Coral Spawning Research Articles

Microphytoplankton variations during coral spawning at Los Roques, Southern Caribbean.

Phytoplankton drives primary productivity in marine pelagic systems. This is also true for the oligotrophic waters in coral reefs, where natural and anthropogenic sources of nutrients can alter pelagic trophic webs. In this study, microphytoplankton assemblages were characterized for the first time in relation to expected coral spawning dates in the Caribbean. A hierarchical experimental design was used to examine these assemblages in Los Roques archipelago, Venezuela, at various temporal and spatial scales for spawning events in both 2007 and 2008. At four reefs, superficial water samples were taken daily for 9 days after the full moon of August, including days before, during and after the expected days of coral spawning. Microphytoplankton assemblages comprised 100 microalgae taxa at up to 50 cells per mL (mean ± 8 SD) and showed temporal and spatial variations related to the coral spawning only in 2007. However, chlorophyll a concentrations increased during and after the spawning events in both years, and this was better matched with analyses of higher taxonomical groups (diatoms, cyanophytes and dinoflagellates), that also varied in relation to spawning times in 2007 and 2008, but asynchronously among reefs. Heterotrophic and mixotrophic dinoflagellates increased in abundance, correlating with a decrease of the diatom Cerataulina pelagica and an increase of the diatom Rhizosolenia imbricata. These variations occurred during and after the coral spawning event for some reefs in 2007. For the first time, a fresh-water cyanobacteria species of Anabaena was ephemerally found (only 3 days) in the archipelago, at reefs closest to human settlements. Variability among reefs in relation to spawning times indicated that reef-specific processes such as water residence time, re-mineralization rates, and benthic-pelagic coupling can be relevant to the observed patterns. These results suggest an important role of microheterotrophic grazers in re-mineralization of organic matter in coral reef waters and highlight the importance of assessing compositional changes of larger size fractions of the phytoplankton when evaluating primary productivity and nutrient fluxes.

Biannual Spawning and Temporal Reproductive Isolation in Acropora Corals.

Coral spawning on the oceanic reef systems of north-western Australia was recently discovered during autumn and spring, but the degree to which species and particularly colonies participated in one or both of these spawnings was unknown. At the largest of the oceanic reef systems, the participation by colonies in the two discrete spawning events was investigated over three years in 13 species of Acropora corals (n = 1,855 colonies). Seven species spawned during both seasons; five only in autumn and one only in spring. The majority of tagged colonies (n = 218) spawned once a year in the same season, but five colonies from three species spawned during spring and autumn during a single year. Reproductive seasonality was not influenced by spatial variation in habitat conditions, or by Symbiodinium partners in the biannual spawner Acropora tenuis. Colonies of A. tenuis spawning during different seasons separated into two distinct yet cryptic groups, in a bayesian clustering analysis based on multiple microsatellite markers. These groups were associated with a major genetic divergence (G”ST = 0.469), despite evidence of mixed ancestry in a small proportion of individuals. Our results confirm that temporal reproductive isolation is a common feature of Acropora populations at Scott Reef and indicate that spawning season is a genetically determined trait in at least A. tenuis. This reproductive isolation may be punctuated occasionally by interbreeding between genetic groups following favourable environmental conditions, when autumn spawners undergo a second annual gametogenic cycle and spawn during spring.

Does the whale shark aggregate along the Western Australian coastline beyond Ningaloo Reef?

Whale sharks (Rhincodon typus) seasonally aggregate at Western Australia’s Ningaloo Reef in the austral autumn and winter, but their occurrence beyond this region during spring and summer remains elusive. The aggregation at Ningaloo Reef coincides with a pulse of productivity following mass coral spawning in early autumn, with the population during this period dominated by juveniles that amass for feeding purposes. To investigate their movement patterns beyond Ningaloo Reef, whale sharks were fitted with SPOT (n = 13) or SPLASH (n = 1) tags between April and September (2010–14). Tagged whale sharks ranged in total length from 3 to 9 m. Each whale shark was also photographed for its subsequent identification using Wildbook for Whale Sharks, and their years of residency at Ningaloo Reef determined. Temporal and spatial observations of whale shark sightings were also determined through the conducting of interviews with people throughout 14 coastal towns along the Western Australian coastline, as well as through historical sightings and the Wildbook database. Satellite tracking revealed that all sharks remained relatively close to the Western Australian coast, travelling a mean minimum distance of 1667 (±316, s.e.) km. Public reports, coupled with satellite tracking, demonstrated that whale sharks inhabit most of the Western Australian coast (from 35°S to 12°S), and that seasonal migrations beyond Ningaloo Reef may be to the north or south and may similarly be associated with areas of increased productivity.

Suspended sediments limit coral sperm availability

Suspended sediment from dredging activities and natural resuspension events represent a risk to the reproductive processes of coral, and therefore the ongoing maintenance of reefal populations. To investigate the underlying mechanisms that could reduce the fertilisation success in turbid water, we conducted several experiments exposing gametes of the corals Acropora tenuis and A. millepora to two sediment types. Sperm limitation was identified in the presence of siliciclastic sediment (230 and ~700 mg L−1), with 2–37 fold more sperm required to achieve maximum fertilisation rates, when compared with sediment-free treatments. This effect was more pronounced at sub-optimum sperm concentrations. Considerable (>45%) decreases in sperm concentration at the water’s surface was recorded in the presence of siliciclastic sediment and a >20% decrease for carbonate sediment. Electron microscopy then confirmed sediment entangled sperm and we propose entrapment and sinking is the primary mechanism reducing sperm available to the egg. Longer exposure to suspended sediments and gamete aging further decreased fertilisation success when compared with a shorter exposure. Collectively, these findings demonstrate that high concentrations of suspended sediments effectively remove sperm from the water’s surface during coral spawning events, reducing the window for fertilisation with potential subsequent flow-on effects for recruitment.

Asynchronous spawning in sympatric populations of a hard coral reveals cryptic species and ancient genetic lineages.

Genetic subdivision within a species is a vital component of the evolution of biodiversity. In some species of Acropora corals in Western Australia, conspecific individuals spawn in two seasons 6months apart, which has the potential to impede gene flow and result in genetic divergence. Genetic comparison of sympatric spring and autumn spawners of Acropora samoensis was conducted to assess the level of reproductive isolation and genetic divergence between the spawning groups based on multiple loci (13 microsatellite loci, the mitochondrial control region and two nuclear introns). Bayesian clustering and principal coordinate analysis of the microsatellite loci showed a high level of genetic differentiation between the spawning groups (F'ST =0.30; P<0.001), as did the sequence data from PaxC and Calmodulin (ΦST =0.97 and 0.31, respectively). At the PaxC locus, the autumn and spring spawners were associated with two divergent lineages that were separated by an evolutionary distance of 1.7% and statistical tests indicate divergent selection in PaxC, suggesting this gene may play a role in coral spawning. This study indicates that the autumn and spring spawners represent two cryptic species, and highlights the importance of asynchronous spawning as a mechanism influencing speciation in corals.

The 2014 coral bleaching and freshwater flood events in Kāne'ohe Bay, Hawai'i.

Until recently, subtropical Hawaiʻi escaped the major bleaching events that have devastated many tropical regions, but the continued increases in global long-term mean temperatures and the apparent ending of the Pacific Decadal Oscillation (PDO) cool phase have increased the risk of bleaching events. Climate models and observations predict that bleaching in Hawaiʻi will occur with increasing frequency and increasing severity over future decades. A freshwater “kill” event occurred during July 2014 in the northern part of Kāneʻohe Bay that reduced coral cover by 22.5% in the area directly impacted by flooding. A subsequent major bleaching event during September 2014 caused extensive coral bleaching and mortality throughout the bay and further reduced coral cover in the freshwater kill area by 60.0%. The high temperature bleaching event only caused a 1.0% reduction in live coral throughout the portion of the bay not directly impacted by the freshwater event. Thus, the combined impact of the low salinity event and the thermal bleaching event appears to be more than simply additive. The temperature regime during the September 2014 bleaching event was analogous in duration and intensity to that of the large bleaching event that occurred previously during August 1996, but resulted in a much larger area of bleaching and coral mortality. Apparently seasonal timing as well as duration and magnitude of heating is important. Coral spawning in the dominant coral species occurs early in the summer, so reservoirs of stored lipid in the corals had been depleted by spawning prior to the September 2014 event. Warm months above 27 °C result in lower coral growth and presumably could further decrease lipid reserves, leading to a bleaching event that was more severe than would have happened if the high temperatures occurred earlier in the summer. Hawaiian reef corals decrease skeletal growth at temperatures above 27 °C, so perhaps the “stress period” actually started long before the bleaching threshold of 29 °C was reached. Hawaiʻi is directly influenced by the PDO which may become a factor influencing bleaching events in subtropical Hawaiʻi in much the same manner as variations in the El Niño Southern Oscillation (ENSO) influences bleaching events at low latitudes in the tropical Pacific. Records show that offshore temperatures measured by satellite will not always predict inshore bleaching because other factors (high cloud cover, high wind and wave action, tidal exchange rate) can limit inshore heating and prevent temperatures in the bay from reaching the bleaching threshold. Low light levels due to cloud cover or high turbidity can also serve to prevent bleaching.

First record of multi-species synchronous coral spawning from Malaysia.

Knowledge about the timing and synchrony of coral spawning has important implications for both the ecology and management of coral reef ecosystems. Data on the timing of spawning and extent of synchrony, however, are still lacking for many coral reefs, particularly from equatorial regions and from locations within the coral triangle. Here we present the first documentation of a multi-species coral spawning event from reefs around Pulau Tioman, Peninsular Malaysia, a popular diving and tourist destination located on the edge of the coral triangle. At least 8 coral species from 3 genera (Acropora, Montipora and Porites) participated in multi-species spawning over five nights in April 2014, between two nights before and two nights after the full moon. In addition, two Acropora species were witnessed spawning one night prior to the full moon in October 2014. While two of the Acropora species that reproduced in April (A. millepora and A. nasuta) exhibited highly synchronous spawning (100% of sampled colonies), two other common species (A. hyacinthus and A. digitifera) did not contain visible eggs in the majority of colonies sampled (i.e., <15% of colonies) in either April or October, suggesting that these species spawn at other times of the year. To the best of our knowledge, this is the first detailed documented observation of multi-species coral spawning from reefs in Malaysia. These data provide further support for the contention that this phenomenon is a feature of all speciose coral assemblages, including equatorial reefs. More research is needed, however, to determine the seasonal cycles and extent of spawning synchrony on these reefs and elsewhere in Malaysia.

High reproductive synchrony of Acropora (Anthozoa: Scleractinia) in the Gulf of Aqaba, Red Sea.

Coral spawning in the northern Gulf of Aqaba has been reported to be asynchronous, making it almost unique when compared to other regions in the world. Here, we document the reproductive condition of Acropora corals in early June 2014 in Dahab, in the Gulf of Aqaba, 125 km south of previous studies conducted in Eilat, Israel. Seventy-eight percent of Acropora colonies from 14 species had mature eggs, indicating that most colonies will spawn on or around the June full moon, with a very high probability of multi-species synchronous spawning. Given the proximity to Eilat, we predict that a comparable sampling protocol would detect similar levels of reproductive synchrony throughout the Gulf of Aqaba consistent with the hypothesis that high levels of spawning synchrony are a feature of all speciose coral assemblages.

Coral spawning in the Gulf of Oman and relationship to latitudinal variation in spawning season in the northwest Indian Ocean.

Despite a wealth of information on sexual reproduction in scleractinian corals, there are regional gaps in reproductive records. In the Gulf of the Oman in the Arabian Sea, reproductive timing was assessed in four common species of broadcast spawning corals using field surveys of gamete maturity and aquarium observations of spawning activity. The appearance of mature gametes within the same month for Acropora downingi, A. hemprichii, Cyphastrea microphthalma and Platygyra daedalea (≥ 75% of colonies, n = 848) indicated a synchronous and multi-specific spawning season. Based on gamete disappearance and direct observations, spawning predominantly occurred during April in 2013 (75–100% of colonies) and May in 2014 (77–94% of colonies). The difference in spawning months between survey years was most likely explained by sea temperature and the timing of lunar cycles during late-stage gametogenesis. These reproductive records are consistent with a latitudinal gradient in peak broadcast spawning activity at reefs in the northwestern Indian Ocean which occurs early in the year at low latitudes (January to March) and progressively later in the year at mid (March to May) and high (June to September) latitudes.

Dispatches

Dispatches

Are cyclones agents for connectivity between reefs?

AbstractAimOur aim was to determine whether cyclone‐modified currents and winds can enhance coral larval transport between geographically separate reefs and whether connectivity could occur within the competency period of coral larvae.LocationThe North West Shelf of Australia.MethodsLarval connectivity was modelled between inshore and mid‐shelf reef zones in two years (1996 and 2001) when cyclones occurred around the time of coral spawning. This was contrasted with 2002, when cyclones were absent during the spawning. The effects of current and wind patterns on the dispersal of buoyant coral eggs was simulated usingGCOM3D, a three dimensional hydrodynamic current model, andOILMAP, a surface wind stress model.ResultsModelling showed that larvae could disperse across the North West shelf and well beyond the parent reefs when influenced by cyclone‐modified wind and current patterns. The dispersal of larvae to reefs separated by 100 km is likely to be frequent, to involve a substantial proportion of the larval population (between 63% and 86%), and to occur within 6 days (the competency period of the larvae). Simulating larval behaviour by including a buoyant phase in the larval model, where larvae are subject to surface winds, reduces connection time between reef areas by 11% to 29%.Main conclusionsCyclones have the potential to increase the distance larvae travel and can act to rapidly connect widely separated inshore and mid‐shelf reefs within the competency period of coral larvae. The buoyant phase of the coral larvae, where larvae are subject to propulsion by surface winds and subsurface currents, may enhance rapid connectivity. Our results show that periodic and extreme climatic conditions could significantly impact connectivity. In combination with larval behaviour they have important implications for connectivity time between inshore and mid‐shelf reefs.

Inorganic nitrogen transformations within permeable carbonate sands

A combination of in-situ push pull tests and a flow through reactor trial were used to quantify the inorganic nitrogen sinks in the permeable carbonate sands of a tropical coral cay (Heron Island – Great Barrier Reef). Addition of dissolved inorganic nitrogen (DIN in the form of nitrate – NO3−, and ammonium – NH4+) directly into sediment porewater resulted in uptake of up to 97% and 60% of added DIN respectively. The initial push pull experiment qualitatively showed that dissimilatory nitrate reduction to ammonia (DNRA), denitrification and nitrification were all active in the sediments. A flow through reactor experiment provided a more detailed approach to quantify these processes and showed that both denitrification and DNRA occurred within the sands at rates of 7.3 and 5.5µmolNcm−3d−1, respectively. Unexpectedly the addition of labile organic material (fresh coral spawn) to the permeable sands did not result in the release of DIN from the reactors, on the contrary it resulted in the increased uptake of both NO3− and NH4+. This was most likely because of the stimulated N uptake associated with the addition of high C:N coral spawn material. The bulk of NH4+ produced via DNRA was found to be adsorbed to sediments within the reactor and was not released with the outlet water. A mass balance over the entire experimental period showed that more inorganic N was retained within the sediments than lost as gaseous products. Our results point to permeable carbonate sands acting as reservoirs of N under the influence of advective flow, even during sudden enrichment periods such as those following coral mass spawning. This implies that permeable carbonate sands may help to buffer coral reefs during periods of extreme oligotrophy.

A continuous, real-time water quality monitoring system for the coral reef ecosystems of Nanwan Bay, Southern Taiwan

The coral reef ecosystems of Nanwan Bay, Southern Taiwan are undergoing degradation due to anthropogenic impacts, and as such have resulted in a decline in coral cover. As a first step in preventing the continual degradation of these coral reef environments, it is important to understand how changes in water quality affect these ecosystems on a fine-tuned timescale. To this end, a real-time water quality monitoring system was implemented in Nanwan Bay in 2010. We found that natural events, such as cold water intrusion due to upwelling, tended to elicit temporal shifts in coral spawning between 2010 and 2011. In addition, Degree Heating Weeks (DHWs), a commonly utilized predictor of coral bleaching, were 0.92 and 0.59 in summer 2010 and 2011, respectively. Though this quantity of DHW was below the presumed stress-inducing value for these reefs, a rise in DHWs in the future may stress the resident corals.

Impact of coral spawning on the biogeochemistry of a Hawaiian reef

We examined the impact of Montipora capitata coral spawning on local biogeochemistry in Kane'ohe Bay, Hawai'i. This event supplied labile, spawn-derived organic matter (SDOM) to the water column, triggering a cascading series of related effects on the biogeochemistry of the reef. Specifically, we measured the isotopic composition and nutrient ratios of spawning material and coral tissues, and utilized these signatures to track pathways of SDOM incorporation into this coral-dominated ecosystem. We observed: (1) shifts in the isotopic signatures of coral tissues after the spawning event, (2) rapid turnover of SDOM within the water column and enhanced deposition of POM to the sediment surface, (3) enhanced sediment efflux of NH4+ after the spawning event that triggered a phytoplankton bloom in the overlying water, and (4) drawdown of dissolved nutrients in the water column after spawning that coincided with the occurrence of a water column phytoplankton bloom. Our results show that single-species spawning events can serve as a source of substantial nutrient input to the water column, contributing in similar ways to storm-driven river nutrient input, and with measurable impact on the biogeochemistry of the reef.

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.

Seasonal, daily and diel N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; effluxes in permeable carbonate sediments

Abstract. Benthic metabolism and inorganic nitrogen and N2 flux rates (denitrification) were measured in permeable carbonate sands from Heron Island (Great Barrier Reef). Some of the N2 flux rates were among the highest measured in sediments. All benthic fluxes showed a significant difference between seasons with higher rates in summer and late summer. There was no distinct response of the benthic system to mass coral spawning. Instead, changes in benthic fluxes over 12 days in summer appear to be driven by tidal changes in water depth and associated changes in phytosynthetically active radiation reaching the sediments. Dark N2 fluxes were strongly correlated to benthic oxygen consumption across all sites and seasons (r2 = 0.63; p &lt; 0.005; slope = 0.035). However, there were seasonal differences with a steeper slope in summer than winter, reflecting either more efficient coupling between respiration and nitrification–denitrification at higher temperatures or different sources of organic matter. Adding data from published studies on carbonate sands revealed two slopes in the dark N2 flux versus benthic oxygen consumption relationship. The lower slope (0.035) was most likely due to high carbon : nitrogen (C : N) organic matter from coral reefs, and associated assimilation of nitrogen by heterotrophic bacteria including enhanced heterotrophic N-fixation, but competition by benthic microalgae or inefficient coupling between respiration and nitrification–denitrification cannot be excluded. The steeper slope (0.089) was most likely due to respiration being driven by low C : N phytodetritus. If the different slopes were driven by the sources of organic matter, then global estimates of continental shelf denitrification are probably about right. In contrast, global estimates of continental shelf denitrification may be over-estimated if the low slope was due to inefficient coupling between respiration and nitrification–denitrification and also due to reduced N2 effluxes in the light associated with competition by benthic microalgae for nitrogen and N-fixation.

Spawning behaviour in Platygyra daedalea off South Africa

Coral spawning has never been observed in South Africa at the southernmost limit of scleractinian distribution along the East African Coast. Corals in the region are exposed to low temperatures (summer means of 26°C) and turbulent conditions (Schleyer et al. 2008), which may infl uence their spawning. Visiting the reefs at night is diffi cult, so 10 colonies of Platygyra daedalea (Ellis and Solander 1986) were collected on Two-mile Reef (27°31′376′′S, 32°41′181′′E) a few days before the Full Moon of February 2012 (07/02/12) and placed in open-water aquaria. The date of spawning in P. daedalea off South Africa was predicted from regular sampling of colonies from 2010. Spawning occurred over three consecutive nights (Full Moon +2 to +4 days) in 6-9 colonies at a water temperature of 26°C. The peak activity in spawning was observed at Full Moon +4days when 9 colonies spawn synchronously. Field sampling showed that in-situ colonies also spawned around these dates. Sperm and egg bundles were released between 19:00 to 22:00, with peak release at 20:30. Individual polyps released single bundles in synchronous waves, every 15-30 minutes. Each bundle of gametes was visible in the polyp mouth for 0.5-2 h before release (Figs 1A, B). During this time, they were rotated inside the polyp mouth, probably to compress the gametes and facilitate bundle expulsion (Fig. 1C). After release, the polyp mouth continued to gape for several minutes (Fig. 1D). This spawning behavior has been reported in several Platygyra species (Babcock et al. 1986). In the present study, the bundles averaged 2.2 mm (SE=0.2, N=25) in diameter and contained an average of 89.2 oocytes (SE=15.9, N=15). Each polyp released 2-3 bundles per spawning night. A single P. daedalea colony with an average diameter of 10 cm could thus re lease up to 10,000 oocytes per night. P. daedalea is known to spawn durSpawning behaviour in Platygyra daedalea off South Africa

Biannual coral spawning decreases at higher latitudes on Western Australian reefs

Seasonal differences in the timing of multi-specific coral spawning between the east and west coasts of Australia may be the result of a genetic legacy or of adaptation to local conditions. Using estimates of the proportions of coral species that spawned in spring and autumn at Ashmore Reef (12°S) and Ningaloo Reef (23°S) in Western Australia, in combination with findings of previous surveys, I examined whether reproductive seasonality varied with latitude. A consistently high proportion of species spawned during the main reproductive season in autumn regardless of latitude. However, there was a clear decrease in the proportion of species spawning in spring, from an average of 49 % at Ashmore Reef (12°S) to 7 % at Ningaloo Reef (23°S). The results of this study suggest that seasonality of coral reproduction in Western Australia reflects environmental gradients and natural selection rather than an inherited genetic legacy.

Coral reef research: advances through the use of SCUBA

Coral reefs are, per unit area, the most productive and diverse ecosystems on the planet. Their complexity and, in some places, fragility provide many complications to scientists conducting high quality research in these environments. Scientific diving has been, for the past six decades, a highly cost-effective and productive tool in coral reef research. Using high impact outputs and based mainly on the research supported through the diving programmes of the Smithsonian Institution, this review outlines some major scientific advances that have been made in coral reef research because of the support or use of SCUBA. The main areas of interest reported here are: algal sexual reproduction on coral reefs, coral spawning, conservation biology, coral diseases and declines, biodiversity, keystone species, biogeography of reef fishes, and DNA barcoding. The review concludes that significant advances in many areas of coral reef research were only achievable through diving-based approaches.

Growth and survivorship of juvenile corals outplanted to degraded reef areas in Bolinao-Anda Reef Complex, Philippines

The worldwide decline of coral reefs due to natural and anthropogenic disturbances necessitates the development of techniques to restore damaged reefs. The potential use of sexually derived coral propagules as a tool in coral reef restoration has recently been actively investigated. This study was conducted to determine growth and survivorship of juveniles of the reef-building coral Acropora valida, outplanted at a degraded reef. These juveniles were products of sexual propagation, i.e. involving coral spawning, gamete fertilization, larval rearing and settlement, and juvenile rearing, at an outdoor hatchery facility. Six-month-old juvenile corals attached to rubble inserted into plastic masonry wall plugs (‘tox’) were outplanted to three experimental bommies at the Bolinao-Anda Reef Complex, northwestern Philippines. Cumulative survival of outplants was 67.5±7.6% during a study period of 183–190 days, with no apparent difference among bommies. The juveniles grew with a 37-fold increase in ecological volume – from 1.25±0.97 cm3 (1.1±0.3 cm mean diameter) at outplantation to 46.19±35.49 cm3 (3.8±1.1 cm mean diameter) at approximately 6 months. The results of this study substantiate the use of sexually derived coral propagules for reef restoration. Cost analysis revealed sexually produced corals to be more expensive than asexual counterparts. Further developments or refinements in the culture technology for sexually derived corals for reef restoration are needed to enhance cost-effectiveness.