Fat Horsemussel Shell, Modiolus capax
Fat Horsemussel Shell, Modiolus capax. Shell collected from within the estuary of the Magdalena Bay complex, Baja California Sur, October 2018. Size: 9.5 cm (3.7 inches) x 4.6 cm (1.8 inches) x 2.7 cm (1.1 inches). Identification courtesy of Bob Hillis, Ivins, Utah.
Fat Horsemussel Shell, Modiolus capax. Shell collected off the beach at Punta Chivato, Baja California Sur, January 2023. Collection, photograph and identification courtesy of Colin Campbell, DVM, Punta Chivato, Baja California Sur.
Phylogeny: The Fat Horsemussel, (Conrad, 1837), is a bivalve mollusk that is a member of the Mussels of the Mytilidae Family. The genus Modiolus is one of twenty-six genera in this family, and there are thirty-nine species in this genus. They are also known as the Capax Horsemussel and in Mexico as Mejillón Huaquilla.
Description: The Fat Horsemussel shell has a somewhat variable profile but is elongated and deeply inflated. The basal margin is widely rounded and the beak and anterior margin of the shell are bluntly rounded. These shells are covered by a dark brown periostracum and the exterior is pinkish red or purple in color and the interior is white with blue or dark purple tinges. Fat Horsemussels reach a maximum of 15.6 cm (6.1 inches) in length and 7.3 cm (2.9 inches) in height.
Habitat and Distribution: Fat Horsemussels are found in groups attached to pier pilings and other hard surfaces, within bays. Lone specimens may found attached to rocks in protected areas along the open coast. On occasion, individuals may be found buried in mud with only their posterior end exposed. Fat Horsemussels live intertidally, and to depths up to 35 m (115 feet). They are a temperate to tropical Eastern Pacific species. In Mexican waters they are found along the entire western coastline, including the Gulf of California.
Ecology and Behavior: Fat Horsemussels are suspension feeders that feed primarily on planktonic algae and suspended organic matter. They are gonochoric and reproduce sexually, through broadcast spawning, with external fertilization. Fat Horsemussels are known to host the protozoan parasite Perkinsus marinus, which can lead to Dermo Disease. There is no mention in the available literature of them engaging in any types of commensal or symbiotic relationships. Fat Horsemussels are edible and are the target of artisanal and recreational fisheries. 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: Modiola capax and Mytilus spatula.