If you live your entire life in the dark do eyes serve much of a purpose? Some species of fish living in subsurface caves have even evolved to become eyeless. But this is not a very conserved trait. Recent research published in Proceedings of the Royal Society B have shown that a number of starfish that live in the deep ocean have functioning compound eyes on the tips are of their arms. Firstly, you must say that wow starfish have eyes? Who knew?
Their shallower water counterparts that live in brighter surroundings have and use compound eyes, but little work has gone into deep sea species. One of the co-authors Anders Garm, a marine biologist from the University of Copenhagen, says “What you see in many other animal groups is that, when you venture deeper into the sea and light intensities get low, either they lose eyes and rely on other senses or the eyes are modified in ways where they are optimized to be very sensitive to low light intensities but with low spatial resolution.” Therefore, why not deep sea echinoderms too?
To investigate deep sea starfish visual capabilities, Garm and his colleagues gathered hundreds of the invertebrates, comprising thirteen species spanning a wide depth range. They collected species residing in the North Atlantic from just under the surface to more than 1,000 meters off the Greenland Coast. They carefully examined the starfish for the presence of eyes and optical and morphological examinations were used to estimate the quality of vision. Additionally, species found in the aphotic zone below 320 m were checked for bioluminescence. All but one species, the infaunal Ctenodiscus crispatus, had eyes on the undersides of the tips of their arms, and two were found to be bioluminescent.
The researchers found that for the species with compound eyes, they left the eyes uncovered and angled the ends of the arms at varying degrees, presumably to vary their gaze direction. Remarkably, one of the species found in the aphotic zone, Novodinia americana, had close to the highest known spatial resolution for starfish compound eyes along with being bioluminescent. Both D. multipes and N. americana were bioluminescent. Garm and his colleagues hypothesize that the animals use their bioluminescent abilities to communicate in the dark, in a place where many animals have completely lost their sight. “It’s a semi-secure line to use if you don’t want everybody to listen in to your signaling,” says Garm. They also hypothesize that N. Americana uses bioluminescent flashes putatively for reproductive purposes.
Examining how bioluminescence and eyes are linked within visual communication in the deep dark ocean needs a lot more investigation, but due to difficulties of the ecosystem, study has been limited to this point. Any input of anthropogenic light could influence the behavior of species of the deep. How the bioluminescent light is perceived and translates into behavior is the next crucial step.
Supplementary figure s3 from Deep-sea starfish from the Arctic have well-developed eyes in the dark
Please see the publication http://rspb.royalsocietypublishing.org/content/285/1872/20172743
Birk MH, Blocher ME, Garm A (2018) Deep-sea starfish from the Arctic have well-developed eyes in the dark. Proceedings of the Royal Society B. DOI: 10.1098/rspb.2017.2743
