Recruitment Of Juvenile Corals Research Articles

Microbes guide corals looking to find a home

Coral reefs are facing unprecedented anthropogenic pressures impacting critical processes such as recruitment of juvenile corals. Through larval choice assays and co-occurrence network analyses, a recent study by Turnlund et al. identified microbial taxa within reef biofilms that positively correlate and therefore have potential key roles in inducing coral settlement.

Resilience potential of an Indian Ocean reef: an assessment through coral recruitment pattern and survivability of juvenile corals to recurrent stress events.

Coral reefs are degraded by the synergistic action of climate and anthropogenic stressors. Coral cover in the Palk Bay reef at the northern Indian Ocean largely declined in the past decade due to frequent bleaching events, tsunami and increased fishing activities. In this study, we carried out a comparative assessment to assess the differences in the recovery and resilience of three spatially distant reefs viz. Vedhalai, Mandapam and Pamban along Palk Bay affected by moderate, severe and low fishing pressure respectively. The assessment was based on the juvenile coral recruitment pattern and its survivability combined with availability of hard substratum, live coral cover and herbivore reef fish stock. The Vedhalai reef has the highest coral cover (14.6±6.3%), and ≥90% of the live corals in Vedhalai and Mandapam were affected by turf algal overgrowth. The density of herbivore reef fish was low in Vedhalai and Mandapam reefs compared to the Pamban reef with relatively few grazing species. The juvenile coral diversity and density were high in the Pamban reef and low in Vedhalai and Mandapam reefs despite high hard substratum cover. In total, 22 species of juvenile corals of 10 genera were recorded in Palk Bay. Comparison of the species diversity of juvenile corals with adult ones suggested that the Pamban reef is connected with other distant reefs whereas Vedhalai and Mandapam reefs were self-seeded. There was no statistically significant difference in the survivability of juvenile corals between the study sites, and in total, ≥90% of the juvenile corals survived the high sedimentation stress triggered by the northeast monsoon and bleaching stress that occurred recurrently. Our results indicated that the human activities indirectly affected the juvenile coral recruitment by degrading the live coral cover and contributed to the spatial variation in the recovery and resilience of the Palk Bay reef. Low species diversity of the juvenile corals will increase the vulnerability of the Palk Bay reef to species-specific endemic threats.

Coral recruitment dynamics and substrate mobility in a rubble-dominated back reef habitat

Some back reef habitats can be characterized by unconsolidated substrate with sand patches that, while potentially valuable as nursery habitat for reef fish species, are typically considered less suitable for settlement by sessile invertebrates. Long-term coral monitoring coupled with short-term rubble movement trials were conducted in a natural rubble field in Biscayne National Park, Florida, to quantify juvenile coral recruitment and species composition, as well as the direction and distance of coral rubble displacement. Mean juvenile density over the 5-yr study ranged from 2.59 to 4.09 juveniles m–2, while juvenile loss (emigration/mortality) was around 50% yr–1. This loss, although higher than nearby reef sites, could be partially explained by rubble movement. During the four observational periods (each 4–7 wk), individual rubble pieces had mean displacement of 33 (SE 8.5) cm during the calmest period and 149 (SE 46) cm during the windiest period. Coral species composition at the natural rubble site consisted of 12–15 species of coral, predominantly brooding species. While coral density, recruitment, and overall coral cover may seem low at the natural rubble site, they are comparable to nearby consolidated reef sites and are significantly higher than those observed at a nearby “restored” rubble field (0.34–1.02 juveniles m–2). Despite the potential importance of rubble fields as source populations for reef species, population dynamics in unconsolidated environments are often overlooked. Thus, studies such as this may provide insight into restoration and management strategies for these understudied habitats.

The Great Diadema antillarum Die-Off: 30 Years Later.

In 1983-1984, the sea urchin Diadema antillarum suffered mass mortality throughout the Caribbean, Florida, and Bermuda. The demise of this herbivore contributed to a phase shift of Caribbean reefs from coral-dominated to alga-dominated communities. A compilation of published data of D. antillarum population densities shows that there has been moderate recovery since 1983, with the highest rates on islands of the eastern Caribbean. On the average the current population densities are approximately 12% of those before the die-off, apparently because of recruitment limitation, but the exact factors that are constraining the recovery are unclear. Scattered D. antillarum cohorts in some localities and aggregation of settled individuals in shallow water have created zones of higher herbivory in which juvenile coral recruitment, survivorship, and growth are higher than they are in alga-dominated areas. Unlike other stressors on Caribbean coral reefs, recent changes in D. antillarum populations progress toward aiding the recovery of coral cover.

Benthic community changes following the 2010 Hainan flood: Implications for reef resilience

In the future, coastal coral reefs in Southeast Asia will be exposed to both climate change and more severe and frequent floods associated with heavy rainfall. Floodwaters have caused severe coral mortality in many near-shore coral reefs throughout the world. However, previous studies have largely ignored the effects of floodwaters on macroalgae and calcified algae and the indirect effects on benthic community interactions following flood events. In October 2010, heavy rainfall of more than 1600 mm in 9 days caused the largest flood on record in eastern Hainan Island. Long-term video transects indicate that the floodwater was responsible for severe benthic community mortality in Wenchang, northeast Hainan Island. Mean live coral cover decreased from 15.2% to 9.8% and showed strongest degradation at stations proximal to areas with a high density of aquaculture ponds (i.e. stations 3S–4S decreasing from 15.0% to 0.8%). Branching corals (Acropora and Pocillopora damicornis) were the most sensitive, while massive corals (e.g. Porites, Galaxea, Platygyra and Favia) were most tolerant of floodwaters. Calcified algae were almost completely removed at all stations; coverage decreased from 28.1% to 1.7%, while macroalgae were not influenced by floodwaters. Thus, remnant coral communities that survived the flooding in Wenchang are likely to be subject to increased competition with macroalgae and benefit less from calcified algae promoting juvenile coral recruitment.

Development of new assessment methods for Acropora coral recruitment using coral settlement devices and holes of marine block

We developed new assessment methods for Acropora coral recruitment using coral settlement devices (CSDs) and holes of a marine block. Both devices were deployed in situ before mass spawning of Acropora corals. The CSDs were sampled after 3–4 months of spawning to measure early recruitment, whereas holes of the marine block were observed underwater or MB was sampled to observe holes after 1 year of spawning to measure the recruitment. By combining the results, we would know the recruitment (visually identified underwater) in reefs, including the amount of early recruitment (identified by stereomicroscopic methods), and the environmental condition of the coral reef. These results would help to predict the future of a reef. Nagura Bay had a high early Acropora recruitment; however, due to competition with algae and sedimentation, the recruitment of juvenile corals was extremely low. The reef would possibly be healthy and sustainable at sites where both the early recruitment and recruitment are high. However, if both are low, the reef will be in a critical condition where urgent restoration will be required.

Coral Bleaching and Mortality on Artificial and Natural Reefs in Maldives in 1998, Sea Surface Temperature Anomalies and Initial Recovery

The bleaching and subsequent mortality of branching and massive corals on artificial and natural reefs in the central atolls of Maldives in 1998 are examined with respect to sea surface temperature (SST) anomalies. SST normally peaks in April–May in Maldives. The UK Meteorological Office's Global sea-Ice and SST data set version 2.3b shows that in 1998 monthly mean SST was 1.2–4 S.D. above the 1950–1999 average during the warmest months (March–June), with the greatest anomaly in May of +2.1°C. Bleaching was first reported in mid-April and was severe from late April to mid-May with some recovery evident by late-May. At least 98% of branching corals (Acroporidae, Pocilloporidae) on artificial structures deployed on a reef flat in 1990 died whereas the majority of massive corals (Poritidae, Faviidae, Agariciidae) survived the bleaching. The pre-bleaching coral community on the artificial reefs in 1994 was 95% branching corals and 5% massives ( n=1589); the post-bleaching community was 3% branching corals and 97% massives ( n=248). Significant reductions in live coral cover were seen at all natural reefs surveyed in the central atolls, with average live coral cover decreasing from about 42% to 2%, a 20-fold reduction from pre-bleaching levels. A survey of recruitment of juvenile corals to the artificial structures 10 months after the bleaching event showed that 67% of recruits (⩾0.5 cm diameter) were acroporids and pocilloporids and 33% were from massive families ( n=202) compared to 94% and 6%, respectively, in 1990–1994 ( n=3136). Similar post-bleaching dominance of recruitment by branching corals was seen on nearby natural reef (78% acroporids and pocilloporids; 22% massives). A linear regression of April mean monthly SST against year was highly significant ( p<0.001) and suggests a rise of 0.16°C per decade. If this trend continues, by 2030 mean April SST in the central atolls will normally exceed the anomaly level at which corals appear there are susceptible to mass bleaching.

Inhibition of coral fertilisation and larval metamorphosis by tributyltin and copper

Fertilisation and larval metamorphosis of reef-building corals are important life history events leading to recruitment of juvenile corals to reef populations. Little is known of the sensitivity of these early life phases to pollution, or their relative susceptibility to certain toxicants compared with established coral colonies. Inhibition of fertilisation and larval metamorphosis of the coral Acropora millepora (Ehrenberg, 1834) was assessed in response to solutions of the antifoulants tributyltin (TBT) and copper (Cu) using laboratory-based bioassays. Nominal concentrations that inhibited 50% fertilisation and metamorphosis (IC 50) were calculated from 4 h fertilisation and 24 h metamorphosis assays and were based on introduced dose. Cu was most potent towards fertilisation with an IC 50 of 17.4 μg/l. TBT however, proved more toxic to larval metamorphosis having an IC 50 of 2.0 μg/l. Inert surfaces coated with either Cu- or TBT-based antifouling paint also inhibited fertilisation and metamorphosis. The degree of inhibition was correlated with surface area of the paint coating. These results indicate fertilisation and metamorphosis of coral can be sensitive to active components of antifouling paints.

Effects of the soft coral Xenia puertogalerae on the recruitment of scleractinian corals

The effects of Xenia puertogalerae on the recruitment of juvenile corals was studied at two levels, involving early and more advanced settlement stages. Inhibition by X. puertogalerae was evident. Terracotta tiles established near soft coral colonies had fewer recruits than those located away from the soft corals. This inhibitory effect may be selective as acroporids were able to settle and survive in the presence of Xenia. This selectivity in allowing recruitment implies an important role for X. puertogalerae in determining succession and community structure. It may affect community structure by favouring the establishment of tolerant species, but effectively deters settlement of other corals in the area.

Patterns of Coral Recruitment and Post-settlement Mortality on Bermuda's Reefs: Comparisons to Caribbean and Pacific Reefs

The recruitment of juvenile corals and post-settlement mortality are important processes for coral population dynamics and reef community ecology. I monitored juvenile coral recruitment and survival on a severely disturbed reef in Bermuda from 1981 to 1989 and on adjacent healthy reefs from 1986 to 1990. Porites astreoides was the dominant recruiting species at all sites, due to the release of brooded planulae that may settle rapidly. The dominant corals on Bermuda's reef, Diploria spp., were poor recruiters, perhaps due to the broadcast mode of reproduction of these species. However, Diploria spp. have lower juvenile mortality rates compared to P. astreoides, which may explain their abundance on Bermuda's reefs. Brooding corals, primarily agariciids, were the dominant recruits on Atlantic reefs compared to high recruitment rates by spawning acroporids in the Pacific, which may be the result of different environmental conditions and/or evolutionary trends in the two oceans. The latter group also suffered high post-settlement mortality compared to brooding coralsin both the Atlantic and the Pacific. Massive corals in both oceans had generally low recruitment rates, related to their spawning mode of reproduction, and low rates of post-settlement mortality. The dominant role of long-lived massive corals on the Atlantic and Pacific reefs can be understood in terms of their life-history strategy in comparison to the relatively short-lived Pacific acroporids and Atlantic agariciids that rely on different strategies to maintain their populations.

Recruitment of juvenile corals onto coral tables preyed upon by Acanthaster plana

Corals of 3 table-forming species of the genus Acropora, whose tissues had been removed by the seastar Acanthasterplanci (L.) during 2 mo prior to a major annual coral-spawning season, acted as substrata onto which juvenile corals settled. Juvenile corals were detected microscopically in the laboratory on samples taken from tables 10 wk after spawning time (mean diameter of juveniles 1.3 mm; density 3.5 per 100 cm2). Previously it had been thought that a delay of at least 1 yr was necessary before corals would settle after predation by A. planci. The same tables were sampled again after the next coral-spawning season (1 yr later). Desplte the presence of algae and other colonizing organisms, new corals had settled at a density of 2.8 per 100 cm! About 34 41 of the previous year's juveniles had survived, and mean diameter of these was now 7.24 mm When tables were searched closely in the field, surviving juveniles and some new recruits (with diameters down to 2.00 mm) could be detected. These results indicate that apparent delay in settlement after predation by A. planci can be due to the small size of newly settled corals, which can only be seen by careful inspection of the substratum during the first year after settlement, and which do not achieve 3-dimensional structure until some time later. A. planci was still present at the slte when these observations were made. Thus re-establishment of the coral community on this reef had begun during the time that the adult community was being preyed upon.