Scallops are a recognizable type of marine bivalve mollusk, found in all of the world’s oceans. They belong to the family Pectinidae, which includes hundreds of different species. Unlike relatives such as clams and oysters, most adult scallops are not fixed permanently to the seafloor. This mobility is one of their most distinctive features, setting them apart as one of the few free-swimming bivalves. Their functions and behaviors, from movement to sensing, allow them to thrive in diverse marine environments.
Swimming and Locomotion
Scallops are one of the few bivalves capable of active swimming, relying on a specialized form of jet propulsion. This action is powered by the rapid contraction of the large, central adductor muscle, which snaps the two halves of the shell together. The closure forces a jet of water out through openings near the shell’s hinge line, propelling the animal forward in the opposite direction.
The resulting motion is an erratic, flapping movement often described as “clapping” or “dancing” flight. This swimming is not used for routine travel but primarily serves as a high-energy defense mechanism. Scallops use this burst of speed to escape detected predators, such as sea stars or crabs, by quickly launching themselves off the seabed.
The powerful adductor muscle has two distinct parts: a fast, striated portion for rapid swimming contractions and a slow, smooth portion that holds the shell tightly closed. The shell’s hinge features a resilient, rubbery pad of protein that stores energy when the shell closes. This elastic rebound springs the shell open once the muscle relaxes, which is crucial for rapid, repeated swimming cycles.
Filter Feeding and Respiration
Like all bivalves, the scallop is a filter feeder, extracting microscopic food particles from the water column. The primary feeding and respiratory organs are the ctenidia, or gills, which are sheets of tissue inside the mantle cavity. These gills serve a dual purpose, absorbing dissolved oxygen for respiration while simultaneously collecting nutrients.
Water is drawn into the mantle cavity, flowing over the gill filaments covered in tiny, beating hairs called cilia. These cilia generate the current that moves water across the gills and trap small organisms, such as phytoplankton, algae, and organic detritus. The trapped food particles are then bound together with mucus to form a continuous string.
From the gills, specialized tracts of cilia move the mucus-bound food toward the labial palps, which surround the mouth. The palps sort the material, rejecting larger or undesirable particles, before directing the usable food into the mouth and digestive tract. This filtering process provides the scallop with its nutritional needs and contributes to clarifying the surrounding seawater.
Sensory Perception and Environment
A scallop’s most remarkable sensory feature is the array of up to 200 tiny, iridescent eyes that line the edge of its mantle, peering out between the two shell valves. These organs operate on a principle similar to a reflecting telescope rather than a lens-based eye like a human’s. Instead of using a lens to focus light, the scallop eye uses a concave mirror at the back of the eyeball.
This mirror is constructed from millions of layered crystals of guanine, which efficiently reflect light onto a double-layered retina. This structure allows the scallop to register sudden changes in light intensity and shadow, which is interpreted as a warning sign of a nearby predator. The dual retina provides a broad visual field, with one layer focusing on objects straight ahead and the other capturing peripheral information.
The scallop also uses numerous short tentacles that fringe the mantle edge for chemical and tactile sensing. These tentacles are sensitive to water movement and chemical cues, allowing the animal to detect changes in water quality or the presence of food and predators. This combination of sight and touch provides the scallop with comprehensive awareness of its immediate environment.
Reproduction and Growth
Scallops reproduce through broadcast spawning, releasing their eggs and sperm directly into the water. Most species are dioecious, meaning they have separate male and female individuals, distinguishable by the color of their gonads—white in males and orange or red in females. A single mature female can release millions of eggs in a spawning event, increasing the chances of successful fertilization.
Once the eggs are fertilized externally, they develop into microscopic, free-swimming larvae called veligers, which drift in the water column for several weeks. This planktonic stage allows for wide dispersal by ocean currents. The larvae then undergo metamorphosis, developing a small shell and transitioning into the juvenile stage, often referred to as spat.
The spat settle onto the seabed, temporarily attaching themselves to objects like seagrass or rocks using fine strands called byssal threads. As the juvenile grows, it detaches, regaining the mobility characteristic of the adult scallop. Scallops grow rapidly, reaching sexual maturity around two years of age and attaining commercially harvestable size within three to five years.