Are two eyes enough?

For humans, yes. For scallops? Apparently, evolution decided they need a few hundred.

Also, they have two retinas per eye, which is a little bit excessive for a creature that objectively doesn’t do very much.

What’s the deal with scallop eye anatomy? Scientists have been curious about this question for a while. Much of the foundational neuroanatomy was done in the 50s and 60s. These researchers did plenty of dissections and discovered scallop eyes have many of the same structures the human eye does - cornea, pupil, lens, retina, all the basic pieces you need to see stuff. But there is one super weird feature of their eyes. 

They use a mirror. Made of crystals.

The back of their eye is a concave mirror that reflects the incoming rays of light onto the retina, which is made up of the neurons that sense light, representing the beginning of the nervous system. 

The more recent findings, outlined in a 2019 publication called “The mirror-based eyes of scallops demonstrate a light-evoked pupillary response,” suggest that, despite the weird organization of their eyes, scallops do share one reflex in common with humans. 

The diameter of the pupil changes in different lighting conditions. In the dark, it gets bigger, which allows the eye to capture more rays of light from the surroundings. And when it gets brighter, the pupil constricts, which allows for better acuity (sharper vision). This pupillary light reflex is an unconscious response that works because of the action of two different muscles. 

The researchers used a stain called phalloidin, which comes from an ominously named mushroom called the Death Cap. The toxin binds to molecules called filamentous actin (f-actin), essentially locking up cellular function. 

In this case, the phalloidin sticks to f-actin found in the muscle cells. And when you see where phalloidin is found, you can see that there are two muscles in the scallop eye, organized in the same way they are in humans. 

So yeah, scallop eyes are a little weird, but they still have much in common with human eyes. More details are provided in the video above.