ATOLL RESEARCH BULLETIN

NO. 166

CROWN OF THORNS (ACANTHASTER PLANCI) PLAGUES: THE NATURAL CAUSES THEORY

by Peter J. Vine

Issued by

THE SMITHSONIAN INSTITUTION

washing to^^, D.C., U.S.A.

Contents - Page

Abstract Introduction Observations Discussion Conclusions Acknowledgements References

Figures (following p. 10)

1 Acanthaster planci removed from hiding and placed in light during daytime soon moves back into shaded conditions. The long light-sensitive terminal podia can be seen.

2 Peter Goreau (son of the late Tom Goreau) studies an Acropora table in the Red Sea. The white region has been predated upon by A. planci and the peripheral darker region is still alive. Twelve A. planci were found under the table, mostly attached-to the basal stem.

3 A. planci moving over sand. 4 Two A. planci revealed by removing an overlying slab

of coral.

CROWN OF THORNS (ACANTHASTER PLANCI) PLAGUES: THE NATURAL CAUSES THEORY

1 / by Peter J. Vine-

ABSTRACT

The history of research on Acanthaster planci prior to 1960 is briefly reviewed and the results of some more recent research are discussed. There is evidence that many "plagues" of A. planci have occurred prior to 1960. Predation by Charonia tritonis on &. planci is unlikely to have been a significant factor controlling its population in any area. The suggestion that artificial damage to coral reefs, by blasting or dredging, has created conditions for population explosions of A. planci is considered in relation to the known habits of larval and juvenile stages. It is suggested that many "plagues" have been aggregations which are a natural and necessary facet of the life of this species. There is some evidence that feeding starfish release a chemical attractant thus producing mini-aggregations. The likely results of destruction of corals by 4. planci are briefly discussed and a possible measure to reduce populations is suggested.

INTRODUCTION

The crown-of-thorns starfish, Acanthaster planci, was known to science long before the recent focus of public attention on destruction of coral reefs which it is causing in the Indo-Pacific. Rumphius mentioned it in 1705 and it was described by Plancus and Gualtieri in 1743. The name Acanthaster planci was created by Linnaeus in 1758. Early knowledge of its widespread distribution can be gathered from the literature; Goa (~ortu~uese india), 1743; Java, 1793; Philippines, 1781; and Mauritius, 1885. It was recorded from Australia's Great Barrier Reef in 1913 lark, 1921) and at Low Isles, where the Great Barrier Reef Expedition was centred in 1928-29 Livingstone, 1932). Other records such as those of' Madsen 11955) indicate a widespread distribution throughout the Indo-Pacific. It has not been recorded from the Caribbean..

--

L'Dcpartment of Zoology, University College, Galway, Eire.

(~anuscri~t received June, 1971--Ed.)

OBSERVATIONS

Early records of large aggregations of A. planci are understandably scarce. The animal prefers a sublittoral habitat and is frequently hidden during the day. Many reef areas were seldom visited and very rarely dived upon. Indigenous people on atolls throughout the Indo-Pacific recognise the starfish, have remedies for stings caused by penetration of its spines, and in some cases are able to report large numbers which have appeared on their reefs in the past. Mortensen, who studied A. planci in the Java Sea in 1929, wrote in 1931: "This species was found rather commonly on the coral reefs at the little island of Haarlem off Batavia, near Onrust......". It was also reported as "common" in Port Galera Bay and Sabang Cove in the Philippines in 1938 (Domantay and Roxas, 1938), as "very common" in the Palaus

I Hayashi,, 1938) and "abundant" around Christmas Island Edmonson, 1946). As Dana (1970) has pointed out it seerns quite probable that these records might have been applied to the modest densities which are now said to constitute plagues (Endean, 1969; Pearson, 1970). 1 have spoken to fishermen in the Solomon Islands who recall large concentrations of them many years ago. It was said of Maringe lagoon, Santa Ysabel, that night-time fishing on the reef was especially hazardous about 40 years ago, because fishermen were afraid of treading on starfish which were present in large numbers. It has been widely held that Green Island experienced the first "plague" on the Great Barrier Reef i.n 1959 (~ndean, 1969) but an experienced professional diver from Townsville, D. Tarca (personal communication and Sunday Mail, September 7, 1969) recalled that there were large numbers on the south-east side of Lodestone reef in 1954.

Attention was drawn to the feeding habits of A. planci by Goreau (1963) who discovered an aggregation in The southern Red Sea. He foresaw the likelihood that: "this species may, under certain condi-tions, be an important factor limiting the growth and. development of coral reefs". A-t that time large numbers of A. planci. were being reported from reefs near Cairns, on xustralials Great Barrier Reef, and destruction of corals had been noticed at the tourist resort of Green Island.

Pearson, the scientist employed by the Queensland Govern- ment to investigate the biology of Acanthaster and the damage which it was causing on the reef, reported further recent and massive destruction of corals in the Innisfail area, which had gone quite unnoticed by the press and public. This suggests that intense destruction of coral by A. planci may have occurred undetected throughout the range of the species many times in th.e past.

Recent research by Pearson and lhdean in Australia, Chesher and a tearn of scientisls at Guam and elsewhere in Lhe

Pacific, Weber and Woodhead in the Pacific, the Cambridge Coral Starfish Expedition in the Red Sea and a large number of individual scientists, who have reported on its distributi.on, have added greatly to our knowledge of this species (see papers by Barnes, 1966; Branham et al, 1971 ; Chesher, 1969 and 1970; Dana, 1970; Dana and Wolfson, 1970; Elderedge, 1970; Roads and Ormond, 1971 ; Vine, 1970; Weber and Woodhead, 1970).

Endean (1969) suggested that removal by shell collectors of the giant triton, Charonia tritonis, which preys upon A. planci, may have removed the predator pressure controlling populations of this starfish. He believed that the larvae had a short planktonic life: "a matter of a few days", and that significant pressure could be exerted on the population by C. tritonis. Mortensen (1931), however, had shown tha-l; the larval - development of A. planci takes two to three weelcs. He succeeded in fertilising eggs and reared the larvae for sixteen days, by which time they had reached the brachiolaria stage. Recently Henderson and Lucas (1971) have shown larval life is 1-1-5 weeks, depending on temperature. Such animals normal1.y produce great numbers of eggs and Pearson has estimated the egg production of a female Acanthaster of average size (30 cms.) to be 12-24 million. Thorson (1950) has shown how such invertebrates undergo great fluctuations in population size, depending on their breeding success and particularly the conditions met by their larvae in the plankton. Charonia tritonis is a so~i~ewhat scarce and sluggish predator. One was recorded as eating on average 0.7 Acanthaster per week when given an unlimited supply (Pearson and Endean, 1969). Experiments have indicated, moreover, that C. tritonis usually prefers to eat Linckia laevigata, a blue starfish which appears -to be more abundant

lanci on many coral reefs which I have visited in the Paclf than lc 7- 1970).

Chesher (1969) suggested that A. planci larvae are heavily preyed upon by live coral polyps and that areas for safe settlement of larvae are severely restricted on a normal coral reef. He poinLs out that mechanical damage to reefs, such as is caused by blasting or dredging, may have reduced predatory pressure on the larvae and increased the area suitable for settlement. Increased survival of larvae would produce localised dense populations of &. planci.. These would increase the area of dead coral by their feeding activities and might thus further reduce predation on the larvae, simultaneously increasing the area suitable for subsequent settlement.

DISCUSSION

This suggestion, that recent mechanical damag~ may have been important, meets with the difficulty that coral reefs have long been subject to cyclone damage. Disturbance of coral by such a natural cause would presumably result in conditions

similar to those postulated as favouring the development of A. planci plagues. Pearson (pearson and Endean, 1969) has shown that larvae can in fact settle in areas of rich coral growth, especially among the basal stems of closely packed branching colonies of Acropora echinata. The larvae may indeed swim for a long time among the coral stems for it seems well established that echinoderm larvae can delay metamorphosis until they discover an area suitable for settlement orte tens en, 1938). The older photonegative larvae (~horsen, 1950) may thus be expected to visit the bottom repeatedly, while being carried by water currents for several days, but it is not clear that they would be vulnerable to predation by coral. If they are photonegative, it is probable that most of their searching is carried out during daylight, when the majority of coral polyps are not feeding. Shaded crevices of coral reefs offer many suitable surfaces, which are free from filter feeders and would provide for the attachment of brachiolaria larvae and the protection of young metamorphosed starfish. In most situations the coral cover of a flourishing reef is only 30-50 per cent (~albot and Talbot, 1971) and the remainder is veneered with hard encrusting red coralline algae forming sheets or patches of cement-like pink calcium carbonate. Larvae reared in the laboratory settled in the presence of encrusting algae, algal detritus, serpulid tubes, fine siliceous sand, litbothamnion, live Pocillopora damicornis, spines -and tube feet of A. planci and recently p r e d a t e d ~ n i c o r n i s (13enderson and Lucas , 1971 ) Mechanical destruction of coral would seem to be unnecessary for survival and metamorphosis of A. planci, and might be unfavourable through reducing the available food. Moreover it does not necessarily follow that a "seed" population would reproduce locally and thus build up in the manner envisaged by Chesher, since the larvae have a planktotrophic life of considerable duration, during which they must be widely dispersed and are probably preyed upon heavily.

Several workers have recently suggested that the present situation regarding populations of A. planci is natural (~eber and Noodhead, 1970; Dana, 1970; Vine, 1970) and it appears likely that the recent outbreak of reports of "A. planci" reflects an increase in the number of divers aware of the starfish. There seems to be no need. to postulate a real increase in the numbers of A. planci in the Indo-Pacific. As Pearson (1969) has pointed gut, prior to 1964, starfish aggregations on the Great Barrier Reef had been overlooked by fishermen and skindivers. In the Red Sea I asked several. fishermen, shell collectors and skindivers where we could find large numbers of Acanthaster. Nobody could direct us to a "plague", although we eventually discovered two reefs on which large numbers were found, both withi-n several miles of Port Sudan (~oads and Ormond, 1971 ) .

The fact that "plagues" have now been reported throughout the range of distribution of - A. planci suggests that these are

i n f a c t n a t u r a l f a c e t s of the l i f e - h i s t o r y of Acanthaster . Work c a r r i e d out by t h e Cambridge Coral S t a r f i s h Expedi t ion i n t h e Red Sea tends t o suppor t t h i s view. Within t h e aggregat ions of A. p l a n c i which were discovered we i d e n t i f i e d "mini-aggrega- tionst'--many s t a r f i s h huddled toge the r , o f t e n wi th arms i n t e r - locked o r d i s c s p a r t l y over lapping, emerging at n i g h t time from t h e i r shaded s i t u a t i o n s under Acropora t a b l e s t o f e e d toge the r on t h e same t a b l e s . Although t h e major i ty were i n a c t i v e and hidden i n daytime, occas iona l i n d i v i d u a l s were observed moving over sandy a r e a s between the hidden mini-aggregations. The presence of o t h e r A. p l a n c i d i d n o t appear t o a t t r a c t t he se moving s t a r f i s h , bu t a t n i g h t A. p l a n c i which had commenced f e e d i n g appeared t o a t t r a c t otGers. Thus t h e number f eed ing on a p a r t i c u l a r t a b l e i nc reased , and i t seems p o s s i b l e t h a t some form of chemo-attraction may be exe r t ed by f eed ing s t a r f i s h . Research by Brauer, Jordan and Barnes (1970) sugges t s tha-t A . p l a n c i may be a - t t r a c t e d by p a r t i c l e s of semi-digested cora l - -- t i s s u e . They demonstrated r e f l e x stomach evers ion by j -nject ion of c o r a l e x t r a c t i n t o t h e wate r . The degree of responsiveness of starfish. t o t h i s sti.mulus was g r e a t e r a t n i g h t than du r ing daytime. Chemo-attraction between ind i~v idua l s has no t been conc lus ive ly demonstrated bu t seenis 1.ikel.y t o occur. Experi- ments c a r r i e d out w i th Y tubes on A. planci. i n the Red Sea i n d i c a t e t h a t indiv:idual.s a r e a t t r a c t e d t o f eed ing s t a r f i s h (R .F .G . Omload, persona l communication, October 1971 ) . I n a Red Sea a r e a where hcan tbas t e r was s p a r s e , observed d i r e c t i o n s of movemen-l; o:E i s o l a t e d iiidi-v:i.duals, i.n r e l a t i o n -to t h e spacing of previous feed ing s c a r s , sugf:;estetl t h e possibj . l i . ty t h a t chemo- a t t r a c t i o n from aggrega t ions may a t t r a c t s t a r f i s h rrom one o r two miles away. Thus mass:ive aggrega-t ions may perhaps form by n~echanisms s i m i l a r t o those which c r e a t e mi.ni aggrega t ions .

The f a c t t h a t f eed ing aggrega t ions develop natura1l .y sugges t s a mc?chani.srn by which massive sudden popul.ation i n c r e a s e s rnay occur. A s l i g h t f l u c t u a t i o n i n l a r v a l s u r v i v a l rnay r e s u l t i n an :i.-ncreased populat;.on densi- ty of s t a r f i s h . Beyond a c e r t a i n :(.eve1 a s ignif ica~z-L i.ncrease [nay occur i n t he number and s i z e of feedirlg aggregat ions which develop. Great ly i nc reased f e r - t i . l i s a t i on >~i.tb.:in these aggregat:i.ons could l ead t o a sudden inc rease i n me-tamorphosed. s t a r f j ~ s i ~ . Thus t h e i n i t i a l change i n condi t ions may be q u i t e small. ( g iv ing r i s e t o a s l - igh t i n c r e a s e %a popu la t ion ) . This t r i g g e r s o f f a n a t u r a l chain of events which may l e a d t o sudden 1ocal:iLsed popula t ion explosj.ons.

An advantage of aggpegations i s tha, t they i.ncrease t h e chances of s u c c e s s f u l f e r t i l . i s a t i o n of eggs. It i s doubt fu l whether i s o l a t e d i n d i v i d u a l s c o n t r i b u t e a t a 3 l t o t h e planktonic popula t ion of l a r v a l A. p l a n c i ( s e e Thorson, 1950). Epidemic spawning has been recorded i n a number of a s t e r o i d s : As t e r i a s rubens (~emmi.11, 1914; Bu l l , 1934) ; A s t e r i a s f o r b e s ' m o f f and Loosanoff, 1939); S o l a s t e r a osz-mill, 1920) ; and Astropecten i r r e g u l a r i s - w n y * Pearson (pearson and Endean, 1969) observed synchronous male spawning i n A . - p l a n c i

and this seems to have triggered off spawning in at least one female a metre away from the nearest spawning male. Probably fertilisation is ensured through some ecto-hormone, as in Asterias (~haet, 1961 ; Kantani et al, 1963; Stebbing, 1967), - and the successful reproduction of A. planci may well rely upon the formation of aggregations.

A fortunate result of the aggregated feeding habits of A. - lanci is that the damage is localised and many large coral 2-

structures are left completely untouched. Where aggregations appear to have been abnormally large (such as at Guam and on a section of the Great Barrier Reef), there is less likelihood of any large coral structures remaining undamaged, excep-t of those species not usually attacked by A. planci (see Pearson and Endean, 1969), but the majority of-reported "plagues" have been of more moderate proportions. It is a characteristic feature of such areas that some coral structures survive undamaged. I have seen examples o:f this at Northeast Bay on Great Palm Island, where isolated outcrops of delicate staghorn Acropora remained untouched whereas at leas-1: 50% of the Acropora - was killed (see also Endean, 1969; Pearson and Endean, 1969). Off Fiji and. in the Red Sea I have observed similar examples. The corals which escape attack are likely .to provide planulae :for settlement and recolonisation of devastated areas. This is a slow process but it has been. observed to occur (Pearson and Endean, 1969).

Suggesti.ons that fishermen will lose thei.r livelihoods, rcefs will erode and a-tolls sink gradua1l.y beneath the waves must be trea-ted. with. more than a little scep-ticism. Apar-t :horn destruction. of corals, few faunistic changes in the ecology of reefs devas-ta-Led by - A. planci have 'been. observed by Pearson and Enclean. Pacific islanders appear -to rely a great deal on 'bottom fishing from deep wa.ter off the edges of reefs, and from fishing for pelagic fish. Neither of these fish popula- tions are :Likely -to be affected by A. planci destruction. There has been. little evidence of serious reef erosion following devastation. by - A. planci and indeed t11j.s seems unlfikely to occur.

Cover by live corals on richly flourishing reefs is remarkably low (~a1bo.t and 'l:albot, 1971 ) and many reefs consist mainly of dead coral material covered by calcareous algae. Indeed, this is a feature of areas most exposed to wave action, such as in shallow water on exposed sides of reefs. Rates of recolonisation and regrowth of coral appear more than enough to compensate for gradual sinking of sorne reefs. In view of the massive thickness of dead coral on most coral reefs, beneath the thin veneer which is alive, it is worth considering the possibility that, far from havj.ng a deleterious effect on the development of coral reefs, dense predation by A. planci may actually promote -their development by killing protuberan-t colonies and thus encouraging firmer

regrowth and cementation b a s a l l y . As growth s t u d i e s on s c l e r a c t i n i a m c o r a l s have shown (Manton, 1932; Motoda,l91$0; Abe, 19110) i n i t i a l r ap id growth r a t e of a colony, o r i nd iv idua l i n t h e case of Fungia, i s fol lowed by a slowi.ng down, leading. t o almost complete c e s s a t i o n . Thus, by providing new sur:faces f o r s e t t l emen t of p lanulae and development and more r ap id growth o f young c o l o n i e s , p reda t ion by - A. e l a n c i - may l e a d t o a n a c c e l e r a t i o n of r ee f growth.

CONCLUSIONS

Although p reda t ion on c o r a l by - A . *- planci. may be a normal. f a c e t of t h e development of c o r a l r e e f s , i n t he s h o r t view massive d e s t r u c t i o n of s c l . e r ac t in i an c o r a l s i s undesirab1.e on r e e f s v i s i t e d by - tour i . s ts . The only prac t ica l . s o l u t j ~ o n so f a r sugges-led i s t o k i l l o r remove Acanthas-Ler :rron~ i n f e s t e d reel's ( ~ n d e a n , 1969). In view of t h e r e s t r i c t e g breedj~ng season of A . G a n c i i n s e v e r a l reg ions a f f e c t e d by plagues (Pearson and - ~ n d e a 5 9 6 9 ) and -the p r o b a b i l i t y t h a t recrui.tment t akes place from aggregated s t o c k s , where f e r t i l i s a t i o n i s more l;.kely i.o occur , i t is suggested t h a t e f f o r t s by sk ind ivers t o remove A . plane' from r e e f s should be concentra ted on aggregati.ons i n - t h e per iod itnmediately p r i o r t o the breed ing season. It i s thus' important -to es tab l i . sh whether a reg:i.onal populati.on e x h i b i t s a r e s t r i c t e d breed ing season such a s t h a t repor ted by Pearson (1970) f o r 4. p l a n c i on. -the Great B a r r i e r Reef, and t o d i scove r -L:he time of t h e main breeding peri~ocl (i3ecetnbe.r l o January i n Queensland wa-ters) . Given cotnpetent d i v e r s i t would be consi.derably more e f f i c i e n t t o c o l l e c t s t a r f i s h a t n igh t time when tnore a r e exposed on c o r a l s . Removal of aggregated s t a r f i s h fo%lowin,y the breed ing season i s u n l i k e l y t o have a s i ~ n i f i c a n t e f f e c t on thc popula-ti.on i ~ n an a r e a , w h i l s t s ea rch ing out and vernovin,g- i s o l a t e d i.nclivi.dua:Ls i ~ s probably nevcl? \\iorth t h e effo:l?t.

I am g r a t e f u l t o Rotary :In:ternati.onal f o r a t r ave l l - i ng fell .owship t o v i s i ~ t lliany a r e a s o:E t he Pa.cifj.c and Lo t h e Cambri-dgc Coral S t a r f i s h Research Expedi-Lion f o r the opportunity t o s tudy i n t he Red Sea. I was r e s i d e n t f o r one yea r a t the James Cook Universi.ty of North Queensland and I wish t o thank P ro fe s so r C . Rurdon-Jones and. Dr R. Kenrly f o r supervi.sion and advice . Present research. i s supported by a g ran t :from Lhe U n i v e r s i l ; ~ of l\rales. For adviice i.n t h e prepara t ion of t h i s paper T .wish Lo thank P ro fe s so r E.W. Knight-Jones.

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FIGURES

1 Acanthaster planci removed from hiding and placed in light during daytime soon moves back into shaded conditions. The long light-sensitive terminal podia can be seen.

2 Peter Goreau (son of the late Tom Goreau) studies an Acropora table in the Red Sea. The white region has been predated upon by A. planci and the peripheral darker region is still-alive. Twelve A. planci were found under the table, mostly attached-to the basal stem.

3 A. planci moving over sand.

4 TWO A. planci revealed by removing an overlying slab of coral.