Did You Know Scallops Have *Eyes*? Me Neither, but Look

Unless you are an avid scuba diver, when you think of scallops, you probably think of linguine and garlic more than oceans and shells. That's because we only eat the muscle of the scallop: You never see them in context. 
 
And so ... one of the most shocking things I discovered researching animal vision was that scallops have eyes! Not only do they have eyes, they have dozens of them along the edges of their shell openings. And the weirdest part? In some species like the bay scallop, the eyes are the prettiest blue color. 
 
See: 
 
scallopeyes.jpg

flickr/Kevin Bryant

Gah! Try and saute up some scallops tonight after seeing that photograph. I dare you.
 
These eyes are not exactly competing with eagle eyes for visual acuity. But they do have some very strange and interesting features, as Sonke Johnsen explains in his surprisingly readable and fun book, The Optics of Life: A Biologist's Guide to Light in Nature. Each of these eyes has a tapeta, which is a biological mirror that sits on the back of the retina. In most nocturnal species, the tapetum (singular form) bounces light back through retina, allowing the photoreceptors in animals like cats and raccoons a second shot at capturing more light, which is key for seeing in very dim conditions. 
 
In scallops, however, the tapeta go above and beyond what they tend to do in other species. They're actually used to focus light, instead of a lens (or lens + cornea combo as in our eyes). Johnsen explains:
 
The tapetum behind the retinas is of such high optical quality that it focuses the light onto the retinas. The lens only appears to compensate for the fact that the mirror is spherical and not a parabola, as a perfect focusing mirror should be. This is one of only two cases in the animal kingdom where an animal has made a mirror of optical quality. Why the eye needs two retinas, why it uses a mirror, and why what is essentially a glorified clam needs fifty to one hundred good eyes are open questions that another of my former students, Dan Speiser, took on. My favorite experiment of his involved showing scallops movies of food (in the form of particles moving on a computer screen). The scallops, held in little seats, would open their shells to feed if the particles were big enough and not moving too fast, suggesting that at least one function of their eyes may be to assess conditions for filter feeding. It was a classic case of an experiment that nobody (including me) ever thought would work, but did anyway.

In case you're wondering, the other animal with an optical quality mirror in its eye is the deep-sea spookfish, Dolichopteryx longipes. Make sure to keep that one in your pocket as the follow-up bit of cocktail party chatter after you tell people about the scallops.

UPDATE!!! A reader sends in this captivating, creepy video of a Martha's Vineyard bay scallop. It was by technician Kathryn Markey of Roger Williams University.


Join the Discussion

After you comment, click Post. If you’re not already logged in you will be asked to log in or register.

blog comments powered by Disqus