pen shells

The bivalve seashells, or shells that come in two parts or halves, can be similar in appearance, but not all bivalves are “clams”. I’m not scientifically minded, but I’ve been learning the difference between the shells I collect and photograph.

I’ve gathered some photos of the more common bivalves I see on beaches and in the saltwater rivers around my area of Florida. A general bivalve list includes: clams, oysters, scallops, cockles and mussels.

Clams and Arks

My photos below in this section contain various types of clam shells. Also, there are many types of Venus clams and loads of coquina clams. Shells called “arks” are also clams.

quahog clam living alive hefty shell — Tagelus clam shell

ark shell with brown stripes — Ponderous Ark

broad ribbed cardita shell underneath — The broad ribbed cardita, measures about an inch in length.

broad ribbed cardita bivalve seashell — The broad ribbed cardita, measures about an inch in length.

Oysters

I rarely collect oyster shells but we see oyster beds while out on our boat. Oyster shells are sharp, and are the scourge of boaters because they can slice up the underside if a bed is accidentally hit while running. While walking the flats in oyster areas I can hear the shells snapping as they feed.

oysters attached to tree roots — Oyster Shells

Scallops

Scallop shells can be some of the prettiest shells. Certain types of scallops have one flat side and the other is normal / rounded. The concave part is in the sand or bottom and the flat part is on top and opens like a flap when the scallop is alive. Scallops propel themselves around by opening and closing their shells.

The Kitten Paw and larger Lion’s Paw shells are also scallops. I found these Kitten Paw shells while visiting Sanibel Island on the Gulf coast. I’ve never seen any where I live on the east coast.

Types of scallop shells — 2 inches across

kitten paw seashells — Kitten’s Paw Scallop Shells

Cockles

Cockles are not clams although they look similar. Because they have an oval shape with high top part (umbo), when a whole cockle is viewed from the side it resembles a heart. For that reason, they are known as the heart shell or heart cockle.

bivalve cockle shells — Atlantic Giant Cockle Shell

inside the giant cockle shell — Atlantic Giant Cockle Shell

Mussels

Mussel shells are something I rarely collect or photograph because they really are not great looking shells. They are dark colored and shaped like thin fans. The pen shell is distantly related and I have a few pictures here. The pen shell can be nearly a foot in length and is more impressive than a mussel.

mussel seashell — Open and empty mussel shell

part of a pen shell on the beach sand — Pen shell next to my son’s foot

shells found at beach — Pen shell next to my son’s foot

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Pen shells (family Pinnidae) are usually gray or brown in color with an iridescent sheen. They are long and tapered in a triangular shape and most of the ones I’ve seen were on the east coast beaches in Florida.

They can be huge! My resource book says up to thirty-one inches in length! But many are found broken since they are a bit more fragile than an ordinary seashell.

The varieties found in Florida include the Saw-toothed pen shell, Stiff pen shell and Amber pen shell. The ones pictured here (photo by me) look like saw-toothed as they are smoother than the Stiff pen shell which has ridges with spines. Amber pen shells are lighter in color (hence the name).

Flag pen shells are wide and rounded on top and are found in the Indio-Pacific region.

The Noble Pen Shell (Pinna nobilis) has an unlikely history. It’s byssal threads (which anchor the shell in the sand when alive) were once used to make fabric. The threads were woven together to make what was known as “sea silk”. This large shell is found in the Mediterranean region and is in danger of extinction due to over use, pollution and decline in places for it to survive.