A velvet belly lanternshark, from Les Poissons (1877).
We know little about how deep-sea organisms capture the sparse sunlight in their environment to use for hunting, hiding and communicating. But an international group of researchers has uncovered a few photon capture strategies of bioluminescent sharks that make their home in the dark oceanic twilight zone (PLOS ONE, doi: 10.1371/journal.pone.0104213).
The researchers analyzed the eye shape, structure and retinal cell mapping in the visual systems of five different species of bioluminescent sharks living in the mesopelagic zone, which is about 200 to 1,000 meters under water. Using a variety of techniques, including light microscopy, stereology-assisted retinal topographic mapping, spectrophotometry and microspectrophotometry, they were able to observe new methods for maximizing photon capture.
In lanternsharks, they found a translucent area in the upper eye orbit that might be part of a reference system for counterillumination adjustment or a spectral filter for camouflage breaking. The team also discovered a new structure—a gap between the lens and the iris that allows extra light into the retina. Calculations of the peak spectral sensitivity of the rod visual pigments averaged 484 nm to 491 nm, allowing the sharks to pick up the largely shortwave light in their environment. These animals also have higher rod densities on their retinas than non-bioluminescent sharks, which could maximize temporal resolution and, perhaps, improve communication during social interactions.
Julien M. Claes, lead author and postdoctoral researcher from the FNRS Université catholique de Louvain, Belgium, remarked, “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.”