Preservation of in situ reef framework in regions of low hurricane frequency: Pleistocene of Curaçao and Bonaire, southern Caribbean

Luxuriant fringing reefs along the southwestern shores of the Caribbean islands of Curaçao and Bonaire (12°N), located outside the most frequent hurricane tracks, are rarely affected by major storms. Consequently, reef growth and long-term preservation are potentially optimal and distinct from reefs experiencing greater hurricane frequency. Hurricane Lenny (November 1999) took an unusual west-to-east track, bisecting the Caribbean Basin north of these islands, but generated heavy waves (3-6  m) that severely damaged reefs along the normally leeward shores. Massive coral colonies >100 years old were toppled, but even at the most severely damaged sites, 82–85% of colonies remained in growth position. Late Pleistocene (125  ka) elevated reefs in the Lower Terrace of Curaçao record even higher proportions of corals in growth position (93%), possibly reflecting a low hurricane frequency during the Pleistocene highstand. In comparison, coeval Pleistocene reefs in regions that today experience a high hurricane frequency (Great Inagua Island and San Salvador, Bahamas) have lower proportions of corals preserved in growth position (79% and 38%, respectively). These results are consistent with the hypothesis that reefs in regions experiencing very low hurricane frequency, like the southern Caribbean, are more likely to be preserved with corals in primary growth position in comparison to regions with higher hurricane frequency.     

The ecology of Millepora

Attention is drawn to the lack of quantitative ecological information on the calcareous hydrozoan Millepora. In spite of their abundance and geological importance on reefs, little attention has been paid to the millepores compared to the dominant corals. The literature on distribution and abundance on reefs is reviewed and attention is drawn to sources of bias in reported estimates of Millepora abundance. Millepora is shown to have many of the same parasites and predators as scleractinians. The sparse data on growth and production are reviewed and compared with that of corals. The physiological and ecological responses of Millepora to storms and other disturbances appear to be similar to those of the scleractinans but the millepores have escaped attack by the starfish Acanthaster and appear to be immune to the common diseases of corals. The morphological similarities and ecological differences between millepores and scleractinians are discussed in terms of life-history strategies.     

Competitive strategies of soft corals (Coelenterata: Octocorallia). II. Variable defensive responses and susceptibility to scleractinian corals

Interactions involving competition for space between several species of alcyonacean and scleractinian corals were assessed experimentally on Britomart Reef, central region of the Great Barrier Reef, Australia. Colonies of three soft coral species, Sarcophyton ehrenbergi Marenzeller, Nephthea brassica Kukenthal, and Capnella lacertiliensis Macfayden Forskal (Coelenterata:Alcyonacea) were relocated within stands of two scleractinian corals, Parités andrewsi Vaughan (= P. cylindrica Dana) and Pavona cactus Förskal (Coelenterata:Scleractinia). Undisturbed scleractinian and relocated alcyonacean controls were also monitored.Alcyonacean corals induced necrosis of tissue in scleractinian corals. Necrosis was significantly more pronounced when colonies were in contact but was also observed in the absence of contact, implicating the presence of active allelopathic agents. Scleractinian coral species varied in their susceptibility to the ill effects of alcyonaceans, with Pontes andrewsi being more susceptible than Pavona cactus. Of the soft corals, Nephthea caused the highest degree of mortality in the two scleractinian corals examined and Sarcophyton the least. Some soft corals appear to retain their toxins while others release them, implying a combination of anti-predatory and anti-competitor roles for the secondary metabolites. Scleractinian corals were often overgrown by soft corals.Both species of scleractinian corals were found to cause approximately equal amounts of tissue necrosis in alcyonaceans. These effects were more pronounced when colonies were in direct contact. The necrotic effects among alcyonacean corals were species-specific. Alcyonaceans also overgrew scleractinian corals and secreted a protective polysaccharide layer in areas proximal to scleractinians. Secretion of this layer was stimulated differentially by the two scleractinian species and also varied in frequency of occurrence among the alcyonaceans.High levels of tissue necrosis were observed in both groups of organisms within 3 wk of initiation of the experiment. Necrosis increased with time in the scleractinian corals and decreased in the alcyonaceans. The development of a protective polysaccharide layer in the alcyonaceans increased with time.     

Climate-mediated mechanical changes to post-disturbance coral assemblages

Increasingly severe storms and weaker carbonate materials associated with more acidic oceans will increase the vulnerability of reef corals to mechanical damage. Mechanistic predictions based on measurements of colony mechanical vulnerability and future climate scenarios demonstrate dramatic shifts in assemblage structure following hydrodynamic disturbances, including switches in species' dominance on the reef and thus potential for post-disturbance recovery. Larger colonies are more resistant to factors such as disease and competition for space, and complex morphologies support more associated reef species. Future reefs are thus expected to have lower colony abundances and be dominated by small and morphologically simple, yet mechanically robust species, which will in turn support lower levels of whole-reef biodiversity than do present-day reefs.     

Changes in coral assemblages during an outbreak of Acanthaster planci at Lizard Island, northern Great Barrier Reef (1995–1999)

Population outbreaks of crown-of-thorns starfish (Acanthaster planci L.) represent one of the most significant biological disturbances on tropical coral reefs and have the potential to devastate coral communities, thereby altering the biological and physical structure of reef habitats. This study reports on changes in area cover, species diversity and taxonomic composition of corals during an outbreak of A. planci at Lizard Island, in the northern Great Barrier Reef, Australia. Mean coral cover declined by 28.8% across ten locations studied. However, densities of A. planci, and their effects on local coral assemblages, were very patchy. Declines in coral cover were mostly due to the selective removal of certain coral taxa (mainly Acropora and Pocilloporidae corals); such that the greatest coral loss occurred at locations with highest initial cover of preferred coral prey. Most notably, coral assemblages in back-reef locations were transformed from topographically complex staghorn Acropora-dominated habitats, to relatively depauperate assemblages dominated by alcyonacean soft corals. Although coral loss was greatest among formerly dominant taxa (especially Acropora), effects were sufficiently widespread across different coral taxa, such that overall coral diversity tended to decline. Clearly, moderate outbreaks of A. planci have the potential to greatly alter community structure of coral communities even if they do not devastate live corals. Recovery in this instance is expected to be very rapid given that all coral taxa persisted, and effects were greatest among fast growing corals.     

Survival of hurricane-generated coral fragments and a disturbance model of reef calcification/growth rates

Summary Hurricane Gerta, with winds reaching 150 km/h, crossed the Belize barrier reef on September 18, 1978. Breakage and scouring of corals occurred in all zones of the reef to a depth of approximately 25 m. Survivorship of storm-generated coral fragments and detached colonies is strongly size dependent, conforming to the power function Y=4.44X^0.66 where Y is the percent of fragments and X is the fragment size. Forty-six percent of detached Acropora palmata branches, which are larger ( =37.6 cm long) than fragments of other species ( =16.7 cm long), survived. Overall, 39% of fragments and detached colonies survived. This high survivorship, which probably increased the total number of colonies present, and redistribution of corals may explain the rapid recovery of reefs from all but the severest hurricanes. Storms appear to prevent coral reefs from reaching a mature state characterized by low calcification and growth rates. Therefore, we suggest that long-term reef calcification and growth rates are highest on reefs periodically distrubed by storms of intermediate intensity.Smithsonian Tropical Research Institute, Box 2072, Balboa, Republic of Panama     

Tiger cowrie Cypraea tigris feeds on coral-competing sponge Rhabdastrella globostellata in an Acropora dominated reef of Gulf of Mannar,India

Gulf of Mannar (GoM) in the southeast coast of India is known for its coral reefs and reef-associated biodiversity. Corals in GoM were affected to a significant extent by climate change-driven coral bleaching in 2016, and are currently recovering. After the bleaching mortality that corals suffered, the competition for space between corals and sponges is obvious in GoM. Rhabdastrella globostellata is a common marine sponge found overgrowing live coral colonies of the patch reefs in GoM at Pattinamaruthoor in March 2019. Underwater assessment of the reef revealed that 60.06% live coral cover was dominated by Acropora corals (81.91%). Among the acroporans 8.23% of colonies were found overgrown by R. globostellata. During the night dives the tiger cowrie Cypraea tigris was observed to feed on R. globostellata. From this observation the present study infers that C. tigris helps the corals fight these sponges, and concludes that tiger cowries should be protected and promoted to tackle climate change implications.     

Predicted disappearance of coral-reef ramparts: a direct result of major ecological disturbances

Two coral cays near La Parguera, Puerto Rico, have large, exposed coral ramparts composed almost entirely of loose pieces of elkhorn coral Acropora palmata (88% of horizontal transects, 98% of vertical transects). The total volume of elkhorn coral in the ramparts of the two cays was estimated at 3600 and 12 800 m³. The present volume of living elkhorn coral on these two reefs was estimated at 7 and 14 m³ and previous volumes at 11 000 and 34 900 m³. White-band disease was found on 8.5% of living elkhorn colonies. Lang’s boring sponge Cliona langae covered 10.8% of the total transect area, overgrowing both dead and living corals. White-band disease and coral-reef bleaching have drastically reduced the populations of elkhorn coral, thus, skeletal coral materials to replenish the plate ramparts are severely reduced, disrupting the process of forming and maintaining these coral reef ramparts. We predict that the next series of major storms striking these prominent cay ramparts will remove them. These disappearances will represent a quick, obvious and permanent consequence of global disturbances. Loss of cay ramparts will modify the environments on and around Atlantic coral reefs. Ramparts may be similarly lost from Indo-Pacific reefs. The lack of any other indisputable definitive indicators of long-term, major disturbances on coral reefs makes the distinct loss of coral-reef ramparts an important physical sign.     

Context-dependent corallivory by parrotfishes in a Caribbean reef ecosystem

This study assesses the patterns of corallivory by parrotfishes across reefs of the Florida Keys, USA. These reefs represent a relatively unique combination within the wider Caribbean of low coral cover and high parrotfish abundance suggesting that predation pressure could be intense. Surveys across eight shallow forereefs documented the abundance of corals, corallivorous parrotfishes, and predation scars on corals. The corals Porites porites and Porites astreoides were preyed on most frequently with the rates of predation an order of magnitude greater than has been documented for other areas of the Caribbean. In fact, parrotfish bite density on these preferred corals was up to 34 times greater than reported for corals on other reefs worldwide. On reefs where coral cover was low and corals such as Montastraea faveolata, often preferred prey for parrotfishes, were rare, predation rates on P. porites and P. astreoides, and other less common corals, intensified further. The intensity of parrotfish predation increased significantly as coral cover decreased. However, parrotfish abundance showed only a marginal positive relationship with predation pressure on corals, likely because corallivorous parrotfish were abundant across all reefs. Parrotfishes often have significant positive impacts on coral cover by facilitating coral recruitment, survival, and growth via their grazing of algae. However, abundant corallivorous parrotfishes combined with low coral cover may result in higher predation on corals and intensify the negative impact that parrotfishes have on remaining corals.     

Experimental fragmentation reduces sexual reproductive output by the reef-building coral Pocillopora damicornis

Natural and anthropogenic disturbances may fragment stony reef corals, but few quantitative data exist on the impacts of skeletal fragmentation on sexual reproduction in corals. We experimentally fragmented colonies of the branching coral Pocillopora damicornis and determined the number and size of planula larvae released during one lunar reproductive cycle. Partially fragmented colonies significantly delayed both the onset and peak period of planula release compared with intact control colonies. Most fragments removed from the corals died within 11–18 days, and released few planulae. The total number of planulae released per coral colony varied exponentially with remaining tissue volume, and was significantly lower in damaged versus undamaged colonies. However, the number of planulae produced per unit tissue volume, and planula size, did not vary with damage treatment. We conclude that even partial fragmentation of P. damicornis colonies (<25% of tissue removed) decreases their larval output by reducing reproductive tissue volume. Repeated breakage of corals, such as caused by intensive diving tourism or frequent storms, may lead to substantially reduced sexual reproduction. Therefore, reef management should limit human activities that fracture stony corals and lead to decreases in colony size and reproductive output. Accepted: 2 February 2000     

Competitive interactions between corals and Trididemnum solidum on Mexican Caribbean reefs

The ascidian Trididemnum solidum competes for space on Caribbean reefs and is capable of overgrowing live scleractinian corals. From 2006 to 2009, we monitored over 30,000 coral colonies and quantified competitive interactions with this ascidian at four reef sites along the Mexican Caribbean. The total number of competitive interactions increased in time, but the mean percentage of coral colonies involved in interactions remained lower than 1% in all reefs. Bottom cover by T. solidum was also low (mean < 0.5%) in all reef sites in all sampling years. We conclude that during the temporal scope of our study, the overall potential effect of T. solidum on the dynamics of Mexican Caribbean coral populations was minimal.     

Changes in gametogenesis and fecundity of acroporid corals that were exposed to elevated nitrogen and phosphorus during the ENCORE experiment

Colonies of two scleractinian reef coral species, Acropora longicyathus and Acropora aspera were transplanted into patch reefs at One Tree Reef, Great Barrier Reef, Australia as part of the ENCORE experiment. These corals and colonies of A. aspera which were naturally present in the patch reefs were exposed to four treatments over two years: controls with normal seawater, elevated levels of nitrogen only, phosphorus only, or nitrogen plus phosphorus. These corals were sampled and used to determine whether gametogenic cycles and fecundity were affected by nutrient enrichment. Acropora longicyathus had a single annual gametogenic cycle. Corals exposed to elevated nitrogen produced significantly smaller and fewer eggs and contained less testes material than those which were not exposed to nitrogen. Exposure to elevated phosphorus only resulted in corals producing more but smaller eggs, and more testes material. Egg numbers of colonies from other treatments decreased as the gametogenic cycles continued, but those of the phosphorus colonies showed almost no reduction in egg numbers between the early and late stages of the gametogenic cycles. These results have important management implications for coral reefs as they demonstrate that small increases in concentrations of nitrogen and phosphorus can have severe effects on reproductive activity in these species of scleractinian corals.     

The influence of colony size and coral health on the occupation of coral-associated gobies (Pisces: Gobiidae)

Fishes of the genus Gobiodon are habitat specialists by their association with Acropora corals. Little is known about the parameters that define host coral quality for these fishes, in particular their breeding pairs. Data were collected in the northern Red Sea using 10 × 1-m belt transects in different reefs and zones. Gobiid density was highly correlated with coral density over all sites and zones, and the more specialized goby species preferred coral species that are less vulnerable to environmental stress. Moreover, the occupation rate of corals by goby breeding pairs significantly increased with colony size and decreased with partial mortality of colonies. Logistic regression showed that both coral size (being most important) and partial mortality are key factors influencing the occupation by breeding pairs. This study provides the first evidence that breeding pairs of coral-associated gobiids have more advanced habitat requirements than con-specifics in other social states. As coral reefs are threatened worldwide and habitat loss and degradation increase, this information will help predict the potential effects on those reef fishes obligatorily associated with live corals.     

Coral reefs in the Southern Barents Sea: habitat description and the effects of bottom fishing

Fifteen Lophelia reefs from offshore to coastal areas off northern Norway were studied using video. Health status of the coral habitat (degree of physical impact, % cover of living tissue, colony size), occurrence of trawl marks and lost fishing gear, height of coral colonies and associated fauna were analysed from 44 video-lines. Fishing impact was more frequent on the offshore reefs (36.5% of the observed areas) than those in the coastal reefs (0.6%). The most visible effects of fishing were broken and displaced coral colonies. At some sites only small scattered fragments of live corals were observed, indicating recent impact. The mean colony height of Lophelia and gorgonian corals at impacted sites was around half the size of those at non-impacted sites. Both species richness and abundance was higher at non-impacted coral habitats compared to impacted. The actinarian Protanthea simplex and unidentified brittlestars were the only taxa with higher abundance on impacted compared with non-impacted habitats. The reefs at the offshore location were protected against bottom trawling in 2009 through the establishment of a marine protected area (MPA), but a general ban against trawling on known coral reefs had already been implemented in 1999. In the MPA, signs of regrowth were observed. Most of the observed damage probably occurred over 10 years earlier. Results show that live and non-impacted cold water coral reefs have an important ecological function by enhancing the local biodiversity and fish abundance. Preventing further damage to impacted reefs may lead to full recovery within a few decades.     

Evolutionary ecology of colonial reef-organisms, with particular reference to corals

Sedentary reef-organisms such as sponges, colonial coelenterates, bryozoans and compound ascidians produce repeated modules (aquiferous systems, polyps, zooids) as they grow. Modular construction alleviates constraints on biomass imposed by mechanical and energetic factors that are functions of the surface area to volume ratio. Colonies thus may grow large whilst preserving optimal modular dimensions. Among corals, optimal polyp size is smaller in the more autotrophic than in the more heterotrophic species. Modular construction allows flexibility of growth form, which can adapt to factors such as water currents, silting, light intensity and proximity of competitors. Modular colonies have great regenerative capacities, even separated fragments may survive and grow into new colonies. All fragments from a parental colony are genetically identical and large branching corals frequently undergo clonal propagation through fragmentation during storms. Soft corals can also fragment endogenously. By spreading the risk of mortality among independent units, the generation and dispersal of fragments lessens the likelihood of clonal extinction. In spite of their ability to propagate asexually, most benthic colonial animals also reproduce asexually. The selective advantages of the genetic diversity among sexually produced offspring seem not to be linked with dispersal, but probably lie in the biological interactions with competitors, predators and pathogens in the parental habitat. Age at first sexual maturity and the proportional investment of resources in sexual reproduction are related to colonial survivorship. Small branching corals on reef flats grow quickly, attain sexual maturity within 1–4 years, planulate extensively, but reach only small sizes before dying. Massive corals are longer lived and have the opposite characteristics of growth and reproduction. Most sessile reef organisms compete for space, food or light. Faster growers can potentially outcompete slower growers, but are often prevented from doing so by several forms of aggression from competitors and by the damage inflicted by storms. Competitive interactions among sedentary organisms on coral reefs are unlikely to be linear or deterministic, and so the co-existence of diverse species is possible.     

Coral recruitment and juvenile mortality as structuring factors for reef benthic communities in Biscayne National Park, USA

Coral communities of Biscayne National Park (BNP) on offshore linear bank-barrier reefs are depauperate of reef corals and have little topographic relief, while those on lagoonal patch reefs have greater coral cover and species richness despite presumably more stressful environmental regimes closer to shore. We hypothesized that differences in rates of coral recruitment and/or of coral survivorship were responsible for these differences in community structure. These processes were investigated by measuring: (1) juvenile and adult coral densities, and (2) size-frequency distributions of smaller coral size classes, at three pairs of bank- and patch-reefs distributed along the north-south range of coral reefs within the Park. In addition, small quadrats (0.25 m²) were censused for colonies <2 cm in size on three reefs (one offshore and one patch reef in the central park, and one intermediate reef at the southern end), and re-surveyed after 1 year. Density and size frequency data confirmed that large coral colonies were virtually absent from the offshore reefs, but showed that juvenile corals were common and had similar densities to those of adjacent bank and patch reefs. Large coral colonies were more common on inshore patch reefs, suggesting lower survivorship (higher mortality) of small and intermediate sized colonies on the offshore reefs. The more limited small-quadrat data showed similar survivorship rates and initial and final juvenile densities at all three sites, but a higher influx of new recruits to the patch reef site during the single annual study period. We consider the size-frequency data to be a better indicator of juvenile coral dynamics, since it is a more time-integrated measurement and was replicated at more sites. We conclude that lack of recruitment does not appear to explain the impoverished coral communities on offshore bank reefs in BNP. Instead, higher juvenile coral mortality appears to be a dominant factor structuring these communities. Accepted: 9 September 1999     

Upper Permian (Murghabian) rugose corals from Oman (Ba’id area,Saih Hatat): Community structure and contributions to reefbuilding processes

Summary Coral-dominated communties are rare in Upper Permian reefs. The study of Murghabian rugose and tabulate corals from allochtonous carbonates (‘Oman exotics’) of the Hawasina Complex and autochthonous carbonates of the Saih Hatat area/Arabian Platform (Oman) provides evidence for a significant contribution of rugose corals to the formation of Late Permian reefs. The corals are described with respect to taxonomy, microfacies and community structure. 8 genera and 7 species were recognized.Monothecalis minor n.sp.,Praewentzelella regulare n.sp. andWentzelella katoi magna n.ssp. are new. The corals represent three communities: (1)Praewentzelella community (Hawasina Complex), (2) cerioid coral community (Hawasina Complex), and (3)Waagenophyllum community (Hawasina Complex and Saih hatat). The corals from the Hawasina Complex and the Saih Hatat flourished in significantly different environments: Rugosa from the Hawasina Complex are representatives of reefs, whereas their counterparts from the Saih Hatat lived in level-bottom communities. Coral-bearing reefal boundstones are characterized by a diverse assemblage of sphinctozoans, inozoans, chaetetids, bryozoans, crinoids,Tubiphytes, Archaeolithoporella and algae. These communities produced bafflestones or framestones and were part of a sponge reef complex. The level-bottom community of the Saih Hatat is low-diverse only comprising rugose and tabulate corals. These of isolated colonies locally acted as bafflers.     

Parrotfish predation on massive Porites on the Great Barrier Reef

Parrotfish grazing scars on coral colonies were quantified across four reef zones at Lizard Island, Northern Great Barrier Reef (GBR). The abundance of parrotfish grazing scars was highest on reef flat and crest, with massive Porites spp. colonies having more parrotfish grazing scars than all other coral species combined. Massive Porites was the only coral type positively selected for grazing by parrotfishes in all four reef zones. The density of parrotfish grazing scars on massive Porites spp., and the rate of new scar formation, was highest on the reef crest and flat, reflecting the lower massive Porites cover and higher parrotfish abundance in these habitats. Overall, it appears that parrotfish predation pressure on corals could affect the abundance of preferred coral species, especially massive Porites spp, across the reef gradient. Parrotfish predation on corals may have a more important role on the GBR reefs than previously thought.     

Severity of the 1998 and 2005 bleaching events in Venezuela, southern Caribbean

This study describes the severity of the 2005 bleaching event at 15 reef sites across Venezuela and compares the 1998 and 2005 bleaching events at one of them. During August and September 2005, bleached corals were first observed on oceanic reefs rather than coastal reefs, affecting 1 to 4% of coral colonies in the community (3 reef sites, n = 736 colonies). At that time, however, no bleached corals were recorded along the eastern coast of Venezuela, an area of seasonal upwelling (3 reefs, n = 181 colonies). On coastal reefs, bleaching started in October but highest levels were reached in November 2005 and January 2006, when 16% of corals were affected among a wide range of taxa (e.g. scleractinians, octocorals, Millepora and zoanthids). In the Acropora habitats of Los Roques (an oceanic reef),no bleached was recorded in 2005 (four sites,n = 643 colonies). At Cayo Sombrero, a coastal reef site, bleaching was less severe in 1998 than in 2005 (9% of the coral colonies involving 2 species vs. 26% involving 23 species, respectively). Our results indicate that bleaching was more severe in 2005 than in 1998 on Venezuelan reefs; however, no mass mortality was observed in either of these two events.