The Tellins

What Puts a Shell in This Family

Pick up a thin, flat, faintly glossy half shell on a Southwest Florida beach, hold it toward the light, and there is a fair chance you are holding a tellin. This is the family that comes aboard in a pocket most often with no name attached, usually because it does not look like anything in particular. No spines, no knobs, no heavy ribs. Nothing a beginner's eye can grab.

The family is Tellinidae Blainville, 1814. WoRMS and MolluscaBase, which are the same database where mollusks are concerned, both carry it as accepted under AphiaID 235, and both place it in the order Cardiida. You will also see it filed under Imparidentia. That is not exactly wrong, it is imprecise: Imparidentia is the superorder, one rank further out, and it holds a great deal besides tellins.

The family is large. Roughly 500 to 550 accepted living species sit in about 100 genera. We are giving you a range rather than a headline number on purpose. The figure shifts every time somebody revises a piece of the family, and this family gets revised.

The Shape, Before the Name

Everything below can be checked on one empty valve, in your hand, without a book.

• Thin. Tellins are built light. Hold one up and many of them pass light through the thinner ground near the margin. That thinness is also why so many of them reach a beach already broken.
• Compressed. Seen end on, the two valves together are flat rather than fat. A cockle seen end on is deep and heart shaped. A tellin is a wafer.
• Equivalve. Left and right match. Set both valves of one animal side by side and neither is deeper than the other.
• Oval to elongate. The outline runs long rather than round, with the beak, the oldest point on the shell, sitting toward the middle of the hinge line.
• Fine concentric sculpture, usually. The lines run parallel to the growing edge and follow the outline of the shell, and on most of the local species they are fine enough to read as texture rather than as ribs. Radial ribs, the kind running from the beak straight out to the margin, are the cockle pattern, not this one.
• A twist at the back end. In the local species listed further down, the posterior end bends consistently to the right. It is subtle, and once you have seen it once you keep seeing it.

One caution on that last point, because it is the kind of detail that gets copied badly. The rightward twist is confirmed for the Southwest Florida species checked for this page. Tellinidae is a worldwide family of several hundred species, and we are not extending that direction to all of them.

The One Muscle That Settles It

There is a feature that, among bivalves, belongs to the tellins: a cruciform muscle. It is a small crossed pair of muscle bundles near the back end of the animal, close to where the siphons emerge, and it leaves a correspondingly small pair of scars inside the shell.

As a family character it is excellent. As a field character it is close to useless, and we would rather say so than send you hunting for it on the beach and let you conclude your eyes are the problem. The scars are tiny. On a fresh, clean valve turned interior up, under a loupe and raking light, an experienced eye can pick them out near the posterior end below the pallial sinus. On a chalky valve that has spent a season in the swash, there is nothing left to find.

So the honest version runs like this. The cruciform muscle is why a specialist can be certain a shell is a tellin. It is not how you will be certain. For that, use the outline, the sinus and the sculpture, and accept a little residual doubt, which is the normal condition of shell identification anyway.

The Pallial Sinus, and Reading Depth Off a Dead Shell

Turn a valve over and look at the inside. Running around the interior a little way in from the margin is a faint line, the pallial line, where the mantle attached. Toward the back end of a tellin that line does not stay near the margin. It swings inward in a deep U or a long tongue, sometimes reaching most of the way to the middle of the shell. That notch is the pallial sinus, and in this family it is deep.

Here is why it is worth learning. The sinus is the space the retracted siphons occupied. A deep sinus means long siphons, and long siphons mean an animal that lived buried well down and still had to reach the surface. So an empty valve, picked up long after the animal died, records how that animal lived. Not many single characters do that much work.

Read it alongside the other interior marks: the beak, the hinge teeth, the two adductor muscle scars. Those are the readable features of any bivalve, walked through on our page about cockles and the bivalve body plan, and the general habit of naming a shell from what your eye can actually see is set out in the Florida shell identification guide.

Two Siphons, Long and Separate

In many burrowing bivalves the two siphons, intake and outflow, are joined along their length into a single tube that works as one unit. In tellins they are not. The two run separately, they are long out of all proportion to the animal, and each moves on its own.

That arrangement is the basis of the family's way of life. Zwarts and Wanink documented the underlying relationship in 1989: across burrowing bivalves, siphon length sets the depth limit. An animal can live only as deep as its plumbing reaches. Tellins have unusually long plumbing, so they live unusually deep, which puts a useful thickness of sediment between them and most of what would like to eat them.

The independence matters too. A fused pair points where the animal points. A separate, mobile intake siphon can be run out of the burrow and swung across the sediment surface above, more or less like a hose, while the outflow siphon carries spent water away somewhere else entirely. Keeping the two apart stops the animal re-breathing what it just expelled.

How a Tellin Feeds, and Why One Word Is Not Enough

Search for what tellins eat and you will be told, confidently and nearly everywhere, that they are deposit feeders. Long intake siphon, sweeps the bottom, takes up settled organic particles, done.

That is half true, and the missing half is the interesting one.

Tellins are facultative feeders. The same animal works both ways. It can extend the intake siphon across the bottom and draw in the fine organic material lying on the surface, which is deposit feeding in the ordinary sense. It can also lift that siphon up into the water above and take in suspended particles, straining them on its gills exactly as a cockle or a venus clam does. Neither mode is a fallback for the other. Both are normal.

What decides it is circumstance, principally food supply and water flow. When current is moving and carrying enough material in suspension, filtering pays, because the water brings the food to the animal. When the water is quiet and the food has settled out onto the bottom, the siphon goes to work on the surface instead.

Why we are being fussy about this: writing that a tellin deposit feeds rather than filtering does not just drop a detail, it drops the explanation. A single fixed strategy would tie the family to a single kind of bottom. A switch lets one body plan work on a still mud flat, on a current swept sand bar, and on everything in between, which is a fair description of where tellins actually turn up. The flexibility is the adaptation.

Six Tellins of This Coast

Every name below is currently accepted on WoRMS and MolluscaBase, and each carries its AphiaID so you can check it yourself rather than take our word for it. The regional list they come from traces to the Bailey-Matthews National Shell Museum and Aquarium on Sanibel, roughly forty to forty-five miles up the coast from us. That is Southwest Florida sourcing rather than a Marco Island survey, and we would rather label it accurately than imply a local species list nobody has published.

Any one of those could carry a page of its own. This one is about what the six have in common, which is the part a species-by-species guide tends to skip past.

The Exception That Shows You the Rule

If you want to understand a family, look hard at the member that breaks the pattern.

The White-crested Tellin, Tellidora cristata, is a tellin by hinge and by sinus and looks almost nothing like the rest of them. Where a typical tellin is a smooth oval wafer with sculpture fine enough to feel rather than see, this one is nearly flat, chalk white, and carries concentric ribs that rise into a row of blunt tooth-like points along both the front and back margins. Held up against the sky the outline reads as serrated. Nobody who finds one mistakes it for anything else.

It is not a common find. We raise it partly because it is the one tellin a visitor tends to remember, and partly because of what it does to the definition. Thin, flat, glossy, faintly sculptured: those are family tendencies, not family rules. The characters that actually hold across Tellinidae are the internal ones, the deep sinus and the cruciform muscle, and those are exactly the ones you cannot see from across a beach.

The Macomas, a Plan Within the Plan

Read older shell literature and you will meet macomas treated almost as their own thing. They are tellins. The macoma build is a variant inside the family, and the Constricted Macoma, Austromacoma constricta, shows it plainly.

Two things set the look. The posterior end is bent, more sharply than in a typical tellin, so the back of the shell reads as folded rather than merely twisted. And the shell carries a visible periostracum, a thin brown skin over the outside, which on a fresh specimen flakes at the margins and looks for all the world as though the shell is dirty.

It is not dirty. The periostracum is a protein layer the animal grows, and it does real work: a 2025 experiment measured roughly a threefold reduction in shell dissolution as coverage rose from about ten percent to about eighty-five percent. Scrubbing it off makes a specimen look tidier and makes it a worse record of the animal. How that layer is built, and how the rest of a shell is assembled, is the subject of our page on how shells are made.

The 2015 Revision, and Why Your Field Guide Is Behind

Look again at the fourth row of that table. Tampaella tampaensis. The species has been on the books since Conrad described it in 1866. The genus Tampaella did not exist until 2015.

That is not a clerical curiosity. In 2015 Huber, Langleit and Kreipl published a substantial revision of this family, and it moved a great deal of it. Species that had sat comfortably in Tellina or Angulus for a century were distributed into genera erected in that same work. Tampaella, Ameritella, Austromacoma, Macoploma, Serratina and Laciolina all date from it. Two of the six shells in our table carry one of those new genus names.

The practical consequence is sitting on a lot of bookshelves. A shell guide printed before 2015, including several very good ones, will show genus names MolluscaBase no longer accepts. The book is not useless. The shells did not move and the photographs are still right. The labels changed.

None of this is unique to tellins. The lightning whelk moved out of Busycon. The horse conch moved out of Fasciolaria. Names change because the evidence changes, usually because somebody finally looked at anatomy or at DNA instead of at outline alone.

Occasionally the specialists disagree outright rather than simply updating. The Florida fighting conch is the local example: MolluscaBase currently treats it as the same species as a closely related conch, while nearly every field guide on the shelf keeps the two separate. We are not going to referee that one. Either way the useful habit is the same. When a name matters to you, check it against MolluscaBase, write down the date you checked, and expect it to move again.

Color Is the Calling Card and the Trap

Color is what makes people pick tellins up. The family runs to clean whites, soft yellows, and above all pinks: a wash of rose over the beak, sharp radiating pink rays on the Candystick Tellin, whole valves in a flat even rose. Local shell crafters favor the Rose Petal Tellin specifically for that color, and it is easy to see why.

Then the same color walks you into a wrong name.

One tellin species can run from white with the faintest wisp of pink at the beak to a saturated rose across the whole valve. The Rose Petal Tellin does exactly that. Two valves off the same stretch of beach, from the same species, can look like two different animals, and a sun-bleached or beach-worn valve has lost whatever color it started with. The reliable characters in bivalves are the countable and structural ones: rib counts, hinge teeth, the shape and depth of the pallial sinus. Color is among the least reliable there is. Coquina makes the point better than any tellin does: a single species that occurs in an enormous range of colors and patterns, and not even in this family.

Use color to notice a shell. Use structure to name it.

Tellins You Will Read About and Probably Not Find Here

Search this family and three names surface quickly: the Sunrise Tellin, the Great Tellin, and the Strigilla species, which are small, round, and marked with fine oblique scratched lines.

All three are documented in the Gulf of Mexico. None of them appears on the regional guide's Southwest Florida list, which is the list this page is built on. So we are putting them where the evidence puts them: wider Gulf and Caribbean relatives, real and worth knowing about, not shells we are going to send a visitor down this beach looking for.

That distinction gets blurred constantly in shelling content, and blurring it costs a reader real time. Occurs in the Gulf of Mexico covers a body of water running from Texas to the Yucatan to the Florida Keys. Occurs on this stretch of Southwest Florida beach is a much smaller claim and a far more useful one. When we cannot support the smaller claim, we say so rather than quietly upgrading the larger one.

Half a Valve Is Still a Record

Half a bivalve is the commonest thing on any beach. The two valves come apart at the hinge once the animal dies and the ligament gives way, and from that point they travel independently. Nobody should feel short-changed by a single valve, because one valve still carries the hinge, the muscle scars, the pallial line and the sinus, which is most of what a name rests on.

Where to look, in general terms: thin light shells sort hydraulically in the swash zone, collect along the wrack line, and pile up on shell bars, on spits, and at the inside bends of passes. Tide state matters more than anything on a calendar, and the timing of it is worked through on our page about when to go looking.

If you keep one, label it. A specimen with a locality, a date, a habitat and a collector's name is data. The same shell in a bowl with no label is a decoration, and there is nothing wrong with a decoration so long as you know which of the two you have. That distinction, and how to store shells without inviting Byne's disease, is covered in our guide to starting a collection. For a second opinion on a name, iNaturalist is credible and keeps your observation attached to a place and a date.

One rule of ours applies aboard the boat whatever any local code says: empty shells only. Nothing with an animal still in it comes home. Trips run three times a day out of Goodland, this is a family we see on the flats we work, and you can reserve a seat here. The rest of the Shell Monographs, taken family by family, starts at the shell families index.