Scallops, a culinary delicacy, have a unique appearance when alive in their natural marine environment. Unlike many bivalves, these mollusks are not static bottom-dwellers. Their distinctive features, including intricately patterned shells, numerous vibrant eyes, and dynamic movement, reveal a complex organism.
The Distinctive Shell and Its Many Eyes
A live scallop’s most striking feature is its shell, typically fan-shaped with a ribbed texture. Shells come in a wide array of vibrant colors, including shades of red, orange, purple, brown, and white, often displaying intricate patterns. The two halves, or valves, join at a straight hinge line, often featuring wing-like projections called auricles. One valve might be more convex, while the other can be flatter, with surfaces ranging from smooth to scaly or knobbed.
Lining the edge of the scallop’s mantle, between the two shells, are dozens to hundreds of small, often bright blue, reflective eyes, known as ocelli. These eyes, which can number from 50 to 200, are specialized to detect changes in light, shadows, and movement. Unlike human eyes, scallop eyes do not form sharp images; instead, they use a concave mirror of guanine crystals to focus light onto a double-layered retina. This optical system, along with their ability to dilate and contract their pupils, allows them to perceive their surroundings and react to threats.
Beyond the Shell: Mantle and Tentacles
When a scallop’s shell is slightly open, its soft, fleshy mantle tissue becomes visible. This frilly or textured membrane lines the inside of the shell and extends to its edges, playing a role in shell formation and respiration. The mantle also serves as a sensory interface with the marine environment.
Protruding from the mantle’s edge, interspersed with the eyes, are numerous delicate sensory tentacles. These tentacles sense the surrounding water, helping the scallop detect food particles and providing early warnings of predators. While typically hidden, a small, muscular foot can be extended for minor movements or for burrowing into the seabed. In juvenile scallops, this foot is more prominent and secretes byssal threads, temporarily anchoring them to surfaces.
Movement and Lifestyle
Scallops are distinctive among bivalves for their ability to swim. They achieve movement through jet propulsion, rapidly clapping their shells together to expel water from their mantle cavity. This action generates thrust, propelling them forward in a jerky, erratic motion, primarily as an escape mechanism from predators like starfish.
Despite their swimming capabilities, scallops often spend periods resting on the seabed. Some species may partially bury themselves in the sediment, while others can attach temporarily to surfaces using byssal threads, especially during juvenile stages. Their reaction to disturbances, such as quickly closing their shells when sensing a threat, shows their dynamic interaction with their surroundings.