South Carolina Oyster Restoration and Enhancement
South Carolina Department of Natural Resources

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This glossary is provided as a reference for terms found on this Web site or that are related to oysters. Many of the words listed below may have multiple usages but are defined here in the context they are used for the South Carolina Oyster Restoration and Enhancement (SCORE) program.

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Adductor Muscle — The muscle that holds an oyster shell closed. When the muscle relaxes, the shell opens. The abductor muscle causes a prominent scar on the inside of the oyster shell.

Anterior — 'Towards the head.' In the case of the oyster, the end closest to the mouth, which is the hinge end.

Anthropogenic — Caused by human activity.

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Benthic — Living in or on the bottom substrate of an aquatic environment. Pertaining to the bottom substrate of an aquatic environment. Oysters are considered benthic organisms.

Biodiversity — The genetic, species, and ecological richness of the organisms in a given area.

Bivalve — Marine or freshwater mollusk that possess a hinged shell and a hatchet-shaped foot. Bivalves are generally sessile or burrow into soft sediment, rock, wood, or other materials. Oysters are sessile bivalves.

Brackish — Having low salt content. Scientist consider water with salinity values ranging from approximately 0.50 to 17.00 parts per thousand (ppt) to be brackish.

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Cilia — Plural for cilium, which is a hair-like structure that moves in a waving motion. Cilia are used for motility in some one-celled organisms and for moving particles or fluids in certain cells of more advanced organisms.

Commensal Organisms — Organisms that live with and rely on a host for a benefit but do not harm or benefit the host. For example, small crabs that depend on oyster reefs for refuge are commensal organisms.

Community — Species that occur together in space and time.

Cultch — Material that larval oysters use as substrate for settlement. The best cultch for oysters is natural oyster shell.

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Diatoms — A major type of phytoplankton that have cell walls made of silica and shaped into two halves.

Dorsal — 'Towards the back.' In the case of the oyster, the side of the body closest to the anus.

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Ecology — The scientific study of the interactions that determine the distribution and abundance of organisms. It is concerned with the life histories, distribution, and behavior of individual species, as well populations and communities.

Ecosystem — The scientific study of the interactions that determine the distribution and abundance of organisms. It is concerned with the life histories, distribution, and behavior of individual species, as well populations and communities.

Epibranchial Chambers — 'Over the gills.' The chambers formed by the fusion of the mantle, visceral mass and base of the gills. The epibranchial chambers house the gills, mouth, and labial palps.

Estuary — A partially enclosed area where freshwater from a river or stream comes into contact with salty oceanic water. It is characterized by water with salt content between that of fresh and marine environments, tidal effects, and populations of animals and plants that are distinct from either the freshwater or oceanic environments. Estuaries are among the most diverse and biologically productive ecosystems on Earth.

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Gametes — Reproductive cells (sperm and egg) that fuse to form a zygote.

Gills — Respiratory organs used by aquatic animals to obtain oxygen from, and release carbon dioxide to, the surrounding water. In oysters, they are the largest organ and consist of four folds. In addition to respiration, oyster gills are directly involved in feeding by creating water currents, collecting food particles, and moving food particles to the labial palps for further sorting. They also serve to separate masses of eggs released from the ovary during spawning into individual ova for efficient fertilization.

Gonad — An organ that produces gametes (eggs or sperm). The oyster gonad lies between the digestive gland and the mantle. The size and thickness of the gonad vary seasonally.

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Habitat — The area or environment where an organism or a community normally lives or occurs. It is often characterized by its physical features or by the dominant plant types. Oyster reefs are habitat for many other estuarine species.

Hatchery — Specialized laboratory where oysters are bred in captivity to produce larvae or spat.

Hemocyte — A cellular component of the blood, especially in invertebrate organisms. There are different types of hemocytes performing a variety of functions including defense and nutrient transport.

Hemolymph — Circulatory fluid found in all mollusks and many other invertebrates.

Hinge — The joining point of the two valves of a mollusk shell at the anterior end.

Husbandry — Application of scientific principles to animal breeding.

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Intertidal — Refers to the environment between high and low tides (mean high water and mean low water) that is alternately exposed to the air and to the sea. Ninety-five percent (95%) of South Carolina oysters live in intertidal areas.

Invertebrate — Animal without a backbone. Oyster are invertebrates.

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Keystone Species — A species whose loss from an ecosystem would cause a greater than average change in diversity or abundance of other species, community structure, and/or ecosystem processes. The original concept of keystone species was applied to predators who could have such impacts. Over time the term has been applied to species at all trophic levels, and new terminology is evolving for these species based on their mode of action or behavior. Under the broadly applied definition, many consider oysters to be a keystone species.

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Labial Palps — Two pairs of large, soft flaps that have a roughly triangular shape and a rough inner surface. The palps direct food towards the mouth, thus controlling the total amount of food ingested. The palps may also sort food before ingestion, perhaps on the basis of particle size or chemical composition.

Larva — A free-swimming and sometimes feeding stage in the early development of certain animals. Oysters have several larval stages including: trochophore, veliger, and pediveliger. Oyster larvae (plural form of larva) are planktonic.

Life Cycle — The sequence of developmental stages through which an organism passes from fertilization to reproductive maturity.

Ligament — In oysters, the fleshy pad between the two shells and located at the hinge end. The ligament forces the shells open when the adductor muscle is relaxed.

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Mantle — Two fleshy folds of tissue that cover the internal organs of mollusks. In the oyster, the mantle's principal role is to form the shells and the ligament. The mantle also participates in other functions such as sensory reception, egg dispersal, respiration, absorption of nutrients, reserve stores, and excretion.

Metamorphosis — An abrupt transition from one developmental stage to another, for example the process of changing from larval to adult form.

Mollusk (Mollusc) — Member of the phylum Mollusca. Invertebrate animals with soft unsegmented bodies, a muscular foot, and a body enclosed in a mantle. Most mollusks have a calcareous outer shell.

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Oyster Reef — The three-dimensional structure created by oysters growing on a firm substrate such as shell, while subsequent generations attach to the older oysters, often forming clusters.

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Pediveliger Larva — The last larval stage of an oyster in which the veliger develops a foot and seeks a substrate on which to settle.

Pericardial Cavity — Cavity containing the heart.

pH (potential of hydrogen) — Relative concentration of hydrogen ions in a solution. A measure of the acidity or alkalinity of a solution. pH ranges from 0 (acid) to 14 (base). A pH of 7.0 is neutral. Seawater is buffered by other ions and thus is slightly alkaline, usually with a pH close to 8.0.

Phytoplankton — Photosynthesizing organisms (plant forms) of plankton, including single-celled algae such as diatoms. Oysters feed by filtering phytoplankton from the water.

Plankton — Small organisms with limited locomotion that passively drift or weakly swim in marine and fresh waters. Oyster have planktonic larvae.

Posterior — 'Towards the rear.' In oysters, the portion of the body furthest from the hinge.

Pseudofeces — Non-food particles that are separated from food, bound in mucus, and rejected by the mouth of oysters. Also spelled pseudofaeces.

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Recruit; Recruitment — Additions to a population, either through birth or immigration. When oyster larvae settle and attach in the vicinity of other oysters, they are 'recruits' into that population.

Refractometer — An instrument which measures the bending (refraction) of light through a liquid. It can be used to measure the salinity of water.

Resident Species — In reference to oyster reefs, organisms that use the reefs as their home.

Restoration — Repair or reconstruction of a damaged ecosystem or habitat.

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Salinity — A measure of the salt content of water, usually expressed in parts per thousand (ppt, o/oo).

Seawater — Water containing salt and other minerals; ocean water. Full strength seawater contains 35 parts of salt for each 1000 parts water (salinity = 35 ppt).

Secchi Disk — A device for measuring water clarity. A Secchi disk (named after its inventor) is a white circle (usually eight inches in diameter) with a black pattern and attached to a rope. The Secchi disk is lowered into water until the pattern is no longer visible. The depth at which this occurs is called the Secchi depth and is a measure of water clarity. Water clarity decreases as turbidity increases.

Sessile — Describes animals that are permanently attached or fixed in position to a surface. Oysters are sessile organisms.

Setting; Settlement — The metamorphosis from the planktonic (free-swimming) larval form to the benthic adult form. When oysters set or settle, they permanently attach to a hard substrate.

Shellfish — A generic term for aquatic organisms with a shell. Can include both crustaceans (crabs, shrimp, etc.) and mollusks (oysters, clams, etc.).

Shucking — Opening an oyster.

Silt — Soil inorganic particles in the 0.002-0.02 mm size range, smaller than sand but larger than clay. Silt, which washes off the land into a body of water, can settle out of the water and smother benthic organisms such as oysters.

Spat — Early juvenile oysters that have settled by attaching to a hard substrate.

Spawning — Producing and releasing gametes (eggs or sperm). Oysters spawn from May to October in South Carolina. Males often spawn first and the presence of sperm in the water is a stimulant to the females. Gametes are released into the water where fertilization occurs.

Substrate — Substance, base, nutrient, or medium in which an organism lives and grows, or surface to which a fixed organism is attached. Oysters attach to hard substrates, preferably oyster shell.

Subtidal — A marine or estuarine environment that lies below mean low-water; always (or almost always) submerged in a tidally-influenced area.

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Tentacles — Soft flexible appendages in aquatic invertebrates that are used to aid in feeding. In oysters, these small organs are located on the edge of the mantle and are used for the detection of environmental stimuli.

Tidal Creek — A creek that is affected by tides, often meandering through salt marsh. In South Carolina, many tidal creeks have oyster reefs along their shorelines.

Tide — The periodic, rhythmic rise and fall of the sea surface that results from the gravity of the moon and sun acting on the rotating Earth.

Transient Species — In reference to oyster reefs, organisms that use the reefs while they are submerged for food and shelter. Transient species leave oyster reefs when the tide goes out.

Trochophore Larva — An early motile (moving) and unshelled larval stage in certain aquatic invertebrates, including oysters, distinguished by a ciliated, pear-shaped body.

Turbidity — A measure of suspended matter or particles in water that block light penetration; cloudy or muddy in physical appearance. Turbidity reduces water clarity.

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Umbo — A knob-like limb arising from the surface of an oyster shell, near the hinge. Often refers to the anterior or hinge end of the oyster.

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Valve — One of the two shells of the oyster.

Veliger Larva — 'Having a velum.' The second and longest planktonic larval stage of the oyster. The name derives from the velum, a distinctive organ used for locomotion and feeding. Early stage veligers are often called 'straight-hinge' larvae or 'D' larvae because of the shell which has a flat hinge and resembles a capital D. Late stage veligers develop a foot and are referred to as pediveligers.

Velum — Ciliated organ used for locomotion and feeding in the veliger larval stage of oysters.

Ventral — 'Towards the front.' In an oyster, the side nearest the mouth.

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Water Clarity — The depth to which light penetrates water. Water clarity can be measured with a Secchi disk and is a relative indicator of turbidity since clarity decreases as turbidity increases.

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Zooplankton — The animal component of plankton that passively drift or weakly swim within the water column. All major animal phyla are represented in zooplankton as adults, larvae, or eggs.

Zygote — Cell formed by the union of egg and sperm.

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