What Eats a Sand Dollar

Why a Sand Dollar Is a Hard Meal

Start with the problem from the predator's side, because it explains most of what follows.

A sand dollar lives buried in the top layer of sand, so the first difficulty is finding one at all. What you would find, if you dug it out, is a disc: wide, flat, and built around a fused plate skeleton of calcium carbonate, in the calcite form, roughly a millimeter thick. That skeleton is not a coating over a body. It is most of the object. The living tissue is a thin layer over and inside it, spread across an area far larger than the volume of meat involved.

Then there is what the animal eats. A sand dollar is a deposit feeder, working fine organic material and diatoms off sediment, and juveniles swallow sand grains on purpose as ballast against being rolled by current. Whatever else is inside one, it is not a package of muscle.

Put those together and you have prey that is hard to locate, awkward to handle, largely mineral by weight, and modest in return. Not impossible. Just expensive.

Buried Is the Real Defense, Not the Test

Most people assume the disc is the armor. It is, up to a point, and the point is sharper than it looks. A low dome of brittle mineral carries evenly spread pressure well and fails at a single point load, which is why a test snaps in a pinch after surviving a winter in the surf. A claw or a tooth plate is a point load by design. Against the one thing the test is worst at, the test is not much of a wall.

What actually protects a sand dollar is position. Buried in the top layer of sand, it is not visible, not silhouetted against light, and not loose in the water where anything passing can take it. A predator has to find it first, then move sediment to get at it, then deal with a wide awkward plate for a small return.

The animal has nothing else. No venom, no speed, no spine that will hurt you. It moves at a pace you measure against a clock rather than a stopwatch. It hides, and it is easy to give up on.

The Kinds of Predator, and the List We Are Not Going to Give You

Search this question and you will get lists. Confident ones, naming fish, crabs, rays, snails and sea stars, usually with no source and usually copied from one another. We are not going to add another, because we cannot back one for the animals in our water.

Here is what can be said honestly. These flats hold plenty of animals equipped to eat hard-bodied prey: snails that drill, crabs with claws built to crush, fish and rays with crushing tooth plates, and invertebrates that swallow prey whole. Those capabilities are not in question. What is in question is which of them actually take living sand dollars here, how often, and at what size.

So this page sorts predators the way the evidence does, by mechanism. There are four ways through the problem. Drill a hole. Crush the disc. Swallow it whole. Or digest it where it lies. Each leaves a different signature, and in two of the four cases no signature at all, which is itself worth knowing before you start reading marks off a test.

Damage One: A Neat Round Bore Hole

This is the mark worth learning, because it is the only one that argues for predation on its own.

What you are looking for is a single round hole with smooth walls and a clean edge, usually off center rather than through the middle, and often slightly wider on the outside than the inside so that it reads as countersunk. The surface around it is undamaged and normal thickness. There is normally one, not several.

Why it counts: drilling is slow, deliberate work, and nothing does it to an empty object. A completed bore hole is the closest thing to a receipt you will find on a beach.

Why it stops there: you cannot name the driller. On shells generally, moon snails bore a beveled, countersunk hole and murexes a straighter-sided one, a distinction covered on our page about how shells are made. That distinction depends on the hole having enough depth to show a profile. Through a wall a millimeter thick there is almost no profile to read. The mark tells you drilled, and then goes quiet.

Damage Two: A Crushed or Chipped Margin

Crushing leaves an arc missing from the rim, edges that are angular and stepped rather than smoothly rounded, and sometimes short cracks running inward toward the center. The break often goes further on one face than on the other.

It is also the least diagnostic damage there is, for two reasons.

The first is ordinary. A bucket, a wave, a rolling whelk and a beach rake all chip rims. Rim damage is the default condition of beach material, not a clue.

The second matters more. A crushing predator that succeeds does not hand you a test with a bite out of it. It leaves fragments, or nothing recognizable. So the crushed tests available to be found are the failures and the near misses, which means beach material systematically under-represents crushing no matter how common it is. You cannot count what worked.

What would raise your confidence a little: a clean arc removed along one line, damage matching on both faces, and a test that is otherwise crisp, unworn and freshly cast. Even then, hedge it.

Damage Three: A Hole Punched Through a Face

A hole through the upper or lower face, irregular in outline, angular rather than round, with edges pushed inward and sometimes short cracks radiating away, is a point load failure. Something concentrated force on one spot until a millimeter of brittle plate gave way.

Plenty of living things can do that. So can a stone in the swash, another shell rolling against it, a foot, a rake tine, and the weight of a heavy shell set on top of it in a bag. On an object that has been in moving water for an unknown length of time, those causes are not separable after the fact.

The temptation is to look at a ragged hole and picture a beak or a jaw. Resist it. A ragged hole is what thin brittle plate does under any concentrated load, and its shape records the physics rather than the culprit.

Which gives a working rule. Round, smooth and countersunk reads as drilled. Irregular, angular and pushed in reads as broken, and the cause stays blank.

Not Predation: The Pocked Tunnelling of Boring Sponges

Sooner or later you will pick up a test that looks moth-eaten: a field of small round pits, similar in size, often in clusters, with intact surface between them, and in a clear example the pits connect to one another below the surface. That is boring sponge work.

Boring sponges excavate calcium carbonate to live inside it. They are common on carbonate material, and they are not predators of the sand dollar in any sense. The sponge is after the mineral, not the meat.

Two qualifications, both honest. We have not verified how often boring sponges attack living sand dollars as opposed to colonizing tests after death, and a wall a millimeter thick gives a borer very little to work in, so heavy sponge damage on a sand dollar is not something we see often. And pitting has other causes worth ruling out first: uneven wear roughens a surface, and the five-petal figure is a genuine field of pores.

Not Predation: Wear, Which Opens Holes All by Itself

The most underrated cause of a hole in a sand dollar is nothing at all. Sand, water, and time.

A test tumbling in the swash zone is being sanded. The petalodium fades, the rim rounds, and the lunule edges soften and widen slightly. The wall thins everywhere, and thinnest where it started thin. Eventually it opens, and what you are holding is a test with a hole in it that nothing ever touched on purpose.

Telling that from a bore hole is easier than it sounds, and it comes down to the material around the hole rather than the hole itself. At a drill hole the wall nearby is normal thickness and the outline is a sharp circle. At a wear hole the wall nearby is already thin, the outline is irregular, and the edge feathers away rather than ending.

The best check costs nothing. Hold the test up to a bright sky. Thin porous calcium carbonate passes light, so a worn test glows through in patches before it opens, and those patches show you exactly where it was going to fail.

The Openings That Were Always There

Before you diagnose anything, rule out the openings the animal grew. This is where most of these questions end.

• The lunules. The slots cut clean through the disc are structural. Five on the Gulf five-slotted sand dollar, Mellita tenuis. Six on the six-holed keyhole urchin. They are supposed to be there.
• The peristome. The mouth opening, at the center of the underside.
• The periproct. The anal opening, on the underside near the margin, which surprises almost everyone.
• The pores of the petalodium. The five-petal figure is a field of paired pores for flattened tube feet used in gas exchange. Under a lens it looks perforated because it is.

All of it is walked through part by part on the anatomy page. And if a test rattles, nothing is living inside it. That is sand, or the five jaw elements of Aristotle's lantern.

Most of It Happened After the Animal Died

Here is the sentence the confident pages leave out. Almost every mark on a beach-cast test was made after the animal was already dead, and tells you nothing about how it died.

Follow the object. The animal dies, for a reason you will never recover. The tissue goes, to scavengers and to decay. The bare test then spends an unknown period in moving water full of abrasive sand and other hard objects, gets sorted and carried like any light flat thing, and lands on a beach that may be mechanically groomed. Collier County does groom the Marco Island beaches.

Now count the opportunities. Every stage in that sequence can mark a test. The one stage during which a predator could have marked it is a single moment at the very beginning. So the base rate runs hard against a predation reading, and the honest default for any given mark is damaged after death, cause unknown. A bore hole earns an exception. Very little else does.

You can at least date a break relative to the beach. A fresh one has sharp, bright, slightly powdery faces and pieces that still mate. An old one has rounded edges and dulled faces. That method is set out on why they break, and it tells you when, not who.

A Healed Margin, and Why It Is the Better Find

There is one mark that is stronger evidence than any hole, and hardly anyone looks for it.

Echinoderms can repair skeletal damage. A margin that was broken and then grew back does not look like a break at all. It looks like a margin with the wrong shape: a shallow bay, or a flattened stretch where the outline should curve evenly, with the plate pattern continuing across it rather than stopping at a raw edge. Nothing about it is sharp.

If you find that, you are holding an animal that was attacked and lived. It is more specific than a hole in a dead disc, because repair only happens in a living animal.

We are not going to tell you how common it is. We have not found a published figure for our species and we will not guess at one. It is worth photographing both faces and keeping.

What a Dense Bed Does to Predation Pressure

Sand dollars do not live scattered. They live in beds: patches of bottom where the animals sit close together, at densities reaching hundreds per square meter in some populations, with bottom holding almost none of them a short distance away.

Two opposite things follow, and both are true as general principles.

A dense patch is a concentrated, predictable food supply, and finding the patch is most of the work. That generally rewards a predator for staying, which argues for higher pressure on a bed than on scattered animals.

At the same time, being one of a crowd lowers any individual's odds of being the one taken. The crowd that attracts attention also dilutes it.

Which effect dominates for Mellita tenuis in our water is not something we can source. Nobody has published a predation rate for beds on this coast, so there is no number here in either direction.

One pressure is not in doubt, because it is ours. The test is about a millimeter thick, the living animal sits directly under a footstep, and wet packed sand transmits weight rather than cushioning it.

The Real Losses Happen in the Plankton

Everything above concerns the adult, the part of the life cycle with a skeleton and a hiding place. It is not where the animal is most at risk.

Sand dollars are broadcast spawners. Eggs and sperm go into open water, and the larva is an echinopluteus, a small armed drifting form that feeds in the plankton before settling to the bottom. A larva has none of the adult's defenses. No burial, no disc, no mineral plate of any consequence. It is loose in open water, which is exactly where the animals that eat plankton are.

For broadcast spawners generally, the overwhelming majority of larvae never reach the bottom. That is a statement about the strategy, not a measurement of our animals, and no percentage is attached to it here.

One result here is worth naming carefully. In 2008, Dawn Vaughn and Richard Strathmann reported in Science that larvae exposed to fish mucus, a chemical cue that a predator is near, responded by cloning themselves into smaller copies. The work was done on Dendraster excentricus, a Pacific sand dollar ranging Alaska to Baja California, which does not occur in Florida. Whether the Gulf species do anything similar has not been tested. The full account of that finding is a page of its own.

What to Record If You Find Something Genuinely Unusual

The difference between an anecdote and a record is about ninety seconds of work.

• Photograph both faces before you move it, with something in frame for scale. A ruler, or a coin. Raking light shows relief that flat light hides, and there is a page on photographing a test.
• Note where it was: which beach, roughly where along it, and whether it came off the wrack line, the wet swash or dry sand. Add the date.
• Note the condition: crisp or worn, edges sharp or rounded, wall thick or thin at the damage, and whether other tests within a few steps carry the same mark. One is an anecdote. Five is a pattern.
• Label it if you keep it. A specimen without a label is a decoration. With locality, date, habitat and collector, it is data.
• Post it where it can be checked. iNaturalist is credible, and the observation stays attached to a place and a date.

If it is dark, spiny and moving, it is alive and not a specimen. Put it back, right side up. Deciding whether one is alive takes about ten seconds, and what the law says about a live one is a page of its own. Our rule aboard is short: empty shells only.

This comes up on the flat most weeks, usually from a child holding a disc with a neat hole in it who wants to know what bit it. The honest answer is that something drilled it, that we cannot tell you what, and that the hole is more interesting for what it does not say. Trips run three times a day out of Goodland, and you can reserve a seat here.