Download Predation of the sea urchin Diadema antillarum Philippi on living

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.
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Predation of the Sea Urchin Diadema antillarum
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Roll P. M. Bak
Guillaume van Eys
Caribbean Marine Biological Institute (Carmabi)
Piscaderabaai
Curacao, Netherlands Antilles