Amber Pen Shell, Pinna carnea

The Amber Pen Shell, Pinna Carnea, is a species of bivalve mollusk belonging to the family Pinnidae. It can be found in Caribbean waters, ranging from southern Florida to the West Indies and Bermuda.

The amber pen shell has a pair of long, thin, and translucent valves and is triangular, fan, or wedge shaped, with a point at one end and a curved and ragged end at the other extremity. The hinge is along the straight side. It has the potential to grow to about 16 inches long though it usually is much smaller than this. It has several low ribs radiating from the pointed end and running the length of the valve. The coloration is a dull orange externally but the inside of the shell is iridescent. It lives with the pointed end and majority of its shell buried in soft sediment and anchored to a rock or firm sediment by a web of byssus threads. As it continues to grow, it can utilize these threads and its retractor muscle to burrow deeper into the sediment. Only a few centimeters are exposed above the seabed.

The amber pen shell occurs in southern Florida, Bermuda and Texas, the Caribbean Sea and south to Brazil. Its habitat is sand, muddy sand, gravel, or seagrass meadow at depths between 6 feet 7 inches and 49 feet 3 inches. Algae and marine invertebrates have a tendency to grow on the exposed portion of the shell, often making it nearly invisible. Sometimes the larva settles on a vertical reef face and grows out horizontally. Here it is much more exposed to wave action and might be colonized by the tree oyster.

As a filter feeder, the Amber Pen Shell draws water into the shell from above and passes it over the ctenidium before expelling it into the open water at its exposed portion of the shell. This avoids the gills clogged with sediment. During this process, oxygen is absorbed and food particles are captured and transferred to the mouth in balls of mucous. If the shell happens to get damaged, it can be repaired by the laying down of further calcareous material.

This mollusk is a hermaphrodite, the gonads producing both sperm and ova. The larvae are planktonic and drift with the currents. When they settle on the seabed, they undergo metamorphosis into juvenile bivalves. These usually then remain in the same spot for the rest of their lives though it’s possible for a shell that has been uprooted in a storm to dig itself in again elsewhere.

Image Caption: Amber Pen Shell, Pinna Carnea. Credit: Luis Ruiz Berti/Wikipedia (CC BY-SA 3.0)