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  • José H. Leal

The Locally Elusive Flat Tree Oyster

For years I’ve been searching for signs of the Flat Tree Oyster in our area. Not a true oyster, Isognomon alatus (Gmelin, 1791) is a rare find along the coast of Southwest Florida, with very few examples present (single digits) among the larger American museum collections. The species is found throughout the Caribbean, the Keys, and the East Coast of Florida. There are a couple of holdings in our collection from Marco Island, but only recently we confirmed the presence of the species in Lee County, thanks to Lorin Buckner, who found and photographed a Flat Tree Oyster near Sanibel (below).

Living Flat Tree Oyster, Isognomon alatus, from Fort Myers, Florida. Photo by Lorin Buckner.

Growing up to 95 mm (3.7 inches), tree oysters attach themselves to hard structures, usually mangrove tree roots, by byssal threads that resemble those of mussels or pen shells. The composite image (below) with views of the shell (from Marathon, Florida Keys) was prepared by Patricia Starkey. The image includes views of the byssus (left), and the white arrow shows where the byssus "emerges" from the shell in the living bivalve; see also how "flat" the bivalve is in the right-most view, near the scale ruler.

The Flat Tree Oyster, Isognomon alatus, from Marathon. Photos by Patricia Starkey.


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