The Crown of Thorns on Coral Reefs
J. M. Branham
The thought of coral reefs arouses a sympathetic response in most of us. It conjures up visions of swaying palms and creamy beaches. Beneath the sur rounding sea strangely shaped and brightly colored animals flit amid the branches of a multihued forest of living coral. Our vision is of a beautiful, intricate, and probably delicate balance of nature, that grew up from the depths, over the eons, through the slow ac-cretion of minerals by coral polyps. It is not surprising, therefore, that a Sydney newspaper article by Graham Gambie aroused public interest in February 1966 when it announced, "Weird Star-fish Eating Miles of Barrier Reef. A plague of giant starfish called the 'Crown of Thorns' has reduced miles of the Great Barrier Reef to rubble."
The article reported that the plague which was first observed in July 1965 near the tourist resort of Green Island, was interpreted as a new occurrence by Robert Endean, then the Secretary of the Great Barrier Reef Committee. He speculated that "something has tipped the balance of nature in the starfishes' favor" and that the reef was being destroyed at an alarming rate. The concern grew over the next 3 years and reached crisis proportions about the time of a report in Science by R. H. Chesher (1969) that the starfish "population explosion" had spread to other areas of the Pacific, and was causing extensive "destruction" of reefs, particularly on Guam. An article in the March 1969 issue of Skin Diver Maga-zine entitled "Divers wage war on the killer star" further inflamed the issue and the popular press responded quickly with articles like Newsweek's (14 July 1969) "Battle of the Coral Sea" which announced that the reefs were being eaten away rapidly and islands were in danger of toppling into the sea. Is-landers were threatened with economic disaster and starvation if the reefs disap-peared (Dixon 1969). In summer of 1969 Chesher organized an extensive survey of the U.S. Trust Territories of the Pacific sponsored by the Westing-house Electric Co. and funded by the Department of Interior (Chesher 1970). The objective was to seek out the menace so that it could subsequently be eradicated. I became involved.
The following account deals with some aspects of the starfishes' biology, the concept of "population explosions" and "plagues" and some effects of the Crown of Thorns on reefs. I hope that it will lead the reader to question some of the conclusions that have been reached, and to generate some productive ideas about coral reef ecology.
The Crown of Thorns (Acanthaster planci) is an extraordinary starfish, usually about the size of a large dinner plate, and bristling with sharp spines (Fig. 1). It can exceed half a meter in total diameter. Pacific specimens usually have 16 rays (or arms), each of which extends out from a central disc for about a quarter of the starfishes' total diameter. Specimens from the Red Sea have about 13 rays (Ormond and Camp-bell 1971). Their color in life is quite variable. Most of the ones that I have seen were green with scarlet-tipped spines, although some were light gray with orange- or yellow-tipped spines. Other authors have reported them to be purple blue, or bluish gray with red-tipped spines (H. L. Clark 1921, Mortensen 1931). The basic coloration can be altered by the ex-tension of red or purple gill-like papulae over most of the aboral surface (H. L. Clark 1921). Partial extension of the papulae makes an animal appear bluish, while more complete extension masks the basic color. Papulae are sometimes extended unevenly over the disc so as to create a distinctive, but transient, pat-tern of concentric circles. When disturbed, the papulae are withdrawn and the animal appears to change color
The prominent spines, up to 5 cm long, are very sharp and, at least in fiction, "alive with a deadly poison" (Roark 1956, p. 231). They readily inflict painful wounds and have con-tributed to the villainous reputation of the Crown of Thorns, particularly in folklore (Miner 1938, Garlovsky and Berguist 1970). The symptoms of star-fish "stings" have been extensively documented, but the existence of a toxin remains uncertain (Fish and Cobb 1954, Pope 1964, Barnes and Endean 1964, Halstead 1965, Odom and Fischermann 1972).
Feeding Theodore Mortensen (1931, p. 29) observed, "This species was found rather commonly on the coral reef at the little island of Haarlem off Batavia, near Onrust [island, 1050 E, 60S], crawling over the top of the madrepora-rian corals on which it feeds, sucking off all the soft substance, leaving the white skeleton of the corals to show where it has been at work." (He also noted that "it is almost impossible to avoid being hurt by its spines" but that did not deter him from his observation on reproduction and larval development.) Feeding does not really involve sucking, but, instead, the gastric sac of the star-fish is everted through the "mouth" and spread over the coral to digest away its living tissue (Goreau 1963). Such activi-ty kills the coral polyps and thus affects the ecology of reefs.
A. planci feed primarily but not exclusively on the soft tissue of reef building scleractinian corals. Food choice apparently involves a complex interaction between the initiation of feeding in response to some nonspecific coral substance(s) (Brauer et al. 1970) and repulsion by the stinging ability of the particular coral colony (Barnes et al. 1970). Some crustacean commensals which live on or in coral colonies apparently can also drive starfish away by pinching their tube feet (Pearson and Endean 1969).
Corals of the genus Acropora seem to be preferred in many places (Weber 1969, Pearson and Endean 1969, Tsuda 1971, Goreau et al. 1972). In Hawaii, where there is presently no Acropora, aggregated sea stars fed almost ex-clusively on another rapidly growing member of the same family (Acropori-dae, Montipora verrucosa), even though it represented but 5% of the coral in the area of the aggregation. The dominant coral in their vicinity (Porites compres-sa) was largely ignored (Branham et al. 1971). The starfish could extend their stomachs far into crevices between projections of Porites to reach small patches of Montipora growing at their base (Fig. 1).
When scleractinian corals were not readily available, as was the case after most of the corals had been killed by anadvancing aggregation, A. planci fed on other cnidarians with more potent stinging mechanisms, like hydrozoan "corals" ("fire coral," Millepora) and alcyonarians, such as "organ pipe coral" (Tubipora), soft corals, and anemones (Pearson and Endean 1969). Interest-ingly, Verwey (1930, p. 326), who was studying the symbiotic relationship between damsel fishes and sea anemones at Haarlem Island in Batavia Bay, ob-served that Acanthaster was "probably one of the worst enemies of anemones, as well as corals." They have also been reported to feed on helmet shells (Cassis cornuta) (A. H. Clark 1950) and giant clams (Tridacna) (Pearson and Endean 1969). I have observed them on rocky bottoms with no coral cover, with their stomachs everted, apparently digesting encrusting organisms. Newly metamorphosed ones prefer coraline algae to corals (Yamaguchi 1972). Captive A. planci can be maintained on various foods, including fish, shrimps, squids, sea urchins, beef bones, and each other.
Diet could have influenced color. The darker green-red combination seemed to predominate when coral was the main food source, and gray yellow when other things were eaten. One green-red animal maintained in our laboratory on a diet of fish and shrimp gradually changed to grayish red after about half a year.
Feeding behavior in nature is rather variable. In the Red Sea, where the A. planci population density was relatively low, feeding was confined to small territories and occurred primarily during the night (Goreau 1963). The starfish did not prefer any particular species of coral but, rather, chose small colonies. I have observed similar situations around Hawaii and in the Marshall Islands. A. ellisii, a related species found in the Bay of California (see Caso 1961, although Sladen 1889, thought all Pacific speci-mens were probably in the same rather plastic species) behaved similarly, except it fed actively during the day (Dana and Wolfson 1970). Starfish in dense aggregations were also active during the day (Chesher 1969, 1970; Pearson and Endean 1969; Branham, et al. 1971, Vine 1972a). Densely aggregat-ed ones either fed selectively with the aggregation remaining in the same area for a long time (Branham et al. 1971). or indiscriminately on most corals, with the aggregation migrating along the reef, leaving large areas of dead coral in its wake, as at Guam and on the Great Barrier Reef (Chesher 1969, Pearson and Endean 1969).
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