Critter News: Scallops, Chitons, Sea Stars and Sea Urchins

giant rock scallop eyeWhat am I?
I am one of about 60 of what you see in the photo on the left. I may help my possessor see danger nearby or good habitat further off. I belong to an animal that is related to snails, octopus, chitons, and abalone. I have both reflective cells and pigmented cells, and am easy to see when a light shines on me. So, what am I? I am the eye of a giant rock scallop!

giant rock scallop

The giant rock scallop’s eyes are embedded in the edge of its mantle, the orange “lip-like” band. Although difficult to see in the above photo, this animal has about 60 tiny iridescent eyes, 9 of which are indicated by arrows.

Scallops have the best vision of any bivalve mollusk. Each eye sits at the end of a tiny stalk projecting from the outer edge of the mantle. A cornea and two retinas are present; one retina is specialized for close-up vision, and the other for distance. Although there is no brain to formulate an image, some scientists think that the eyes perceive vague shapes of objects as well as light and dark. Scallops that can swim at all life stages have the best vision, probably to locate suitable habitat. Only small juveniles of the giant scallop to the right, can swim, so the vision of this species is about mid-range for a scallop.

ChitonThe chiton on the right, possibly a lined chiton, Tonicella lineata, is a mollusk relative of the giant rock scallop.

Chitons have hundreds of eye-like structures with lenses made of aragonite, a type of rock (most animals have eye lenses of protein and chiton). The lenses are on the top surface of the chiton’s 8 plates. The lenses of another species of chiton, native to Florida, were recently studied and found to focus light well enough for the chiton to see simple shapes, not just respond to light and dark. This may also be true for the lined chiton.

The vision of humans, by the way, is about 1,000 times more acute.

Red urchinThe vision of sea stars is well known (simple lenses that perceive light and dark), but other marine invertebrates, such as sea cucumbers and sea urchins, also have some vision.Sea cucumbers have light sensitive cells around their oral tentacles that respond negatively to bright light, such as camera flashes, and possibly to shadows.

Sea urchins have photoreceptors all over their bodies, located on the tip and base of each tube foot. It is thought that the urchin’s entire body may act as a huge compound eye, giving a shadowy directional vision to the animal.

To learn more about these animals and to get a closer look, come visit the Life on the Edge exhibit at the Aquarium!

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