Taxonomy
- The word oyster is used as a common name for a number of distinct groups of bivalve molluscs. True oysters are members of the family Ostreidae. This family includes the oysters.
Species
of Commercial Interest
- There are several genera of oysters cultured in the world, but Ostrea, Crassostrea, and Saccostera are the most important genera in aquaculture.
Distribution
- Oysters are found in all marine waters of the world except those near the north and south poles. Specific ranges differ between species.
- For example, The eastern oyster (Crassostrea virginica) inhabits intertidal and subtidal locations in estuaries where salinities range from 5 to 18 ppt. The Pacific oyster (Crassostrea gigas) thrives in waters with salinities of 23 – 28 ppt or higher on flats composed of firm mud, sand, or gravel, but they also can grow on rocks. The European flat oyster (Ostrea edulis) is typically found in highly productive, shallow, coastal waters with salinities of 20 – 28 ppt. This species can grow on firm bottoms of almost any texture. The Sydney rock oyster (Saccostrea commercialis) is native to Australia and is found in coastal zones of Victoria and Queensland. This species occurs on rocks in the intertidal zone in estuaries near river mouths.
Biology
- Oysters are one of several groups of bivalve (2-shelled) molluscs. They orient to the bottom and are capable of attaching to a variety of substrates via several long, rope-like byssal threads. They feed by filtering particulate matter our of the water, and as such are very amenable to aquaculture. They require little “feed” and actually help purify the water of their environment. It has been suggested that large oysters in Chesapeake Bay can filter up to two gallons of water an hour during warm summer months.
- Under natural conditions, oysters spawn as water temperature rises in the spring. Oysters are known as “broadcast spawners” because they release eggs and sperm into the water column. A fertilized egg develops into a planktonic, free-swimming trochophore larva in about 6 hours. The trochophore larva does not feed and develops into a fully shelled veliger larva within 12 to 24 hours. The veliger has a thin shell and actively feeds in the water column. A veliger larva remains planktonic for up to 3 or 4 weeks. Towards the end of this period, it develops a foot and settles to the bottom to seek a hard substrate. When a suitable surface is located, the larva attaches itself by byssal threads and metamorphoses into adult form. Oyster larvae will attach themselves to many types of substrate, but they seem to prefer molluscan shells and other materials which provide hard, firm surfaces.
- This complex reproductive cycle is what has allowed oysters to colonize much of the world's oceans.
Sustainability
- IUCN Red List Status: Not Evaluated (NV); There are some species of endangered oyster, but none contained in the three most important commercial genera.
- MBA Seafood Watch:
- Farmed oysters account for 95 percent of the world's total oyster consumption. Most oyster farming operations are very well managed and produce a sustainable product. Farming oysters brings little risk of pollution or escapees, and habitat effects from the farms are minimal. In addition, oysters minimally impact marine resources as they don't rely on wild-caught fish - in the form of fishmeal or fish oil - for food. And, thanks to the oyster's filter-feeding action, oyster farms can actually benefit the surrounding coastal waters.
Potential
Health Concerns
- Most filter-feeding bivalves are susceptible to biotoxins, usually accumulated through the uptake of toxic plankton (dinoflagellates). The toxin is not harmful to the oyster but can cause a condition known as Paralytic Shellfish Poisoning (PSP) in humans. Commercial shellfish is normally government-inspected so the risk of paralytic shellfish poisoning (PSP) is extremely minimal.
