Wrinkled Pen Shell, Pinna rugosa

Wrinkled Pen Shell, Pinna rugosa. Size: 16 cm (6.3 inch) x 5.5 cm (2.2 inches). Shell collected from within the estuary of the Magdalena Bay complex, Baja California Sur, October 2018.

Phylogeny: The Wrinkled Pen Shell, Pinna rugosa G.B. Sowerby I, 1835, is a bivalve mollusk that is a member of the a Pinnidae Family of Pen Shells. The genus Pinna is one of three genera in this family, and there are thirty-one species in this genus. They are also known as the Rugosa Pen Shell and in Mexico as Hacha Larga, Nacre Rugosa, and Pina Hacha Larga.

Description: The Wrinkled Pen Shell is long, triangular, and thin. They are equivalve and moderately inflated. They are sculpted with radiating folds that form scales or short spines. The interior of both valves have two nacreous lobes, separated by a narrow groove. Their exterior is yellowish brown to dark brown in color.  Wrinkled Pen Shells reach a maximum of 43 cm (17 inches) in length and  20 cm (8 inches) in height.

Habitat and Distribution: Wrinkled Pens are found in shallow mudflats, often in lagoons and mangrove forests. They live in the intertidal zone, and at depths up to 100 m (330 feet). They are a subtropical to tropical Eastern Pacific species that are found in all Mexican waters of the Pacific Ocean with the exception that they are absent from north of the San Ignacio Lagoon, Baja California Sur, along the central and northwest coasts of the Baja Peninsula. Some sources extend this range north to the Coronado Islands, Baja California.

Ecology and Behavior: Wrinkled Pens are suspension feeders, which feed on plankton and suspended organic matter. They are gonochoric and reproduce sexually, through broadcast spawning. They are known to host the Pearl Oyster Shrimp, Pontonia margarita, in their mantle cavity, in what appears to be a commensal relationship. Otherwise their engagement in any type of  parasitic or symbiotic relationship has not been formally documented. From a conservation perspective the Wrinkled Pen has not been formally evaluated. However, they are fairly common with a relatively wide distribution and should be consider to be of Least Concern.

Synonyms: Pinna (Pinna) rugosa