The male dragonfish’s eyes increase in size in order to attract mates, representing a unique case of sexual dimorphism in vertebrate evolution, according to research findings.
Despite their small size, male dragonfishes are fierce predators, seemingly willing to do anything for romance or even just to find a partner.
A recent study conducted by researchers from Boston College revealed that male dragonfishes develop bigger eyes for the purpose of mate attraction, making them a distinctive case in vertebrate evolution, as reported in the Royal Society journal Biology Letters.
Like many organisms living in the dark depths of the ocean, dragonfishes have evolved several adaptations, including the ability to produce bioluminescent light. However, female dragonfishes don’t shine as brightly as their male counterparts, according to Christopher P. Kenaley, the lead researcher on the study titled “Sexually Dimorphic Eye-size in Dragonfishes, a Response to a Bioluminescent Signaling Gap.”
“Our findings indicate that male dragonfishes have evolved larger eyes to detect females that emit less light,” Kenaley explained. “This is quite remarkable and provides critical insight into how these lesser-known species manage to survive and flourish in the deep ocean.”
Kenaley also noted that sexual dimorphism in the visual systems of vertebrates is exceedingly rare, adding, “This may be only the second known instance within fish, the largest group of vertebrates.”
The deep-sea is recognized as the largest biome on Earth, characterized by darkness and sparse life. The distances between individuals residing in this environment are often significant, according to Kenaley. Many creatures in these depths generate their own light through bioluminescence, using it to attract prey and find partners. Numerous deep-sea fish exhibit sexual dimorphism in this aspect, with males possessing larger photophores—the organs responsible for producing light—than females.
This presents a challenge when it comes to locating a mate, Kenaley remarked.
“This creates a paradox: if bioluminescence is crucial for sexual signaling and locating one another, then this dimorphism leads to a situation where females can spot males at shorter distances compared to vice versa,” he said. “In essence, there’s a gap in bioluminescent detection. We proposed that to bridge this gap, males may have developed larger eyes to capture more light.”
Kenaley and a team of undergraduate students from Boston College, in collaboration with Stockholm University biologist Valentina Di Santo, examined two species of deep-sea dragonfishes—Malacosteus niger and Photostomias guernei—that exhibit sexual dimorphism in their photophore sizes. The researchers analyzed the distance at which males and females of each species could perceive members of the opposite sex, based on eye size and the amount of light produced by the opposite gender.
The research revealed a significant detection gap in both species, ranging from a few meters to over 100 meters, as detailed in the journal. Additionally, they found that males of both species possess larger eyes, which help to narrow the gap by approximately 5 meters. Although this is a subtle enhancement, it is crucial for finding a rare mate in such an expansive environment.
In earlier studies, some deep-sea biologists had suggested the existence of a bioluminescent detection gap. The Boston College-led study is the first to measure this gap’s extent and, importantly, examine whether these species have developed larger male eye sizes.
Kenaley mentioned that future research may investigate why male dragonfishes are brighter than females.
“Currently, we do not fully understand the reason behind luminescent dimorphism in these and other deep-sea species,” Kenaley noted. “It could serve as a method for signaling to one another that a potential mate is nearby. Addressing this question will necessitate additional studies similar to ours that identify patterns of how detection distances fluctuate with varying levels of dimorphism.”
