INTERSPECIFIC AGGRESSION AMONG STONY

BULLETIN OF MARINE SCIENCE, 65(3): 851–860, 1999

CORAL REEF PAPER

INTERSPECIFIC AGGRESSION AMONG STONY CORALS IN EILAT, RED SEA: A HIERARCHY OF AGGRESSION ABILITY AND RELATED PARAMETERS Avigdor Abelson and Yossi Loya ABSTRACT Stony corals use various aggressive mechanisms to compete for space, which is often considered a limiting resource on coral reefs, particularly at undisturbed sites. Interspecific aggression among stony corals has been studied in Eilat [Northern Gulf of Aqaba (Eilat)], using random SCUBA traverses and line transects. The resultant hierarchy, constructed by ranking the abilities of species to damage neighboring corals, reveals intransitive interactions which occur mostly within the intermediately-aggressive species. Possible relations of the aggression rank of the species studied to ecological or biological parameters (i.e., polyp (calyx) size, and species abundance and distribution) were examined. No apparent correlation was found between aggression and species abundance. Other parameters (i.e., polyp size and species distribution) show significant links with the species’ aggression rank. Aggression hierarchies are among the ecological parameters used to compare coral reefs of different geographical regions. The present paper provides a first record of the aggression hierarchy for Red Sea corals, which may be compared to hierarchies from other regions. The constructed aggression hierarchy may serve as a step towards a better understanding of the ecology of coral reefs in the Gulf of Aqaba.

Aggressive interactions, which are deleterious to neighboring reef organisms, are considered to be a form of competition for space (Lang and Cornesky, 1991). Many studies dealing with competition for space on coral reefs are concerned with stony corals, the main frame builders of the reefs (e.g., Lang, 1971; Connel, 1976; Sheppard, 1979, 1981; Logan, 1984; Dai, 1990; for review, see: Lang and Chornesky, 1991). The basic suggestion of these studies is that corals compete for space by damaging or killing their neighbors and growing over them. It was suggested that this competition by means of aggression is one of the life history traits of corals affecting their community structure (Lang, 1971; Connell, 1976, 1978; Sheppard, 1979, 1981, 1982, 1985; Wellington and Trench, 1985; Karlson and Hurd, 1993). Following the potential role of coral aggression in coral-reef ecology, diverse studies have been made in an effort to better understand the aggression ranking of corals in given sites. A common way to determine the coral aggression rank in a given community is to construct a hierarchy in which species higher up the scale are mostly capable of damaging those beneath them. The species are ranked in a hierarchy according to the frequencies of success versus failure events in which each species is involved. The first aggression hierarchy was built by Lang (1973) who noticed a transitive hierarchy of digestive dominance in an aquarium. Based on this, Lang proposed a mechanism by which slow growing, rare species outcompete fast growing abundant species in the Jamaican coral reefs. Later, Connell (1976, 1978) demonstrated two kinds of hierarchy, transitive and intransitive, and suggested their possible role in the high species diversity of coral reefs. Likewise, aggression abilities were used to explain species abundance (Loya, 1976; Porter, 1976; Sheppard, 1979; Dai, 1990) and persistence (Wellington and Trench, 1985).

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Since the discovery of varied aggression abilities of stony corals, attempts have been made to relate aggressiveness ranks of coral species to various parameters which may partially explain the possible ecological or evolutionary implications of aggression. Such attempts were made to relate aggression ability to species diversity (e.g., Sheppard, 1982; Logan, 1984), species abundance (e.g., Lang, 1973; Sheppard, 1979), species distribution (Sheppard, 1979), and polyp size (Sheppard, 1981). Likewise, using aggression hierarchies, there has been an attempt to compare coral reefs of distinct geographical zones (Cope, 1981; Logan, 1984). One prominent reef-coral bearing sea which has not yet received much attention in the context of stony-coral aggression ranks and their potentially related parameters, is the Red Sea. The present study was conducted with the following objectives: (1) to evaluate the relative aggression ability of Red-Sea stony corals, and to classify them in an aggression hierarchy for the coral reefs of Eilat; (2) to characterize the model of hierarchy—whether transitive (simple hierarchy) or intransitive (network); (3) to examine possible relations of certain ecological or biological parameters (i.e., species abundance, species distribution, body morphology and polyp size) to the aggression rank of the coral species examined. MATERIALS AND METHODS The study was carried out along the coast of Eilat (the northern-most part of the Gulf of Aqaba, Red Sea), during the years 1985–1987. Observations were made at depths between 0 and 40 m. Previous work at the same sites recorded 100 stony coral species exhibiting very high species diversity and species richness (Loya and Slobodkin, 1971; Loya 1972). Interactions between neighboring coral colonies were recorded in the field by 50 random SCUBA traverses and a series of 54 line transects. The traverses were carried out by means of random explorations in shallow water (reef table to 10 m depth) and deep water (10 to 40 m depth), during which interactions were located, classified and recorded. The line transects were carried out in three fixed stations in the coral reef reserve of Eilat. In each station, the transects were run along depth contours parallel to the shore from the reef table to 30 m depth in six depths of ca 0.5, 3, 5, 10, 20, and 30 m depth. Each transect was 10 m long, and any coral observed under the line which was in interaction with a neighboring coral, was recorded (for further description of the line transect methodology, see Loya, 1972). Four types of interactions were recognized: (1) dead margins on the subordinate coral (easily recognized in the field due to bleaching of tissue or settling of algae on the recently exposed skeleton); (2) overgrowth of the subordinate coral by physical contact of the margins of the dominant one; (3) unnatural deviation of branch growth, or abnormal morphology that fits neighboring colonies with no physical contact; (4) no evident reaction, or mutual damage with no obvious winner between colonies located within 1 cm of each other (considered as a stand-off). In order to assess the aggression mechanisms involved among the various species night observations were made in the field and corals were placed in aquaria for continuous observations. Ten replicates of induced interactions by means of colony transplantations were carried out in the field to clarify indistinct field observations involving certain species of special interest. An attempt was made to match equally sized corals for each pair. In order to mimic the natural conditions of interaction as much as possible, the transplanted corals were positioned on plastic stands at their own natural depth and with a gap of 1 cm between the opponents. Calyx diameter is used as an index for polyp size (Sheppard, 1981). The calyx diameter of coral species exhibiting large calyx size was measured to a 0.5 mm precision, using a caliper. The measurement of calyx size of species with fine calices was done by scale under a dissecting microscope. For each coral species, nine measurements were taken from three specimens. For each species, the largest calyx size was considered for the calyx size class (Table 1).

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Table 1. List of the species hierarchy, their aggression rank, mean calyx diameter, calyx size-class (1 = 0– 2.0 mm, 2 = 2.1– 5 mm, 3 = 5.1– 10 mm, 4 = 10.1– 15 mm, 5 = >15.1 mm), and abundance rank (1 = rare, 2 = uncommon, 3 = sporadic, 4 = common, 5 = abundant, 6 = dominant). Species Plerogyra sinuosa Fungia fungites Galaxea fascicularis Lobophyllia sp. Cynarina lacrymalis Goniopora savignyi Favia doryensis Echinopora gemmacea Hydnophora contignatio Pocillopora damicornis Pocillopora verrucosa Echinophyllia aspera Acropora hyacinthus Platygyra lamellina Hydnophora microconus Montipora granulata Favites pentagona Goniastrea pectinata Cyphastrea chalcidicum Pavona varians Favites abdita Montipora erythraea Montipora lobulata Favia favus Acropora variabilis Acropora hemprichi Turbinaria sp. Stylophora pistillata Millepora sp. Stylophora prostrata Porites alveolata Porites lutea Astreopora myriophthalma Seriatopora angulata

Aggression Rank 17 17 16 16 15 15 15 14 13 12 12 11 11 10 9 8 7 7 6 5 5 5 4 4 4 4 3 2 2 2 1 1 1 1

Calyx size (mm) 16 50 8 50 30 6 3 6 5 1 1 11 1 8 5 1 10 6 2 2 12