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| A scallop sitting on a rock. Benjamin Hollis, CC BY 2.0. |
Similarly, scallops sometimes live under the sand, but they also rise to the surface and swim away. Next to the squid, octopuses, and the other cephalopods, scallops are the “cleverest” mollusk...and they don’t have brains. But they do have neuron clusters called ganglia. They have three pairs of these, each pair roughly corresponding to parts of a brain. One usually fused pair controls the viscera and gills, another usually fused pair controls movement and touch sensations in their foot, while the cerebral pair controls decision making, such as when to swim. This pair also controls the mouth and the sense of where they are in their environment—their position and orientation.
Altogether their ganglia are sort of like a primitive brain that’s divided into either four or six pieces, but they’re interconnected and are able to work together. Unlike other bivalves, scallops also have additional ganglion associated with vision and their sense of smell.
We have ganglia too, but they act as relay stations for signals going to and from our brain. Some of ours are visceral, motor, and cerebral, as are those of a scallop, and we also use some of the same neurotransmitters, such as dopamine, serotonin, and norepinephrine. We have additional ganglia that scallops don’t have, but those aren’t of interest here.
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| A scallop with blue eyes and a closeup from another scallop. Top: Rachael Norris and Marina Freudzon. Bottom: Matthew Krummins, CC BY 2.0. |
Depending on the species, scallops have between thirty and a couple hundred of eyes scattered around their mantle fringe, along with their tentacles, near the edge of their shells. Each eye is at the end of its own tentacle and they’re usually, but not always, bright blue. Each has a pupil, which is coordinated with the others so that they all open and close at the same time. In other words, they blink.
Their eyes give the scallop a minimum 270-degree horizontal view, but whether they see a combined panoramic view like we do, or whether they process the input from each eye separately, we don’t yet know. The latter offers faster response times with less processing, so having a panoramic vision might not be as useful to a bivalve. Still, they can see a wide-angle view, with a blind area behind them of a quarter of the full visual field. In addition, a few of the eyes in the top side are curved to look up, while some on the bottom side look down, which extend their view vertically.
Each eye uses a curved mirror made up of millions of square and rectangular reflective crystal plates that fit together like tiles. This is unusual since these crystals don’t naturally form into squares. Somehow the scallops get them to take that shape. The mirror focuses light on their two retinas. There are some other animals with mirrors in their eyes, but rarely achieve as clear an image as scallops do.
One of the two retinas detects peripheral movement in dim light and the other distinguishes movement in brighter light in the center of its field of view, along with variations in light intensity. Both would be useful in spotting predators, while the latter could help them when seeking a good spot to settle. They are also able to spot drifting food that is only six one-hundredths of an inch wide (1.5 mm). They usually rely on their well-developed sense of smell to detect food, but vision seems to let them know when to investigate, which caused one scientist to suspect they experience curiosity.
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| Swimming scallops. NOAA, CC BY 2.0. |
On noticing a predator, scallops are quick to respond. They’re the only bivalve that can swim and they do this in two ways. By clapping their shells together they produce jets of water that shoot out of either the front or the back next to one side of the hinge or the other. When they squirt it out the front, they shoot backwards hinge first a few feet in a straight line. Repeated claps can move them pretty quickly. When they jet water out the back on each side of their hinge, they can go straight forward, or they can alternate jets on each side of the hinge. This produces a forward-moving zigzag motion, going to the left, then to the right, then the left again. In general, they can swim at a rate of about fourteen inches (36 cm) per second.
Unlike other bivalves, scallops don’t have siphons, but by changing the shape of their mantle to direct the jet of water, they can move in almost any direction. They can shoot upward off the sand and can quickly change direction. No matter which way they go, their movements resemble fleeing pairs of clapping castanets.
Sea stars are one of their primary predators and scallops don’t have to see or touch one to know it’s there—they can smell it. And the slightest whiff will send the scallop jetting away. Their other predators include fish, crabs, and lobsters.
By the way, since starfish aren’t fish, marine biologists and naturalists instead call them sea stars, just as jellyfish are now called jellies, but they haven’t gotten around to changing the names of most of the other things that are similarly misnamed.
For some unknown reason, most bivalves—such as oysters and mussels—don’t have eyes, and once they’re adults, they can’t swim either, but they can hear and smell.
Clams are also unable to swim, so they dig down to escape, or if it’s too late, they can clamp tightly shut for long periods of time, hoping the predator gives up and goes away. Scallops have one powerful muscle for closing their shells, where clams and mussels have two, but unlike those two animals, scallops can’t completely close their shells, so they’re much less protected if a predator catches them.
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