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Related FAQs: Sea Stars 1, Sea Stars 2, Sea Stars 3, Sea Stars 4, Sea Stars 5, Brittle Stars, Seastar ID 1, Seastar ID 2, Seastar ID 5, Seastar ID 6 & Seastar Selection, Seastar Compatibility, Seastar Behavior, Seastar Systems, Seastar Feeding, Seastar Reproduction, Seastar Disease, Seastar Disease 2, Seastar Disease 3, Star Disease 4, Star Disease 5, & Asterina Stars, Chocolate Chip Stars, Crown of Thorns Stars, Fromia Stars, Linckia Stars, Linckia Stars 2, Sand-Sifting Stars,

Related Articles: Echinoderms, An Introduction to the Echinoderms: The Sea Stars, Sea Urchins, Sea Cucumbers and More... By James W. Fatherree, M.Sc. Brittle Stars, Asterina Stars, Crown of Thorns Seastars, Marine Scavengers,

Sea Stars, Class Asteroidea

part 1 of 11

To: Part 2, Part 3, Part 4, Part 5, Part 6, Part 7, Part 8, Part 9, Part 10, Part 11

By Bob Fenner

Don't touch! Acanthaster plancki

Sea Stars: Completely marine echinoderms of obviously overall pentaramous symmetry. Live oral side down, feeding on a myriad of sedentary invertebrates and detritus, detrital infauna.

Consider the Reticulated Seastar (Oreaster reticulatus), a large starfish common in the Turtle grass beds of the tropical west Atlantic. Growing to twenty inches in diameter, it consumes mollusks, even oysters in heavily calcified, tightly shut shells, methodically and with a voracious appetite. Like others in this fascinating Class, the Reticulated Seastar possesses a cleverly evolved arsenal of hydraulic tube feet connected to an elaborate water-vascular system that encircles the animal's mouth and extends via five radial canals down the center of each arm. Below: an Oreaster in Belize's shallows, an individual at night off Cozumel, and an aquarium specimen.
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Numbering in the hundreds on each arm, tube feet can attach to a food object, such as an oyster or mussel, and with relative ease, pry open even the most defensively tightened set of bivalve shells. With even a mere crack of an opening available, the Sea Star can force its slippery stomach into the shell of a mollusk. There it secretes digestive enzymes that rapidly turn the animal's flesh into a puree that the Sea Star promptly absorbs. Momentarily satisfied, the asteroid retracts its stomach, releases its grip, and glides away, leaving an intact set of bivalve shells stripped as if an alien force had cleaned them, leaving no evidence of forced entry.

Not for nothing have the invertebrates been called "spineless wonders". Some species of Sea Stars can make fascinating and appropriate aquarium subjects, and many of the Brittle Stars as well, can serve as energetic, if cryptic scavengers in reef systems.

Orange Marble Starfish (Fromia monilis): boldly appealing and amongst the most appropriate species for marine aquarium systems that lack big predators such as Triggerfishes and large crustaceans. Variation in Gili Air, Lombok, Indonesia.


Sea Stars and Starfishes make up the Class Asteroidea. Asteroids typically have five arms radiating outward from their central disks where their mouths open toward the bottom. All have podia or tube feet projecting down along the grooves on the undersides of the arms. There is no brain as such, only one or more rings of nerve tissue surrounding the esophagus to lend some coordination to the animal's movements.

Tube feet close-up. The ambulacral system in action.
Approximately 1,600 species of Sea Stars are known. They are found free-ranging worldwide in marine environments; over and under rocky, sandy and muddy sea bottoms. The five living asteroid orders are divided on the basis of structural differences in their water-vascular systems and ossicles (endoskeletal elements). A close up of the surface of a Batstar, Patiria miniata reveals interlocking elements.

"Your number of arms may vary". Akin to gas mileage estimates, there are Seastars with 5,6,7,8,9, 11,13,15,22,27 and more/less arms, and even a variable number per species. Here's a "handy" species: Pycnopodia helianthoides, a Sun Star or two off Baja California, Mexico. Up to two feet across, and an eating machine!

What makes a Seastar go? It's (and other echinoderms) "water vascular or ambulacral system"... a series of interconnected channels, tubes, valves and pumps that coordinate to move the animal, some with such strength that they can pries shellfish apart. Here is a close-up view of the aboral (top) side of a Fromia star showing the Madreporite (circular, off-center, light-colored), and anus.


Specimens of several genera are commonly offered in the trade, and more unusual species may be special ordered through dealers. What to look for or look out for? The individual in question should be lively, moving and turgid-bodied, with tube feet visible in the case of Sea Stars. A good test is to turn the animal on its back and see if it rights itself. A limp or weak individual is a poor aquarium prospect. Some may eventually recover, but many do not. Most losses of these, and other spiny-skinned animals is subsequent capture and handling trauma.

Other warning signs are dark or whitish necrotic matter and vacuolations (missing areas). Lost arms are common and will eventually heal over, but it is unwise to buy a specimen that is freshly wounded as infection and rapid decline may follow. Note: not at all rare are "comets", detached single arms that are regenerating new bodies. This is seen as a large arm with a small body and a set of small arms at one end. These are often very good specimens.

These Linckias are healthily active, with exposed tube feet actively holding on to the side of their tank. The discolored, sunken area evidenced by looking at the oral side of this Sea Star portends trouble. Don't be put off on purchasing a Sea Star or Brittle Star with missing, or smaller limbs if they're otherwise healthy.
A warning sign. This Seastar (in the wild) is exhibiting a likely internal parasite... NOT likely to live in captivity.

Echinoderms are notable as the only animal phylum with no parasitic members. They are hosts to many parasites themselves, however, particularly copepods and gastropods. You should check a prospective buy for any attached or obvious internal parasitic problems.

Some Fave Groups, Species for Marine Aquarium Use:

The most common error in selecting Sea Stars is acquiring species that get too large or are ravenous omnivorous predators. Not only will many species attack various types of reef invertebrates and fishes, they often can't get enough to eat even this way! Unless you are willing to make a special effort to house and feed the larger, predatory species, it is best to star with "reef safe" choices. Among the industry favored species are the very attractive Sand Sifting Star, Archaster typicus, the Little Red Starfish and Orange Marble Starfish (Fromia elegans and Fromia monilis respectively), Blue Starfish (Linckia laevigata), and Purple "Linckia", Tamaria stria. A note of clarification here: these are not necessarily the best suited species for aquarium use. "Regular" Linckias/Linckias in particular are generally short-lived. The genera and species below are our choices for most suitable.

Good looking, hardy and utilitarian, a Sand Sifting Star, Archaster typicus Müller & Troschel, 1840. As with dealing with all sizable burrowing animals, make sure your rocky habitat is securely placed on the bottom of the tank (not the substrate). And beware re stocking these subterranean sifters... they are rapacious feeders on all interstitial fauna... denuding systems of only hundreds of gallons. NOT to be used in reef systems where you want to have good populations of these beneficial organisms. <Might also note that, because these animals do denude even large systems of all interstitial fauna (their food source) they almost always eventually and inevitably starve to death. -Sara M.>

To: Part 2, Part 3, Part 4, Part 5, Part 6, Part 7, Part 8, Part 9, Part 10, Part 11

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