Invertebrates
Oyster
Ostreidae · also called true oyster, rock oyster
An oyster spends about three weeks swimming, then glues itself to a rock or another oyster shell and never moves again. From that spot it filters up to 190 litres of water a day, cleaning the estuary around it, and it switches sex back and forth as it grows. Pile enough of them up and they build reefs — habitat now recognised as one of the most degraded on Earth.
Overview
Oysters are the family Ostreidae, rough-shelled bivalve molluscs of temperate and tropical coasts and estuaries, in the intertidal and shallow subtidal. They are filter feeders, straining plankton and fine particles across gills that also serve as their lungs, and a large one processes 90–190 litres of seawater a day.
Cemented, and building reefs
An oyster larva swims for two to three weeks, then settles and cements one shell permanently to a hard surface — most often an older oyster shell. Because larvae keep settling on old shell, oysters stack up over generations into reefs metres thick and kilometres long, providing hard habitat for fish, crabs, worms, and hundreds of other species in soft-bottomed estuaries.
Sex-changers, and not the pearl kind
Most true oysters are protandric hermaphrodites — male first, then female, often switching back and forth by year. The lustrous gem pearls come from a separate group, the pearl oysters (Pteriidae); edible oysters only rarely make a small dull one.
Status
Farmed species like the Pacific and eastern oysters are produced in the billions and in no danger, but wild oyster reefs are in serious decline — an estimated 85% lost globally, over 99% in some bays, from overharvesting, disease, siltation, and the removal of settlement shell. Native oysters like the European flat and Olympia oysters are down to remnants, and large-scale reef restoration is now widespread.
Oyster: common questions
How much water can an oyster filter?
A single adult oyster filters roughly 90 to 190 litres of seawater a day, drawing water in over its gills, trapping plankton and fine particles in mucus, and passing them to the mouth while pumping the cleaned water back out. Because oysters occur in dense reefs, the combined effect is large. Historically, the oyster population of Chesapeake Bay was estimated to be able to filter the entire volume of the bay in about three to four days; today, with the population a small fraction of that, it takes closer to a year. This filtering removes suspended sediment and excess nutrients, improves water clarity, and is one of the main reasons oyster reef restoration is promoted as a tool for cleaning up estuaries.
Do oysters feel pain, and are they alive when eaten?
Oysters have a simple nervous system with nerve ganglia but no brain and no specialised pain-processing centres, and there is no evidence that they experience pain in the way animals with central nervous systems do, which is part of why some people who otherwise avoid animal products will eat them. A raw oyster served on the half shell is indeed alive, or very recently killed, when eaten, and freshness is judged by whether the oyster is still tightly closed or closes when tapped, since a gaping oyster that does not respond is dead and should be discarded. Cooked oysters are killed by the heat.
Is it true you should only eat oysters in months with an R in them?
The old rule of avoiding oysters in May, June, July, and August has some basis but is largely outdated. The warm months are when many oysters spawn, which makes them thin, watery, and less pleasant to eat, and warm water also raises the risk of the bacterium Vibrio, which can cause serious illness. Modern refrigeration, water-quality monitoring, and the availability of triploid farmed oysters that do not spawn have made summer oysters safe and palatable in most markets, so the R-month rule is now more a matter of quality and tradition than a strict safety requirement, though care with raw shellfish in hot weather is still sensible.
What is the difference between an oyster and a clam or a mussel?
All three are bivalve molluscs, with a body enclosed between two hinged shells, but they live differently. A clam has two matching, usually smooth, oval shells, is not attached to anything, and burrows into sand or mud using a muscular foot. A mussel has two matching, elongated, often dark shells and anchors itself to rock or rope with a bundle of tough threads called a byssus, which it can release and re-spin to move a little. An oyster has two mismatched, rough, irregular shells, cements one of them permanently to a hard surface, and never moves once settled. Oysters and mussels attach and clump; clams bury and stay loose.