Bi-directional sex change in a coral-dwelling goby

Bi-directional sex change has recently been reported among obligate coral-dwelling gobies of the genus Gobiodon. However, neither the functional role of this pattern of sex change nor the frequency of sex change in either direction in natural populations is known. We investigated the social structure and pattern of sex change of Gobiodon histrio at Lizard Island on the Great Barrier Reef. The social structure of G. histrio within coral colonies usually consisted of a single juvenile or a heterosexual adult pair. The size of adult social groups was not constrained by coral colony size. In contrast to expectations for pair-forming species, G.␣histrio was primarily a protogynous hermaphrodite. All immature G. histrio were females and sex change from female to male occurred readily when two mature females were placed in a coral colony. In addition, male G. histrio were able to change back to females when two mature males were placed in a coral. Sex change from female to male, however, occurred with over twice the frequency of sex change from male to female. Where two males were placed in a coral colony, heterosexual pairs were most frequently re-established by immigration of females from outside the treatment population. This pattern might be predicted if sex change from male to female is more expensive than sex change from female to male for G. histrio. Where sex change is expensive, movement may be favoured over sex change, particularly where coral densities are high and movement among corals incurs little mortality risk. Received: 10 November 1997 / Accepted after revision: 16 May 1998     

Effects of sheltering fish on growth of their host corals

Stony corals are the foundation species of tropical reefs, and their structures can harbor a diverse range of mutualist taxa that can confer important benefits, including provision of nutrients. Prominent among the associates of branching coral in the genus Pocillopora are groups of zooplanktivorous damselfishes that take refuge in the coral to avoid their predators. In field and laboratory experiments, we explored the effects of colonies of resident damselfishes on growth of their host corals. Laboratory studies revealed a positive relationship between biomass of fish and output of ammonium. In the field, levels of ammonium were significantly elevated in the water surrounding the branches of Pocillopora occupied by colonies of damselfish, particularly in time periods following active feeding by the fish. Experimental manipulation of the presence of fish on host corals during a month-long field experiment revealed that corals hosting fish grew significantly more than those that lacked fish, and coral growth was positively correlated with the biomass of resident fish. The Pocillopora colonies in the field experiment varied in the degree of openness of their branching structure, and dye studies indicated that this affected their ability to retain waterborne nutrients. Together with biomass of resident fish, colony openness explained 76% of the variation in coral growth rate during the experiment. Corals can exhibit considerable morphological variability, and mutualistic fish respond to colony architecture during habitat selection, with some species preferring more open-branched forms. This makes it likely that corals may face tradeoffs in attracting resident fish and in retaining the nutrients they provide.     

Effects on growth and reproduction of the coral Stylophora pistillata by the mutualistic damselfish Dascyllus marginatus

Although coral dwelling fishes are common on coral reefs, the nature of their effect on the host corals is poorly understood. The present study, conducted in the Gulf of Eilat (Red Sea) between July 1989 and August 1990, demonstrated that the branching coral Stylophora pistillata (Esper) benefits, in two components of coral fitness, from the presence of the damselfish Dascyllus marginatus (Rüppell), an obligate coral dweller. The growth rate of damselfish-inhabited corals was significantly higher than that of corals without damselfish. This was observed, using two growth assessment methods, in long-term (>7 mo) comparisons between: (1) corals where the damselfish were experimentally removed versus corals with unaltered fish groups; and (2) naturally inhabited versus non-inhabited corals. The presence of damselfish did not affect the coral's specific (per surface area) reproductive output, whether it was assessed by the number of female gonads per polyp or by the number of planulae released cm^-2 surface area d^-1. However, the more rapid increase in branch size in damselfish-inhabited corals resulted in an apparent increase in the total reproductive output, with age, in growing corals. These findings demonstrate that the association between the damselfish D. marginatus and its host coral, S. pistillata, is mutualistic.     

Stimulation of fat-body production in the polyps of the coralPocillopora damicornis by the presence of mutualistic crabs of the genusTrapezia

A mutualism exists between the xanthid crabs of the genusTrapezia and their host corals,Pocillopora damicornis. It has previously been established that these obligate coral residents benefit the coral hosts by defending them against echinoderm predators and by increasing the survival of polyps located deep between the coral branches. In turn, the corals apparently benefit the crabs by producing lipid-filled structures on which the trapezid crabs feed; these fat bodies may contain some of the lipid which in previous studies of coral metabolism has been termed excess. It was determined by experiments conducted at the Hawaii Institute of Marine Biology that the presence of crabs in colonies ofP. damicornis stimulates the polyps to produce the lipid-filled fat bodies; removal of crabs causes corals to cease producing fat bodies. A structure very similar to the fat bodies ofP. damicornis has been reported inAcropora durvillei. Both of these coral genera ordinarily possess xanthid-crab mutualists. This association between branching corals and crustaceans may have evolved because corals of these genera provide shelter among their branches and because these shallow-water corals are evidently capable of releasing lipid which is excess to the corals' metabolic needs, but which can be utilized by the crabs.     

The effects of partial mortality on the fecundity of three common Caribbean corals

The recent intensification of human disturbances in the Caribbean has increased the prevalence of partial mortality on coral colonies. Partial mortality can change colony size by directly shrinking colonies or by splitting colonies into fragments. A reduction in colony size can also adversely affect fecundity and fitness as internal resources shift away from reproduction toward colony maintenance. This study aimed to determine whether three Caribbean coral species, Siderastrea siderea, Montastraea faveolata, and Diploria strigosa, along the reef tract in Puerto Morelos, Mexico (20^o52′N, 86^o51′W), continued to dedicate resources to reproduction when colonies were fragmented to pre-maturation size. Contrary to expectations, eggs were found in colonies that were smaller than the maturation size and had been subjected to partial mortality. The continued dedication of resources toward reproduction, even in the smallest colonies, suggests that resource trade-offs away from reproduction are not as rigid as previously suggested in stressed corals.     

Implications of geometric plasticity for maximizing photosynthesis in branching corals

Reef-building corals are an example of plastic photosynthetic organisms that occupy environments of high spatiotemporal variations in incident irradiance. Many phototrophs use a range of photoacclimatory mechanisms to optimize light levels reaching the photosynthetic units within the cells. In this study, we set out to determine whether phenotypic plasticity in branching corals across light habitats optimizes potential light utilization and photosynthesis. In order to do this, we mapped incident light levels across coral surfaces in branching corals and measured the photosynthetic capacity across various within-colony surfaces. Based on the field data and modelled frequency distribution of within-colony surface light levels, our results show that branching corals are substantially self-shaded at both 5 and 18 m, and the modal light level for the within-colony surface is 50 μmol photons m^?2 s^?1. Light profiles across different locations showed that the lowest attenuation at both depths was found on the inner surface of the outermost branches, while the most self-shading surface was on the bottom side of these branches. In contrast, vertically extended branches in the central part of the colony showed no differences between the sides of branches. The photosynthetic activity at these coral surfaces confirmed that the outermost branches had the greatest change in sun- and shade-adapted surfaces; the inner surfaces had a 50 % greater relative maximum electron transport rate compared to the outer side of the outermost branches. This was further confirmed by sensitivity analysis, showing that branch position was the most influential parameter in estimating whole-colony relative electron transport rate (rETR). As a whole, shallow colonies have double the photosynthetic capacity compared to deep colonies. In terms of phenotypic plasticity potentially optimizing photosynthetic capacity, we found that at 18 m, the present coral colony morphology increased the whole-colony rETR, while at 5 m, the colony morphology decreased potential light utilization and photosynthetic output. This result of potential energy acquisition being underutilized in shallow, highly lit waters due to the shallow type morphology present may represent a trade-off between optimizing light capture and reducing light damage, as this type morphology can perhaps decrease long-term costs of and effect of photoinhibition. This may be an important strategy as opposed to adopting a type morphology, which results in an overall higher energetic acquisition. Conversely, it could also be that maximizing light utilization and potential photosynthetic output is more important in low-light habitats for Acropora humilis.     

Habitat preferences of coral-associated fishes are altered by short-term exposure to elevated CO2

Rising atmospheric carbon dioxide (CO₂) concentrations are causing additional CO₂ to be absorbed by the oceans. Recent studies show that exposure to elevated CO₂ causes olfactory impairment in reef fishes; however, the ecological consequences of this impairment are largely unknown. This study examined the effects of short-term exposure to elevated CO₂ on habitat preferences of coral-dwelling gobies. Adult gobies collected from the reef at Lizard Island (Great Barrier Reef, Australia) were exposed for 4 days to ambient CO₂ (440 μatm) or elevated CO₂ (880 μatm). Habitat preferences were then tested in laboratory and field experiments at ambient conditions. In olfactory preference tests, Paragobiodon xanthosomus displayed a strong preference for odour cues of their sole host coral Seriatopora hystrix; however, this preference was absent in gobies exposed to elevated CO₂. Habitat choice experiments conducted in the field showed that Gobiodon histrio placed on dead coral colonies located preferred live habitat within 24 h; however, gobies exposed to elevated CO₂ associated with both preferred and non-preferred habitats in approximately equal frequency. Preferred habitats are known to confer fitness advantages to coral-dwelling gobies. Consequently, these results suggest that future elevated CO₂ levels might affect the ability of habitat specialist fishes to select favourable habitats.     

Three party symbiosis: acoelomorph worms,corals and unicellular algal symbionts in Eilat (Red Sea)

Epizoic worms were found to occur on certain coral colonies from reefs off the coast of Eilat (Red Sea). We identified 14 coral species infested by acoelomorph worms at a depth range of 2–50 m. The host corals were all zooxanthellate and included both massive and branching stony corals and a soft coral. Worms from all hosts were identified as belonging to the genus Waminoa and contained two distinct algal symbionts differing in size. The smaller one was identified as Symbiodinium sp. and the larger one is presumed to belong to the genus Amphidinium. Worm-infested colonies of the soft coral, Stereonephthya cundabiluensis, lacked a mucus layer and exhibited distinct cell microvilli, a phenotype not present in colonies lacking Waminoa sp. In most cases, both cnidarian and Acoelomorph hosts displayed high specificity for genetically distinctive Symbiodinium spp. These observations show that the epizoic worms do not acquire their symbionts from the “host” coral.     

Predators selectively graze reproductive structures in a clonal marine organism

Although the fitness consequences of herbivory on terrestrial plants have been extensively studied, considerably less is known about how partial predation impacts the fitness of clonal marine organisms. The trophic role of Caribbean parrotfish on coral reefs is complex: while these fish are important herbivores, as corallivores (consumers of live coral tissue), they selectively graze specific species and colonies of reef-building corals. Though the benefits of parrotfish herbivory for reef resilience and conservation are well documented, the negative consequences of parrotfish grazing for coral reproductive fitness have not been previously determined. We examined recently grazed colonies of Montastraea annularis corals to determine whether grazing was positively associated with coral reproductive effort. We measured gonad number, egg number and size, and proportional reproductive allocation for grazed and intact coral colonies 2–5 days prior to their annual spawning time. We found that parrotfish selectively grazed coral polyps with high total reproductive effort (number of gonads), providing the first evidence that parrotfish selectively target specific tissue areas within a single coral colony. The removal of polyps with high reproductive effort has direct adverse affects on coral fitness, with additional indirect implications for colony growth and survival. We conclude that chronic grazing by parrotfishes has negative fitness consequences for reef-building corals, and by extension, reef ecosystems.     

Population structure of an exploited benthic cnidarian: the case study of red coral (Corallium rubrum L.)

Octocorals are an important part of many ecosystems as they add three-dimensional complexity to the benthos and thereby increase biodiversity. The Mediterranean red coral (Corallium rubrum, L. 1758) is a longevous octocoral that is harvested commercially, yet natural and anthropogenic influences on its population size structure are little understood. This study found that some harvested red coral populations had a significantly different size structure when compared to populations at the nearby Marine Protected Area (MPA) of Medas Islands at the Spanish Costa Brava (NW Mediterranean). Eighty-nine percent of the red corals in the harvested Costa Brava area are less than 10 years old and 96% of all colonies have not yet grown more than second-order branches. The size/age distribution of the harvested population is notably skewed towards younger and smaller colonies. Thus, although red coral is still abundant, its population structure is strongly distorted by harvesting. The results confirm that MPAs are useful to distinguish between anthropogenic and natural influences on population structure. However, 14 years of protection appears to be an insufficient recovery time for a longevous octocoral population such as red coral.     

In situ thermal dynamics of shallow water corals is affected by tidal patterns and irradiance

We studied the diel variation of in situ coral temperature, irradiance and photosynthetic performance of hemispherical colonies of Porites lobata and branching colonies of Porites cylindrica during different bulk water temperature and tidal scenarios on the shallow reef flat of Heron Island, Great Barrier Reef, Australia. Our study presents in situ evidence that coral tissue surface temperatures can exceed that of the surrounding water under environmental conditions typically occurring during low tide in shallow reef or lagoon environments. Such heating may be a regular occurrence on shallow reef flats, triggered by the combined effects of high irradiance and low water flow characteristic of low Spring tides. At these times, solar heating of corals coincides with times of maximum water temperature and high irradiance, where the slow flow and consequent thick boundary layers impede heat exchange between corals and the surrounding water. Despite similar light-absorbing properties, the heating effect was more pronounced for the hemispherical P. lobata than for the branching P. cylindrica. This is consistent with previous laboratory experiments showing the evidence of interspecific variation in coral thermal environment and may result from morphologically influenced variation in convective heat transfer and/or thermal properties of the skeleton. Maximum coral surface warming did not coincide with maximum irradiance, but with maximum water temperature, well into the low-tide period with extremely low water flow in the partially drained reef flat, just prior to flushing by the rising tide. The timing of low tide thus influences the thermal exposure and photophysiological performance of corals, and the timing of tidally driven coral surface warming could potentially have different physiological impacts in the morning or in the afternoon.     

Variation in colony geometry modulates internal light levels in branching corals, <Emphasis Type="Italic">Acropora humilis</Emphasis> and <Emphasis Type="Italic">Stylophora pistillata</Emphasis>

Colonial photosynthetic marine organisms often exhibit morphological phenotypic plasticity. Where such plasticity leads to an improved balance between rates of photosynthesis and maintenance costs, it is likely to have adaptive significance. To explore whether such phenotypic plasticity leads to more favourable within-colony irradiance for reef-building branching corals, this relationship was investigated for two coral species Acropora humilis and Stylophora pistillata, along a depth gradient representing light habitats ranging from 500 to 25 μmol photons m⁻² s⁻¹, during 2006 at Heron Island, Great Barrier Reef (23.44°S, 151.91°E). In the present study changes in flow-modulated mass transfer co-varied with light as a function of depth. In low-light (deep) habitats, branch spacing (colony openness) in A. humilis and S. pistillata was 40–50% greater than for conspecifics in high-light environments. Also, branches of A. humilis in deep water were 40–60% shorter than in shallow water. Phenotypic changes in these two variables lead to steeper within-colony light attenuation resulting in 38% higher mean internal irradiance (at the tissue surface) in deep colonies compared to shallow colonies. The pattern of branch spacing was similar for S. pistillata, but this species displayed an alternate strategy with respect to branch length: shade adapted deep and cave colonies developed longer and thinner branches, allowing access to higher mass transfer and irradiance. Corals in cave habitats allowed 20% more irradiance compared to colonies found in the deep, and had a 47% greater proportion of irradiance compared to colonies in the shallow high-light environment. Such phenotypic regulation of internal light levels on branch surfaces partly explains the broad light niches of many branching coral species. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Plasticity in size and age at maturity in a monogamous fish: effect of host coral size and frequency dependence

The number and maximum body size of the gobioid fish, Paragobiodon echinocephalus, increase with the size of its obligate host coral, Stylophora. Only the largest two individuals breed monogamously in each coral head, and the reproductive success of each spawning is positively correlated with body size. In this study, the plasticity in size and age at maturity in P. echinocephalus was examined. We analyzed life history data from gobies 15–20 mm TL (total length) at their initial marking. Gobies found in larger corals were of lower rank in size order and began to breed later at a larger size, usually upon moving to other corals. The size at maturity ranged widely from 17.2 to 36.0 mm TL. After maturation, growth rates decreased. Mortality, however, was not affected by the timing of maturation. The host coral size did not affect the growth and mortality of marked fish, but the mortality rate of juveniles prior to marking appeared to be higher in smaller corals. The estimated lifetime reproductive success did not differ between the gobies inhabiting corals of different size. Thus the plasticity in size and age at maturity in this species may be maintained by frequency-dependent selection in choosing a host coral size that affects an individual’s social status. Received: 5 April 1995/Accepted after revision: 18 February 1996     

Interplay of host morphology and symbiont microhabitat in coral aggregations

The Belizean reef coral Agaricia tenuifolia Dana forms aggregations in which rows of thin, upright blades line up behind each other. On average, the spacing between blades increases with depth and hence with decreasing ambient irradiance. We designed and built a small, inexpensive light meter and used it to quantify the effect of branch spacing on light levels within colonies at varying distances from branch tips. Concurrently, we measured photosynthetic pigment concentrations and population densities of symbiotic dinoflagellates (zooxanthellae) extracted from coral branches of colonies with tight (≤3 cm) vs wide (≥6 cm) branch spacing, collected at 15 to 17 m and from colonies with tight branch spacing collected at 1 to 2 m. Light levels decreased significantly with tighter branch spacing and with distance from the branch tips. Total cellular pigment concentrations (chlorophylls a, c ₂ and peridinin) as well as chlorophyll a:c ₂ and chlorophyll a: peridinin ratios all increased significantly with distance from the branch tip, indicating very localized differences in photoacclimation within individual branches. Zooxanthellae from colonies with widely-spaced branches displayed significantly lower chlorophyll a:c ₂ and chlorophyll a:peridinin ratios, and were present at significantly higher population densities than those from colonies with tightly-spaced branches collected at the same depth (15 m). Tightly-spaced colonies collected from shallow environments (1 to 2 m) displayed pigment ratios similar to those from widely-spaced colonies from deeper water (15 m), but maintained zooxanthellae populations at levels similar to those in tightly-branched colonies from deeper water. Thus, variation in colony morphology (branch spacing and distance from branch tip) can affect symbiont physiology in a manner comparable to an increase of over 15 m of water depth. These results show that a host's morphology can strongly determine the microhabitat of its symbionts over very small spatial scales, and that zooxanthellae can in turn display steep gradients in concordance with these altered physical conditions. Received: 12 June 1997 / Accepted: 24 June 1997     

The effects of an abnormal decrease in temperature on the Eastern Pacific reef-building coral Pocillopora verrucosa

Coral bleaching events are associated with abnormal increases in temperature, such as those produced during El Niño. Recently, a breakdown in the coral–dinoflagellate (genus Symbiodinium) endosymbiosis has been documented in corals exposed to anomalously cold-water temperatures associated with La Niña events. Given the ecological significance of such events, as well as the threat of global climate change, surprisingly little is known about the physiological response of corals to cold stress. This study evaluated some physiological effects of continuous temperature decline in colonies of the eastern Pacific reef-building coral Pocillopora verrucosa. Twenty days of incubation at 18.5–19.0 °C resulted in a substantial decrease in holobiont lipid and Chla content, as well as an increase in Symbiodinium density. These observations suggest a combination of symbiont acclimation due to the temperature decline and reallocation of carbon toward algal growth as opposed to translocation to the host coral. With a decreased availability of symbiont-derived carbon, the coral likely catabolized storage lipids in order to survive the stress event. Despite this stress and some tissue necrosis, no mortality was noted and corals recovered quickly when returned to the ambient temperature. As these results are in marked contrast to similar studies investigating elevated temperature on this coral from this same location, Pocillopora in the Mexican Central Pacific may be more prone to long-term damage and mortality during periods of ocean warming as opposed to ocean cooling.     

The energetic cost of protogynous versus protandrous sex change in the bi-directional sex-changing fish Gobiodon histrio

To investigate the relative cost of protogynous versus protandrous sex change we induced sex change in each direction in Gobiodon histrio (Gobiidae) and then compared growth, body condition and biochemical condition between sex-changed and non-sex-changed fish in treatment and control groups. Sex change in each direction was induced by establishing pairs of adult males and pairs of adult females on isolated coral colonies. Heterosexual pairs were used as controls. For both males and females, growth and body condition did not vary between fish that changed sex and those that did not. The relatively low cost of sex change, compared to the likely costs of searching for a mate of the correct sex, appears to explain the evolution of bi-directional sex change in coral-dwelling gobies. Lipid concentrations in the liver of males and females that changed sex were reduced by similar amounts (34% and 41%, respectively) compared to male and female controls that did not change sex. Therefore, changes in biochemical condition were approximately equal for fish that sex change in each direction and cannot explain the unequal frequency of sex change in each direction in natural populations of G. histrio.     

Population structure and feeding deterrence in three shallow-water antarctic soft corals

Alcyonium paessleri and Clavularia frankliniana are numerically abundant soft corals in the nearshore (12 to 33 m depth) benthic communities of eastern McMurdo Sound. They are much less abundant in western McMurdo Sound where a third species, Gersemia antarctica, co-occurs in low numbers. The body tissues of these three species are comprised mainly of organic material (53 to 70% dry wt), which is primarily dervied from NaOH-soluble protein and refractory material. The energetic contents of the whole-body tissues of A. paessleri, C. frankliniana and G. antarctica are 15.9, 17.3, and 14.5 kJ g^-1 dry wt, respectively. The mean biomass per individual is 1.81, 0.008, and 45 g dry wt for each respective species. Based on population densities of 7.3, 1337.3, and 0.04 soft corals m^-2 for A. paessleri, C. frankliniana and G. antarctica, respectively, the population energetic densities are estimated to be 210.1, 185.1, and 26.1 kJ m^-2. Despite the relatively rich energetic content of the tissue and apparent vulnerability to predators, very little predation occurs on these soft corals. Two potential predators, the antarctic sea stars Perknaster fuscus and Odontaster validus, exhibited significant chemotactic defensive tube-foot retractions to hexane, chloroform, methanol, and aqueous methanol extracts of each soft coral. In addition, wholebody tissue of each soft coral was rejected by the demersal fish Pseudotrematomus bernacchii and the cryopelagic fish Pagothenia borchgrevinki. In contrast, whole soft-coral tissues sequentially extracted in four increasingly polar solvents were readily ingested by these antarctic fishes, indicating that sclerites do not play a significant role in deterring predators. Our results indicate that these antarctic soft corals contain bioactive compounds which deter common predatory seastars and fishes.     

Proximity to competitors changes secondary metabolites of non-indigenous cup corals, Tubastraea spp., in the southwest Atlantic

Competition for space changes species’ distributions and community organization on tropical rocky shores, and the presence of secondary metabolites in the tissues of non-indigenous species may aid them in establishing and expanding their range through negative competitive interactions. The aim of this study was to describe the range of chemical substances produced by the non-indigenous cup corals Tubastraea coccinea and T. tagusensis and to test whether they varied in the field when the corals were placed in proximity to two local competitors. Cholest-5-en-3β-ol and 9-octadecanoic acid were two common secondary metabolites found in the tissues of Tubastraea. In the competition interaction experiment, necrosis was detected on the tissues of the coral Mussismilia hispida, and this species induced variation in sterol, alkaloid, and fatty acid production in Tubastraea tissues. In contrast, a sponge overgrew Tubastraea colonies. These results indicate that chemical defense may contribute to the ability of these non-indigenous corals to invade native communities.     

Bacterial colonization and endocytosis on the gill of a new limpet species from a hydrothermal vent

Calcification rates in different fragments along branches of the hermatypic coral Stylophora pistillata were tested in the laboratory using a new technique, the optic glassfiber method. By this method, the tested colony remains constantly in dark conditions while a narrow beam of light, transferred through the optic fiber, illuminates a small distinct point of coral tissue (on a branch tip or base). The selected illuminated portion of the branch serves as the experimental fragment, while all the other parts of the same colony serve as the dark controls. The results indicate that significantly more calcium is incorporated in the tip fragments than in the bases, both in light and in dark conditions (4.1 to 13.2 times more). Illumination of the tips or the bases did not stimulate or enhance the calcification rates of these fragments. Thus, in all colonies tested, the calcification rates of the illuminated fragments were not significantly different from the average rates of other similar, non-illuminated fragments of the same colony. It is suggested that light does not directly enhance calcification in hermatypic corals, but rather, that light enhances O₂ production, which consequently stimulates coral metabolism. Our preliminary results indicate that calcification rates recorded in aerated dark experiments are significantly higher than calcification rates of non-aerated dark controls.     

Marginal tentacles of the corallimorpharian Rhodactis rhodostoma. 2. Induced development and long-term effects on coral competitors

Polyps of the corallimorpharian Rhodactis rhodostoma (Ehrenberg, 1934) form aggregations that monopolise patches of space on the shallow reef flats of some Red Sea coral reefs. Some of these polyps bear specialised bulbous marginal tentacles (BMTs) where they contact cnidarian competitors. BMTs differ from the normally filiform marginal tentacles (FMTs) of R. rhodostoma, and appear to develop from them. However, their morphogenesis and long-term impacts on spatial competition with reef corals are unknown. We experimentally induced contacts between R. rhodostoma polyps and colonies of the branching stony coral Acropora eurystoma on a shallow coral reef at Eilat, northern Red Sea. During the first 24 d of contact, the A. eurystoma colonies extruded mesenterial filaments that damaged the tissues of the corallimorpharian polyps. After 18 d,>90% of R. rhodostoma individuals had developed BMTs, which resulted in a reversal in the direction of competitive damage. During the subsequent 1.5 years of observation, the corallimorpharians maintained well-developed BMTs, unilaterally damaged the tissues of A. eurystoma, and in some cases moved onto the stony coral skeletons and partially overgrew them. BMTs developed from FMTs in a series of four distinct stages, accompanied by significant changes in their morphology, cnidom, and density of nematocysts. Isolated control polyps did not develop BMTs or show any signs of damage. In contrast, corallimorpharian polyps transplanted into contact with colonies of the massive stony coral Platygyra daedalea began to develop sporadic BMTs, but were unilaterally and severely damaged by the corals, and started to disappear within 21 d, after the corals developed sweeper tentacles. We conclude that long-term outcomes of competition between R. rhodostoma and reef-building corals depend largely on the relative aggressive reach of the competitive mechanisms developed by each species. As a consequence, this corallimorpharian is an intermediate competitor in the aggressive hierarchy among Indo-Pacific reef corals. This study confirms that R. rhodostoma polyps may actively damage and overgrow some stony corals, leading to the formation of an almost continuous blanket of polyps in large patches of some shallow reef flats. Received: 15 July 1998 / Accepted: 24 March 1999