Annual Shell Show This weekend: The four types of pen shells found on Marco beaches

A pen shell with stone crab and sea cucumber. Submitted

A pen shell with stone crab and sea cucumber. Submitted

A pen shell with stone crab and sea cucumber. Submitted

A pen shell with stone crab and sea cucumber. Submitted

A pen shell with byssal threads. Submitted

A pen shell with byssal threads. Submitted

What a photographers dream! It took a while and help to pry the shell open to extract his finger. Submitted

What a photographers dream! It took a while and help to pry the shell open to extract his finger. Submitted

A pen shell with stone crab. Submitted

A pen shell with stone crab. Submitted

Pen shells are sometimes called fan mussels and are wedge shaped. The shell is thin, brownish, tan or clear, and rather fragile with an iridescent lining inside. Many varieties are found in warm seas and at least four are found here.

They live in the sand, pointed end down and are attached to some small bit of rubble by byssal threads. They are suspension feeders, rather sedentary, have no siphons or eyes, and have large gills. The spiny exterior helps anchor the pen shell in the mud or sand and also serves as a holdfast for barnacles, oysters, tube worms and algae. They live in colonies.

At very low tide out on the flats you might see the tops of some of living shells sticking out of the sand. Should you find a pen shell alive and pick it up, be somewhat cautious so you won’t get your finger caught like this gentleman did. He innocently picked up a pen shell on the beach to examine it as it was slightly open. Thinking it was dead, or not thinking at all, he had his finger inside the shell and the pen shell took offense and closed on his finger. What a photographers dream! It took a while and help to pry the shell open to extract his finger.

Both the gentleman and the pen shell were well after the encounter!

There is a small commensal crab (Pinnotheres) that sometimes lives inside a pen shell. It is protected inside the shell and feeds on surplus food particles. Many pen shells also have soft bodied pen shrimp living inside.

On the beaches when you find a number of pen shells, empty and dead, be curious, and look inside the bivalve (a shell that has two parts). You may be surprised at what has been using the empty pen shell as a house.

Sometimes you will see a sea cucumber and a small stone crab living inside a pen shell found on our beach or perhaps a small octopus living inside. You never know what you might find if you look.

Some cultures find the pen shell muscle delicious like eating scallops. The other parts and organs are shopped up for soup , stuffing and sauces. Bon appetite!

When preparing your pen shell be sure to check for small black pearls as you are chopping up the tissue and remove them. They are used frequently for making jewelry.

You have probably heard of the Golden Fleece. In ancient times the threads of the Noble Pen Shell (Pinna Nobilis) found in the Mediterranean were woven into fabric to produce small items of clothing like gloves and socks for very rich clientele.

The byssal threads that attach the shell itself to rubble to keep it relatively sedentary are removed and cleaned. In Egypt only royalty were allowed to wear byssal garments and it was called sea silk. Cloth found in their tombs had 152 threads in the warp (the lengthwise run of the cloth) in one inch. Our finest cotton today has 88 threads. The old Golden Fleece was indeed a very fine textured cloth. The art of making cloth from byssus had been lost over time. It was last seen in Sardinia in the 1920s.

Today many crafters clean the threads and use them for the hair on dolls and other items to keep the craft item made totally from material from the sea.

The four types of the pen shells we find here are: Sawtooth pen shell (Atrina serrata); half-naked pen shell (Atrina seminuda); stiff pen shell (Atrina rigida); and occasionally the amber pen shell (Pina carnea).

The sawtooth pen shell has about 30 radiating ribs bearing hundreds of short, hollow prickles. The shell is tannish-gray both inside and out. The exterior is covered with many even rows of spines with an undulating pattern. It is about 10 inches in length and moderately common at low tide up to 20 feet.

The half-naked pen shell has about 15 radiating ribs bearing a few to dozens of long tubular spines. Their posterior (fan end) muscle scar is completely within their pearly area. The mantle is pale yellowish. They are common in water up to 70 ft. and grow to nine inches. The shell is yellowish gray to dark brown.

Stiff pen shells are darker and broader than the half-naked pen shells and have their posterior muscle scar outside the shiny nacre. The mantle is bright orange near the hind end. They are common in shallow bays and grow to 10 inches.

The amber pen shell grows to six inches, is similar to the half-naked and is not found north of Collier County. It has a light tan, amber, or pinkish-orange shell.

Enjoy your shelling. Be sure to put the live shells back.


The Marco Island Shell Club’s Annual Show

When: Thursday, Friday and Saturday, March 14, 15, and 16, from 10 a.m. to 4 p.m.

Where: United Church of Marco Island, 320 N Barfield Drive, Marco Island.

Description: The show includes juried contests for scientific shell specimen displays and Artistic Shell Craft displays. In addition to the exhibits there will be a large sale of artistic shell crafts created by the members of the Shell Club. These include beautiful shell flowers and shell flower arrangements, picture frames decorated with shells, novel creatures (animals, caricatures) created with shells, jewelry, and holiday and home décor.

Proceeds benefit the Marco Island Shell Club Scholarship Fund. For more information check the website or

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