Giant Bittersweet Shell, Glycymeris gigantea

Giant Bittersweet Shell, Glycymeris gigantea. Size: 10.0 cm (3.9 inches) x 10.0 cm (3.9 inches). Shell collected from the beach of Bahía Concepción, Baja California Sur, April 2005. Shell collection, photograph and identification courtesy of Bob Hillis, Ivins, Utah.

Phylogeny: The Giant Bittersweet, Glycymeris gigantea (Reeve, 1843), is a bivalve mollusk that is member of the Glycymerididae Family of Bittersweet Shells. The genus Glycymeris is one of four genera in this family, and there are sixty species in this genus. They are known in Mexico as Almeja de Arca and Almeja Indio.

Description: The Giant Bittersweet Shell is large and solid. It has a nearly round profile, with a slightly flattened dorsal margin and a fairly high beak. The surface of the Giant Bittersweet shell is smoother than other bittersweets found in the area, with fine, radiating lines. There are around 50 crenulations on the inner ventral margin. The exterior is white with reddish brown blotches, zig-zags, or chevrons. The interior is white to cream, with some dark brown blotches. The periostracum is brown and velvety. The Giant Bittersweet Shell reaches a maximum of 10.5 cm (4.1 inches) in length and 10.5 cm (4.1 inches) in height.

Habitat and Distribution: Giant Bittersweets are found on, and within, sand and mud substrates within the intertidal zone and to depths up to 60 m (200 feet). They are a subtropical to tropical Eastern Pacific species that are found in Mexican waters  from Magdalena Bay, Baja California Sur south to Acapulco, Guerrero. They are found in the Gulf of California from Bahía Willard, Baja California to Cabo San Lucas, Baja California Sur.

Ecology and Behavior:  Giant Bittersweets are infaunal suspension feeders that feed primarily on planktonic algae and suspended organic matter. They are gonochoric and reproduce sexually, through broadcast spawning, with external fertilization. There is no mention in the available literature of them engaging in any types of parasitic, commensal, or symbiotic relationships. These beautiful shells were used for making rings, bracelets, and other jewelry by the indigenous people of the region. They were used a trade items and have been found in archaeological digs as far away as the American southwest. 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: Pentunculus concinnus and Pentunculus gigantean.