[This diary was originally posted to Daily Kos on August 25, 2006.]
The Bay Scallop (Aequipenctin irradians) is a Molluscan maverick. The species has succeeded by breaking or bending the rules of bivalvery at almost every turn. Think Han Solo, Zaphod Beeblebrox and Captain Hilts from "The Great Escape" all stuffed between two calcareous shells.
The shells are identical (actually mirror images of one another) except for the color. The top shell is darker, often with bands of alternating brown and cream. The bottom shell is white. Both shells are strongly ribbed, with up to twenty raised lines radiating from the base. Scallops cannot dig as clams do, so the dark top shell helps it to camouflage as it sits on the dark, muddy bottom. Weeds and sponges grow on it as well, completing its disguise.
The shells are more lightweight than clams or oysters, and the reason for this is that the scallop is the only type of bivalve in New England that can actually swim. Tiny yellow hair-like projections stick out between the shells when the animal is at rest. These are sensitive to chemicals in the water. When they sense a predator, such as a sea star, the scallop opens its shells wide, drawing in water, then clamps the shells quickly shut. As the shell closes, the mantle, which is a thin sheet-like organ that actually creates the shell, seals the opening. This forces all of the water to shoot out through the back hinges of the shells, moving it forward through the water, and away from the predator, via jet propulsion.
Swimming can last anywhere from a few seconds to a couple of minutes, depending on water currents and the severity of the threat. A potential predator that the scallop perceives as more of an annoyance, such as a sea urchin, may cause the scallop to just give itself a short, quick burst to move it away from the danger. The scent of a sea star, probably the deadliest predator an adult scallop is likely to face, will cause it to go ballistic as it desperately tries to put as much distance between itself and the enemy as possible.
An obviously posed pile of scallops swim away from a couple of sea stars.
The decision to swim away from a predator or close its shells tightly depends on the type of predator that is near it. If the predator sensed is able to open the shell, the swimming defense kicks in. If the predator cannot then the scallop simply seals its shells up until the danger passes.
A bivalve's strength comes from muscles called adductors. Most have two, and these work together to clamp the shells tightly shut. To open the shells slightly, as they do when feeding, these muscles are simply relaxed a bit. You can tell how many adductors a species has by looking for the muscle scars on the inside of the shell. The clam below has two.
Anterior adductor scar is on left. Posterior scar is to the right.
Our maverick scallops only have one, but it is a whopper. This is where the power comes from which allows this animal to swim. In the diagram below (courtesy of www.food.gov.uk) you can see the enormous size of this muscle, as well as some of the other anatomical features. Notice especially the size of the gonads.
Scallops are hermaphrodites. Each animal is both male and female, although they do not self-fertilize (the sex cells of each individual mature at different rates). When the gonads are fully formed they are simply released into the water column to mix by chance with that of other individuals. Water temperature cues trigger the release so that all gravid scallops in an area spawn at the same time. This greatly increases the odds that the eggs will be fertilized, and scallops have only one chance at this so they need to make it count. All bay scallops spawn when they are two years old. Then they die.
When I was about ten I interned at a state-run scallop hatchery. We would take the adults and place them in little glass bowls of seawater. We would then run cool water around the bowls and follow this by a warm water bath. There was a temperature formula used, but this was thirty years ago so I forget what it was exactly. The point is, we could manipulate the water temperature and induce all the scallops to fill their little bowls with gametes. We'd then mix them all up to fertilize the eggs and raise the larvae on cultured plankton and release them all into the bay to help restock the struggling population. (Because they are heavily fished and have a short life span, this species is very susceptible to wide yearly fluctuations in its population.)
Larval scallops are planktonic and in a few days they will form a rudimentary shell (this stage is called a "spat") and settle to the bottom to grow. However there is a catch. Scallops live in muddy bays and harbors and a tiny spat that lands on the mud will suffocate from the silt. The larva must find something suspended above the mud to hang on to until it is big enough to resist being silted to death. Eelgrass is the predominant plant (and it is a true plant) in scallop habitats and is a vitally important substrate for the young mollusk to attach itself to.
Like mussels, scallops can produce byssal threads using a small gland at the base of the shell. The spat uses these thin, sticky threads to hold on to the blade of the plant. Adults also can produce byssus and the threads, when the scallop needs to swim, can be instantly snapped at will.
During the 1930's a fungus known as "eelgrass blight" decimated the plant all along the Atlantic coast. It took decades for the grass beds to recover. During these years the scallop fishery literally vanished.
Now, about those eyes. When the scallop's shell is gaped you can see two rows of tiny, bright blue eyes. This species has forty of them. And these aren't the simple light sensitive eyes that most mollusks have (discounting, of course, the incredible sight possessed by cephalopods). They can also sense motion, so a crab or fish trying to sneak up on one and grab some flesh before the shells close will be seen before it gets even close. And if the scallop loses an eye it simply regenerates a new one in its place.
Here's an ultra closup of the eye. The color hasn't been tampered with.
Fun Fact: Unlike most edible bivalves, the only part of the scallop that can be eaten is the adductor muscle. Here's a plateful of them.
Other diaries in this series can be found here.