Enigmatic sea creature: The many blue eyes of the scallop

Die Kammmuschel Argopecten irradians has bright blue eyes, and a few dozen of them. Much like the European scallop, its East Coast relative known as the “bay scallop” is exceptionally mobile for a clam. Squeezing water forwards or sideways from between its shells, it hops backwards or swims in zigzags to advance. For example, she can escape from a hungry starfish – provided she has noticed it in time. The scallop’s eyes, which peek out from under the shell on short stalks, are small, about a millimeter in diameter, but well endowed:

Instead of a lens, there is a kind of concave mirror behind the light-sensitive cells of the retina, which focuses the incident light. Long, thin tentacles also protrude from between the eyes, which can use their tactile and olfactory sensory cells to explore suspicious objects more closely.

Biologists led by Daniel R. Chappell from the University of South Carolina in Columbia recently discovered that the optical information provided by several dozen mussel eyes from different angles is obviously combined to form a panoramic view. In the laboratory, the researchers observed how small scallops, lying quietly on the ground and using their gills to filter nutritious particles from the water, reacted to visual stimuli. In the absence of such stimulation, the mussels usually extend some tentacles, but do so in no particular direction.

Just wait, then flee

However, if the researchers let a vertical dark stripe circle around the scallop, it usually showed a corresponding dynamic reaction: A wave of stretching tentacles followed the optical stimulus at the specified speed. On the other hand, when confronted with an immobile stripe, the mussels tended to extend their tentacles in that direction unless the stripe appeared just behind the hinge that connects the two halves of the shell, where there are no eyes.

Detailed view of a scallop

Image: David Liittschwager

Observations like these suggest that scallops, with their many small eyes, can see a panorama of at least 270 degrees, the researchers write in the Proceedings of the Royal Society B. Each individual eye achieves a spatial resolution of about two degrees. Its field of vision is 90 to 100 degrees and greatly overlaps with neighboring eyes. According to Chappell and his colleagues, the scallops first have to evaluate the signals from the sensory cells of each individual eye for the panoramic view. The task then is to bring the information together in such a way that the mussel can stretch out its sensory tentacles in a targeted manner.


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