Science Graphic of the Week: See What’s Going on Inside the Eye of a Glowing Shark
- 12:00 pm |
Hundreds of feet underwater, where dim rays of sunlight mix with flickers from bioluminescent fish, lanternsharks rely on their sensitive eyes to survive. The image above shows how their retinas have adapted to life in the twilight zone.
The top row shows the density of photoreceptive rods in the retinas of three different species of lanternshark (red corresponds to more receptors). The jagged ridges around each retina are the rips created when a curved retina is squished onto a flat microscope slide.
The bottom row shows the density of ganglion nerve cells, which aggregate the signals from the rods. Each species has a different pattern of dense red spots and sparse blue ones, marine biologist Julien Claes of the Université catholique de Louvain and colleagues report in a new paper in PLOS ONE. Claes and his colleagues speculate that these patterns reflect the different lifestyle, coloration, and feeding habits of each species.
Each lanternshark has bright marks on its flanks, enabling its friends to recognize it and coordinate schooling and hunting. E. Lucifer has an extremely long lateral mark, and its eyes appear to be tuned to catch it. By having a cluster of ganglion cells on each side of its eye (bottom row, middle), the shark gets an extra-wide range of vision.
During the day, E. Spinax and E. Lucifer dive down to scan the sandy sea floor for smaller glowing fish to eat. That requires a good view of a horizontal plane, and corresponds to a dense horizontal band of receptors (top row, left and middle). E. Splendidus remains higher up where light might enter its eyes from any direction, requiring more evenly distributed rods (top row, right).
The three species also have something important in common: they use bioluminescence for camouflage. In a strategy called counterillumination, the underside of each shark gently glows to hide its silhouette from predators below. To enable the delicate modulation of its glow in response to changes in ambient light, each lanternfish ganglion cell is hooked up to twice as many photoreceptors as in any species of shark previously observed.