Acanthaster

Related Articles: Sea Stars, Asterina Stars, An Introduction to the Echinoderms: The Sea Stars, Sea Urchins, Sea Cucumbers and More... By James W. Fatherree, M.Sc.

/A Diversity of Aquatic Life

Acanthaster, Crown of Thorns Sea Stars

Bob Fenner

An Acanthaster in Malaysia

Of the several Sea Stars known by marine aquarists the genus Acanthaster is probably the most familiar and notorious for it's celebrated coral-eating destructiveness. This article offers pertinent information on the selection and care of this group of species and some biological notes of interest.

Classification: Taxonomy

Sea Stars aka Starfishes are part of the phylum Echinodermata, the "spiny-skinned animals", a group characterized by pentaramous (five-armed) radial symmetry, a water-vascular system (ambulacral) used primarily for locomotion and a true body cavity (coelom). All are marine with internal calcareous skeletons. About 5,300 described species.

Sea Stars/Starfishes make up the class Asteroidea. The other four living classes should be familiar to you: the Ophiuroidea are the Brittle Stars, the Holothuroidea are the Sea Cucumbers, the Echinoidea comprise the Sea Urchins, and the Crinoidea are the Sea Lilies and Feather Stars. Asteroids are typically pentaramous with arms thickening toward their central disks where their mouths open toward the bottom. Approximately 1,600 known species. Free-ranging over rocky and over and in sandy and muddy bottoms. The five living asteroid orders are divided on the basis of structural differences in their water-vascular systems and ossicles (endoskeletal elements).

Acanthaster ellisii , the Panamic Crown of Thorns Seastar; from the Sea of Cortez to Peru. An A. ellisii in the Sea of Cortez.

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The images in this table are linked to large (desktop size) copies. Click on "framed" images to go to the larger size.

Acanthaster plancii (Linnaeus 1758), the Crown of Thorns Seastar common species in its respective range. Here the two species are shown. A. plancii in Moorea, French Polynesia

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Relation With Other Groups

Acanthaster Stars share many/most characteristics with other asteroids; need for pollution-free water, carnivorous feeding habits, physiology, behavior, embryology... Their big shortcoming/point of interest is their strict coral-eating diet.

Selection: General to Specific

Specimens of this genus are offered from time to time in the trade, or may be specially pre-ordered through dealers. What to look for, or look out for? Behaviorally that the individual is "lively", with tube feet visible, moving, turgid. A good bioassay is to turn the animal on it's back (after acclimation) and watch to determine if it can/does right itself. Other warning signs are dark or whitish, necrotic matter and vacuolations (missing areas). Echinoderms are notable as the only animal phylum with no known parasitic members. They are host to many parasites themselves however.

Of particular import, I'd like to stress that these starfishes are toxic to the touch; yes, I mean it: THEY ARE VENOMOUS.

Acanthasters should be handled only with a net and then only carefully. The spines evident in the accompanying photos are sharp and accompanied with a hemolytic (blood-cell splitting) toxin. This is a similar situation to that posed by the handling of many commercial sea urchins that many people handle haphazardly. Be careful! Here is a close up of a specimen in Pulau Redang, Malaysia... you get my point?

Environmental: Conditions

Habitat

Something nice in the way of adequate space and coral, rocky space and arrangements to behaviorally adjust for light and circulation.

Chemical/Physical

Echinoderms are used as biological assays for pollution, quality of synthetic salt mixes and other as an indication of their sensitivity. They are among the first to be mal-affected by metabolite and/or metallic problems.

Filtration

These stars and all other echinoderms have been maintained under all available system modalities, with natural and synthetic waters. Frequent partial water changes and monitoring are encouraged.

Behavior:

Introduction/Acclimation

Is very simple. Once a suitable specimen has been selected and transported to your site, drip-acclimate it over a half to full hour or so and net into the desired space.

Predator/Prey Relations

Due to obvious spininess and possibly known crunchy toxicity asteroids are generally left alone. Other than the Triton Mollusk Charonia (Pictured) they seem to enjoy about predator-free existence in the wild. I would not put it past certain Triggers, Angels, Puffers, large crustaceans among others to try out stars (or most anything else) out of boredom or hunger. Certain polychaete and other worms are known to feed on Sea stars.

Reproduction:

By sexual means (all echinoderms are dioecious, = two houses; boys and girls), and fragmentation. Hence the lack of logic in destroying individuals by cutting, hammering... Programs have run their course in the P.I. (fancy acronym for Philippine Islands), Australia et al. in mano a mano (hand to hand) collection/removal attempts. Natural mechanisms have prevailed.

Examples of what a scourge Acanthaster plancii can become. Here is a wave of individuals scouring Acroporid beds in the Philippines and about the only effective human intervention to date: sticking them and removing to the land to dry and die.

Locomotion:

Typical water-vascular, optic-cup aided, tube foot crawling/pulling.

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

Like most sea stars Acanthasters are carnivorous. Their feeding mechanism is almost notorious; everting their stomach outside the body over "poor, defenseless" living coral polyps, doing the old extra-cellular digestion right then and there. Oftentimes Crown of Thorns population explosions/"infestations" are attributed to human removal of predators, most notably the Giant Triton Charonia tritonis L. 1758, as well as "Cod" and some large Groupers. There are stop-gap measures in place in several countries restricting the taking of these controlling influences.

Though they seem to prefer certain live coral polyps, Acanthaster are opportunistic omnivores eating algae, encrusting invertebrates, dead fish and other aquatic protein, even other starfishes! They have been trained in captivity to accept prepared and fresh foods in the place of expensive Scleractinians.

Disease: Infectious, Parasitic

Most notably a fungus (Branchiomycetes sp.) and Vibrio bacterial infections are primary sources of infectious disease mortality. Proper selection (see above) and appropriate environment are not all a hobbyist can employ to assure on-going success. The use of a fresh water bath/dip goes a long way in freeing sea-stars (and most all other aquarium marine invertebrates) of undesirable parasitic, infectious and vector matter.

Helwig offers Griseofluvin at 250 mg per twenty gallons as a sure cure for fungal problems and furan compounds and antibiotics for bacterial difficulties. It bears repeating that all such treatments must be administered out of the main bio-system with provision for monitoring/ameliorating water quality.

Once one or all related organisms in the phylum begin dissolving/otherwise dying in the system, one must act very quickly, make that immediately to arrest total wipe-out. Changing 30% plus of the system water, removing affected stock to treatment quarters is strongly indicated.

Close: So my peek at this genus of asteroids goes. Yes, they can be gorgeous, like this purple specimen in the Andaman Sea off of Thailand's western coast... Acanthaster are beautiful and interesting, but dangerous! If you deal with them, handle with care.

Bibliography/Further Reading:

Birkeland, Charles & John S. Lucas. Acanthaster planci: Major Management Problem of Coral Reefs. 1990. CRC Press.

Esterbauer, Hans. 1995. The Crown-of-Thorns Starfish. TFH 11/95.

Fenner, Bob. 1995. Acanthaster, Crown of Thorns Sea Stars. FAMA 1/95.

Fenner, Robert M. 1998. The Conscientious Marine Aquarist. Microcosm, VT. 432pp.

Herwig, Nelson. 1980. Starfish, Sea Urchins and Their Kin. FAMA. RCM Publications.

Kenchington, R.A. & B. Morton. Two Surveys of the Crown of Thorns Starfish Over a Section of the Great Barrier Reef. 1976. Aust. Govt. Publ. Svc., Canberra.

Oliver, A.P.H. 1980. The Larousse Guide to Shells of the World. Larousse and Co., Inc. New York.

Raymond, Robert. Coral Death & the Crown of Thorns Starfish Wars. McMillan Comp. of Aust. Pty. Ltd. Melbourne 1986.

Walls, Jerry G. 1979. The Creepy Crawly Coral Cruncher. TFH 1/79.

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