Bay Scallops - General Information

BACKGROUND

In prehistoric times, scallops could be found from West Palm Beach to Pensacola, but in recent decades, that range has contracted considerably. Although bay scallops were once plentiful throughout Florida's west coast, they have virtually disappeared in some areas. Today bay scallops occur in discrete populations scattered along the coast of Florida. Dense aggregations of bay scallops are found only in the area between Tarpon Springs and Port St. Joe.

The stability of the Florida bay scallop population is defined by the existence of multiple, healthy, local populations. On the west coast of Florida, these local scallop populations are isolated from one another by areas of inhospitable adult habitat (e.g., salinity < 20 parts per thousand, areas of no seagrass). Because bay scallops are an annual species, local populations periodically collapse due to natural events. Natural restoration of a collapsed population requires transport of larvae from neighboring populations.

ANATOMY

The bay scallop is a member of Phylum Mollusca in the class Bivalvia. Bivalves have two valves, or shells joined by a hinge. The bay scallop's upper valve is a dark mottled color, and its lower valve is typically white. Occasionally both shells will be a bright yellow or orange, but these individuals are rare. Bay scallops may reach a shell height of 90 mm (3.5") and live two years. In Florida the bay scallops rarely exceed 75 mm (3") or live longer than one year. The bay scallop feeds continuously by filtering small particles of algae and organic matter from the water.

The water enters along the ventral and anterior margin (the open side of the two shells) and is funneled over the gills. Nutritious particles are skimmed out of the water. The filtered water, rejected particles, and digestive wastes exit posterior (near the hinge). The amount of food available and the surrounding water temperature influence growth rate.

Scallops open their valves when feeding or breathing and close them when predators approach. The shell can also be shut to avoid silt, which can clog the animal's delicate gills. The scallop can only close its shell to protect its gills for a short period of time, about two hours.

Many tiny, blue eyes arrayed along the outer rim of the shell detect movement near the animal and serve as a warning system. When threatened, the scallop can swim backwards by contracting and expanding its large adductor muscle, clapping its valves together and expelling water rapidly.

REPRODUCTION

A bay scallop has the remarkable ability to develop both female and male sexual organs; consequently, the scallop produces both eggs and sperm. In the final stages of reproductive development, scallops move energetic reserves from the muscle and digestive system to the gonad. This type of energy partitioning leaves the scallop vulnerable to predation and disease after spawning and many do not survive to spawn a second time.

A rapid change in water temperature generally triggers spawning. In Florida, most spawning occurs in the fall when the temperature drops, or in the spring when water warms. Each scallop is capable of producing millions of eggs at once, but the mortality rate is extremely high. Only one egg out of 12 million may survive to adulthood.

In Florida waters, bay scallops appear to spawn only once, generally during fall. Larval scallops are pelagic (living in the water column rather than on the bottom) for 10-14 days. During that time they may be dispersed a considerable distance from the source population.

It takes about 36 hours for fertilized eggs to become tiny larvae, known as veligers, that drift in the water for about two weeks. At this time, larvae transform into juvenile scallops, commonly called spat, and attach to seagrass blades. The spat gradually move up the seagrass blades, out of the reach of bottom-dwelling predators such as crabs. Even then, survival is uncertain. Approximately 90% of the spat will die within six weeks of latching on to seagrass. Those that do grow large enough to avoid consumption by predators will eventually drop off and fall to the bottom, where they remain the rest of their lives.

Most scallops of each local population only live for 12-18 months, although a small proportion of each year class may survive for 24 months or more. Each year a local population must produce enough offspring to replace itself, or receive offspring from neighboring populations. Because individual scallops only live for about one year, population fluctuations are extreme and the collapse of local populations is a natural feature of bay scallops in Florida.

For information on the bay scallop recreational harvest, view the article on the current bay scallop season.