Scallops are marine bivalve mollusks found throughout the world’s oceans, making them one of the few bivalve groups that are primarily free-living. They are distinct from oysters and clams due to their unique anatomy and active lifestyle on the seafloor.
The Iconic Shell and Coloration
The most recognizable feature of the scallop is its shell, which serves as protection and aids in locomotion. The shell, or valve, is generally circular with a distinctive fan shape and a pair of flat, triangular projections near the hinge, often called auricles or “ears.” Many species feature prominent, radiating ribs that extend from the hinge to the shell’s edge, giving the surface a corrugated texture that adds structural integrity.
The coloration of the shell is highly variable and directly relates to the scallop’s ability to blend into its surroundings. Shells can display a wide spectrum of colors, including vibrant reds, deep purples, mottled browns, yellows, and oranges. These complex color patterns function as effective camouflage against the mixed-color gravel, sand, and debris of the ocean floor. The shell’s composition is primarily calcium carbonate, which the mollusk secretes over time, resulting in a durable and colorful exterior.
Amazing Vision: Scallop Eyes
The scallop has a sophisticated visual system. Along the edge of the mantle, which lines the opening of the shell, hundreds of minute eyes are arranged in a continuous ring. These eyes often have an iridescent, bright blue color and number up to 200 in some species, providing a nearly 360-degree view of the surrounding water.
Unlike the eyes of humans and most other animals, scallop eyes do not use a lens to focus light onto a retina. Instead, they operate using a concave mirror located at the back of the eye, which is composed of thousands of precisely arranged square-shaped guanine crystals. This mirror technology reflects light onto a double-layered retina, functioning much like a miniature reflecting telescope.
This unique visual arrangement allows the scallop to detect changes in light intensity, shadows, and the motion of objects in its environment. This ability is particularly effective for spotting the shadow cast by a predator, such as an approaching starfish, which then triggers a rapid escape response. The eyes serve as an early warning system for survival rather than forming sharp, detailed images.
Movement and Habitat
Scallops are typically found resting on soft ocean bottoms, favoring substrates of sand, gravel, or mixed shell debris. While they are often seen on the seafloor, most species are not permanently fixed in one location, setting them apart from many of their bivalve relatives. Juvenile scallops, and some adult species, may anchor themselves temporarily to a firm object, like a rock or seagrass, using fine, silky strands called byssal threads.
The most notable behavior of the scallop is its ability to swim using a form of jet propulsion. When a threat is perceived, the scallop rapidly opens and closes its valves using its powerful adductor muscle. This action forcefully expels jets of water from either side of the hinge, propelling the animal forward in a swift, erratic, and fluttering motion.
This swimming is generally a short-distance, evasive maneuver, lasting only a few moments before the scallop settles back down onto the ocean floor. Their streamlined shell and large muscle mass are adaptations that support this active, free-living existence in their benthic marine habitat.