NC's *Atrina* Pen Shells

I know of three pen shells here in NC.

Sawtooth pen shells Atrina serrata is the easiest to ID because it's the most ribbed (30 narrow riblets) and has hundreds of short hollow prickles (Witherington)

The other two are very hard to tell apart for me and I'm not sure many RG observations are correct because people don't realize there are two very similar species in the area.

First we have stiff pen shells Atrina rigida. My Audobon field guide says its 5-11 inches long with 15-20 radiating ribs. The upper ribs are large and bear "many erect, hollow, sometimes tubular spines"

The field guide goes onto say that the half-naked pen shell Atrina seminuda was formerly confused with A. rigida. It says the half-naked pen shell is "slightly smaller, thinner, more narrow, and lighter in color"

My beachcomber's guide says half-naked pen shells "have about 15 radiating ribs bearing few to dozens of long tubular spines"

The best way to ID those two that I know of requires pictures of the inside of the shell. The half-naked pen shell A. seminuda has the posterior muscle scar is "wholly within the pearly area" (FGS) and Witherington says it's "completely within the shiny (or cloudy) nacreous area"

The stiff pen shell A. rigida has a "large muscle scar protruding above edge of pearly layer" (FGS) and (Witherington) says in their words that the posterior muscle scar is "outside the shiny nacre"

I also wanted to note that the Beachcomber's Guide by Witherington and Witherington mentions their habitat and I wonder if that's useful in IDing as well. It says sawtooth and half-naked pen shells live in colonies out to 20 ft. It says stiff pen shells A. rigida live in bays and sounds.

In summary:

Stiff Pen Shell A. rigida
-Darker
-Boader
-Muscle scar outside of nacre
-15-20 ribs
-Lives from low-tide line to water 90' deep (Audubon FGS)
-Lives in bays and sounds (Witherington)

Half-naked pen shell A. seminuda
-Thinner
-Narrower
-Smaller muscle scar within nacre
-15 ribs
-Lives in colonies buried in soft sediment out to 20 ft

Posted on December 5, 2022 07:22 PM by aureleah_aurita