🐚 At a glance
- Scientific name
- Atrina serrata
- Shape
- Large, thin fan; broad at the open end, pointed at the hinge
- Ribs
- About 30, each lined with fine scaly projections
- Decisive tell
- Scales finer than the stiff pen — not coarser
- Where & when
- Sandy Gulf flats off the boat-only islands
How to identify a sawtooth pen shell
Start the same way you would with any pen shell: a long, thin, dark triangular fan, broad and squared at the open end, tapering to a point at the hinge, brittle enough to flake at the edges. Then go to the ribs. Count them — a sawtooth pen carries about thirty — and run a fingertip along their length. On the sawtooth, each rib is crowded with small, fine, closely-spaced scales that give it a delicate, file-like texture rather than the rough, widely-spaced spines of its sturdier relative.
That texture is the whole game. Two pen shells can look nearly identical in silhouette and color, so shellers learn to trust their fingertips here, not just their eyes.
The SW Florida pen trio
Three pens turn up along our coast, and the sawtooth sits in the middle. The stiff pen (Atrina rigida) is the rugged one, with coarser, heavier scales. The sawtooth pen (Atrina serrata) wears those same scales but finer and more numerous. And the half-naked pen (Atrina seminuda) is the rare third, named for ribs that are partly smooth. Once you've handled the finer scaling, the sawtooth is the easiest of the three to call.
| Pen shell | Scaling on the ribs | Notes |
|---|---|---|
| Sawtooth (A. serrata) | Fine, small, crowded | ~30 ribs; delicate file-like feel |
| Stiff (A. rigida) | Coarse, heavy, spaced | The rugged, common pen |
| Half-naked (A. seminuda) | Partly smooth ribs | The rarer one of the trio |
Where we find them
Like the rest of their family, sawtooth pens live anchored in the sandy and grassy Gulf flats and wash onto the open barrier-island beaches. On our shelling tours we work Keewaydin, Kice Island and the Ten Thousand Islands, and the best whole pens of any kind arrive after a calm stretch when the surf isn't smashing them on the way in. Collier County is empty-shells-only, so admire and photograph any with a living animal and bring home the empties.
From the Helm: hunting the saw-tooth pen shell in the back creeks off Goodland
A stretch of brisk wind out of the north is what gets our attention for this one. It works the current harder through the back creeks off Goodland, and that extra push is often enough to roll a saw-tooth pen shell loose from the oyster rubble it likes to wedge against. We’re not looking at open grass flats here — this one tends to sit tucked along the edges of oyster bars in the tidal creeks, angled into the current rather than buried flat. It’s a different kind of ground than the open pen shell flats we work elsewhere, and it rewards patience more than volume.
We work these creek edges from the boat on an incoming half-tide, poling along slow enough that a guest can lean over the gunwale and scan the oyster clumps instead of blind-digging in mud. You’re looking for a fan standing nearly upright against the rock rather than lying flat, which is often the giveaway before you can even confirm the ribbing. Pull one free and it comes out lighter in the hand than you’d guess from its size, the ribs catching softer against a thumb than the coarser cousin most people picture when they hear the words pen shell.
If the oyster bars are running too skinny that day, or the current’s dead slack and nothing’s moving loose, we’ll shift to a nearby back creek mouth where the bottom drops off faster and pens tend to stack instead of scatter. It’s a smaller move than switching islands entirely, and usually enough. Compare what you find here against a straight-up pen shell from the backcountry flats and the difference in ground alone — oyster rock versus open grass — tells you almost as much as counting ribs does.
We won’t pretend this one turns up every outing. Most creek walks net you the common stiff pen shell more often than the saw-tooth, and that’s just the honest ratio out here. On a good push of current after a few days of wind, our captains have better luck than on a dead-calm week, but neither guarantees anything specific. Whatever you pull loose, remember Collier County rules mean it comes home empty — leave anything with a tenant still inside right where the current put it.
Bring a pair of light gloves if your hands are sensitive; oyster rock is sharper on the knuckles than sand ever is, and it’s the main reason we work these creeks from the boat rather than wading them barefoot. Ask your captain before the trip if you want time built in for this kind of creek-edge hunting specifically — most of our standard runs favor the open flats, so a back-creek detour is something we plan for rather than stumble into on any of our regular shelling tours.