Atrina rigida, commonly known as the rigid pen shell, is a bivalve mollusc found from the North Carolina

North Carolina

North Carolina is a state located in the southeastern United States. The state borders South Carolina and Georgia to the south, Tennessee to the west and Virginia to the north. North Carolina contains 100 counties. Its capital is Raleigh, and its largest city is Charlotte...

 coast through the West Indies and to Brazil

Brazil

Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...

. It has attracted research interest in studies of calcification

Calcification

Calcification is the process in which calcium salts build up in soft tissue, causing it to harden. Calcifications may be classified on whether there is mineral balance or not, and the location of the calcification.-Causes:...

 in its nacre

Nacre

Nacre , also known as mother of pearl, is an organic-inorganic composite material produced by some mollusks as an inner shell layer; it is also what makes up pearls. It is very strong, resilient, and iridescent....

 as well as other aspects of its ecology.

Description

As is typical with the Pinnidae

Pinnidae

Pinnidae is a family of large saltwater clams sometimes known as pen shells. They are marine bivalve molluscs in the order Pterioida.The shells are fragile and have a long and triangular shape, and in life they are anchored in sediment using a byssus...

, the shell is elogated and wedge-shared, 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

Mantle (mollusc)

The mantle is a significant part of the anatomy of molluscs: it is the dorsal body wall which covers the visceral mass and usually protrudes in the form of flaps well beyond the visceral mass itself.In many, but by no means all, species of molluscs, the epidermis of the mantle secretes...

 of the animal is orange and does not fill the shell entirely. The similar A. seminuda

Atrina seminuda

Atrina seminuda, or the half-naked pen shell, is a species of bivalve mollusc in the family Pinnidae. It can be found along the Atlantic coast of North America, ranging from North Carolina to Texas and south to Argentina....

has the adductor scar entirely within the nacreous area.

A. rigida may grow to 30 centimeters in length, though typical specimens are somewhat shorter.

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

Byssus

Byssus means both a silky filament by which certain molluscs attach themselves to hard surfaces, and a rare fabric, also called sea silk and its fibre source.-Word:...

. 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

Commensalism

In ecology, commensalism is a class of relationship between two organisms where one organism benefits but the other is neutral...

 crustacean

Crustacean

Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...

s may live in this opening, apparently feeding upon excess food; barnacle

Barnacle

A barnacle is a type of arthropod belonging to infraclass Cirripedia in the subphylum Crustacea, and is hence related to crabs and lobsters. Barnacles are exclusively marine, and tend to live in shallow and tidal waters, typically in erosive settings. They are sessile suspension feeders, and have...

s 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 Pleuroploca gigantea.

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

Mexico

The United Mexican States , commonly known as Mexico , is a federal constitutional republic in North America. It is bordered on the north by the United States; on the south and west by the Pacific Ocean; on the southeast by Guatemala, Belize, and the Caribbean Sea; and on the east by the Gulf of...

 coast; the large posterior adductor is similar to the meat of the scallop

Scallop

A scallop is a marine bivalve mollusk of the family Pectinidae. Scallops are a cosmopolitan family, found in all of the world's oceans. Many scallops are highly prized as a food source...

. Black pearl

Pearl

A pearl is a hard object produced within the soft tissue of a living shelled mollusk. Just like the shell of a mollusk, a pearl is made up of calcium carbonate in minute crystalline form, which has been deposited in concentric layers. The ideal pearl is perfectly round and smooth, but many other...

s are produced which are suitable for jewelry but extremely rare.

Scientific interest is the species is extensive. Of late it has been the subject of molecular biology

Molecular biology

Molecular biology is the branch of biology that deals with the molecular basis of biological activity. This field overlaps with other areas of biology and chemistry, particularly genetics and biochemistry...

 studies of its calcification

Calcification

Calcification is the process in which calcium salts build up in soft tissue, causing it to harden. Calcifications may be classified on whether there is mineral balance or not, and the location of the calcification.-Causes:...

 enzymes and its myosin

Myosin

Myosins comprise a family of ATP-dependent motor proteins and are best known for their role in muscle contraction and their involvement in a wide range of other eukaryotic motility processes. They are responsible for actin-based motility. The term was originally used to describe a group of similar...

. It is also studied due to its regenerative capabilities and for the ecology of its commensal crabs and shrimp.