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Related FAQs: Spiny-Skinned Animals, Sea Lilies, Feather Stars, Sea Stars 1, Brittle & Basket Stars, Sea Urchins, Sea Cucumbers

Related Articles: An Introduction to the Echinoderms:  The Sea Stars, Sea Urchins, Sea Cucumbers and More... By James W. Fatherree, M.Sc. Sea Cucumbers, Seastars,  Brittle & Basket Stars, Crown of Thorns (Acanthaster), Crinoids, Urchins, Survey Articles on Invertebrates, Water Flow, How Much is Enough,

Spiny-Skinned Animals, Phylum Echinodermata

By Bob Fenner

  A Culcita genus Biscuit Star 

Considered as the best characterized and distinctive phylum of the Animal Kingdom, the sea urchins, sand dollars, sea stars, basket stars, sea lilies and sea cucumbers make up the Echinodermata (='spiny-skinned).

Many of these are attempted as marine aquaria specimens. Few specimens survive very long due to being inappropriate, poorly handled or forced into unsuitable habitats. Many species are outright dangerous to the their fellow tankmates and you, the hobbyist!

Whether you "inherit" Echinoderms with "live" rock or plunk down your hard-earned cash on purpose for a specimen or two this article will inform and inspire you re these prickly metazoans.

Classification: Taxonomy, Relation With Other Groups

All members of the phylum are readily distinguished by:

1) The presence of a water-vascular system (tube feet) used in locomotion and food manipulation. Pictured is a close-up of an Urchin, Tripneustes ventricosus, in the shallows of Belize, showing its defensive and manipulating tube feet. And a close-up of the five-rowed arms of a Seastar at S.I.O.'s Birch Aquarium.

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2) A sub-epidermal (internal) microstructured calcareous skeleton. A close up of the surface of a Batstar, Patiria miniata reveals interlocking elements.

3) A pentaramous (5-radial) body symmetry, meaning they can be approximately divided into five equal parts from a central point. All living species share these characteristics; and yet the group has considerable diversity. They are the only phylum with no known parasitic members, though they are often hosts themselves. A red Fromia showing radial symmetry.


For instance? All classes have representatives with a greatest diameter of less than 10 mm! One urchin has a test of over 20 cm (8 in.) diameter and there is a sea cucumber more than 2 meters in length.

There are about six thousand described species distributed worldwide in shallow to deepest seas. Living echinoderms are divided into five Classes (some other schema lump the Brittle and Sea Stars, but we won't). They are:

Class Asteroidea: Sea stars or Starfishes have arms that are supported by elements in the body wall, not separable from a central disc. They usually have five arms though some have six to a large number. This Class is sometimes combined with the Brittle and Basketstars with these two Starfish groups divided as Subclasses by science.

Class Ophiuroidea: Brittle and Basket stars have slender, often branching arms capable of tremendous flexion. These arms have internal structures for support and are abruptly discontinuous from the animals' central discs. Also unlike Seastars, they use their entire arms, not just tube feet for locomotion.

Class Crinoidea: The Sea Lilies and Feather Stars, characterized by having 1) A stalk and being attached to the substrate (at least in the early post-metamorphic stages), & 2) Erect feeding structures (brachioles &/or arms) on their upper surfaces. They have upward and outward extending arms supported by calcareous plates. 3) Of the five extant Classes of Echinoderms, Crinoids are the only ones with their mouths, water-vascular system and anus on the body surface away from the substratum. 4) Semi-rigid bodies restricting them to suspension feeding.

Class Echinoidea: Urchins and Sand Dollars are marked having body walls composed of plates with moveable spines and no arms. Their overall shape is globular to oval.

Class Holothuroidea: Sea Cucumbers bear feeding tentacles extending from their circum-oral feeding ring. They show a wide range of reduction in endoskeletons.

Selection: General to Specific

Here's hoping you're ready for some large to smaller generalizations sure to please few and torque off most all collectors, dealers and their intermediaries. Most of this is reiterated/re-enforced in subsequent smaller taxonomic articles on the group.

Most broad: 

1) Don't buy "new" specimens. Leave them at least a few days at the shop if at all possible. Echinoderms can and do die quickly, mysteriously, without prior indication. Most of the massive mortality with the group occurs within a day or two of arrival. Wait.

2) Avoid at all costs specimens with whitish, necrotic spots and patches. These are most likely infected with bacteria, fungi and possibly debilitating parasites.

3) This may seem contradictory with #2 but don't necessarily disqualify a specimen on the basis of broken spines, or even chipped mouth parts. This group has a renowned regenerative capacity. If the individual is otherwise healthy and placed in a proper setting, it will repair.

Crinoids, Sea Cukes and most Urchin species should not be tried by any but the steady of hand/mind, large of wallet, most careful, serious-most aquarists. Sea Lilies are sometimes expensive finicky eaters... Cucumbers are notorious for "getting agitated" eviscerating (polite scientific term for throwing up their guts, gonads,... you get the idea), otherwise dissolving and taking everything with them in the entire co-mixed system, fast!... Some Urchins are very dangerous to handle as mechanical devices, toxic stinging, venomous, not-so-nice-and-innocuous as they may appear...

Many Seastars are okay. There are some notorious exceptions like the Crown of Thorns (Acanthaster) and a few others. What a fun group, eh?

For the most part this is the group with the most demanding need for high and consistent water quality. No toxic metal concentration. They will be the first to show ill effects and kick.

Environmental: Conditions

Echinoderms are aerobic organisms; although some may stand extended periods of low levels of oxygen or exposure to the air, others do not. Many, many members of this phylum are lost to simple anoxia. Keep their systems well-aerated and circulated.


Some study of the bio-type of your charges is encouraged. Approximating the physical make-up, lighting, circulation et al. will contribute to your success. Such information is accessible through accurately researched periodical literature, hobby, trade and scientific publications available through the pet trade, public libraries and colleges.


As previously alluded to, the term "steno" (=narrow) versus "eury" (=broad), and steady define the tolerance and necessary consistent parameters in keeping most of the commonly available species. Several urchins and stars could qualify for the miner's "canary in a cave" as ready bioassay organisms. Pre-mixing synthetic water mixes, matching such "windows" as pH, KH, temp. are more critical...


These "lesser deuterostomes" are amazing in many aspects. They are the next most closely related group to that which includes (gulp!) us, the chordates. Their embryology, genetic development and outright physiology is very similar to sea squirts, fishes, other notable quadrupeds and the aforementioned mammal.

There is even a swimming sea cucumber. Watch out! And strange goings on with vast numbers of Stars, "walking" Sea Cukes, Urchins et alia spiny suckers virtually covering the abyssal bottoms in some areas of the seas.


Critical. Though they're admittedly slow-moving, Echinoderms can ingest, digest and egest a prodigious amount of material, and excrete a great deal of ammonia in the process. You've heard me and other's state it before. Get and use a protein skimmer!


See above re researching the natural habitat. Most commonly offered varieties are shallower water, more tolerant varieties that do well provided some opportunity to "choose" their own environment behaviorally; that is move to more/less lit, vigorously circulated, aerated...part of their system. Some of the relatively expensive species, like the "Linckia Seastars are quite shy of the limelight, and the whole phylum is more retiring than bold.

Behavior: Territoriality

Behavior: Territoriality  

Spend some time in the library, your fish club and chatting with Fellow-Marine-Sufferers (FMS's) before devoting the big coin. There are some of these animals that mix beautifully with their own kind and anybody elses. There are examples of chemical/physical warfare/engagements the likes of which would make the civil servant pentagon-types feel down-right puny.


A very important time for both of you. As much as I'd like to avoid the lowest ring of Dante's Inferno/Hades (reserved for hypocrites, and rightly so), I must reverse one of my most steadfast rules, dear reader. Yes, this is one of those instances where adding, even mixing (!) shipping water (if perceived to be not outright contaminated) into your system with the new livestock is to be considered.

Allow me to semi-vindicate myself from this break with "thou shalt not..." demagoguery. There is a whopping lot of evidence that echinoderm water and presence have a therapeutic effect on other organisms and the system itself. Hey, this isn't hocus-pocus, I'm serious.

Predator/Prey Relations

As Ricky Ricardo might put it, "these thins will eat everythin' and anythin', bobbalooo". They are nature's surest answer to "who's going to clean up"?. Otherwise it's the same old song of tropical marine relations; whose going to eat who first.

Burrowing forms don't appear obvious to wanna-be predators. Exposed forms are too crunchy, spiky, poisonous (eat), venomous (touch), odd-shaped to consume. There are fishes (triggers, puffers, wrasses and basses...) crustacea (large crabs, lobsters, some notable star-eating shrimp), and large marine snails that will try almost anything.


Is almost exclusively sexual with individuals being male or female. They produce small eggs, broadcast into the sea, external fertilization, turning into planktotrophic larvae, or brooded (internally or externally) with direct development.

Asexual reproduction can occur either through parthenogenesis, fission or autotomy. The last two are a matter of degree of breaking into two more or less equal pieces (in the Ophiuroids, Asteroids, Holothuroids) or just a fragment (as celebrated so oft in the Asteroids) in the latter.

Parthenogenesis may be familiar to you as recounted at times concerning brine shrimp and "Amazon" Mollies. It is a form of reproduction in which eggs develop without fertilization; that is without union with a sperm cell... this is thought to have survival value in situations of low fecundity &/or low population density. Understanzee? Gute.


These critters are not going to win any races. Perhaps as a function of their radial symmetry (or secondarily derived bilateral symmetry in burrowing Urchins and Cukes O' the Sea), Spiny-Skinned Animals are slow. Most of the Sea Lilies, being attached, are at a dead stop. All other classes get about by means of tube feet and pushing with the help of their spines, as present. Some Sea Cucumbers approximate an undulating swimming at, and for short spurts, above the bottom.

Feeding/Foods/Nutrition: Types, Frequency, Amount, Wastes

Unlike groups of organisms with a rigid exoskeleton, like insects and crustaceans, the endoskeleton of echinoderms allows them to actually re-sorb (shades of osteoporosis!) calcium, getting smaller and even changing shape. Know that if insufficient limestone (calcium carbonate) is not provided intentionally in/as food or as substrate (coral et al. skeletons, gravel...) your specimens will not grow and may shrink!

Feeding mechanisms vary: Crinoids are basically passive suspension feeders relying on environmentally produced water movement. Capture of food items is via tube feet.

Sea Stars, Brittle Stars and Urchins, all having their mouths and water-vascular system facing the ground, allows them to take advantage of benthic food sources.

Basket stars

Basket stars sweep their slender arms around and over the substrate. Their tube feet, like Crinoids, lack suckers. Some are predatory and carrion-feeding carnivores, the remainder are microphagous detritivores, feeding on small particles in/on the bottom or wafting about in the water column.


Asteroids are profoundly affected by the relative immobility of their more tightly secured arms. This is due to skeletal support that unlike the Basket Stars, whose is internal, is in the body wall. Therefore Seastars must move to get themselves to food. They feed on macro-prey. Their suckered feet are not necessary so much to capture or manipulate prey as they are to scale vertical impediments and resist wave/current energies. Probably the most renowned aspect of Seastar biology is extra-oral feeding with the pushing out of the "stomach" into the environment into/over prey (Geez, I could eat the whole pizza).


Urchins feeding involves the use of inter-digitating mouth parts called the "Aristotle's lantern". This is an arrangement of 5 teeth, muscles & supporting articulating structures. They feed on benthic organisms, drift food and carrion.

Holothuroids, aka Sea Cukes 

Holothuroids use their feeding tentacles extending from an anterior circum-oral feeding ring to "mop-up" particulate food either in suspension or deposited, or to capture free-living prey.

If your spiny livestock doesn't seem to be super-interested in feeding don't be overly concerned. Feeding rate is influenced by physical (salinity, temp., light) and biological (size, physiological state, food preferences, quality and size of food) variables. There are instances of many months of apparent non-feeding. As previously noted destruction of sections of the animal including mouth parts does not necessarily spell doom.

When/where in doubt (Ubi dubi?) make more algal food available; soft or calcareous, fresh or prepared.

Disease: Infectious, Parasitic

There are two types of wipe-out infections that are common. One is bacterial, the other fungal. Both are avoidable more than treatable by careful collection/selection and meticulous maintenance procedures. Specific actual treatments are covered in the sub-phylum pieces that follow. Most therapeutics are to be avoided for the individual hobbyist as being dangerous, worthless and economically inappropriate.

Steer clear of specimens with obvious bulges and vacuolations (depressions, gaps). The group as a whole sponsors a large host of internal/external symbionts/mutuals/parasites.


What a group! And you thought they were dumb and backward. Be aware and careful with this phylum. As you now understand a little better, they are interesting but little bothered in the wild for good reasons.

Bibliography/Further Reading: See individual Classes and sub-divisions, species of echinoderms coverage

http://home.att.net/~ophiuroid Marine Hitchhiker/Critter ID (Maughmer, Toonen, Tompkins)

Erhardt, Harry & Horst Moosleitner. 1998. Marine Atlas, v.3 Invertebrates. MERGUS, Germany. 1,326pp.

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