Arc Oyster Shell, Undulostrea megodon

Arc Oyster Shell, Ostrea megodon. Shell collected from within the estuary of the Magdalena Bay complex, Baja California Sur, October 2018. Size: 6.7 cm (2.6 inches) x 5.6 cm (2.2 inches).

Phylogeny: The Arc Oyster, Ostrea megodon (Hanley 1846), is a bivalve mollusk that is a member of the Ostreidae Family of True Oysters. The genus Ostrea is one of sixteen genera in this family, and there are twenty species in this genus. They are known in Mexico as Ostra Plegadais.

Description: The Arc Oyster’s shell is crescent shaped with deeply folded margins. There are four such folds in each valve that align with the folds on the other valve. The external surface of the shell has a rough texture. The shell has a gray exterior and a white interior, with a green tint near the interior margin.  The Arc Oyster Shell  reaches a maximum of 10.3 cm (4.1 inches) in length and 7.2 cm (2.8 inches) in height.

Habitat and Distribution: Arc Oysters are found on hard substrates, in the intertidal zone, and at depths up to 110 m (360 feet). They are a tropical Eastern Pacific species. In Mexican waters they range from Bahia de San Bartolome, Baja California to Guatemala. In the Gulf of California they are found from Loreto southward to Cabo San Lucas, Baja California Sur.

Ecology and Behavior:  Arc Oysters are suspension feeders that feed primarily on phytoplankton and other suspended organic matter. Their main predators are sea stars and gastropods. They are protandric hermaphrodites, maturing first as males and then becoming females in subsequent seasons. Reproduction is sexual, through spermcast spawning, with internal fertilization. The eggs are brooded in the gills of the female, and released as veliger larvae. They are known to host endoparasitic protists and protozoa, and may host kleptoparasitic pea crabs. The are edible and harvested for human consumption. From a conservation perspective they have not been formally evaluated however they are fairly common with a relatively wide distribution and should be consider to be of Least Concern.

Synonyms: Ostrea cerrosensis, Ostrea gallus, and Undulostrea megodon.