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Oecologia (Berl.) 20, 111--115 (1975) 9 by Springer-Voting 1975 Predation of the Sea Urchin Diadema antillarum Philippi on Living Coral Rolf P. M. Bak and Guillaume van Eys Caribbean Marine Biological Institute (Carmabi), Piscaderabaai, Curapao, Netherlands Antilles Received April 8, 1975 Summary. On the coral reefs of Curapao a n d Bonaire (Netherlands Antilles) the sea urchin Diadema antillarum is a major coral predator. I n areas with high coral cover, up to 8.2% of the Diadema population (with a density of 8.5 animals/m 2) was feeding on living coral surfaces a t night. Acropora species are the most heavily attacked corals. For reasorts such as the largely unknown causes of death of corals and the impact of Acanthaster planci recently in the Pacific, predation on stony corals (Scleractinia) has been the subject of much research lately. Few animals are known to eat living coral in the Caribbean. A recent summary (Glyan, 1973) names only five invertebrate predators. On Barbados, only two coral predators are of consequence (Oft and Lewis, 1972). I n addition to invertebrates, some fishes are known to eat coral. Our field observations are in accord with those of Randall (1967) in t h a t fish only occasionally feed on living coral colonies, although in other areas, their effect is more important (Glynn, 1973). Over the past year we have noticed scars on coral colonies that could not be explained Uy predation of the fire worm, Hermodice carunculata or the gastropod, Coralliophila abbreviata (apparently most common invertebrate coral predators in Curagao). Night diving with SCUBA gear, revealed t h a t the sea urchin Diadema antillarum is a very common predator on various coral species. I t is commonly regarded as a herbivore (Lewis, 1964; Randall etal., 1964; Sammarco etal., 1973, 1974) but it also feeds on detiitus and sediment and occasionally as a carnivore (Lewis, 1964; Quinn, 1965). None of the authors mentions Diadema as feeding on living coral. The strong and extendable chewing apparatus of Diadema scrapes off the living tissue with part of the underlying calcium carbonate skeleton. The urchins are normally found in one of three locations on coral heads: 1. on the boundery between the living coral and the bare dead skeleton, 2. on the edge of an existing scar on the living tissue, and 3. on a fresh scar on the coral surrounded by living tissue (Fig. 1). Examination of urchin gut contents within an hour after collection showed coral tissue, mucus, nematocysts and zooxanthellae to be present. We did our quantitative observations on size and distribution of the Diadema antillarum population and the number of coral eating urchins and their prey species, on a reef on the southwest coast of Cln'apao. Due to the complexity of the substrata on the deeper reef, our observations are restricted to the shallow 8 Oecologia (Berl.), Vol. 20 112 1%. P. M. Bak a n d G. v a n Eys Fig. 1. (a) A Diadema feeding on the edge of a large scar in the tissue of Montastrea annularis. Coral tissue and corallites are scraped off. (b) A feeding .Diadema is removed to show the lesion in the living coral tissue. This small scar in the living coral surface of Acropora palmata (see arrow) shows how Diadema bites off the living tissue with p a r t of the underlying skeleton. Scale bar represents 1 cm Predation of ~he Sea Urchin Diadema antillarum 113 Zone r [ ] Zone E Zone -o 12 ~ o ~ 4 N t J Day Night Cordl feeding[IOn} Coral Rock Sand 50 ~ 50 u 40 Fig. 2. (a) Mean numbers of Diadema antillarum in the different reef zones during day and night and the number of coral feeding animals ( x 10). (b) Percentage of reef zone covered by living coral, coral rock and sand. Vertical lines represent standard deviation. Values significantly different (t tests, P<0.05) from other two zones are marked with an asterisk reef platform ( ] 2 - 2 m depth). This platform is 40-50 m wide and extends from the first dropoff to the shore zone (Bak, in press). The platform can be divided in three main zones: zone I is the deepest zone with a b u n d a n t coral cover; zone I I intermediate in depth with m u c h sediment; and zone I I I is the shallow Aeropora palmata zone. We did three types of census along one meter wide transects. One census established the n u m b e r and distribution of Diadema during the d a y ; another provided these d a t a for the noctural feeding population; and a third census gave the numbers of Diadema found feeding on living coral and their p r e y species. A line transect m e t h o d (Loya, 1972) was used to quantify the types of the substrata (living coral, rock or sand) over the reef. The results are shown in Fig. 2. The d~ta on coral feeders are m i n i m u m figures because Diadema starts to move when disturbed b y diver-induced water m o v e m e n t s and light (both inherent in SCUBA diving at night). Many more fresh scars were found t h a n the n u m b e r of animals actually feeding. The greatest supply of benthic algae, the normal food of Diadema, is found on the reef rock which is most a b u n d a n t in zone I I I . The significantly higher density of urchins in this zone at night suggests a nightly m o v e m e n t to this area for feeding. Coral is fed u p o n in all zones but m o s t l y in the zones where living coral is most abundant. The percentages of the population feeding on coral in zone I, I I and I I I are respectively 8.2, 4.5 arid 6.5 %. The only other active predator we observed were five specimen of Hermodice caruneulata. I n zone I, the coral diversity is high and almost all species are attacked b y the urchins, including the H y d r o z o a n Millepora 8* 114 R . P . M . Bak and G. van Eys species. Most Diadema feeding on corals were f o u n d on Madracis mirabilis, Agaricia agaricites a n d Montastrea annularis. These are also t h e m o s t c o m m o n species in this area. I n zone I I , coral cover is low; m a n y species are p r e s e n t a n d m o s t are a t t a c k e d . Acropora cervicornis, in p a r t i c u l a r , is i n v a r i a b l y fed u p o n when present. I n zone I I I , Acropora palmata is the d o m i n a n t coral a n d 86% of t h e coral-feeding Diadema were f o u n d on this species. The i n d i v i d u a l fresh scars are u p to 10 cm in d i a m e t e r , because urchins r e p e a t e d l y feed a r o u n d t h e same lesion, t h e t o t a l a r e a of a scar can be e n l a r g e d to cover m o s t of a b r a n c h of an A. palmata colony. The a v a i l a b l e d a t a in t h e l i t e r a t u r e ( R a n d a l l etal., 1964; S a m m a r c o et al., 1973, 1974; Smith, 1973) suggest t h a t t h e p o p u l a t i o n d e n s i t y of Diadema antillarum is e x t r e m e l y high in our s t u d y area. A food shortage m i g h t e x p l a i n t h e change in feeding b e h a v i o u r from a p r i n c i p a l l y h e r b i v o r o u s a n i m a l to a f a e n l t a t i v e coral feeder. The o n l y o t h e r k n o w n e c h i n o d e r m p r e d a t o r on living coral is Acanthaster planci, a n d it r e c e n t l y b e c a m e well k n o w n as a coral d e s t r o y e r when t h e r e were p o p u l a t i o n explosions of t h i s starfish in t h e Pacific. The increase in p o p u l a t i o n s densities of Acanthaster was a t t r i b u t e d to a n u m b e r of causes, a m o n g s t which p o l l u t i o n a n d o t h e r h u m a n a c t i v i t y are p r o m i n a n t (Endean, 1973). As we o b s e r v e d h e a v y p r e d a t i o n on t h e r e l a t i v e l y u n p o l l u t e d reefs of Bonaire, p o l l u t i o n does n o t seem to be necessarily responsible for pred a t i o n on living coral b y Diadema. I n Bonaire, as well as in Curagao, Acropora species a p p e a r to be t h e corals m o s t h e a v i l y a t t a c k e d . This p a t t e r n m a y e i t h e r be because of a preference of Diadema for this prey, or because Acropora species are d o m i n a n t in certain h a b i t a t s . I t should be n o t e d t h a t despite similarities b e t w e e n p o p u l a t i o n s of Diadema a n d Acanthaster (e.g. t h e r a r i t y of small specimens on t h e reef (Yamaguchi, 1973), Acanthaster p o p u l a t i o n s feed p r i n c i p a l l y on corals, while Diadema m u s t be classified as a f a c u l t a t i v e coral p r e d a t o r . Acknowledgements. We thank Drs. J. Lawrence, S. V. Smith and L. Vlijm for their comments on the manuscript and our diving friends at Carmabi for their assistance in the field. References Bak, R. P. ]V[.: Ecological aspects of the distribution of reef corals in the Netherlands Antilles. Amer. Ass. Petroleum Geologists Mere. (in press) Endean, R.: Population explosions of Acanthaster planci and associated destruction of hermatypic corals in the Indo-West Pacific Region. In: Biology and geology of coral reefs, vol. 2, O. A. Jones, R. Endean, eds., p. 389-438. New York: Academic Press 1973 Glynn, P. W.: Aspects of the ecology of coral reefs in the Western Atlantic Region. In: Biology and geology of coral reefs, vol. 2, A. O. Jones, R. Endean, eds., p. 271-324. New York: Academic Press 1973 Lewis, J. B. : :Feeding and digestion in the tropical sea urchin Diadema antillarum Philippi. Canad. J. Zool. 42, 549-557 (1964) Loya, Y.: Community structure and species diversity of hermatypic corals at El|at, Red Sea. Mar. Biol. 18, 100-123 (1972) 0tt, B., Lewis, J. B.: The importance of the gastropod Coralliophila abbreviata (Lamarek) and the polychaete Hermodice carunculata (Pallas) as coral reef predators. Canad. J. Zool. 50, 1651-1656 (1972) Quinn, B. G. : Predation in sea urchins. Bull. Mar. Sci. 15, 259-264 (1965) Randall, J. E.: Food habits of reef fishes of the West Indies. Stud. Trop. Oceanogr. ~, 665-847 (1967) Predation of the Sea Urchin Diadema antillarum 115 t~andall, J. E., Schroeder, R. E., Stark II, W. A. : Notes on the biology of the echinoid Diadema antillarum. Carib. J. Sci. 4, 421-433 (1964) Sammarco, P. W., Levinton, J. S., Ogden, J. C. : Studies on the activity and food of the echinoid Diadema antillarum Philippi on a West Indian patch reef. Special publication 2, West Indies Laboratories, St. Croix, 65 (1973) Sammareo, P. W., Levinton, J. S., Ogden, J. C. : Grazing and control of coral reef community structure by Diadema antillarum Philippi (Eehinodermata: Echinoidea): A preliminary study. J. Mar. Res. 82, 47-53 (1974) Smith, D. P. B. : Daily migrations in the long-spined sea urchin Diadema. Proc. AIMLC 9, 24 (1973) Yamaguchi, M. : Early life histories of coral reef asteroids, with special reference to Acanthaster planci (L.). In: Biology and Geology of coral reefs, vol. 2, 0. A. Jones, R. Ende~n, eds., p. 369-387. New York: Academic Press 1973 Roll P. M. Bak Guillaume van Eys Caribbean Marine Biological Institute (Carmabi) Piscaderabaai Curacao, Netherlands Antilles