Japanese Oyster Shell, Magallana gigas
Japanese Oyster Shell, Magallana gigas. Shell courtesy of the commercial fishermen of Leyes de Reforma, Asuncion, Baja California Sur, April 2025. Size: 9.3 cm (3.7 inches) x 6.3 cm (2.5 inches) x 3.3 cm (1.3 inches). Photographs and identification courtesy of Colin Campbell, DVM, Punta Chivato, Baja California Sur.
Japanese Oyster Shell, Magallana gigas. Shell courtesy of the commercial fishermen of Leyes de Reforma, Asuncion, Baja California Sur, April 2025. Size: 9.3 cm (3.7 inches) x 7.0 cm (2.8 inches) x 3.6 cm (1.4 inches). Photographs and identification courtesy of Colin Campbell, DVM, Punta Chivato, Baja California Sur.
Japanese Oyster Shell, Magallana gigas. Shell collected in the greater San Diego area, San Diego California, April 2012. Size: 37 cm (14.6 inches) x 32 cm (12.6 inches). Collection, photograph and identification courtesy of Bob Hillis, Ivins, Utah.
Phylogeny: The Japanese Oyster Shell, Magallana gigas (Thunberg, 1793) is a bivalve mollusk in the Ostreidae Family of True Oyster Shells. The genus Magallana is one of sixteen genera in this family, and there are thirteen species in this genus. They are also known known as the Giant Pacific Oyster, the Miyagi Oyster, Pacific Cup Oyster and the Pacific Oyster Shell and in Mexico as Ostion Japones.
Description: The Japanese Oyster Shell is large in stature and highly variable in shape, from elongated oval, to teardrop-shaped. They appear to have a layered construction with a highly arched left valve and a relatively flat right valve. A percentage of the shells have strong visible radial ribs but others do not. Likewise, some have frilled margins and others do not. The exterior of the shell is a mottled white, gray, and purple, often with radial banding. The interior is white, with lighter colored adductor muscle scars. Japanese Oysters reach a maximum of 45 cm (18 inches) in length and 35 cm (14 inches) in height.
Habitat and Distribution: Japanese Oyster Shells are found attached, by their left valve, to rocks and other hard substrates. They can also attach to the shells of other Japanese Oysters and eventually form oyster reefs. They live in the intertidal zone, and to depths up to 6 m (20 feet). They are native to the Asian Western Pacific but were transplanted and commercially farmed by the Japanese all over the world commencing 100 years ago. Some oysters escape these farms and naturalize. Japanese Oyster Shells have been cultivated at several locations along the Baja Peninsula including Magdalena Bay and La Paz Bay, Baja California Sur but escapees can be found in numerous other locations. In some locations they have become highly invasive displacing and destroying native oyster populations. At this time, their range in Mexican waters appears limited to the west coast of the Baja Peninsula and the entire Gulf of California.
Ecology and Behavior: Japanese Oyster Shells are suspension feeders that feed primarily on phytoplankton and other suspended organic matter. Their main predators are sea stars, crabs, and gastropods. They are protandric hermaphrodites, maturing first as males and then becoming females in subsequent seasons. Reproduction is sexual, through broadcast spawning. They are known to host endoparasitic protists, protozoa, bacteria, and copepods. Externally they host epibionts such as microalgae, bivalves, gastropods, flatworms, bryozoans and tunicates. Where they have been introduced they can have negative impacts on the environment by altering habitats (oyster reefs), harming water quality, displacing native species, and spreading other non-native species. They are edible, harvested, and farmed 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: Crassostrea (Magallana) gigas, Crassostrea gigas, Crassostrea laperousii, Crassostrea posjetica, Crassostrea talienwhanensis, Dioeciostrea hispaniola, Lopha (Ostreola) posjetica, Lopha (Ostreola) posjetica var. beringi, Lopha (Ostreola) posjetica var. newelskyi, Lopha (Ostreola) posjetica var. zawoikoi, Ostraea cymbaeformis, >Ostraea rostralis, Ostrea chemnitzii var. elongata, Ostrea gigas, Ostrea gigas var. tientsiensis, Ostrea gravitesta, Ostrea laperousii, Ostrea posjetica, and Ostrea talienwhanensis.