Range: Temperate and tropical marine waters worldwide Habitat: Underwater surfaces Size: 3-13 inches (7.6 to 33cm) long Diet: Plankton and particulate matter Threats: Birds, marine mammals, humans, and sea stars Life span: Varies by species; Up to several decades

Transcript:

Today on Animal Fact Files we’re discussing oysters. Specifically we’ll be discussing “true” oysters which are the ones most commonly associated with human consumption. Other oysters include thorny oysters, saddle oysters, and pearl oysters who are specifically bred for their pearls. Technically, all bivalves have the potential to create pearls, but jewelry pearls are derived from oysters classified in a separate family to today’s subjects.

There are dozens of true oyster species, and they can be difficult to tell apart. Their shell shape is influenced by where they live, so two species living in the same area may look quite similar. Their shells are often covered by other organisms such as barnacles and even other oysters, which lends to more misclassification. The number of oyster species has fluctuated throughout the years due to these difficulties in identification. More research could lead to a more definitive count of the number of living true oyster species.

Oysters can be found in temperate and tropical marine waters throughout the world. Oysters are molluscs and, more specifically, they are bivalves. Bivalves have two shells, or valves, that form a watertight barrier around their bodies. Adult oysters are asymmetrical. Their left valve is almost always larger than their right valve. The left valve acts as a cup that holds the oyster’s organs. This is usually the “bottom” shell of the oyster and is the section that attaches to substrates. These molluscs live along continental coastlines where they cement themselves to a surface, not unlike the barnacles we discussed in our last episode.

Oysters are able to osmoregulate meaning they can handle changes in water salinity. This means that they may survive in nearly freshwater areas like estuaries, to high salinity places. They can also tolerate temperature changes. Those living in colder climates may slow their growth and feeding until the weather warms up. On the flip side, warmer climate oysters grow faster than their cooler climate relatives.

In size, oysters range from 3 to 13 inches in length (7.6 to 33cm). They are protandric meaning they start life as males but as they age they switch into females. Both sexes release their gametes, or eggs and sperm, into the water column for broadcast spawning. The eggs are fertilized externally in the water and quickly hatch into free-swimming larvae. These grow for about two to three weeks until they search out a spot to settle. They often chose a spot on or near other oysters as they are attracted to the calcium in the shells of adult oysters. This is how huge oyster reefs form! Other animals, like fish, crabs, and other molluscs use these reefs for protection. They also act as erosion barriers. Along with this, oysters clean surrounding sea water by filtering as much 50 gallons (190 liters) of water in a single day, so they can keep their environments healthy!

Along with filtering water for cleanliness, they filter for food. Oysters catch plankton and other particles on their mucus covered gills which is then moved to the mouth. Their valves are opened using a large muscle, called the adductor muscle. Inside, the oyster’s soft organs are protected and they may even house other animals like pea crabs! Even with their hard shells, these animals may be predated by birds, seafaring mammals - including people - and sea stars. It takes a few years for most oyster species to reach adulthood and some are known to reach decades in age.

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