BULLETIN
OF MARINE SCIENCE, 31(3): 514-522, 1981
COMPETITION FOR SPACE AMONG CORAL-REEF
ORGANISMS AT EILAT, RED SEA
SESSILE
Y. Benayahu and Y. Loya
ABSTRACT
The major benthic organisms occupying space in the coral reefs of the northern Gulf of
Eilat are stony corals, soft corals and algae, Living coverage of these three groups served
as the criterion for determining their relative utilization of space, in nine reef localities. Stony
corals and algae are the major space utilizers on the reef flats, while soft corals represent a
minor group. It is suggested that on the reef flats, stony corals and algae are the major
competitors for space. Stony corals constitute the major group occupying space in all upper
fore-reef zones (1-4 m depth), whereas living coverage of soft corals and algae is lower.
Space partitioning among the three groups, measured in one locality, in a series of transects
to a depth of 29 m, indicates that below the reef flat soft corals and stony corals are the
major components occupying space (approximately 70% of the available substrate), while
algal cover is negligible. Living coverage of stony corals consistently increases from ]0% at
a depth of 4 m to 70% at 29 m. The most abundant group of soft corals at a depth greater
than 3 m is the Xeniidae. The xeniids exhibit a pattern of successive decrease of living cover
with depth (from 50% at 4 m to 5% at 29 m).
The most abundant species among the xeniids of Eilat is Xenia macrospiculata. This
species is the first among the soft corals to colonize artificial substrates and denuded areas.
A unique mechanism of active movement is described for this species. It is suggested that
such active movement enables the coral to regulate its population size, maximize substrate
utilization and hence, successfully compete for space with other benthic organisms.
The coral reef ecosystem provides a substrate for a large variety of organisms.
Space in the light is one of the most important limiting resources on a coral reef,
resulting in competitive interactions among co-occurring populations (Connell,
1973; Lang, 1973). Most of the studies on competitive processes among sessile
reef communities have concentrated on stony corals, while only vague information is given on competitive capacities of other space utilizers (Glynn, 1973; Maragos, 1974; Jackson and Buss, 1975; Potts, 1977; Maguire and Porter, 1977; Sheppard, 1979).
The major faunistic and floristic components occupying space among the coral
reef benthic communities of the northern Gulf of Eilat (Red Sea) are stony corals,
soft corals and algae (Benayahu and Loya, 1977a). Quantitative studies concerned
with distributional patterns of these components on the coral reef ecosystem are
scarce (Faure, 1974; Drew, 1977; Morrissey, 1980). Some studies deal with competitive relationships between stony corals and benthic algae (Banner and Bailey,
1970; Maragos, 1972; Bak et al., 1977; Morrissey, 1980), while interactions of soft
corals with the other two groups have been almost neglected.
In the present work, we present quantitative results of a multi-group study
demonstrating space partitioning among stony corals, soft corals and algae in
various localities along the northern Gulf of Eilat. Other benthic organisms such
as sponges are scarce in the areas studied and hence not included in the present
study.
A unique mechanism for occupation of space by active movement is described
for the soft coral Xenia macrospiculata (Alcyonacea, Xeniidae) and its competitive significance is discussed.
514
BENA YAHU AND LOY A: REEF SPACE COMPETITION
515
100
.0
.0
10"
Figure I.
flats.
Percent living cover per transect of stony corals, soft corals and benthic algae on the reef
METHODS
The study was carried out at 9 localities (Fig. 1) along the eastern coast of the Gulf of Eilat, from
the coral reef of the Nature Reserve of Eilat to Ras el Burqa', approximately 50 km south of Eilat
(for detailed map of exact localities see Benayahu and Loya, 1977a). The living coverage of stony
corals, soft corals and algae was recorded on the reef flats and the upper fore-reef zones (1-4 m
depth) by series of line transects (10 m each). In one station, Center Muqebla', the reef was surveyed
to a depth of 29 m with a depth interval of I m between adjacent transects. The line transect methodology used in this study follows that of Loya and Siobodkin (1971).
Details on exact location of studied areas, dates of sampling, systematic lists of the species of stony
corals, soft corals and algae in each locality, number of transects surveyed in each station and within
each reef zone, methodology for measuring annual living coverage of algae and statistical procedures
involved in the analysis of the data are given in Benayahu and Loya (1977a; b).
One of the most abundant soft corals found in the study areas is Xenia macrospiculata. In order
to study some aspects of the population dynamics of this species, colonies found in high-density
patches were tagged with a plastic DYMO tape, with individual reference numbers. The tags were
attached to the corals by piercing a stainless suture-wire through the basal stalk of the colonies
without causing any damage to the colonies. PVC plates were put near the bases of the corals.
Periodical underwater photographs of the tagged colonies were taken by a Nikonos III camera (28
mm lens with close-up attachment).
RESULTS
Figures 1 and 2 are a graphical presentation of detailed data given in Benayahu
and Loya (1977a) summarizing the percentage of living cover of stony corals. soft
corals and algae in the reef flats and upper fore-reef zones at the northern Gulf
of Eilat.
516
BULLETIN
OF MARINE SCIENCE. VOL. 31. NO.3,
1981
I
'Or
I
I
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lr
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I
Figure 2. Percent living cover per transect of stony corals, soft corals and benthic algae on the
upper fore-reef zones.
Figure 1 indicates that stony corals and algae are the major space utilizers on
the reef flats, while soft corals are a minor group. Thus, in all localities the
cumulative coverage of stony corals and algae is always higher than that of soft
corals. It is striking that on the reef flats of Ras el Burqa' and the Nature Reserve,
where the algal cover is extremely high, the living cover of stony corals is poor.
Stony corals constitute the major group occupying space in all the upper forereef zones, reaching a maximum of almost 50% in Ras el Burqa'. Living cover
of soft corals and algae in these areas is considerably lower. The average percent
cover of soft corals on the reef flats and on the upper fore-reef zones is approximately the same, with few exceptions. The highest living cover was recorded on
the reef flat at km 202 (17%, Fig. 1) and on the upper fore-reef slope of South
Muqebla' (15%, Fig. 2).
Figure 3 demonstrates the depth distribution of stony corals and soft corals in
a series of line transects run from shallow water to a depth of 29 m, at the coral
reef of Center Muqebla'. Algal cover is omitted in the figure, since it is negligible
at depths below 4 m. Each point within a given depth represents the results of
a 10m long transect.
Since xeniids are the most significant contributors to living coverage within the
soft corals, their distributional pattern is presented separately from that of other
soft corals in Figure 3. When living coverage of all soft corals (excluding xeniids)
is examined, an even pattern of low coverage persists all along the depth gradient
studied. The curve of total cover represents the summation of living cover of
5]7
BENAYAHU AND LOYA: REEF SPACE COMPETITION
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Figure 3. Space utilization of stony corals, xeniids and all other soft corals at Center Muqebla' along
a depth gradient. Each point within a given depth represents the percentage of living cover per IO-m
transect.
stony corals, xeniids and all the other soft coral groups. It is evident from Figure
3 that a decrease in living cover of xeniids was recorded from a maximum of 50%
at 4 m depth to 5% at 29 m depth. An opposite pattern is exhibited by the stony
corals, where living cover is only about 10% at a depth of 4 m, and consistently
increases to a maximum cover of 70% at 29 m depth (Loya, 1972). The combined
total cover of stony corals and all soft corals indicates high utilization of space
by these components, yet it does not exceed 100% (see Discussion).
The most abundant species among the Xeniidae in depths below 4 m in Center
Muqebla' is Xenia macrospiculata. However, other common Red Sea xeniids
such as X. membranacea,
X. hicksoni, X. b/umi, Heteroxenia fuscescens,
H.
ghardaqensis, Anthelia glauca and A. fishelsoni seldom appear in this area.
The low cover of soft corals other than xeniids is contributed by several
species: Lithophyton arboreum is common at a depth of 5-10 m. Pareythropodium fulvum fulvum and Paralemnalia thyrsoides appear along the whole
depth range. Sinularia, Lobophytum and Sarcophyton species which are common
in shallow reef zones are rare in reef areas below 4-6 m.
Preliminary experiments on colonization rate of Xenia macrospiculata on artificial and natural denuded substrates indicated that this species was the first to
densely colonize these areas. The most rapid colonization was evident on PVC
plates placed in close neighborhood to Xenia colonies, in densely populated
areas. These artificial substrates were covered by Xenia colonies within a period
of 5-8 days, which is undoubtedly too short a period for the time required for a
planula larva to settle and develop into an adult colony. We have observed that
the colonization of the newly available space is achieved by an active movement
of Xenia colonies. This is the first reported evidence for such a movement in soft
corals.
Figure 4 demonstrates the main stages in the sequence of movements of a
colony from its stand on a natural substrate to a nearby PVC plate. Field observations have indicated that colonies of X. macrospiculata gradually detached
518
BULLETIN OF MARINE SCIENCE, VOL. 31, NO.3, J981
Figure 4. The sequence of movements of a colony of Xenia rnacrospiculata from its stand on a
natural substrate to a nearby PVC plate: (a) Day I, a PVC plate is placed near the basis of X.
rnacrospiculata (No. 11); (b) Day 2, the colony bends towards the PVC plate; (c) Day 5, the colony
is partly detached from the natural substrate and most of its basis is attached to the introduced plate;
(d) Day 8, the colony has completed its movement and settled on the plate.
BENAYAHU AND LOYA: REEF SPACE COMPETITION
Figure 5. Settlement of Xenia macrospiculata
of a whole colony.
519
on Seriatopora caliendrum through active movement
from their previous place and simultaneously
adhered to adjacent skeletons of
dead stony corals. Active movement is used by this species for climbing and
settling on living tissue of stony corals (Fig. 5).
DISCUSSION
Space in the light is essential for corals as well as algae, for successful settlement and development of their propagules (Dahl, 1973; Maguire and Porter, 1977).
Since space is one of the limiting factors in the coral reef ecosystem, overlap in
the utilization of space may result in competitive interactions among co-occurring
populations. Stony corals and algae are the major space utilizers in shallow water,
where soft corals are relatively less abundant (Figs. 1, 2). However, in reef zones
deeper than 4 m, living coverage of algae is minute, while soft corals are significant space utilizers together with the stony corals (Fig. 3).
In all studied areas, space utilization in shallow water by the three components
varies widely but does not exceed 100% (Figs. 1,2). In deeper reef zones (4-30
m), approximately 70% of the surface is covered by corals (Fig. 3). Some other
benthic organisms may be found on the reef surface, such as bryozoans, sponges,
sea anemonies and hydrozoans, but their space utilization at the coral reefs of
the northern Gulf of Eilat is negligible. The rest of the vacant reef surface is a
potential site for colonization.
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BULLETIN OF MARINE SCIENCE, VOL. 31, NO.3,
1981
The complexity involved in the interrelationship between benthic algae and
corals has been widely discussed in the literature. Algae can suppress coral survivorship by various mechanisms. Some investigators claim that filamentous algae
prevent settlement of coral larvae (Stephenson, 1930; Vine, 1974). Algae may
cover or overtop stony corals and consequently cause their death (Banner and
Bailey, 1970; Potts, 1977). Turf algae, which are common in the coral reef environment, rapidly colonize newly denuded surfaces or vacant substrates (Dahl,
1972; Benayahu and Loya, 1977a; Bak et aI., 1977; Morrissey, 1980). The relative
success of algae in colonization is probably a result of their biological and ecological attributes, such as short generation time, rapid growth rate, opportunistic
life histories and wider tolerance ranges (Morrissey, 1980). On the other hand,
grazing activity by sea urchins and other herbivores suppress algal monopolization of space (Benayahu and Loya, 1977b).
Ecological studies dealing with soft corals are scarce. In spite of their high
abundance in Indo-Pacific coral-reef regions, they usually constitute a minor part
in general descriptions of coral reef surveys (Nishihira and Yamazato, 1974; Veron et aI., 1974). Some preliminary observations have been made by Garrett
(1974) on competition for space in the light by large and small colonies of stony
corals and soft corals. From the limited data obtained on experimental plots,
massive encrusting species of Lobophytum, Sarcophyton and Sinularia appear
to inhibit growth of hard coral recolonizers, such as Acropora, Porites, Galaxea
and Stylophora, especially when these corals are still small. Large scleractinian
corals, such as Acropora (especially plate forms), encrusting Montipora, Poci/lopora, Tubastrea and some faviids seem to be able to inhibit growth of alcyonarian colonies smaller than themselves, such as Lobophytum, Sarcophyton and
xeniids.
In the study areas soft corals create aggregations on the reef flats and on the
upper fore-reef zones. Many soft corals belonging to the family Alcyoniidae, such
as Sinularia, Lobophytum and Sarcophyton species dominate these shallow areas
(Benayahu and Loya, 1977a). In the reef flat of km 202 (Fig. 1) and the upper
fore-reef zone of South Muqebla', space is highly monopolized by such aggregates. In other localities their coverage is relatively low (Figs. I, 2).
There are many gaps in our knowledge on the competitive strategies of soft
corals. Some evidence is given by Maragos (1974) and Nishihira and Yamazato
(1974) that soft corals can overgrow stony corals and cause their death. In addition, their rapid growth rate enables them to capture empty space (Endean,
1976). It is interesting to note that in Center Muqebla' where 70% of the substrate
is covered by stony corals and soft corals, the relative contributions to living
cover in this reef interchanges between stony corals and xeniids at 10-12 m depth
(Fig. 3). That is, where living cover of soft corals is high, the living cover of
stony corals is relatively low and vice versa. A similar pattern of distribution has
been described by Kinzie (1973) in his study of gorgonian populations on West
Indian coral reefs. He found that diversity of gorgonians was inversely related to
the area of living coral.
In this study we provided for the first time evidence for active movement of
a soft coral (Fig. 4). This capability of X. macrospiculata acts both as a mechanism for regulation of its population size and as a unique competitive feature.
Presence of vacant space near Xenia colonies stimulates them to quickly occupy
the available substrate. Physical and biological disturbance in the coral reef environment may cause death of benthic organisms. Thus, space is continuously
opened for settlement. The ability to quickly seize available substrate enables X.
macrospiculata to maximize its space utilization, hence preventing settlement
BENA YAHU AND LOY A: REEF SPACE COMPETITION
521
and growth of other benthic organisms. In addition, the active movement is used
by this species for climbing and settling on living colonies of stony corals (such
as Seriatopora caliendrum, Stylophora pistillata, Porites lutea) and competitively
excluding them (Fig. 5).
Experiments dealing with colonization capacities of xeniids revealed that the
movement phenomenon is a wide-spread and common feature in these soft corals.
In Xenia macrospiculata we observed this phenomenon year around. Since sexual reproduction and planulae shedding of this species is limited to 2-3 months
only (Benayahu and Loya, in preparation), the movement capability ofthis species
increases its chances of successfully competing for space with other sessile reef
organisms.
ACKNOWLEDGMENTS
We are grateful to Dr. J. Verseveldt, Zwolle, The Netherlands, for identifying the soft corals and
to Professor L. Fishelson, Tel Aviv University, for his helpful advice during the study. Professor M.
Pichon and Drs. C. Wallace and J. D. Collins from James Cook University of North Queensland,
improved the manuscript by their good comments. We thank Ms. L. Maman, Mr. A. Shoob and Mr.
Z. Wolodarski for taking the photographs. This research was supported by the United States-Israel
Binational Science Foundation, (B.S.F.), Jerusalem, Israel.
LITERATURE
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362-382.
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and --.
1977b. Seasonal occurrence of benthic-algae communities and grazing regulation
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R. Endean, eds. Biology and geology of coral reefs. Vol. II. Academic Press. 480 pp.
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1973. Surface area in ecological analysis: quantification of benthic coral reef algae. Mar.
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1981
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DATE
ACCEPTED:
April
13, 1981.
ADDRESS:
Department of Zoology, The George S. Wise Center for Life Sciences, Tel Aviv University, Tel Aviv, Israel.