Before humans harvested them, oysters lived longer, grew larger, and produced denser quantities of offspring. Scientists compared oyster shells from Pleistocene-age oyster reefs with those from Native-American archaeological sites and modern harvests. Pre-human contact oysters lived as long as 30 years, while oysters since human colonization never live longer than 6 years. Pleistocene oysters grew up to 10.2 inches, pre-historic oysters from Native-American middens grew to 7.4 inches, and modern oysters reach 6.1 inches. Native-Americans harvested oysters in a sustainable way, but populations of oysters since European colonization have been reduced by over 99%, despite restoration efforts. Pollution and overharvesting have destroyed oyster numbers. This is unfortunate because oyster reefs are a productive natural community, providing habitat for at least 303 species that have co-evolved with oysters over the past 135 million years, ever since these bivalves first evolved. Scientists estimate the original oyster population of Chesapeake Bay was capable of filtering the entire contents of this estuary in just a few days, so they help clean the water as well. Modern day estuaries are suffering without more abundant populations of oysters.
Ancient oyster midden.
Pelican in front of a Georgia oyster reef at low tide.
A representative of every species living in oyster reefs could fill a big city aquarium. Barnacles, mussels, clams, and bryozoans attach themselves to the reefs and live out their lives filter feeding just like their hosts. Mud crabs (Eurypanopeus depressus) graze on the algae and detritus that accumulates on the reefs and sometimes feed upon the smaller oysters. Oyster pea crabs (Pinnotheres) depend upon reefs for their very survival. The seashore springtail (Anurida maratima), unusual salt water insects, prey on microorganisms living on the reefs. Amphipods, worms (Polydora and Polychaetas), anemones, mites, and hydroids are commensal animals dependent upon the existence of oyster reefs. Boring sponges (Cliona) and starfish directly prey on the oysters.
The seashore springtail is a true insect that lives on oyster reefs.
The depressed mud crab grazes on algae, detritus, and small oysters on oyster reefs.
Many small species of fish swim in and around oyster reefs during low tide because the structure affords protection from predators. Species of fish commonly found in Georgia oyster reefs include in order of abundance naked goby (Gobiosoma bosci), feather blenny (Hypoblennius hentzi), skilletfish (Gobiosox strumosus), seaboard goby (Gobiosoma ginsbingi), striped blenny (Chasmodes bosguianus), oyster toad fish (Opsanus sp.), and the crested blenny (Hypleurochilus geminatus). During high tide larger fish such as sheepshead, black drum, and croakers move in and feed upon the shellfish and smaller fish living on the reef.
The naked goby is the most common fish living in Georgia oyster reefs. They feed upon worms, crustaceans, and dead open oysters.
The skillet fish clings to oysters with its sucking mouth.
Land vertebrates forage oyster reefs during low tide. Raccoons and wading birds find many a meal on the reefs. Oyster catchers (Haematopus palliatus) specialize on feeding upon the oysters and other bivalves growing here. Even boat-tailed grackles exploit oyster reefs–they eat the amphipods and pea crabs crawling over the reef.
The American oystercatcher thrives on oyster reefs.
Oysters have a complex life cycle. They expel sperm and eggs into the ocean water, and when these sex cells meet by chance they form larva. (Oysters change sexes, so that males become females and vice-versa. Some individuals are hermaphroditic and expel sperm and egg at the same time.) The larva lives in the zooplankton until they develop a foot. The oyster senses pheromones from other oysters on a reef and will attach its foot to the structure where it will remain for the rest of its life, filter feeding upon diatoms, dinoflagellates, inorganic particles, bacteria, and marsh plant detritus.
Oyster reefs also have life cycles. When oysters begin colonizing an area it is known as the clustering phase. Oysters attach to each other and on old dead oyster shells during the accretionary stage, building reefs. Eventually, oysters reach a vertical limit and start building the reef horizontally during the senescent phase. Large reefs block sediment and shell debris carried by tidal currents and this action can create islands. Little Egg Island in the middle of the Altamaha River mouth is an example of an island created by an oyster reef.
Bahr, Leonard, William Larsen
“The Ecology of Intertidal Oyster Reefs of the S. Atlantic Coast: A Community Profile”
U.S. Geological Survey 1981
Lockwood, R.; K. Kusperck, S. Bonanani, and Gratt, A.
“Reconstructing Population Demographics and Paleoenvironment of Pleistocene Oyster Assemblages: Establishing a Baseline for Chesapeake Bay Restoration”
North American Paleontological Convention 2014
Rick, Turbin; et. al.
“Millenial-scale Sustainablity of the Chesapeake Bay Native American Oyster Fishery”
The Natural Environments of Georgia
Georgia Department of Natural Resources 1978