Here is a good quote about scallop vision from an amazing book, An Immense World: How Animal Senses Reveal the Hidden Realms Around Us by Ed Yong
“Daniel Speiser never thought he would spend his career trying to empathize with scallops. When he started graduate school in 2004, he thought about them the same way most people do—“ as lumps of meat on a plate,” he tells me. But those appetizing pan-seared lumps are merely the muscles that scallops use to close their shells. Look at a full, living scallop, and you’ll see a very different animal. And that animal will see you, too. Each half of a scallop’s fan-shaped shell has eyes arrayed along its inner edge—dozens in some species, and up to 200 in others. In the bay scallop, the eyes look like neon blueberries. Speiser finds them “funny and horrifying and charming,” all at once. It is strange enough that scallops have eyes when most other bivalves like mussels and oysters do not. It’s even stranger that those eyes, as Mike Land showed in the 1960s, are complex. Each one sits at the end of a mobile tentacle. Each has a little pupil: “It’s wild and creepy to see all of them opening and closing at the same time,” Speiser says. Light passes through the pupil and hits the back of the scallop’s eye, where it is reflected by a curved mirror. The mirror is a precisely tiled array of square crystals that collectively focus light onto the scallop’s retinas. That’s retinas, plural. There are two per eye, and they are about as different as two animal retinas could be.[* 12] Between them, they have thousands of photoreceptors, which gives them enough spatial resolution to detect small objects. “Their optics are really good,” Speiser says.[* 13] But why? When scallops are threatened, they can swim away, opening and closing their shells like panicked castanets. Beyond these rare moments of action, though, they mostly sit on the seafloor, sieving edible particles from the water. They’re “glorified clams,” according to Sonke Johnsen. Why do they need such a complicated eye, let alone dozens or hundreds of them? What does a scallop use its vision for? To find out, Speiser ran an experiment that he called Scallop TV. He strapped their shells to small seats, placed them in front of a monitor, and showed them computer-generated movies of small, drifting particles. It was such a ridiculous setup that no one seriously thought that it would work. But it did: If the particles were large enough and moving slowly enough, the scallops opened their shells, as if ready to feed. “It was the craziest thing I’ve ever seen,” Johnsen tells me. At the time, Speiser thought that scallops must use their eyes to spot potential food. Now he thinks something else is happening. Interspersed between their eyes are tentacles that scallops use to smell molecules in the water. Speiser thinks they use smell to recognize predators like starfish and vision to detect things that are simply worth an investigative sniff. When they opened their shells in response to Scallop TV, they weren’t trying to feed but were seeking to explore. “My guess is that we were seeing scallops being curious,” Speiser says.
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