Aptly named lanternsharks are dotted with luminescent markings called photophores that light up their bodies in the dark depths where they live. According to a new Royal Society Open Science study, these glowing sharks glow for sex.
Researchers have yet to understand the chemistry behind the light production of bioluminescent sharks from the genus Etmopterus, though they do know that the glow is controlled by hormones and neurons. Recent studies have also suggested that their mysterious photophore aggregations play a role in communication, warding off predators (like lightsabers), and a phenomenon known as camouflage by counter-illumination. That’s when the glow from their bellies perfectly matches up with the faint light coming down from above, National Geographic explains, so that predators lurking below can’t see their silhouettes.
This might explain the belly (or ventral) photophores, but not the flank (or lateral) markings. To test the communication idea, a team led by Julien Claes from the Catholic University of Louvain collected adult velvet belly lanternshaks (Etmopterus spinax, pictured below) from Raunefjord in Norway and housed them in seawater tanks in a dark room kept at a cold 4 degrees Celsius. They used a luminometer to record luminescence intensities from eight live specimens, and then they created visual models based on these measurements.
Flank markings, they found, work as a medium range signal for the detection and recognition of other lanternsharks of the same species. With their flank markings, potential mates can detect each other from 2.8 to 4.4 meters away. And without the photophores, a shark is detectable from 0.9 to 1.4 meters away. Also, males and females glow from different parts of their bodies, Science reports.
Furthermore, the team found that there’s a surprisingly high number of species in the Etmopterus group. That’s not the case for other bioluminescent shark groups that have few or no species with flank markings. The team thinks that these flashy, glow-in-the-dark patterns – which differ from species to species – may have provided a method of reproductive isolation: The sharks find and mate with their own kind. Keeping species so firmly separated may have helped with the group’s rapid diversification.
“The very first glowing sharks probably only used the light organs only for counter-illumination,” Claes tells National Geographic. “Then, some genetic change allowed some glowing organs to move a bit up to the side, and that allowed the owners to see members of their own species more easily. When you’re living in the permanent darkness of the deep sea, it’s a big advantage to be able to signal your presence to [others of your kind].”