Eyes for love
A small but ferocious predator, the male聽dragonfish will apparently do anything for love. Or at least to find a mate.
A new study by researchers at Boston College found the eyes of the male聽dragonfish grow larger for mate-seeking purposes, making the dragonfish an聽anomaly in vertebrate evolution, the team today in the Royal聽Society journal Biology Letters.
Like many creatures that inhabit the dark depths of the sea, dragonfish survive thanks to numerous adaptations, including glowing bioluminescence. But females are not quite as bright as males, according to Boston College聽biologist Christopher P. Kenaley, lead author of the report 鈥淪exually聽Dimorphic Eye-size in Dragonfishes, a Response to a Bioluminescent聽Signaling Gap.鈥�
Boston College biologist Christopher Kenaley
鈥淲e鈥檝e found that these male dragonfishes have evolved larger eyes to find聽females who produce less light,鈥� said Kenaley. 鈥淚t鈥檚 rather stunning and a聽really important insight into how these poorly known species exist and thrive聽in the deep sea.
鈥漇exual dimorphism in the visual systems of vertebrates is extremely rare,聽Kenaley added. 鈥淭his is perhaps the second known case in fishes, the largest聽group of vertebrates.鈥�
The largest biome on Earth, the deep sea is a dark, barren place and the聽distance between individuals that live there can be vast, Kenaley said. Most聽animals in this realm produce their own light, bioluminescence, using it to聽lure prey and often to find mates. Many deep-sea fishes are sexually聽dimorphic in this regard鈥攎ales have larger photophores, the organs that聽produce light, than females.
That can present a problem when it comes to finding a mate, Kenaley said.
鈥淭his creates a paradox: if bioluminescence is important in sexual signaling聽and finding each other, this dimorphism results in a scenario in which聽females can detect males at shorter distances than the reverse,鈥� he said. 鈥淚n聽short, there's a bioluminescent detection gap. We hypothesized that, to聽close this gap, males might have evolved larger eyes to capture more light.鈥�
A species of deep-sea dragonfish known as Malacosteus niger.
Kenaley and his team of Boston College undergraduates, working with聽Stockholm University biologist Valentina Di Santo, studied two species鈥�Malacosteus niger听补苍诲听Photostomias guernei鈥攐f deep-sea dragonfish that聽are sexually dimorphic in the size of their photophores. The team modeled聽the distance at which males and females of each species could see members聽of the opposite sex. This was based on the eye size and how much light a聽member of the opposite sex could produce.
The researchers found there is a substantial detection gap in both species聽ranging from just a few meters to more than 100 meters, they report in the聽journal. They also found that males of both species have larger eyes and聽that this dimorphism closes the gap by about 5 meters or so. It's a modest聽improvement, they noted, but significant to the difficult task of finding a rare聽mate in such an open system.
In previous work, a few deep-sea biologists have asserted that a聽bioluminescent detection gap exists. The Boston College-led team is the first聽to assess the extent of this gap and, more importantly, whether these聽species might have evolved large male eye size.
Kenaley said future work may focus on why the bioluminescence of males is聽brighter than female dragonfish.
鈥淲e don't yet know exactly why luminescent dimorphism exists in these and聽other deep-sea species,鈥� said Kenaley. 鈥淧erhaps it's a way of signalling to聽one another that a member of the opposite sex is nearby. Answering this聽question will require more studies like ours that establish a pattern of how聽detection distances vary with levels of dimorphism.鈥�