more importantly, whether these species may have evolved large male eye sizes.
Kenaley said future work could focus on why the bioluminescence of males is brighter than that of female dragonfish.
“We don’t yet know exactly why luminescent dimorphism occurs in these and other deep-sea species,” says Kenaley. “Perhaps it is a way to let each other know that a member of the opposite sex is nearby. To answer this question, more studies like ours are needed that establish a pattern of how detection distances vary with the degree of dimorphism.”
