Eastern Oyster Biology
The Eastern oyster can be found in estuarine environments, where there is a mixture of fresh and salt water. They are commonly found in shallow water or in intertidal areas fringing off mangrove islands. While oysters prefer to live in a brackish environment, they can tolerate periods of extended saline conditions. Oysters are able to tolerate short pulses of freshwater conditions as they can close their shell during stressful conditions. However, low salinities for a long period of time will eventually kill the oyster.
Oysters will settle on almost any hard substrate such as pilings, docks, seawalls, mangrove proproots, substrates used for oyster restoration, and other oyster shells. When juvenile oyster spat settles on a hard substrate on or near other oysters, that is how reefs are formed. As the reef is formed, it becomes excellent habitat, shelter, and potential nursery grounds for commercially important species.
Eastern Oyster Anatomy
Eastern oysters are a part of the phylum Mollusca in the class Bivalvia. Bivalves have two valves, or shells, that are connected by the hinge. Oysters are inequivalve with a shell composed to two unequal valves. The left valve is the larger of the two valves and is the valve that is cemented to substrate that it has settled on. The right valve is typically smaller and flatter.
The hinge is a rubbery ligament that both keeps the two shells attached but also pushes the two shells apart like a spring. There are two types of adductor muscle; one that pulls the shell shut quickly to avoid predators (the large creamy portion of the muscle) or simply hold to hold the valves closed during low tide (the tough white portion of the muscle). When the valves are open, tiny cilia on the gills work like paddles to pump water across the gills allowing the oyster to feed by filtering out plankton and organic particles from the water as well as allowing the oysters to breathe by drawing oxygen out of it using their gill membranes. Oysters can survive up to several years in the wild but have survived to up to 20 years in captivity. Typically, they grow to 3 to 5 inches in length, but can reach lengths of 8 inches.
Eastern Oyster Reproduction
Oysters typically spawn during the warmer months of the year. The larvae settle on hard substrates and other oyster shells to form dense clusters called oyster reefs or beds.
Eastern oysters are protandric hermaphrodites, meaning they typically start out their life cycle as males and have the ability to change to females. During this change it is possible to see both sexes present at the same time.
Oysters in Florida spawn from Spring through Fall using environmental factors like water temperature and salinity as cues. A single female can produce millions of eggs, but most eggs will be eaten by predators or washed away to unfavorable habitats before settling as a spat, and most of the juveniles that do settle will also be eaten by predators before they reach adulthood. When the eggs are fertilized in the water column they develop into free-swimming, planktonic, trochophore larvae. While drifting in the water column they develop into fully shelled veliger larvae and after about 2-3 weeks into a pediveliger larvae. During this time the larvae settles and, by using its newly developed foot, searches for a hard substrate to attach too. After it settles, the left valve is cemented to the substrate where the oyster spat will spend the rest of its life. See additional information on how FWRI biologists monitor oyster reproductive efforts monthly.