Scallops are exclusively marine mollusks that belong to the family Pectinidae under the class Bivalvia. They are defined by two fan-shaped shell valves hinged together, a structure used not only for protection but also for their movement. By clapping these valves, scallops eject water jets near the hinge, consequently swimming actively. Housed between the valves is a single, central adductor muscle that controls the opening and closing of the shell.
Along the edge of the mantle, these mollusks bear up to 200 tiny eyes containing concave, parabolic mirrors made of guanine crystals rather than a lens.
These animals are found in all oceans of the world, with most species living in relatively shallow waters, where they filter phytoplankton and organic particles using their gills.
The average diameter of the adductor muscle of small sea scallops is 0.5 to 0.75 in (1.3 to 2 cm), whereas larger species measure around 2 in (5 cm).
The maximum shell diameter of most scallops reaches around 8 in (20 cm).
These mollusks are encased in a fan-shaped shell comprising two valves, a left (upper) valve and a right (lower) one, divided by a plane of symmetry. Most species rest on the right valve, and hence, this valve is deeper and more concave than the left. The two valves meet at a hinge line on the dorsal margin of the shell. A series of growth rings originates at the center of the hinge from a spot called the beak. The beak is surrounded by a raised area called the umbo.
They have two flat, wing-like extensions called ears (also called auricles), located on either side of the umbo along the hingeline.
A small, V- or U-shaped notch, known as the byssal notch, lies on the ventral margin of the shell, very close to the hinge line. This notch marks the place where juvenile scallops once extended byssal threads for attachment to the substrate. Although most adults do not generally use byssal threads, the notch remains as a vestigial structural feature. It helps establish the anterior–posterior orientation of scallops by marking the anterior side, while the opposite side is considered the posterior.
All scallops, at some stage in their life cycle, bear a comb-like row of small teeth, the ctenolium, on the anterior edge of the right valve next to the byssal notch, a distinctive feature that defines this group.
Being bivalves, scallops have an open circulatory system characterized by three-chambered hearts (having two auricles and one ventricle).
Their hemolymph is bluish-green in color, owing to the presence of the copper-based respiratory pigment, hemocyanin. The hemolymph flows through open spaces called sinuses that bathe the organs directly.
They possess a pair of large, highly-folded gills that function in both gaseous exchange and filter feeding. The surface of these gills is lined with tiny cilia, whose rhythmic movements draw water across the folds, allowing the scallop to trap and direct food particles into the body.
The particles trapped in the gills are digested in a specialized organ, the digestive gland, which envelops part of the esophagus, the entire intestine, and the stomach. After digestion, the remaining fecal matter passes through the intestine and is expelled through the anus.
Nitrogenous waste is expelled from the hemolymph through a two-step filtration process. First, the hemolymph passes through the pericardial glands located in the pericardial cavity, where initial filtration occurs. It then enters the paired nephridia, which complete the processing of the waste. Once filtered, the nitrogenous waste is expelled through small openings called nephridiopores into the mantle cavity. From there, water currents produced by the gills carry the waste out of the body into the surrounding water.
They have a single central adductor muscle, which holds the two valves together. This muscle is composed of striated and smooth muscle fibers. While the striated fibers generate rapid contractions for swimming, smooth fibers maintain a sustained contraction to keep the shell closed.
Like all other bivalves, scallops lack a brain or a true centralized nervous system. Instead, they possess three paired ganglia: the cerebral (or cerebropleural), the visceral, and the pedal ganglia.
The cerebral ganglia are connected to the visceral ganglia via long cerebro-visceral connectives, whereas they link to the pedal ganglia through short cerebro-pedal connectives. These ganglia control the mouth and labial palps through the palp nerves. They also connect to statocysts (balancing organs), enabling equilibrium and spatial orientation.
The visceral ganglia, the largest and most complex of the three pairs, form an almost fused mass located near the center of the body. These ganglia give rise to the branchial nerves, which control the movement of the gills. They also innervate the eyes and tentacles.
The pedal ganglia give rise to the pedal nerves, which innervate the muscular foot.
These animals possess up to 200 small eyes (around 1 mm in diameter) arranged along the edge of the mantle. These eyes are unusual among molluscs because they rely on a concave, parabolic mirror (instead of a lens) made of guanine crystals to focus light. When light hits the eye, the guanine reflects shorter wavelengths (particularly blue), giving the eyes their striking blue or blue-green appearance.
Their eyes also have a double-layered retina, with the outer layer sensing high-intensity light and the inner layer detecting an abrupt decrease in light intensity.
The family name ‘Pectinidae’ comes from its type genus, Pecten, which in turn derives from the Latin word pecten, meaning ‘comb,’ referring to the comb-like ctenolium on their shell.
About 300 species of scallops are grouped under 4 confirmed subfamilies (there is 1 undefined group as well) and 70 extant genera (approximately, depending on the classification). The following list includes 67 extant genera.
The earliest records of true scallops, which include species of the genera Pleuronectis and Praechlamys, date back to the Triassic Period, approximately 200 million years ago. In fact, scallops were the most diverse bivalve family in the Mesozoic Era. However, they came to near-decline by the end of the Cretaceous Period.
These bivalves are found worldwide, ranging from tropical oceans to the poles. Their highest diversity is observed in the Indo-Pacific region.
Most species are found in shallow waters, typically between the low tide line and depths of around 328 ft (100 m). However, some species, such as the Atlantic deep-sea scallop (Placopecten magellanicus), have been occasionally observed at depths of 557 to 590 ft (170 to 180 m).[1]
They are found beneath rocks, among corals, seagrass, and kelp, as well as within sandy or muddy sediments.
Scallops are filter feeders that mainly consume microscopic particles in the water, particularly phytoplankton and microalgae. They also take in small amounts of zooplankton, such as crustacean larvae, along with bacteria and detritus when these particles are present in the water column.
Unlike most bivalves that stay close to the substrate, scallops are typically free-living, active swimmers. However, a few species, such as members of the genus Mimachlamys, are not free-living as adults and remain attached to the substrate through byssal threads.[2]
These animals swim by rapidly opening and closing their valves (a form of jet propulsion). When the striated fibers of the adductor muscle contract, they snap the valves shut, forcefully expelling water from the mantle cavity through openings along the edge of the mantle. This jet of water pushes the scallop in the opposite direction, allowing it to travel in short bursts. When the muscle eventually relaxes, the hinge ligament automatically reopens the shell, drawing fresh water back into the mantle cavity, making the animal ready for another contraction.
Some scallop species, such as Chlamys hastata, host epibionts like sponges on their shells. This forms a mutualistic relationship in which the sponge gets a stable surface to grow on, while the scallop receives protection, as the sponge interferes with the adhesion of starfish tube feet, making it harder for these predators to grasp the scallop’s shell.[3]
While some species, such as the bay scallop (Argopecten irradians), survive only 12 to 26 months, others, like the Atlantic deep-sea scallop, have a maximum longevity of about 20 years in the wild.[4][5]
Although most scallops are simultaneous hermaphrodites (having both male and female sex organs at once), many species, like Crassadoma gigantea, are dioecious, having separate sexes. Many others, such as Chlamys varia, are protandrous hermaphrodites, beginning as males and switching to females over time.[6]
In females, the egg-producing gonad (roe) is red in color, while males have white roe that produces sperm. The gametes are released freely into the water (broadcast spawning, which typically occurs in late summer and early autumn), where they undergo external fertilization.
Typically, within 22 to 36 hours, the fertilized eggs hatch into tiny, planktonic larvae called spat. In 4 to 7 weeks, the larvae settle on the ocean floor (spatfall) and attach to it using byssal threads. They gradually develop shells, lose their byssal threads, and eventually transform into juveniles.
Juveniles become sexually active adults in about 2 years, but they begin producing functional gametes at around 4 years.
Their primary predators are starfish, particularly species like the common starfish (Asterias rubens) and the purple sea star (Pisaster ochraceus), which pry open the scallop’s shell using their tube feet and digest their inner tissues. Other predators include certain crabs, lobsters, and large snails, such as whelks.
Juvenile scallops are preyed upon by fish, including cods, haddocks, and flounders.
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