Coral reefs are constantly being built up and simultaneously torn down, but Crown of Thorns Starfishes throw that equation out the window. Left to their own devices during population outbreaks, Acanthaster planci devour coral polyps and devastate reef habitats. Herewith: Crown of thorns starfish facts.
IF CROWN OF THORNS STARFISHES HAD A THEME SONG, it would be Darth Vader-type music. So destructive are these rogue echinoderms that their very name is synonymous with bad news for coral reefs.
Even their appearance, even in good times, is menacing. Not only do their stiff, prickly “thorns” hurt if they jab a victim, they’re imbued with a venom that adds to the jabbee’s pain.
CROWN OF THORNS STARFISH FACTS 1: EVERYONE HAS A GOOD SIDE
Still, it would be wrong to see crown of thorns starfishes – COTS to their friends – as entirely evil. They do devour coral polyps they extract from their calcium carbonate shells, but a healthy reef is a dynamic of simultaneous growth and destruction. In normal times – that is, when they’re present in limited numbers – COTS play a beneficial role in maintaining reef diversity.
Unfortunately, COTS populations periodically break out in massive densities that can overwhelm and decimate a coral reef. Scientists spend a lot of time and energy working on controlling COTS populations. They’re particularly destructive of branching and table corals.
CROWN OF THORNS STARFISH FACTS 2: INDO-PACIFIC BASIN DENIZENS
Crown of thorns starfishes (Acanthaster planci) are a phenomenon of the Indo-Pacific basin, found in tropical and sub-tropical waters from the Red Sea and Indian Ocean across the Pacific to the west coast of the Americas. Their name, of course, derives from the resemblance to the Biblical Crown of Thorns of the stiff spines that adorn their arms and upper surfaces.
They’re starfishes with basic starfish architecture, but anatomical distinctions. The phylum name Echinodermata means “spiny skin,” but that reference is to the bony plates underneath their skin. COTS’ “thorns” are stiff, sharp spines atop their arms and upper surfaces.
Crown of thorns starfishes may begin life with starfishes’ five-way symmetry but they lose that as they mature, growing as many as 23 arms. They’re more flexible than most starfish, and each arm is equipped with two rows of echinoderm tube feet, the better to grip with. In size, A. planci are larger than other starfishes, growing to as much as 10-14 in/25-35 cm in size. They come in a range of colors, including reds, blues, purples, grays and tans.
And, they’re especially fertile. They reproduce by broadcast spawning. A large female can launch as many as 50 million eggs into the surrounding waters.
SEEING STARS: OTHER ECHINODERMS ON POSEIDON’S WEB – EXPLAIN THE REEF!
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- Brittlestars: Well-Armed, Laid-Back and Full of Tricks
- All Arms, All the Time: Feather Stars
- Sea Cucumbers – Superheroes of the Seas
- The Sea Urchin’s Tale
CROWN OF THORNS STARFISH FACTS 3: THORNY PROBLEMS
The thorns in crown of thorns’ armory are basically defensive, not offensive. Stiff and sharp spines, they easily penetrate soft tissue. They’re imbued with toxins called saponins. The spines don’t eject the material. Pain is inflicted when spine tissue breaks off and stays in the wound.
Predator fishes have been observed engaging in “mouthing” behavior that suggests an adverse meeting with a thorn. Also, it’s said not to taste good.
The pain of A. planci toxin is a problem for divers engaged in trying to remove troublesome COTS specimens. A human stabbed with a COTS thorn experiences immediate sharp pain that can last for hours and swelling that can last for a week. It’s a problem not just for potential predators but for humans trying to manage a COTS population outbreak.
CROWN OF THORNS STARFISH FACTS 4: PLAN OF ATTACK
First of all, A. planci are typically nocturnal, staying out of sight during daytime hours.
Like all starfishes, they dine out on other animals by everting their stomachs, injecting digestive enzymes into the corals’ hard skeletons to liquefy solid tissue inside and then ingesting the nutrients. But they have a significantly large ratio of stomach surface to body mass, covering the coral prey’s surface to nearly their own diameters.
Each crown of thorns starfish can devour an area its body diameter in size in the course of a night. A large, busy COTS can ravage a reef area as large as 110 sq ft/10 m2 in the course of a year.
Since COTS find themselves able to grip to and wrap themselves around extended shapes, they prefer branching and table corals for their dining pleasure. If those are in short supply, they will prey on rounded coral structures like mounding star corals, but those offer more limited surface areas to exploit. If necessary, they’ll seek out soft corals for dinner. Also, sponges, algae (especially red coralline encrusting algae) and other organisms.
CROWN OF THORNS STARFISH FACTS 5: PREDATORS
Does anything seek out COTS for dinner? A few predators, most notably the triton’s trumpet conch (Charonia tritonis). They tear the starfish apart with their hard radula, or rasping beak. Unfortunately, they’re prized for their beautiful shells and subject to human harvesting.
Some species of pufferfishes and triggerfishes have been observed eating COTS but it’s unclear whether they are committed predators of the starfish. A small shrimp, Hymenocera picta, and a polychaete worm (Pherecardia striata) are also known to sometimes prey on A. planci.
Some crab and shrimp species are known to be able to drive crowns away from the corals they live with, but not to attack them for food.
CROWN OF THORNS STARFISH FACTS 6: THE BEST OF TIMES, THE WORST OF TIMES
As mentioned, in normal times and low densities, crown of thorns starfishes can help support the diversity of life on a reef by keeping fast-growing branching corals in check, giving slower-growing corals space and time to become established and healthy. In the best of times, coral growth can keep up with crown of thorns starfish destruction.
But periodically, in time spans generally lasting years, COTS experience population surges and myriads of the voracious creatures descend on a reef, devouring its coral polyps.
The healthy density for the starfish is one per hectare (about 2.5 acres). A population outbreak is generally considered to be a density of 30 or more per hectare, or when the population is so large they consume corals faster than the corals can regrow. An eight-year outbreak on the Great Barrier Reef in the 1970s peaked with some 1,000 COTS per hectare left 150 reefs stripped of living coral and 500 more reefs damaged.
CROWN OF THORNS STARFISH FACTS 7: CONSEQUENCES
A population outbreak leaves behind bare coral skeletons that are quickly covered by algae, changing the nature of the reef. The abundance of species that depend on healthy corals for food and shelter diminishes and reef productivity drops. Branching corals are weakened to the point of collapse, creating a reef dominated by dead rubble. Sponges and soft corals may move in where hard corals once thrived. An algae-dominated reef changes the habitat to an emphasis on herbivorous grazers, especially among fishes.
Why do outbreaks happen? The answers are not clear, although scientists believe major a factor is runoff of land-based freshwaters laden with agricultural nutrients. This increases the density of phytoplankton in reef waters, fostering better COTS larval survival. Warming seas resulting from climate change would also contribute to increased phytoplankton density. Other factors are believed to be overharvesting of triton conchs, overfishing of fish predators and destruction of predator habitats.
CROWN OF THORNS STARFISH FACTS 8: CONTROLLING COTS THREATS
For organizations like Australia’s Great Barrier Reef Marine Park Authority, controlling crown of thorns starfish populations is a major priority. The GBR has recorded four major outbreaks since the 1960s. A outbreak typically lasts for some 10 years. The reef system is still dealing with one that started in 2010.
A number of techniques for control have been tried. One was simply to cut them into pieces, but COTS’ regenerative powers appears to have rendered that ineffective. Theoretically, at least, the various pieces of a starfish could regenerate into an even larger number of the creatures.
Other approaches have included removing them and burying them on shore, baking them in the sun, injecting them with compressed air and injecting them with toxins. Physical removal is labor- intensive, and carries with the hazard of divers being injured by the spines.
More recently, the GBR authority’s preferred approach involves injecting substances called bile salts into the crowns in place on the reef. It kills the starfishes without harming surrounding corals and other life.
PRINCIPAL SOURCES: Reef Creature Identification, Tropical Pacific, Paul Humann, Ned DeLoach; Indo-Pacific Coral Reef Field Guide, Gerald Allen, Roger Steene; Coral Reef Animals of the Indo-Pacific, Terrence Gosliner, David Behrens, Gary Williams; Crown of Thorns Starfish, Reef Resilience Network; Crown-of-thorns starfish control program, Great Barrier Reef Marine Park Authority; Crown of Thorns Starfish, Australian Institute of Marine Science; Crown of Thorns Starfish (COTS), Living Oceans Foundation; Crown of thorns starfish, Oceana; Great Barrier Reef: Crown-of-thorns starfish eating their way through coral in major outbreak, Australian Broadcasting Network; Crown of thorns starfish, Wikipedia.