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An Introduction to the Echinoderms:

The Sea Stars, Sea Urchins, Sea Cucumbers, and More...

An Introduction to the Echinoderms:
The Sea Stars, Sea Urchins, Sea Cucumbers, and More...

By James W. Fatherree, M.Sc.

What on Earth could Sea Stars, Brittle Stars, Serpent Stars, Basket Stars, Sand Dollars, Sea Biscuits, Sea Urchins, Sea Lilies, Sea Cucumbers, and Sea Apples all have in common?  They're all Echinoderms, that's what.  Thus, they are all members of the biological group known as the Phylum Echinodermata (Latin for “spiny skin”), which is comprised of over 6,000 marine species, and are all cousins of sorts.  Many of them may seem as different as night and day, like a Sand Dollar and a Serpent Star, but as you'll see in a minute, they aren't so different in a few fundamental ways.  We'll take a look at just how much they really are alike.

Similarities aside, the differences between them have led to the division of the phylum into five living classes.  So, while they are all cousins, we can call various sorts of them either Asteroids, Ophiuroids, Crinoids, Echinoids, or Holothuroids.  We'll take a look at all of these separately, too.

There are WAY TOO MANY  Echinoderms available to hobbyists to go through the habits, diets, and care requirements of all of them, or even all of the popular ones, but I'll give you enough information to gain a good understanding of what it takes to be an Echinoderm, and the general characteristics of the classes.  I'll also make note of the various sorts that you should probably avoid trying to keep in aquariums.  So, let's get started...

The Things They Share

Various Echinoderms obviously come in an amazing variety of forms considering they are all cousins.  However, if you know what to look for you can usually spot a few physical characteristics that are typical of the phylum's members.  What you'll find is that they really do share a lot of features, despite their varied shapes and lifestyles.

This urchin test clearly shows penta-radial symmetry.  The dark bands divide the test into five equal sections.  Also note that the test is made of fused calcite plates, many of which have attachment points for spines.

The first thing to note is the symmetry that the living Echinoderms share.  While we'll see some exceptions in a moment, in general one of the easiest things to note about Echinoderms is that their body parts are arranged around a central axis.  It doesn't matter if they have arms/tentacles or not, they still have a round or nearly round body with body parts radiating from its center in an orderly fashion.  This is called radial symmetry and it is the same body plan that the members of the Phylum Cnidaria have (corals, anemones, jellyfish, etc. are all cnidarians).  Just picture a Sea Star's or Jellyfish’s body and the numerous arms/tentacles radiating from the center.  You could cut up either into pie-shaped pieces and all the pieces would be roughly identical.  Conversely, we humans exhibit bilateral symmetry, which is characterized by having a left side and a right side that roughly mirror each other and that don’t radiate from a round central axis.

On top of developing a radial form, an Echinoderm’s body is also divided into 5 (or a multiple of 5) roughly equal parts.  Thus, the Echinoderms are properly said to have five-fold or penta-radial symmetry rather than just radial, making them distinct from the Cnidarians, which have an even number of body divisions (like the 6-fold hexacorals, which have hexa-radial bodies and 6 tentacles, or a multiple of 6, or the 8-fold octocorals, which have octo-radial bodies, etc.).  


Exceptions to Penta-Radial Symmetry

Photo James W. Fatherree

Like so many different sorts of organisms, there are exceptions to the rules with Echinoderms.  For example, there are quite a few Sea Stars that have 6 or 7 arms (or some other number that isn't a multiple of 5).  The reasons for these anomalies aren't known, but they aren't mutations of Penta-symetric species.

This Sea Star is a great example of this exception.  It has seven arms, which is normal for this species.


When looking at the underside of a Sea Star,  hundreds of tube feet can be seen emerging from the grooves that run down the arms.  You can also see this star's eversible stomach protruding a bit in this photo.

Ehinoderms also share a unique body system called the water-vascular system, which no other group of animals possess.  This is a complex system of muscles, canals, pouches, bladders, tubes, and suckers that allow Echinoderms to move around and to eat.  You can't see it on the inside of an Echinoderm without opening it, of course, but if you take an up close look at any Sea Star , you’ll immediately see that they have lots and lots of suction cup-tipped “tube feet” that emerge from the grooves on their undersides. Through coordinated movement of these tube feet, the owner can move about surprisingly quickly.  They can also grab hold of a seemingly impenetrable clam and open its shell wide enough to make a meal of it, or climb right up the side of your aquarium.  Likewise, other Echinoderms use their tube-feet, or similar extensions of the water-vascular system to capture planktonic foods, or to burrow through substrates, etc.  Again, it doesn't matter what they look like, they all have a W. V. S.

Echinoderms are great at regenerating body parts.  This unfortunate Sea Star lost three of its five arms in a run-in with a powerhead intake, yet it managed to survive and eventually re-grow all three missing arms.  *Note that in this extreme case, as I did do some hand feeding

While it is not an exclusive Echinoderm trait, they are also well-known for their ability to regenerate damaged or lost body parts.  Sea Stars, Brittle Stars, Serpent Stars, and the likes are typically predated upon by a wide variety of animals, but in many cases the prey can survive and recover.  All of these can survive if one or two, or sometimes even more arms have been lost, as they can re-grow completely new ones in as little time as a few weeks.  Also, like so many lizards that drop their tails on purpose, some Echinoderms can even shed a wriggling arm intentionally to distract or confuse a predator while they escape to shelter.  It'll grow back.

Lastly, Echinoderms share one more key characteristic, although like their ability to regenerate body parts it is not exclusive feature.  They all have a skeleton of some sort which is composed of the mineral calcite and is covered by an epidermis (outer skin).  In the case of Sea Stars and other similar Echinoderms, the calcitic skeleton is made up of thousands of little plates which are held together by special connective tissues that can change from very soft to very firm.  This combination allows the owner to be very flexible when the tissues between the plates are loose, or to be extremely tough when these tissues stiffen up.

Other Echinoderms like Sea Urchins and Sand Dollars also have a skeleton made of plates, but in their cases the plates are fused together to form a shell which is properly called a test.  If you look at a cleaned up test of either, you’ll see that the whole thing is comprised of these plates which have grown together.  In fact, they are joined in much the same way as the plates that make up your cranium.  Permanently bound by fusion joints and very strong.

In still other Echinoderms like the Sea Cucumbers, the skeleton is rudimentary, being made up of nothing more than a number of tiny, oddly-shaped calcitic plates which are embedded in their thick skin of connective tissue.  Again, in this case the combination allows cucumbers to become stiff as a board on demand despite the fact that they are usually rather "squishy."

That covers most of the features that define the Phylum.  So, you should now have a good idea of what it takes to belong in the Phylum Echinodermata, and we can take a closer look at the 5 individual classes.  Onwards!

The Class Asteroidea

We’ll start with the most popular first... This class contains all of the true Sea Stars (which we all call Starfish despite the fact that they obviously aren't fish) which are easily recognized as having a relatively thick body that spreads out into arms.  All of them are also mobile, and almost all of them have 5 arms.  But, as I mentioned before there are also number of exceptions with some Asteroids may have as many as 40 arms!  They move about by using the tube feet on the undersides of their arms, but they don’t really use the arms themselves for crawling.  In other words, the arms tend to stay straight out, even when they are on the move.

Like the rest of their relatives the Asteroids can find food by chemical detection.  They "smell" things nearby, and can even find prey that is buried well below the surface.  Many are simply opportunistic scavengers that crawl from here to there, sniffing the water and running into scraps of food, algal mats, or an occasional whole deceased animal.  Others are active predators that will attack and eat clams, snails, sponges, corals, anemones, and just about anything else they can get a hold on with their tube feet. The majority are therefore not invertebrate/reef-safe by any means.

Sadly, dried specimens like this one are an all too common sight at beach town souvenir shops.  This is the top side (left) and bottom side (right) of a typical Sea Star, with a thick body and arms.  Note the prominent grooves in the arms, where the tube feet emerge.

What's really neat about these critters is how they eat once they find a meal.  Almost all Asteroids have a relatively large, eversible stomach that they can extend well beyond their mouths. The stomach is made of very thin material that is folded in an orderly fashion and packed in the body, but when needed it can be unfolded to digest food outside of the body itself.  Some Asteroids will simply unfold their stomachs over a rock or soft bottom and digest anything growing on it.  The common Chocolate Chip Starfish is a good example of an Asteroid that does this, because they can often be seen feeding over an area of glass in an aquarium where algae is growing, simply digesting the algae right off the glass.  Of course they can also scavenge leftover fish food and wastes off the bottom and I’ve even seen seemingly clever ones turning upside down and everting their stomachs directly in front of a pump or filter return so that there is a constant flow of water (and food particles) flowing over the digestive tissue.

There are also real predators in the group which are best known for using their tube feet and eversible stomachs to eat various shellfish.  The Sea Star gets a good grip on both halves of a clam, scallop, or oyster with its arms and suckers and slowly pulls the victim open just enough to slip in its stomach.  The stomach then begins to digest the prey while it is still alive and as it gets closer to death and loses strength the shell will open more and more.  Eventually the shell will open fully and everything inside will be eaten.

A few other Asteroids are suspension feeders.  Oddly enough these Sea Stars catch plankton from the water for food.  These unusual Asteroids produce a coating of mucous on their upper surfaces that traps food in flypaper fashion, after which the food-laden mucous is reeled into the mouth on the underside and eaten.  Odd indeed!

What's important to remember is that most of the Sea Stars available to hobbyists are scavengers and/or predators, meaning they are best left out of reef tanks unless you are absolutely sure that the particular species you are interested in is okay (yes, homework).  However, some smaller varieties do well in non-reef aquariums without needing any specific care, while larger ones need only an occasional feeding.  Most will eat small bits of meat placed nearby on the bottom of the tank, or the animal can be placed on top of the food.  Care must be taken to prevent the food from being snatched by fishes before the Sea Star goes to work on it.

The Class Ophiuroidea

This class contains all of the Brittle and Serpent Stars which are generally (and incorrectly) thought to be nothing more than skinny Sea Stars.  The beautiful and unusual basket stars are also Ophiuroids, and all together these three make up the largest group of all the Echinoderms.  While they may superficially look like Sea Stars at first there are actually a few fundamental differences between the Ophiuroids and the Asteroids (which is why they are different classes, of course).  So, let's take a look.

First off, Ophiuroids have long thin arms that are clearly distinct from the body, or disc, which is typically rather small and somewhat flat compared to that of an Asteroid.  The Brittle Stars and Serpent Stars are also restricted to having only 5 arms which are used for locomotion.  In addition, unlike the Asteroids the Ophiuroids don’t use their tube feet for locomotion, but literally crawl around using the arms themselves.  This gives them considerably more speed than Asteroids, and some can even move their arms quickly enough to make it off the bottom and swim a bit.

The Ophiuroids also lack parachute-like eversible stomachs and thus cannot eat clams or many other things that Asteroids can either.  However, many are still successful scavengers and predators that eat a variety of worms, snails, crustaceans, and small fishes.  Some can even use their arms to hold their disc off the bottom while they sit and wait for a small fish, or other prey animal to swim or crawl under it.  The trap is then sprung and the arms close down and quickly move the disc downward onto the prey.  The victim thus ends up under the mouth where it is consumed.

Others are deposit feeders that move around over the bottom picking up bits of this and that, while others burrow through the sediment extracting what they can.  Still others, like the Basket Stars are filter feeders that open their unique thin arms into the current and snare anything that bumps into one of them.  Anything from large plankton to small fishes are caught this way, and food bits are subsequently passed to the mouth and eaten.

Brittle Stars (Left) and Serpent Stars (Right) both have small bodies and long slender arms.  Brittle stars are easily identified by the spines or other adornments on their arms.  Serpents have smooth arms that are rarely covered by projections.

Again, if you know what to look for it’s usually pretty easy to tell the three basic types apart. Many Brittle and Serpent stars may initially appear to be very similar, but can easily be distinguished by the general lack of fancy projections from the arms of a serpent star.  The arms of Brittle Stars are much “fancier” and may be covered with numerous spines, spikes, and/or clubs, while those of a Serpent Star are relatively smooth and are typically undecorated.  They often look almost like snake skin up close.  Many aquarists and dealers incorrectly use the terms "Brittle" or "Serpent stars" interchangeably, so beware of confusion.  To add to the occasional confusion, there are actually a few Ophiuroids that look like they are somewhere in between Brittle and Serpent Stars with a row or two of relatively small projections coming off the arms.


Basket Stars are beautiful ophiuroids with unique branching arms, but they aren't well-suited for aquarium life.  This one is just starting to unroll its arms, the complex branching of the arms is obvious and distinguishes it from Brittle Stars and Serpent Stars.

Basket Stars on the other hand, have exceptionally long, thin arms which are branched, branched again, and then branched again, etc., and form an efficient net to capture food.  Sometimes the arms are tightly rolled up, but when dinnertime comes they are unfurled to cover as much area as possible.

As far as keeping these fascinating animals in aquariums, Brittle Stars and Serpent Stars may be fine, but Basket Stars are off limits.  Because they live on a diet of suspended plankton and other small critters they are not well-suited for life in captivity.  Even if you use "plankton-in-a-bottle" or some other means of providing suspended foods, they are not likely to live for long.  Brittle Stars, on the other hand, are great additions to non-reef and reef tanks because they can usually find enough food for themselves, while not bothering anyone else.  Many of the burrowing varieties are also prized for their use in reef tanks that utilize deep sand beds due to their constant churning and cleaning of the upper layers of sand.  [Editor's note:  Some authors assert that burrowing stars prey on beneficial sand critters while only turning over and cleaning the sand as a coincidence.]

A word of caution: many Serpent Stars have a habit of getting big.  There are large species that typically come in orange or red that can grow to be a foot across, sometimes even more, and they get hungry!  These may not eat anything else that you keep in your reef tank, but then again they might depending on exactly what you have around.  Some can catch fish, and many others will gobble up small crabs, shrimps, and snails if given the opportunity.  I had a very large green one that would immediately come out from underneath the coral head it called home at feeding time, then stand on two arms and prop itself up on the front pane of the tank waving the rest of its arms for food.  A few sinking shrimp pellets were usually all it took to keep it happy and growing, but then I did come up with a missing damsel every once in a while, too.  Regardless of its diet, I wouldn't have put it in a reef tank anyway, for fear that it might bowl over anything and everything in the tank!  So, it is obviously important to use a little common sense when deciding what’s safe and what’s not in a reef tank.  Yes, more homework before you buy.  Not after!

The Class Crinoidea

Much like the Asteroids and the Ophiuroids, most Crinoids are free-living animals that have a central body with numerous arms radiating from it.  Crinoids however, have far more arms than their cousins and these arms are exceptionally thin and are covered with tiny branches. Their plume-like arms have also led to another common name "Feather Star." There are also others that unlike the free-living species are attached to the bottom by long stalk-like structures. This shape gives the impression that they are some sort of large sea-dwelling flowers, and earns them the common name "Sea Lily". 

Free-living or not, all of the Crinoids are filter-feeders and their arms and the branches that run their length are covered with tube foot-like extensions which do the food collecting.  Particles are then passed down the arms to the mouth and ingested.  These arms are long and very flexible, but the body (called the calyx, like the base of a flower) is made of fused calcite plates that are arranged in five-fold geometric patterns, giving it strength.

Crinoids have unique arms designed for filter feeding.  Unfortunately, these interesting creatures are unlikely to survive long in an aquarium.  Photo by Anthony Calfo

On the underside of the calyx, free-living varieties have a ring of short arms that are used for crawling around and for holding on to surfaces.  However, they can also actively swim about by rhythmically waving their arms.  They are also typically nocturnal and will come out after dark to perch upon a coral or other structure.  Then they spread out their arms perpendicular to the current to feed.  The stalked species, whose feeding parts sit high off the bottom also turn their “plumage” into the current to feed, but are restricted to one spot.

Unfortunately, as is the case with the Basket Stars, it is this dependence on suspended food that makes Crinoids generally unsuitable for aquarium life.  In every case of the half-dozen or so hobbyists that I know personally that have tried to keep them alive in reef tanks, all have eventually failed.  I admit that even I tried one and did everything I could, but that wasn't enough.  All have died within a few weeks to months (the Crinoids, not the hobbyists!).  I know there are at least a few people out there that have kept them alive for extended periods, but these cases are very infrequent, and I suggest that you leave Crinoids in the sea to live out their days.

The Class Echinoidea

On to something else, and some happier memories.  Unlike the Asteroids, the Ophiuroids, and the Crinoids, Echinoids have no arms.  These are better known as the Sea Urchins, Sea Biscuits, and Sand Dollars, and while they still have five-fold radial symmetry, tube feet, and a calcite skeleton made of solidly fused plates – they’re all body.

This Long-Spine Sea Urchin is a typical example of a regular Echinoid.  Underneath all those spines is a penta-radial test.

Echinoids can be loosely split into two groups:  the urchins, which are covered with relatively long spines and live on the surface of the seafloor or substrate, and Sand Dollars and Sea Biscuits which don’t have long spines and live under the surface of the seafloor.  Instead of long spines, Sand Dollars and their fat cousins the Sea Biscuits are covered with numerous tiny moving structures that make them look like they have a scruffy beard growing over the surface of their tests.  These, with the help of short tube feet, help them to burrow through soft sediments where they feed on buried organic material.

Sea Urchins typically have relatively long spines that cover their bodies and don’t burrow.  Because they exhibit radial symmetry they are referred to as regular Echinoids.  Their bodies are radial as can be and they move in any direction they wish with no preference for which side goes first.  They also typically feed on algae rather than buried organic matter – which would be rather difficult to get to with all those spines.  They also have tube feet with suction-cup tips that are longer than the spines, allowing them to reach out and grab things beyond the spine tips.  An urchin’s long spines are usually sharp, (although there are several exceptions) and are attached to the test in a ball-and-socket fashion with muscles at their bases.  This allows all of the spines to be rotated in one direction or another to best defend against a predator, while at the same time, allowing the urchin to maneuver itself around obstacles or in a confined space.


Exceptions to the Radial Symmetry Rule

This Sand Dollar (Left) and Sea Biscuit (Right)  are both irregular Echinoids.  These bleached tests make it easy to see that they still have five sections even though they have become a little elongated and bilateral.

Some Sand Dollars and Sea Biscuits are exceptions to the radial symmetry rule.  One of the unusual things about the bearded echinoderms is that they actually have bilateral symmetry like you and I.  It's superimposed on their radial symmetry and these critters are thus called the irregular Echinoids.  If you look at their lifestyle and structure, it's not hard to understand why, though.  Most animals that have a preferred direction of motion (whether flatworm, shrimp, snake, elephant, or  Sea Biscuit) also have bilateral symmetry with an anterior end at the “front” and a posterior end at the "rear."  Since these echinoderms live by moving in a preferred direction through the sediment instead of aimlessly roving around over surfaces, they have become somewhat elongated in one direction and thus have two equal halves instead of being perfectly round.  They still have five-fold bodies that can be seen very easily upon inspection, so their form is indeed quite unique.   


Despite their spines, Sea Urchins can still move around just fine as they have tube feet that can reach  beyond their spine tips.

As far as keeping Echinoids goes, yes for Urchins, but no for Biscuits and Dollars.  Irregular Echinoids get fairly big and depend on lots of buried organic material for food, and that’s something that you should hope to never have in your aquarium.  So, like some of their Crinoid cousins they should be avoided due to their specialized diet.  Many reef hobbyists make the same recommendation concerning regular Echinoids.  Most eat algae of all sorts and can be kept in non-reef or reef tanks, as long as there is algae growing in the tank or added by the keeper, but many will also eat your prized coralline algae.  So, if you put an urchin in your reef aquarium full of coralline-encrusted live rock you are likely to find interesting bright white tracks winding over surfaces where the Urchin has scraped away a meal.  Still, in a big tank the damage is typically minor enough that you don’t have to worry about it too much.  Again, be sure to do some homework and find out what you can about a particular species.

The Class Holothuroidea

All right! I saved the strangest for last. Now we get to the Holothuroids, commonly known as Sea Cucumbers.  Again, unlike the Stars and Lilies these have no arms.  Instead, they have a number of highly-branched fleshy tentacles that can be extended from their oral end.  They crawl on their sides, too.  "Huh?" you say. Whereas Sea Stars and such have a "top side" and a "bottom side" where the mouth is found, Holothuroids have their anus at one end and mouth at the other.  They are stretched out from end to end, from the mouth to anus rather than from side to side from arm tip to arm tip.  Thus, in a sense they really are crawling on their sides when they move around.

Not much to look at, but a useful scavenger.  Most common sand- sifting Sea Cucumbers look more like feces than cucumbers, which has earned some varieties the common name "donkey dung."

Despite their elongate shape, Holothuroids do have five-fold symmetry. When these animals are observed from the “end” looking along the length of the body, it becomes apparent that they have 5 (or a multiple of 5) body sections or rows of tube feet that run the length of the body.  As was mentioned earlier, the skeleton is also quite unique in that it has been reduced to nothing more than a few small calcite rods, spines, and plates called ossicles that help to give some extra support to their thick leathery skin.

These are the elaborate filter-feeding tentacles of a Sea Apple.  Even though these Holothuroids are colorful and interesting, their feeding requirements make them poorly suited to aquarium life.

The Holothuroids can also be placed into two basic groups.  There are those that get their food from the sediment and those that filter feed from the water column.  The majority live in the sediment or right at the surface and feed earthworm-style by forcing sediment through their digestive tract, or by picking out bits of this and that with their tentacles, but the rest are filter feeders that use their 10 to 30 highly-branched tentacles to snare floating food particles.  The tentacles are then pulled into the mouth and slurped clean and re-extended afterwards, one at a time. 

Unfortunately, like the other filter feeders, these may have a very hard time finding food in an aquarium and will not fare well without special care.  Many of these are marvelously colored and are irresistible to unknowing aquarists who won't be able to keep them alive.  They are also sold as Sea Apples, rather than Sea Cucumbers.  So, to sound completely like a broken record...  research your Echinoderm choices before purchase!  This is the only way to separate the safe and useful choices from those that are dangerous or doomed to starve.


Calfo, A. and R. Fenner.  The Natural Marine Aquarium Series: Reef Invertebrates, An Essential Guide to Selection, Care, and Compatibility. Pennsylvania:  Reading Trees, 2003.

Barnes, R.D. and  E. E. Rupert.  Invertebrate Zoology, 6th Ed. Texas:  Saunders College Publishing, 1994

WWM about Spiny Skinned Animals

Related Articles: Echinoderms, Sea LiliesSea Stars, Brittle StarsSea Urchins, Sea Cucumbers,

Related FAQs: Sea Lilies, Feather Stars, Sea Stars 1, Brittle & Basket Stars, Sea Urchins, Sea Cucumbers


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