Atrina rigida

Atrina rigida
Scientific classification
Kingdom:	Animalia
Phylum:	Mollusca
Class:	Bivalvia
Order:	Pterioida
Family:	Pinnidae
Genus:	Atrina
Species:	A. rigida
Binomial name
Atrina rigida (Lightfoot, 1786)
Synonyms
Pinna rigida

Atrina rigida, commonly known as the rigid pen shell, is species of large bivalve mollusc which is found from the North Carolina coast south through the West Indies and south from there to Brazil. This species has attracted research interest in studies of calcification in its nacre as well as in other aspects of its ecology.

Description

As is typical with the Pinnidae, the shell is elongated and wedge-shaped, coming to a point at the anterior end. There are fifteen or more ridges running lengthwise, and spines arise from these towards the posterior edge. The exterior of the shell is an olive brown. The interior is nacreous to about half its length, and there is a conspicuous scar in the center of the shell at the border of this area, where the posterior adductor is attached. The mantle of the animal is orange and does not fill the shell entirely. The similar A. seminuda has the adductor scar entirely within the nacreous area.

A. rigida may grow to 30 centimeters in length, though typical specimens are somewhat shorter. it also produces perfect black pearl but after 20 years they can produce the pearl as natural.

Ecology

Like most pen shells, A. rigida lives in soft bottoms, with the majority of the shell buried, point down, leaving only a few inches exposed, held in place by an extensive net of byssal threads. This pen shell tends to prefer shallower waters, whereas related species prefer greater depths.

It is a suspension feeder, taking in water and nourishment through the gap between the valves at the posterior (upper) edge. Commensal crustaceans may live in this opening, apparently feeding upon excess food; barnacles and other creatures may attach themselves to the exposed exterior of the shells.

This species is preyed upon by starfish, and other carnivorous gastropods, including the horse conch Triplofusus papillosus.

A. rigida is noted for its ability to heal breaks and holes made in its shell.

Uses

Pen shells are harvested locally for food, especially along the Mexican coast; the large posterior adductor is similar to the meat of the scallop. Black pearls are produced which are suitable for jewelry, but are extremely rare.

Scientific interest in the species is extensive. Of late it has been the subject of molecular biology studies of its calcification enzymes and its myosin. It is also studied due to its regenerative capabilities and for the ecology of its commensal crabs and shrimp.

References

• "Stiff Pen Shell". eNature.com. Archived from the original on 2011-05-16. Retrieved 2008-02-01.
• Carpenter, Kent E., ed. (2002). The Living Marine Resources of the Central Western Atlantic (PDF). American Society of Ichthyologists and Herpetologists. Retrieved 2008-02-01. p. 79
• "Guide to Shelf Invertebrates - Gulf of Mexico: Family Pinnidae" (PDF). Retrieved 2008-02-01.
• Grave, Benjamin H. (1909). "Anatomy and Physiology of the Wing-Shell Atrina Rigida". Bulletin of the Bureau of Fisheries. Govt. print. off. xxix. Retrieved 2008-02-01.
• Mitchell, Patricia B. "American Stiff Pen Shell: Strength and Rigidity". Archived from the original on 20 December 2007. Retrieved 2008-02-01.