There are challenges to living far beneath the ocean’s waves and researchers are looking into the glowing eyes of sharks that call this dark part of the ocean home. The smalleye pygmy shark, viper shark, velvet belly lantern shark, splendid lanternshark, and blackbelly lanternshark all call the twilight zone home and swim about 200-1,000 meters below the sea. With very little natural sunlight the animals at this depth rely on bioluminescence to help them survive.Because humans are not adapted to travel to these depths easily, researchers are continually finding new ways in which the creatures in the twilight zone use bioluminescence. A recent study by Dr. Julien Claes of the Université Catholique de Louvain in Belgium has discovered differences in the structure of the eyes of these five sharks that help them adapt.
In the lantern shark, researchers found that the area in the upper eye orbit contained a translucent area. They believe this adaptation uses bioluminescence for camouflage allowing the shark to use counter-illumination, or the ability to match the light of its background. This method of camouflage is used by other animals including squid.
Further showing that deep-sea shark eyes are different, the scientists found a gap between the lens and the iris allowing more light to the retina. The sharks studied also had a higher rod density in their eyes when compared with other shark species. This allows for better temporal resolution and aids the shark in communication.
Much is unknown about life in the twilight zone, but researchers studying bioluminescence have found many special adaptations that animals have developed to live in this zone. “Every bioluminescent signal needs to reach a target photoreceptor to be ecologically efficient. Here, we clearly found evidence that the visual system of bioluminescent sharks has co-evolved with their light-producing capability, even though more work is needed to understand the full story,” said Dr. Claes.
Information for this article is from the following paper: http://dx.plos.org/10.1371/journal.pone.0104213
Citation: Claes JM, Partridge JC, Hart NS, Garza-Gisholt E, Ho H-C, et al. (2014) Photon Hunting in the Twilight Zone: Visual Features of Mesopelagic Bioluminescent Sharks. PLoS ONE 9(8): e104213. doi:10.1371/journal.pone.0104213
Do you want to learn more about bioluminescent creatures read A Day in the Deep by Kevin Kurtz, illustrated by Erin Hunter?
Also coming this fall to Arbordale Animal Eyes by Mary Holland explores eyes and adaptations of animals on land and in the water.