Chestnut Hill, Mass (07/24/2024) – The male dragonfish, a small yet formidable predator, appears to go to great lengths in pursuit of love—or, at the very least, to find a mate.
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Dragonfish
According to a recent study from Boston College, researchers have discovered that male dragonfishes develop larger eyes specifically to enhance their ability to locate potential mates. This adaptation sets the dragonfish apart in vertebrate evolution, as detailed by the team in today’s publication in the Royal Society journal Biology Letters.
Dragonfishes, like many other creatures inhabiting the dark abyss of the sea, rely on various adaptations for survival, including their bioluminescent glow. However, it’s been observed that females emit less light than males, noted Christopher P. Kenaley, a biologist at Boston College and the lead author of the study titled “Sexually Dimorphic Eye-size in Dragonfishes, a Response to a Bioluminescent Signaling Gap.”
“We’ve identified that male dragonfishes have evolved larger eyes to detect females who emit lower levels of light,” said Kenaley. “This finding is quite striking and offers valuable insights into the survival strategies of these lesser-known species in the deep sea.”
Kenaley further explained that sexual dimorphism in the visual systems of vertebrates is exceedingly rare. “In fact, this may be only the second documented case in fishes, which are the largest group of vertebrates.”
The deep sea, the largest biome on Earth, is a dark and desolate place where individuals are often widely dispersed, Kenaley added. Many creatures here produce bioluminescence to attract prey and locate potential mates. This sexual dimorphism in bioluminescence—where males typically have larger photophores than females—creates a challenge in mate-finding.
“This creates a paradox: bioluminescence is crucial for sexual signaling and finding partners, yet this dimorphism results in females being able to detect males from shorter distances than vice versa,” Kenaley explained. “In essence, there exists a bioluminescent detection gap. Our hypothesis was that males may have evolved larger eyes to compensate for this gap and capture more light.”
Collaborating with Stockholm University biologist Valentina Di Santo, Kenaley and his team studied two species of deep-sea dragonfishes—Malacosteus niger and Photostomias guernei—that exhibit sexual dimorphism in photophore size. Their research involved modeling the visual detection distances of males and females of each species based on eye size and light emission capabilities.
The findings, published in the journal, revealed a significant detection gap between the sexes, ranging from a few meters to over 100 meters. Additionally, the researchers confirmed that males of both species have larger eyes, which helps to reduce this gap by approximately 5 meters. While modest, this improvement is crucial in the challenging task of locating a rare mate in such a vast and open environment.
Previous studies had hinted at the existence of a bioluminescent detection gap among deep-sea organisms, but Kenaley’s team is the first to quantify its extent and propose that male eye size may have evolved in response.
Looking ahead, Kenaley suggested future research could delve into why male dragonfish exhibit brighter bioluminescence compared to females.
“The exact reason behind this luminescent dimorphism in deep-sea species, including dragonfishes, remains unclear,” said Kenaley. “It might serve as a means of signaling presence to potential mates. Answering this question will require further studies like ours that establish patterns in detection distances influenced by dimorphism levels.”
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