Scallops can have up to 200 eyes
© ShaneKato / iStock
Scallops can have up to 200 eyes, although scientists still don't know exactly how they all work together to help the mollusks see.
The word "scallop" usually evokes a juicy, round adductor muscle - a seafood delicacy. So it isn't widely known that scallops have up to 200 tiny eyes along the edge of the mantle lining their shells. The complexities of these mollusk eyes are still being unveiled. A new study published in Current Biology reveals that scallop eyes have pupils that dilate and contract in response to light, making them far more dynamic than previously believed.

"It's just surprising how much we're finding out about how complex and how functional these scallop eyes are," says Todd Oakley, an evolutionary biologist at the University of California, Santa Barbara.

The optics of scallop eyes are set up very differently than our own ocular organs. As light enters into the scallop eye, it passes through the pupil, a lens, two retinas (distal and proximal), and then reaches a mirror made of crystals of guanine at the back of the eye. The curved mirror reflects the light onto the interior surface of the retinas, where neural signals are generated and sent to a small visceral ganglion, or a cluster of nerve cells, whose main job is to control the scallop's gut and adductor muscle. The structure of a scallop's eye is similar to the optics systems found in advanced telescopes.


Comment: It is the comparisons to machines and technology - that can only be assembled with purposeful intent and an actual blueprint - that the above mention of advanced telescopes reminds us of. Molecular biologist Michael Behe has been successfully countering conventional beliefs about evolution with his concept of irreducible complexity. Behe would further say that:
Irreducible complexity is just a fancy phrase I use to mean a single system which is composed of several interacting parts, and where the removal of any one of the parts causes the system to cease functioning.
How then can we reconcile the incredible complexity of the scallop's eye with natural selection - or mere adaptation to the organism's environment - when it clearly compares to the human's most advanced technologies?? You can't. An incredible amount of science had to be understood and engineered and built together to make an advanced telescope. And we are supposed to believe that the scallop's eye just happened to correctly "assemble" its visual apparatus out of an imperative to adapt??


For many years, the physics and optics of the scallop eye posed a perplexing problem. "The main retina in the eye gets almost completely unfocused light because it's too close to the mirror," says Dan Speiser, a vision scientist at the University of South Carolina and the senior author of the new study. In other words, any image on the proximal retina would be blurry and out of focus. "That just seems so unreasonable to me," Speiser says.

The new study sheds some light on this mystery. The researchers found that the scallop pupils are able to open and contract, though their pupillary responses aren't as quick as our own. A scallop pupil's diameter changes by about 50 percent at most, and the dilation or contraction can take several minutes. Their eyes don't have irises like our eyes do, and instead, the cells in the cornea change shape by going from thin and flat to tall and long. These contractions can change the curvature of the cornea itself, opening the possibility that the scallop eye might change shape and respond to light in a way that makes it possible to form crisper images on the proximal retina.

"It really changes the ability of that eye and ultimately the organism to be able to have the type of resolution to see its environment," says Jeanne Serb, a vision scientist at Iowa State University.

Now, Speiser is working to understand if the scallops are able to change the curvature of the mirror and the eye as a whole, which would enable it to adjust the focus of the image even further. "The eyes' dynamic structures open up some new possibilities for what you can do with a mirror-based eye like this," Speiser says.


Adaptive mirrors aren't the scallop eye's only mystery. "It turns out that scallop eyes have three times as many opsins as we do," Serb says. Opsins are light-sensitive proteins found in the photoreceptor cells of the retina that mediate the conversion of light into electrochemical signals. Scientists don't know whether all 12 scallop opsins are expressed in every single scallop eye or if the eyes subspecialize in different channels of the visual spectrum. Some opsins may be expressed in the proximal retina while others are in the distal retina.

Serb's team at Iowa State studies the opsins in scallops, clams and other animals. Bivalves-mollusks that live inside two matching cupped shells connected by a hinge-have evolved some form of eye multiple times. Some clams even have compound eyes, or eyes with multiple visual units, though they differ from the better-known compound eyes of insects. By studying the different opsins outside of the animals, Serb can measure their absorption and ultimately understand how they work in the different animals.

Eyes have probably evolved at least 50 or 60 times across all animals, and in many cases, the molecular underpinnings of vision - the proteins that translate light signals to electrical signals - vary quite a bit. "The big evolutionary question for me is, how do these proteins evolve to sample light? And then, how does it become specified to the different types of light environments that the animals can occur in?" Serb asks. She believes that the opsins, in most cases, are being repurposed from some other function within the animal to be used in the eyes.


Comment: This is where Serb's good questions need to be taken a step further by questioning the premises of conventionally understood
evolution - and being open to other possibilities: intelligent design.

Michael Behe isn't the only scientist to question the neo-Darwinian ideas so liberally sprinkled throughout this article. Charles Pritchard was another. And he happened to question the complexity of the eye well over 100 years ago:
I cannot understand how, by any series of accidental variations, so complicated a structure as the eye could have been successively improved. The chances of any accidental variation in such an instrument being an improvement are small indeed. Suppose, for instance, one of the surfaces of the crystalline lens of the eye of a creature, possessing a crystalline and a cornea, to be accidentally altered, then I say, that unless the form of the other surface is simultaneously altered, in one only way out of the millions of possible ways, the eye would not be optically improved. An alteration also in the two surfaces of the crystalline lens, whether accidental or otherwise, would involve a definite alteration the the form of the cornea, or in the distance of its surface from the centre of the crystalline lens, in order that the eye may be optically better. All these alterations must be simultaneous and definite in amount, and these definite amounts must coexist in obedience to an extremely complicated law. To my apprehansion then, that so complex an instrument as an eye should undergo a succession of millions of improvements, by means of a succession of millions of accidental alterations, is not less improbable, than if all the letters of the "Origin of Species" were placed in a box, and on being shaken and poured out millions on millions of times, they should at last come out together in the order in which they occur in that fascinating and, in general, highly philosophical work. (Pritchard, Charles. 1866. The Continuity of the Schemes of Nature and Revelation, p 33.)

Although there is a diversity of eye morphologies and of photoreceptors across animals, the building blocks - the genes that control eye development - are remarkably similar. For example, Pax6 is a developmental gene that is critical for eye development in mammals, and it plays a similar role in the development of scallop eyes. In a recent study preprint, Andrew Swafford and Oakley argue that these similarities belie the fact that many types of eyes might have evolved in response to light-induced stress. Ultraviolet damage causes specific molecular changes that an organism must protect against.

"It was so surprising that time and time again, all these components that are used to build eyes, and also are used in vision, have these protective functions," Oakley says. In the deep history of these components are genetic traits that trigger responses to light-induced stress, such as repairing damage from UV radiation or detecting the byproducts of UV damage. Once the suite of genes involved in detecting and responding to UV damaged are expressed together, then it may be just a matter of combining those parts in a new way that gives you an eye, the researchers suggest.


Comment: Even if these scientists don't come out and directly attribute the incredible intricacies of the eye to a designer, they sure do lean in that direction with the language used in the above paragraph!


"The stress factor can bring together these components maybe for the first time," Swafford says. "And so the origins of the interactions between these different components that lead to vision are more attributable to this stress factor. And then once the components are there, whether it be pigments or photoreceptors or lens cells, then natural selection acts to elaborate them into eyes."


Comment: ... but then quite often capitulate to the orthodoxy of natural selection.


However they were made, scallop eyes have some impressive functionality, warping their internal mirrors to bring light into focus like a telescope. So next time you are enjoying some garlic scallops, try not to imagine the mollusks staring back at you.