More than 200 species of scallops live in the oceans and bays, and many of their varied colored shells can be found in the sand along the beaches. Though some shells bear scars and holes from animals that attached to them or bored holes in their surfaces, most are pristine because they were protected by one of two types of sponges. The most common is Myxilla incrustans, which is a yellow or orange sponge. The other is Mycale adherens, which is purple or brown. These sponges are only found on scallops. The sponge and scallop share a special relationship where one protects the other.
I've Got Your Back
By covering the scallop's shell, the sponge makes it difficult for predators like the sea star, or starfish, to attach its suction feet to the scallop, the first step in making the scallop his lunch. This gives the scallop time to open and quickly close its shell, which shoots out water and propels the scallop up to 3 feet. By covering the scallop's shell completely, the sponge leaves no room for boring sponges, which bore holes in scallops and can stunt their growth or kill them.
He Never Saw it Coming
The sponge has another weapon against predators. As a threat approaches and comes close enough to the scallop to touch the sponge, the sponge emits a toxin so quickly that the predator doesn't see it or expect it. Sensing it, however, the predator quickly backs away. This helps protect even an immobilized scallop that can't get away. It doesn't do the sponge's predators any good, either.
Catch Me if You Can
Although the scallop has two rows of bright blue "eyes," it can't really see images with the eyes, it only senses changes in light and dark. Luckily, the scallop is one of only a few bivalves that can move on its own. Just as the scallop moves away from its own predators by opening and quickly snapping shut its shell, it moves away from the sponge's predators, too, carrying the sponge along with it and out of danger. Sponges live on other living things or structures such as rocks, so they're unable to move themselves out of danger. That's OK, because the scallop does it for them.
Making a Clean Break
In its movements through the water, the scallop often takes the sponge into clean water. It propels itself or rides on the currents, which the sponge would not be able to do for itself. This keeps the sponge from becoming foul and rotted, as it might if it were attached to immobile things such as rocks, or animals that stay in the mud or dirt in more stagnant water.
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