Simulation of recovery of pre-disturbance size structure in populations of Porites spp. damaged by the crown of thorns starfish Acanthaster planci

Outbreaks of the crown of thorns starfish Acanthaster planci (L.) have caused high levels of mortality and injury in corals on the Great Barrier Reef. In surveys conducted in 1985 and 1986, it was estimated that a quarter of the massive Porites spp. corals surveyed at five reefs — John Brewer, Rib, Potter, Feather and Green Island — had been killed outright. In addition, there was minor to severe injury to colonies throughout the coral's size range. A population model was used to evaluate this damage. The evaluations were based on simulations of the time necessary for prior abundances of large colonies (>2 m diam) to be re-established and on simulations of the number of additional disturbances the populations could withstand before becoming locally extinct. The affect of recurrent disturbances on populations receiving recruits was also simulated. Assuming no further disturbance, the model predicts minimum recovery times in excess of 50 yr for most of the populations, and 9 to 100 yr for sub-populations within reefs. For populations subjected to repeated disturbance every 10 to 30 yr, it predicted that all large colonies would soon be lost unless there was both low background mortality and a major recruitment of Porites spp. between outbreaks. In the worst-case scenario of failed recruitment and high background mortality, four of the five populations could withstand as few as two to three additional disturbances equivalent to those earlier in this decade.     

Influences of habitat and natural disturbances on contributions of massive Porites corals to reef communities

We compared densities, distributions and size frequencies of massive corals in the genus Porites on five relatively exposed, mid-shelf reefs (50 km offshore) in the central Great Barrier Reef with those on a sheltered inshore reef (10 km offshore). Data included various transect and mapping studies between 1984 and 1990, estimates of size-dependent damage from the crown-of-thorns starfish Acanthaster planci, estimated densities of herbivorous sea urchins (potential predators of juveniles), and observations of size-specific effects of tropical cyclones. Assemblages of Porites spp. on mid-shelf reefs were dominated by small colonies (2 to 10 cm diam) established either from planula larvae or from small tissue remnants that had survived A. planci predation in the early to mid-1980s. Large colonies (up to 10 m diam) were scarce, except for localized aggregations on terraces at the base of reef slopes (6 to 12 m deep). Extensive space suitable for settlement by coral larvae can be attributed to recurrent cyclones and A. planci outbreaks. Despite low sea urchin predation, the slowly growing Porites juveniles are likely to die from overgrowth by numerous, much faster growing corals. On the sheltered inshore reef, the coral community was dominated by very large (>5 m diam) Porites colonies, several centuries old; recruitment was mainly by fragmentation of large colonies; there was little space available for settlement, and probabilities of juvenile mortality from grazing urchins were high. Differences in settlement and early survival of Porites spp. are exacerbated by different regimes of storm damage. A model is proposed that links wave climate with the size and age reached by corals before dislodgement by storm waves, and which is consistent with observed densities and size-frequency distributions of Porites in sheltered and exposed areas.     

The reef environment and competitive success in the Corallimorpharia

Competitive success within coral reef communities is controlled by various factors. In addition to competitive abilities in direct interactions with a contestant, external influences such as disturbance caused by nutrient input may determine the outcome of antagonistic interactions. We examined the competitive success of corallimorpharians on coral reefs by investigating their distribution patterns within reefs and how well they perform in interference competition with staghorn corals in different environments. Substrate composition and corallimorpharian growth were examined on three reefs in Tanzania under different disturbance regimes using the line-intercept transect and point techniques. A transplant experiment was conducted in which staghorn corals (Acropora formosa) were exposed to the polyps of Rhodactis rhodostoma to establish how competition between corals and corallimorpharians affects their respective distributions. Within reefs corallimorpharians seemed to be more competitive in shallow waters. This could be due to both environmental factors as well as varied competitive abilities depending on surrounding benthos that changed with depth. Reef environment also seemed to influence corallimorpharian growth among reefs as they had the highest densities in the areas with the highest nutrient loads. The transplant experiment revealed that the corallimorpharians had a competitive advantage over the corals, and in comparisons of reefs influenced by different degrees of disturbance, corallimorpharians were most competitive in the area with the highest nutrient content. Hence, stress on coral reefs in the form of raised nutrient loads may favour the competitive success of corallimorpharians.Communicated by P.W. Sammarco, Chauvin     

Seasonality of coral reproduction in the Dampier Archipelago,northern Western Australia

Coral spawning in Western Australia (WA) occurs predominantly in the austral autumn in contrast to the Great Barrier Reef (GBR) on Australia’s east coast where most spawning occurs in spring. Recent work, however, suggests a second spawning period in northern WA with at least 16 Acropora spp spawning in spring or early summer. This discovery has initiated a re-examination of reproductive seasonality in northern WA, particularly on inshore reefs adjacent to large development projects, such as the site of this study in Mermaid Sound, in the Dampier Archipelago. Three locally abundant taxa, Porites spp, Pavona decussata and Turbinaria mesenterina were sampled monthly from September 2006 to May 2007 to determine sexuality, the mode of reproduction and the time of gamete maturity. All three taxa were gonochoric broadcast spawners. Porites spp. colonies were mature in November and December, P. decussata in March and April. In contrast, most colonies of T. mesenterina contained mature gametes for up to 5 months beginning in November, suggesting either individuals are releasing gametes on multiple occasions, or they retain mature gametes for more than 1 month. Field surveys to determine the reproductive status of the remaining coral assemblage were conducted prior to the full moon in October 2006 and March 2007. Only four species contained mature gametes in October 2006. In contrast, 55 species contained mature gametes in March 2007. We conclude that the major spawning season of corals on shallow-inshore reefs in the Dampier Archipelago is autumn, although taxa that spawn in spring and summer include Porites spp., Acropora spp. and possibly T. mesenterina that are numerically dominant at many of these sites. Consequently, management initiatives to limit the exposure of coral spawn to stressors associated with coastal development may be required in up to five months per year.     

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.     

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     

Coral polyp budding is probably promoted by a canalized ratio of two morphometric fields

Phenotypic plasticity is the ability of an organism to alter its developmental modes in response to changes in the environment. A major challenge, however, is to explore the existence of a stabilized "morphometric code", a morphological set of buffering rules used repeatedly by members of any species, regardless of environmental factors. Budding of polyps in reef-building corals, an accretive process of the asexual growth form, is thought to be a plastic developmental characteristic. However, we found that intra-tentacular polyp budding in the coral genus Favia from the Red Sea (6 colonies, 37 polyps during a period of 3 years) is not correlated with polyp size. It is initiated after reaching an allometric threshold ratio of two stereotypic, radially symmetrical morphometric landmarks, consisting of the surface area of the polyp and its perimeter.     

The effects of temperature on the growth of juvenile scleractinian corals

Tropical reef corals are well known for their sensitivity to rising temperature, yet surprisingly little is known of the mechanisms through which temperature acts on intact coral colonies. One such mechanism recently has been suggested by the association between the growth of juvenile corals and seawater temperature in the Caribbean, which suggests that temperature causes a transition between isometric and allometric growth scaling in warmer versus cooler years, respectively (Edmunds in Proc R Soc B 273:2275–2281, 2006). Here, this correlative association is tested experimentally for a cause-and-effect relationship. During April and May 2006, juvenile colonies (8–35 mm diameter) of massive Porites spp. from Moorea, French Polynesia, were incubated at warm (27.8°C) and cool (25.7°C) temperatures for 15 days, and their response assessed through the scaling of growth (change in weight) with colony size. The results reveal that the scaling of colony-specific growth (mg colony⁻¹ day⁻¹) was unaffected by temperature, although growth absolutely was greater at the cool compared to the warm temperature, regardless of colony size. This outcome was caused by contrasting scaling relationships for area-specific growth (mg cm⁻² day⁻¹) that were negatively allometric under warm conditions, but independent of size under cool conditions. In April 2007, a 22 days field experiment confirmed that the scaling of area-specific growth in juvenile Porites spp. is negatively allometric at a warm temperature of 29.5°C. Based on strong allometry for tissue thickness, biomass, and Symbiodinium density in freshly collected Porites spp., it is hypothesized that the temperature-dependency of growth scaling in these small corals is mediated by the interaction of temperature with biomass.     

Size-dependent reproduction in the Caribbean gorgonian Pseudoplexaura porosa

Reproductive success among clonal taxa is often portrayed as a simple function of clone size, but reproduction in Pseudoplexaura porosa (Houtuyn), a common Caribbean gorgonian, reflects a more complex relationship between size and gamete production. Tagged colonies were sampled at two reefs in the San Blas Islands, Panama during the 1995 spawning season to determine the colony size at first reproduction, size-dependent polyp fecundities, and whole colony gonad production, and to follow the gametogenic cycle. Additional data were obtained during non-spawning months in 1984 and 1985. Of 120 colonies ranging in height from 20 to 250 cm, only colonies taller than 50 cm were reproductive. An average of 4.33 oocytes polyp⁻¹, 560 to 800 μm in diameter, were released during monthly synchronous spawning events from June to September. Oocytes released each month developed from a group of intermediate size oocytes present at the end of the preceding month. Almost all oocytes >560 μm were released during each spawning event. Spermatogenesis was a shorter process than oogenesis. During each spawning month, mature spermaries developed from a group of <125 μm spermaries. Virtually all spermaries >190 μm were released each month, and most of the spermary volume in mature male polyps was generated anew each month of spawning. Among reproductive colonies, polyp fecundity increased with colony height from 50 to 200 cm. Per capita fecundity of >200 cm tall female colonies was lower than for 150 to 200 cm colonies, but whole colony fecundity of large colonies was greater due to the exponential increase in the number of polyps that occurs with increasing colony height. In male colonies, both polyp and colony reproductive output increased with colony height. The large amounts of sperm produced by large male colonies probably contributes to the high rates of in situ fertilization observed in P. porosa. Differences in fecundity as a function of colony size and sex suggest differences in some combination of the cost of reproduction and/or allocation of resources to reproduction. Received: 9 September 1998 / Accepted: 7 June 1999     

Reproductive biology of the antipatharian black coral Antipathes fiordensis in Doubtful Sound, Fiordland, New Zealand

The reproductive biology of Antipathes fiordensis Grange, a species endemic to south-western New Zealand, was followed from April 1994 to May 1995. Ten colonies were individually tagged in Doubtful Sound and sampled on a monthly basis in order to determine their reproductive activity. The fecundity of each of the five tagged female colonies was determined by estimating the total number of polyps per colony from photographs of each colony and by planimetry, the proportion of gravid polyps per colony, and the mean number of oocytes per gravid polyp. In addition, 56 colonies were sampled in March 1995 to estimate the sex ratio, height at sexual maturity, and mean sizes of females and males. A. fiordensis was found to be a dioecious species which, in the absence of gonads in the polyps, has no obvious external morphological differences between the sexes. Broadcast spawning of gametes is the likely mode of reproduction. Gametogenesis began in November 1994 and was highly synchronous within and between colonies, with spawning occurring in March 1995. The sex ratio in adults was 1:1. Colonies reached sexual maturity between the heights of 70 and 105 cm which, based on existing estimates of growth rate, corresponds to a minimum age for sexual maturity of about 31 yr. The largest oocytes measured ranged from 100 to 140 μm in size. Female colonies produced between 1.3 and 16.9 million oocytes, with the larger colonies dominating the reproductive output of the population. Received: 23 June 1997 / Accepted: 1 August 1997     

A basidiomycete isolated from the skeleton of<Emphasis Type="Italic"> Pocillopora damicornis</Emphasis> (Scleractinia) selectively stimulates short-term survival of coral skeletogenic cells

Endolithic fungi bore through the extracellular calcium carbonate skeleton of reef-building scleractinian corals, both healthy and dead, and effect net erosion of coral reefs. Potential fungal interactions with coral tissue were investigated using an in vitro approach suggested by earlier observations of skeletal repair cones at the site of fungal perforation in Porites sp. A fungal strain was isolated from the skeleton of a long-term culture of healthy, tissue-covered, Pocillopora damicornis Linnaeus colonies maintained in a recirculating system in Monaco. As coral soft tissue spontaneously dissociated in vitro, the skeleton became exposed and hyaline hyphae emerged radially from 15% of the total clipped branches. In this study, which was performed between January 2001 and March 2003, 35 skeleton–hypha explants were embedded in agar-based solid medium, yielding 60% hyphal growth. A fungal strain (F19-3-1) of the dominant (80%) morphology was isolated and propagated in agar-based solid medium. The strain was identified by 18S and 26S rDNA gene sequence analysis as a basidiomycete in the genus Cryptococcus. Co-cultures were used to provide experimental exposure of coral soft tissue to the fungus. The fungus extended the survival of coral cells by 2 days, selectively maintaining skeletogenic cell types. This effect may be interpreted as stimulation by the fungus of a short-term coral defense response.Communicated by J.P. Grassle, New Brunswick     

Evidence for broadcast spawning as well as brooding in the scleractinian coral Pocillopora damicornis

The reproductive activity of 88 colonies of Pocillopora damicornis at Rottnest Island, Western Australia, was examined over 12 mo (20 August 1988 to 20 August 1989). Larvae were found close to the three new moons of February, March and April, 1989, but in large numbers only during March; 67% of all corals produced larvae. There was no obvious connection between gametogenesis and the production of brooded larvae; however, those corals which did not produce, larvae also did not produce ova but did produce, sperm, and must be considered male colonies. All colonies which produced ova went on to produce planulae. Spermatogenesis occurred throughout the year, but oogenesis occurred between September 1988 and April 1989. Dramatic declines in the percentage of polyps containing eggs and sperm occurred in early February and early April 1989. On these dates the amount of lipid in the tissue of the corals decreased, which suggests that the decline in the percentage of polyps with sperm and eggs was not due to the gametes being resorbed by the polyps. Therefore, P. damicornis at Rottnest Island appears to be both a brooder and a broadcaster with asexual reproduction through brooded larvae and sexual reproduction through spawning of gametes. This accounts for earlier evidence in the literature that some input from sexual reproduction had been occurring in the populations of P. damicornis at Rottnest Island. There is no direct evidence for spawning, as broadcasting of gametes has not been observed, but the indirect evidence provides a strong argument for the existence of this dual reproduction.     

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.     

Annual density banding in massive coral skeletons: result of growth strategies to inhabit reefs with high microborers’ activity?

Porites and Montastraea are the major reef-building massive coral genera in the Indo-Pacific and Atlantic oceans, respectively. They are also the most commonly used genera in sclerochronological studies. Despite the marked differences in the way these genera use calcareous material to construct their skeletons (growth strategies) and in their skeletal architectural structure, they form annual high and low density bands in their skeletons, that result from the positive relationship of coral calcification rate with sea surface temperature and seasonal changes of the latter. Evidence in the literature suggests that the different growth strategies allow these organisms to construct denser skeletons far from terrigenous inputs, on reefs where microborers’ activity is high. It seems quite probable that this has consequences for the evolution, diversity, distribution and abundance of reef corals.     

Marginal tentacles of the corallimorpharian Rhodactis rhodostoma. 1. Role in competition for space

The corallimorpharian Rhodactis rhodostoma (Ehrenberg, 1934) forms aggregations that dominate patches on some coral reef flats in the Red Sea. The outcomes and mechanisms of competition for space between this corallimorpharian and other sessile organisms are poorly understood. Polyps of R. rhodostoma were observed to overgrow zoanthids, hydrozoan corals, sponges and encrusting macroalgae on a fringing reef at Eilat, northern Red Sea. R. rhodostoma polyps also damaged, and in some cases overgrew, reef-building corals in the families Poritidae, Acroporidae and Pocilloporidae, most of which form branching colonies with small polyps that are subordinate in coral competitive hierarchies. In contrast, most stony corals in the families Faviidae and Mussidae had standoff interactions with R. rhodostoma, in which they prevented the corallimorpharians from damaging them or approaching closer than 1 to 3 cm. The latter corals are ranked at the top of competitive hierarchies for Indo-Pacific corals, and they form massive colonies of large polyps which may develop aggressive organs termed sweeper tentacles. Some soft corals that exude allelopathic chemicals also avoided overgrowth by the corallimorpharians. Tentacles along the oral disk margin of R. rhodostoma polyps were swollen and bulbous during contacts with cnidarians. These bulbous marginal tentacles had significantly thicker ectoderm and a higher proportion of holotrichous nematocysts than did the normally filiform marginal tentacles of R. rhodostoma polyps. It is concluded that, on the reef flat at Eilat, this corallimorpharian damages and overgrows a variety of sessile competitors, including branching stony corals, via the application of specialised marginal tentacles filled with penetrating nematocysts. R. rhodostoma is an intermediate competitor in the aggressive hierarchy among Indo-Pacific Anthozoa, including the reef-building corals. Received: 1 July 1998 / Accepted: 24 March 1999     

Reef coral reproduction in the eastern Pacific: Costa Rica,Panama, and Galapagos Islands (Ecuador)

Reproductive ecology of two major reef-building corals in the eastern Pacific [Pocillopora damicornis (Linnaeus) andPocillopora elegans Dana] was investigated between 1984 and 1990 in Costa Rica, Panama (Gulf of Chiriqui and Gulf of Panama), and the Galapagos Islands (Ecuador) following the 1982–83 El Niño disturbance. Mature spermatocytes and oocytes were found in both species and were usually present in the same polyp in nearly equal ratios. Numerous zooxanthellae were observed in mature, unfertilized oocytes. Although spawning in field populations was not observed, it is likely that both pocilloporids are simultaneous hermaphroditic spawners, as evidenced by the disappearance of mature gametes after full moon. This is in contrast with most known pocilloporid corals that brood and release planula larvae. Corals were reproductively most active in the thermally stable environments of Costa Rica and the Gulf of Chiriqui (Panama) where 32 to 90% of all colonies contained gametes. In the moderately varying thermal conditions in the Galapagos Islands, 16 to 40% of colonies contained gametes, and in the pronounced seasonal upwelling environment of the Pearl Islands (Panama) only 6 to 18% of colonies contained gametes. Year-round reproduction occurred in Costa Rica and the Gulf of Chiriqui, whereas reproduction was confined to warm periods in the seasonally varying environments of the Galapagos Islands and the Gulf of Panama. Pocilloporid corals in Costa Rica and the Gulf of Chiriqui demonstrated lunar spawning activity, with mature gametes present a few days before and after full moon. Some limited spawning may have occurred also at new moon. While frequent gamete maturation has been demonstrated in this study, the relatively low rates of larval recruitment occurring on eastern Pacific coral reefs disturbed by the 1982–83 El Niño suggest that the recovery of important frame-building corals could be greatly prolonged.     

Coral mortality following extreme low tides and high solar radiation

Extreme tidal events are one of the most predictable natural disturbances in marine benthic habitats and are important determinants of zonation patterns in intertidal benthic communities. On coral reefs, spring low tides are recurrent disturbances, but are rarely reported to cause mass mortality. However, in years when extremely low tides coincide with high noon irradiances, they have the potential to cause widespread damage. Here, we report on such an event on a fringing coral reef in the central Great Barrier Reef (Australia) in September 2005. Visual surveys of colony mortality and bleaching status of more than 13,000 corals at 14 reef sites indicated that most coral taxa at wave-protected sites were severely affected by the event. Between 40 and 75% of colonies in the major coral taxa (Acropora, Porites, Faviidae, Mussidae and Pocilloporidae) were either bleached or suffered partial mortality. In contrast, corals at wave-exposed sites were largely unaffected (<1% of the corals were bleached), as periodic washing by waves prevented desiccation. Surveys along a 1–9 m depth gradient indicated that high coral mortality was confined to the tidal zone. However, 20–30% of faviid colonies were bleached throughout the depth range, suggesting that the increase in benthic irradiances during extreme low tides caused light stress in deeper water. Analyses of an 8-year dataset of tidal records for the area indicated that the combination of extended periods of aerial exposure and high irradiances occurs during May–September in most years, but that the event in September 2005 was the most severe. We argue that extreme low-tide, high-irradiance events are important structuring forces of intertidal coral reef communities, and can be as damaging as thermal stress events. Importantly, they occur at a time of year when risks from thermal stress, cyclones and monsoon-associated river run-off are minimal.     

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.     

Intra-colonial variability in light acclimation of zooxanthellae in coral tissues of Pocillopora damicornis

We investigated heterogeneity of light acclimation of photosynthesis in sun- and shade-adapted coenosarc and polyp tissues of Pocillopora damicornis. The zooxanthellar community within P. damicornis colonies at Heron Island is genetically uniform, yet they showed a large degree of plasticity in their photo-physiological acclimation linked to light microclimates characterised by fibre-optic microprobes. Microscale scalar irradiance measurements showed higher absorption in polyp than coenosarc tissues and higher absorption in the more densely pigmented shade-adapted polyps than in sun-adapted polyps. The combination of an O₂ microelectrode with a fibre-optic microprobe (combined sensor diameter 50–100 μm) enabled parallel measurements of O₂ concentration, gross photosynthesis rate and photosystem II (PSII) quantum yield at the coral surface under steady-state conditions as a function of increasing irradiances. Lower O₂ levels at the tissue surface and higher compensation irradiance indicated a higher respiration activity in sun-adapted polyp tissue as compared to shade-adapted polyps. Shade-adapted coenosarc and polyp tissues exhibited lower maxima of relative electron transport rates (rETR_max) (84±15 and 41±10, respectively) than sun-adapted coenosarc and polyp tissues (136±14 and 77±13, respectively). Shade-adapted tissues showed stronger decrease of rETR at high scalar irradiances as compared to sun-adapted tissues. The relationship between the relative PSII electron transport and the rate of gross photosynthesis, as well as O₂ concentration, was non-linear in sun-adapted tissues over the entire irradiance range, whereas for shade-adapted tissues the relationship became non-linear at medium to high scalar irradiances >200 μmol photons m⁻² s⁻¹. This suggests that rETR measurements should be used with caution in corals as a proxy for photosynthesis rates. The apparently high rates of photosynthesis (oxygen evolution rates) suggest that there must be a considerable electron transport rate through the photosystems that is not observed by the rETR measurements. This may be accounted for by vertical heterogeneity of zooxanthellae in the tissue and the operation of an alternative electron pathway such as cyclic electron flow around PSII.