The Sand Dollar Larva That Clones Itself, on the Pacific Coast

Start With the Species, Because the Species Is the Point

The research on this page was done on Dendraster excentricus, the Pacific sand dollar. Its range runs from Alaska to Baja California. It does not occur in Florida, it does not occur anywhere in the Gulf of Mexico, and it is not the animal you picked up off a sandbar this morning.

That sentence has to come first, because this finding travels badly. Strip the species name off it and you are left with "sand dollar larvae clone themselves when they smell a predator," which sounds like a fact about sand dollars in general. It is not. It is a documented result in one species, on the other side of the continent.

So read everything below as a report about a close relative that lives in a different ocean. The five-slotted disc on a Gulf beach is Mellita tenuis. Nobody has published a test of this behavior in Mellita, and nobody has published one in Encope michelini, the notched sand dollar that also turns up here. That is not caution on our part. It is the state of the literature.

Sand Dollars Spawn Into Open Water

To understand why a sand dollar larva is out there on its own in the first place, start with how sand dollars reproduce. They are broadcast spawners. Males and females are separate animals, and neither one carries the young. Both release gametes, sperm and eggs, straight into the water above the sand.

Fertilization happens out there, in open water. There is no pairing, no nest, no burrow full of eggs, and no parental care of any kind. The adult's entire contribution ends the moment the gametes leave its body.

This is one of the reasons sand dollars live packed together. Sperm dilutes fast in moving water, and a spawning animal with neighbors a few inches away has better odds than one sitting alone across a bare flat. The dense aggregations you find on a sand bottom are, among other things, a fertilization strategy. They get their own treatment on the sand dollar beds page.

What comes out of a successful fertilization is not a small sand dollar. It is something else entirely.

The Echinopluteus Looks Nothing Like a Sand Dollar

The larva of a sand dollar is called an echinopluteus. It is microscopic, it is close to transparent, and if you saw one under a scope with no label attached, you would not connect it to a flat white disc.

It carries long slender arms held out from a small body. Each arm is stiffened inside by a rod of calcium carbonate, the same mineral the adult test is built from, and fringed along its edge with cilia, the fine beating hairs that do two jobs at once. They keep the larva swimming, and they sweep single-celled algae down toward a mouth. The larva feeds itself. It is not coasting on yolk.

The strangest part is the symmetry. An adult sand dollar is pentaradial, built in five parts around a center, which is what the petals on top are counting out. The larva is not. It has a left and a right, a front and a back, the way you do. Five-part symmetry is something an echinoderm develops into at metamorphosis, not something it starts with. The anatomy page walks through the adult version of that body.

Weeks in the Plankton, Mostly Being Hunted

Once it is swimming, the larva lives in the plankton: the drifting community of things too small or too weak to hold a position against a current. It stays there for a period usually measured in weeks rather than days, then settles to the bottom and turns into something you would recognize. Nothing about that stretch is safe.

There is nowhere to hide in open water. No sand to burrow into, no rock, no test. The main defense a planktonic larva has is being difficult to see, and many of them are built of tissue so clear that light passes through with very little left to catch a hunting eye.

The hunters that matter here are the visual ones. Small fish find prey by seeing it, and a visual hunter is limited by what it can resolve against the background of the water. Size drives that. A larger object is detectable from further away, which means a larger larva is effectively surrounded by a wider circle inside which a fish will pick it out. Everything in the finding below turns on that one relationship.

The Experiment: A Predator That Was Not Actually There

The cue used in the work was fish mucus. Fish shed mucus from their skin constantly and it dissolves into the surrounding water. For something small and edible, chemical traces of fish are about as useful an early warning as the ocean offers, because they arrive before the fish does and they do not require seeing anything.

The setup is the part worth slowing down for. Larvae of Dendraster excentricus were exposed to that cue with no fish present. Just the chemistry. Nothing chased them and nothing ate any of them. The only thing that changed was the information in the water.

The larvae responded by cloning themselves, producing smaller copies. That is the result. A drifting larva, detecting a chemical signal that something with a mouth was in the neighborhood, did not flee, did not sink and did not harden. It divided part of itself off into a new and genetically identical individual.

The animal's answer to a predator was to reproduce.

What Cloning Means When a Larva Does It

Cloning here means asexual reproduction. The larva produces a new individual out of part of its own body, with no second parent and no egg or sperm involved. The result is genetically identical to the original, and it is a separate animal from that point forward. It has to feed for itself, grow for itself, and get through metamorphosis on its own.

Two things about that are odd enough to sit with for a minute.

The first is that this is reproduction by something that has never been an adult. The larva has not built a test, has not grown a jaw, has not settled into sand and has not spawned. It reproduces anyway, at a stage most animals treat purely as a phase to survive.

The second is that the copies are smaller. A larva that clones is not conjuring new tissue, it is dividing what it already built, so each individual that results is smaller than the one that started. That size change is what the leading explanation rests on, and it is also one of the figures this page deliberately declines to put a number on, for reasons a couple of sections down.

The Leading Explanation: Small Things Are Harder to See

The interpretation this finding usually arrives with is a detection argument, and it is a clean one. If the thing that is going to kill you hunts by sight, the most useful property you can change is your size. Shrink the target and you shrink the distance at which a fish can pick it out of the water.

A larva that responds to a predator cue by splitting into smaller copies has done exactly that. Where there was one object of a given size, there are now several, each smaller than the original, each harder to detect from a distance.

There is a second half to the logic that is worth stating out loud. Because the copies are genetically identical, spreading the same genes across more individuals is a hedge in its own right. A fish that finds one of them has not removed that genotype from the population. In a single move the larva has made itself both less visible and less concentrated.

That is a good explanation. It is also an explanation, which is a different kind of thing from an observation.

The Readings That Are Also Honest

An experiment that shows a response does not, by itself, show what the response is for. The cloning happened. Why it happened, and whether it helps, are separate questions, and there are readings other than the standard one that the observation alone does not rule out.

• Cloning may be a general reaction to stress or disturbance rather than an anti-predator strategy specifically. Fish mucus is a strong biological signal, but it is not the only strong signal an animal could react to.
• Fish mucus says "fish." It does not say "predator." Not every fish eats larvae, and a cue that reliably announces a large animal nearby could be read more broadly than the anti-predator framing suggests.
• Getting smaller has costs. A smaller larva has more growing left to do before it can settle, which means longer in the plankton, which means more total time exposed to the very hunters it is hiding from. Whether that trade pays is a calculation, and this experiment did not run it.
• A dose of a cue in a controlled container is not an ocean, where signals are patchy, diluted and mixed in with everything else.

None of that makes the result shaky. Larvae exposed to a predator cue cloned themselves, and it was published in Science, which is not an easy place to publish. What stays open is the why, and how much good it does.

Who Did the Work, and When

The paper is by Dawn Vaughn and Richard Strathmann, published in Science in 2008. Vaughn followed it with further work on larval cloning in 2009.

Two details in that citation are worth noticing if you are deciding how much weight to give a finding you are reading about secondhand, on a boat company's website.

The first is the journal. Science is a general research journal rather than a specialist one, which means an editor decided the result would interest people who do not study larvae for a living. A larva that clones itself when it detects a predator clears that bar without much trouble.

The second is that there is a follow-up. One arresting result is a headline. A result the same investigator went back to, extended and published on again is the start of a line of work, and it is a fair signal that the first paper was not a one-off oddity.

If you want the primary source instead of a summary, and a summary is all this page is, search the authors' names together with the year. We would rather point you at the citation than at somebody's retelling of it.

The Three Numbers This Page Will Not Give You

Retellings of this finding tend to show up with three numbers attached: how much smaller the clones were, how many of them a larva produced, and how much better they survived. You will not find any of the three here.

That is not because no figure exists somewhere. It is because we have not verified those figures against the original work, and this site does not print numbers it has not checked. A number is the easiest thing in the world to copy and the hardest thing to walk back once it is loose.

If you land on a page that hands you all three in one confident sentence with nothing to check them against, treat it the way you would treat a shell label reading Mellita quinquiesperforata on a Gulf beach: possibly copied from something older, possibly right, definitely unexamined. That label is wrong here for the same sort of reason, and we take it apart on the Mellita page.

The finding does not need the decimals. Larvae met a predator cue and cloned themselves into smaller copies. That is already the strange part.

Nobody Has Tested This on a Gulf Sand Dollar

Here is the qualifier that falls off nearly every retelling.

Dendraster excentricus is a Pacific animal, Alaska to Baja California. The five-slotted sand dollar on our beaches is Mellita tenuis, H. L. Clark, 1940, described from a type locality at Sanibel Island. The notched sand dollar that also occurs on both Florida coasts is Encope michelini. Those are different genera, in a different ocean, in water holding a different set of fish.

No published study tests predator-cue cloning in either of ours. Not a negative result. No result at all.

There are two ways to get that wrong and they are mirror images of each other. One is to assume the Gulf animals must do it because a relative does. The other is to assume they must not, because nobody has found it. Both are statements about data that has not been collected.

What is fair to say is narrower. Sand dollars across the group share a body plan and share the echinopluteus larval form, so it is a reasonable question to ask. Asking a reasonable question is not the same as holding the answer. Until somebody runs the experiment on a Gulf Mellita, this page will keep saying "unstudied," which is a real answer and not a dodge.

What It Changes About the Disc in Your Hand

Every white test that turns up on a Gulf beach was once a nearly invisible thing with arms, drifting somewhere out in the water column, eating algae too small to see, weeks away from having a body anyone would recognize. That much is true of the local animals. It is true of sand dollars generally, and it is worth carrying around with you.

Whether the larva that eventually became the disc in your hand ever met the situation Dendraster's larvae met in a laboratory in 2008, nobody can tell you. The honest shape of what is known runs like this: open-water spawning, a long larval drift, settlement into sand, then years living in a bed. How many years is its own hedged question, and we handle it on the lifespan page, where the best data again comes from the Pacific species and again does not carry across.

The rest of the animal, the test, the petals, the jaw pieces and the honest version of what you may take home, sits on the main sand dollar page and across the Sand Dollar Codex. If you want to see the flats these animals actually live on, that is where we spend most days out of Goodland, and you can book a trip from the home page.