Basic morphology

Oysters belong to the Class Bivalvia in Phylum Mollusca and possess a soft body encased in two calcareous shells, or "valves".  The lower valve is usually larger and somewhat cupped, and the top valve acts as a lid.  When an oyster larva finds a suitable site, it secretes an adhesive substance and the lower valve is attached or cemented onto a hard substrate. On an oyster reef, the larva attaches to another oyster shell, thereby contributing to the biomass and structure of the reef.

Feeding and respiration

	General anatomy of the Eastern oyster, Crassostrea virginica. Illustration from The Eastern Oyster (redrawn from Galtsoff, 1964), with permission of Maryland Sea Grant.

Oysters generate a flow of water over their gills and mantle surface by means of microscopic cilia.  This allows the oyster to move considerable amounts of water across its gills and up to its mouth, where algae and small organic particulate matter (detritus) in the water are further sorted and filtered.  Phytoplankton (microscopic algae) and detritus of acceptable size (3 to 12 microns) are then shunted into the esophagus and stomach and used by the oyster for energy.  Rejected particles are bound up in mucus and discharged in a stream of matter termed "pseudofeces", which is either forcibly expelled from the mantle cavity via rapid closure of the valves or passively shunted out via water currents.  Through this process oysters filter relatively large volumes of water, estimated at 1 to 8 gallons per hour (Loosanoff and Nomejko 1946; Jordan 1987; Choi et al. 1993), depending on the size of the oyster and environmental factors. Because one of the benefits of oysters is enhanced water clarity, decreases in oyster populations have resulted in more turbid water conditions in many locations.  One scientist estimated that in the early 19th century the oysters in Chesapeake Bay could have filtered the whole bay in less than a week, but now it would take about a year because of the depletion of oyster reefs (Newell 1988).

As water passes across and through the gills, oxygen is absorbed across the gill tissues and taken up by the "blood" (hemolymph), which is colorless in oysters, and distributed via the circulatory system to other tissues.  At the same time, carbon dioxide is released from the gill membranes into the water.  Oxygen uptake is greatly facilitated by a counter-current system whereby the water flows in one direction and hemolymph in gill blood vessels flows in the opposite direction.  Circulation is further aided by a three-chambered heart, which helps pump the hemolymph throughout blood vessels in the body.

Reproduction

	Oyster spat attached to shell used as spat collector in Mullica River, N.J. Photo credit: Dr. Gustavo Calvo, New Jersey Dept. of Fish and Wildlife, Bureau of Shellfisheries.

Most adult oysters reproduce in the spring and summer, when water temperatures rise.  Eastern oysters are called protandric hermaphrodites because they are usually males first, when they are small, and females when they are larger (some oysters as young as 6 weeks post-settlement have been reported to spawn).  Depending on their sex, oysters produce either sperm or eggs, which are released directly into the water column, and fertilization is external in most species.  The Olympia oyster, and others in the group Ostreidae, is an exception.  These oysters are hermaphrodites (reproductive sytem contains both eggs and sperm) and also have internal fertilization. 

The chemical stimulation of one oyster spawning creates a chain reaction that causes others to release their gametes.  Extensive clouds of gametes might develop in the water as oysters on a reef release eggs and sperm in unison.  The fertilized egg quickly develops into a swimming larval form called a "trochophore," and then into a "veliger" with a fully-formed shell and a velum (a ciliated structure for swimming).  The larva remains in the water column, or is said to be "planktonic," for about 3 weeks, then develops a foot (at this stage it is called a "pediveliger") and settles to the bottom because of the increasing weight of its shell.  On the bottom it seeks a suitable hard substrate for attachment, produces an adhesive substance to cement itself to the substrate, and becomes a small oyster ("spat").  When spat attach to other oysters and shells, grow, and form dense aggregations, an oyster reef is formed.

Predators, pests, and diseases

	Oyster drill (Thais haemastoma) an oyster predator that drills into the shell. Picture from the Galtsoff Collection (NOAA North East Fisheries Science Center, Historical Photo Archives), R.O. Smith, photographer.

Oysters are prey to a host of predators, including stone crabs (Menippe mercenaria and M. adina) and blue crabs (Callinectes sapidus); gastropods or oyster drills, e.g., Southern oyster drill (Stramonita [=Thais] haemastoma), Atlantic oyster drill Urosalpinx cinerea), and thick lipped drill (Eupleura caudata); and fish (e.g., oyster toad fish and black drum).  In addition, some flatworms or "oyster leeches" (e.g., Stylochus sp. and Pseudostylochus sp.) can either rasp through the shell and consume the meat of a small spat, or enter through the opening between the shells and consume the meat from the inside.  Some other animals are pests that may not directly kill an oyster, but they can penetrate the shell, which requires the oyster to expend energy to secrete additional shell and prevent penetration into the tissue.  For example, polychaete worms (Polydora sp.), boring clams (Diplothera smithii), and boring sponges (e.g., Cliona sp.) can infest an oyster's shell and erode it, making it brittle and vulnerable to attack by predators. 
   
Predators and pests take their toll on oyster reefs in a fairly predictable manner.  However, diseases have virtually decimated oyster reefs in some areas.  In the 1950s, oysters in Chesapeake Bay and Mid-Atlantic states began to experience unprecedented mass mortalities from a disease organism that was finally identified as a haplosporidium (protozoan) parasite (Haplosporidium nelsoni).  The parasite was dubbed "MSX," an acronym for multinucleated sphere X (or unknown).   Some researchers believe MSX was accidentally introduced into Chesapeake Bay in the 1930s with some experimental plantings of the Pacific oyster (Crassostrea gigas), which apparently is resistant to the disease.  Losses due to MSX were exacerbated by the presence of yet another disease caused by a protozoan parasite called Dermo  (from the original scientific name, Dermocystidium marinum), which is also found in oysters from the northeastern Gulf of Mexico (Florida, Alabama, Mississippi, Louisiana, and Texas).  These parasites mainly affect Crassostrea virginica, and some states have worked for decades to breed disease-resistant strains.  Dermo causes extensive "summer mortalities" in Gulf Coast oyster populations.  Although the effects of MSX and Dermo are most pronounced in higher-salinity waters, drought conditions in Maryland and Virginia in the past few years have allowed the parasites to flourish farther up Chesapeake Bay.  To save the oyster industry in Chesapeake Bay, oyster biologists in Virginia and Maryland are giving strong consideration to introducing a different species, the Suminoe oyster (Crassostrea ariakensis), which may be resistant to one or both of these diseases.
   
On the West Coast, the Pacific oyster (Crassostrea gigas) is also susceptible to some diseases, but not nearly to the extent of oysters along the Gulf of Mexico or Atlantic Coasts.  Denman Island disease, first discovered in British Columbia oysters, also occurs in oysters in Puget Sound, Washington, but apparently has not caused the significant mortalities of MSX and Dermo in Chesapeake Bay.  However, a phenomenon called "summer mortality" affects the Pacific oyster on the West Coast.  Summer mortalities, which can be substantial in some locations, may be related to depletion of energy stores (glycogen) during the warm summer months when oysters are spawning, or it may be caused by some as-yet-unknown parasite.  At this time, no disease agent has been definitively linked to the mortalities.