Flat Tree Oyster (or Atlantic Tree Oyster)
Phylum: Mollusca, Class: Bivalvia, Order: Ostreida, Family: Isognomonidae, Genus: Isognomon, Species: alatus · Isognomonidae (Tree Oyster family) · Bivalve (fragment); compressed/fan-shaped irregular shell
Species
Isognomon alatus
Shell Type
Bivalve (fragment); compressed/fan-shaped irregular shell
Family
Isognomonidae (Tree Oyster family)
Size
Approximately 1.5 - 2 cm (0.75 inches). This is a small fragment or juvenile; adults can reach 7-10 cm (3-4 inches).
Color & Pattern
Off-white to creamy base with faint yellowish-orange staining. The interior shows a dull nacreous (pearly) luster. The exterior appears weathered and sun-bleached, losing the typical dark purplish-grey or brown streaks of living specimens.
Rarity
Very Common; these are abundant in mangrove ecosystems and leur fragments are frequently found on nearby beaches.
Habitat
Found in the intertidal zone, typically attached by a byssus to the aerial roots of red mangroves, rocks, or pier pilings in shallow, brackish, or marine waters.
Geographic Range
Tropical Western Atlantic, ranging from Southern Florida and the Gulf of Mexico through the Caribbean (including Cancun) to Brazil.
Description
This is a weathered fragment of a Flat Tree Oyster. The shell is naturally very thin and brittle, composed of multiple laminated layers of calcite and aragonite. It features an irregular, foliated (leaf-like) edge and a somewhat scaly exterior texture.
Key Features
Thin, compressed profile; irregular 'winged' shape; laminated shell structure; numerous small pits on the surface caused by boring sponges or erosion; dull pearly interior.
Collector Value
Minimal. While the species is biologically interesting, beach-worn fragments are common and usually only kept as mementos rather than high-value specimens. Intact, paired valves with rich color are preferred by collectors.
Condition Notes
Poor to Fair. This is a beach-worn fragment lacking the hinge ligament and the dark pigmentation of a live specimen. It shows significant erosion and holes likely made by Cliona (boring sponges).
Interesting Facts
Unlike true oysters, tree oysters are more closely related to pearl oysters. They are called 'Tree Oysters' because of their unique habit of growing in dense clusters hanging from mangrove roots above the muddy seafloor.
Ecological Role
Filter feeders that improve water quality by straining plankton. They provide habitat for small crabs and worms within their clusters. They are prey for predatory gastropods and shorebirds.
Similar Species
Isognomon radiatus (Lister's Tree Oyster), which is usually more elongated and found on rocks/corals rather than mangrove roots.
Beach Finding Tips
Look near mangrove thickets and in the 'wrack line' (debris line) after high tide. Because they are light and flat, they are often pushed high up onto the sand in Cancun after storms.
Notes
Cancun