Flat Tree Oyster, Isognomon alatus

The Flat Tree Oyster, Isognomon alatus, is a species of bivalve mollusk belonging to the family Isognomonidae. It can be seen along the Atlantic coast of North America, ranging from southern Florida to Brazil and Bermuda.

This oyster has two thin and irregularly shaped valves that are joined by a long straight hinge. The exterior is sculpted by a large number of rough, concentric rings with loose flakes and varies in color from a pale brownish olive to a purplish black. The nacre on the inside is lustrous and cream colored and shaded with a purplish brown. The shell is attached to the substrate by a byssus thread and grows to about 3 to 3.7 inches in length.

It’s found in southern Florida, the Caribbean Sea and Gulf of Mexico, Colombia, Costa Rica, Cuba, Jamaica, Lesser Antilles, Venezuela, Puerto Rico, San Andres and Brazil. It’s one of the three closely related species that are found, sometimes in dense patches, on the roots of mangroves, particularly the red mangrove and on shallow rocky areas, down to depths of 49 feet or more. On coral reefs, it occurs on rock ledges and exposed rocks in areas of high sedimentation, creating mats of hundreds of individual animals. It also grows on the shells of the Atlantic thorny oyster.

As a filter feeder, it draws water into its shell and passes it though its gills, extracting plankton and small organic particles in the process.

Breeding occurs after heavy rain has lowered the salinity of the water. The majority of the adult oysters in the area participate in a mass spawning even when they all liberate their gametes at the same time. Fertilization occurs in the water column and the larvae are planktonic. When they have passed through several developmental stages, they settle to the seabed, undergo metamorphosis and attach themselves with byssus threads as miniature adults.

Image Caption: Isognomon alatus (Bivalvia: Pterioida) del Golfo de Cariaco, estado Sucre. Credit: Veronidae/Wikipedia (CC BY-SA 3.0)