Corals

BSc International Land and Water Management Wageningen University

Supervisors:
WU Supervisor: Klaas Metselaar
Host supervisor: Sabine Engel

Abstract
Mangrove forest systems and coral reefs are vital ecosystems with ecological, economic, and societal importance. However, they face threats such as excessive sediment inflow from human activities, endangering their health and resilience. This report explores sediment-related issues in the Lagun Bay catchment area on Bonaire and proposes dams as sediment control measures. Potential soil erosion was assessed and dams were designed to reduce sediment transport effectively. However, maintenance is essential for dam functionality. Though not definitive, these dam designs offer potential solutions. Sustainable land and coastal management, coupled with improved data and regular maintenance, can protect these valuable ecosystems and preserve biodiversity. Further research is needed to enhance data quality and expand the study area.

https://onlinelibrary.wiley.com/doi/abs/10.1111/mec.17072

Abstract

The plastic ability for a range of phenotypes to be exhibited by the same genotype allows organisms to respond to environmental variation and may modulate fitness in novel environments. Differing capacities for phenotypic plasticity within a population, apparent as genotype by environment interactions (GxE), can therefore have both ecological and evolutionary implications. Epigenetic gene regulation alters gene function in response to environmental cues without changes to the underlying genetic sequence and likely mediates phenotypic variation. DNA methylation is currently the most well described epigenetic mechanism and is related to transcriptional homeostasis in invertebrates. However, evidence quantitatively linking variation in DNA methylation with that of phenotype is lacking in some taxa, including reef-building corals. In this study, spatial and seasonal environmental variation in Bonaire, Caribbean Netherlands was utilized to assess relationships between physiology and DNA methylation profiles within genetic clones across different genotypes of Acropora cervicornis and A. palmata corals. The physiology of both species was highly influenced by environmental variation compared to the effect of genotype. GxE effects on phenotype were only apparent in A. cervicornis. DNA methylation in both species differed between genotypes and seasons and epigenetic variation was significantly related to coral physiological metrics. Furthermore, plastic shifts in physiology across seasons were significantly positively correlated with shifts in DNA methylation profiles in both species. These results highlight the dynamic influence of environmental conditions and genetic constraints on the physiology of two important Caribbean coral species. Additionally, this study provides quantitative support for the role of epigenetic DNA methylation in mediating phenotypic plasticity in invertebrates.

This thesis focuses on understanding the connectivity patterns of coral larvae in the area around Aruba, Bonaire and Curacao and their implications for population dynamics and conservation efforts. The study aims to analyze the spatio-temporal variability of larval dispersal and identify key factors influencing larval connectivity among different reef systems. Connectivity is studied by simulating currents with the 2D version of the Second generation Louvain-la-Neuve Ice-Ocean Model SLIM coupled with a Lagrangian Particle Tracker (LPT). Using a combination of field observations, numerical modeling, and graph theory analysis, we investigated the larval dispersal patterns of four coral species during their respective spawning seasons, the behavior of another species during 6 different months and the connectivity of a fictional species with low mortality rate and no competency loss. The results revealed distinct species-specific differences in larval connectivity, with some species exhibiting higher levels of connectivity within a short distance from their spawning sites, while others displayed broader dispersal capabilities. We also highlighted a strong seasonal variability in the area. Inter-island connectivity have been found for the fictional species and intra-island pattern have been focused on. Furthermore, using other tools from graph theory, the analysis of connectivity matrices and computation of connectivity indicators helped identifying zones of high interest for protection and restoration measures. The research outcomes contribute to the understanding of larval connectivity dynamics in the South-Eastern Caribbean Sea and provide insights for coral reef management and conservation. The results highlight the need for targeted conservation efforts to protect critical larval dispersal pathways and promote genetic exchange among reef systems.

http://hdl.handle.net/2078.1/thesis:40210
 

Abstract  Evolutionary tradeoffs between life-history strategies are central to animal evolution. However, because microbes can influence aspects of host physiology, behavior, and resistance to stress or disease, changes in animal-microbial symbioses have the potential to mediate life-history tradeoffs. Scleractinian corals provide a highly biodiverse and data-rich host system to test this idea, made more relevant by increases in coral disease outbreaks as a result of anthropogenic changes to climate and reef ecosystems. Identifying factors that determine coral disease susceptibility has therefore become a focus for reef conservation efforts. Using a comparative approach, we tested if coral microbiomes correlate with disease susceptibility across 425 million years of coral evolution by combining a cross-species coral microbiome survey (the “Global Coral Microbiome Project”) with long-term disease prevalence data at multiple sites. Interpreting these data in their phylogenetic context, we show that microbial dominance and composition predict disease susceptibility. We trace this dominance-disease association to a single putatively beneficial bacterial symbiont, Endozoicomonas, whose relative abundance in coral tissue explained 30% of variation in disease susceptibility and 60% of variation in microbiome dominance across 40 coral genera. Conversely, Endozoicomonas abundances in coral tissue strongly correlated with high growth rates. These results demonstrate that the evolution of microbial symbiosis in corals correlates with both disease prevalence and growth rate. Exploration of the mechanistic basis for these findings will be important for our understanding of how microbial symbiosis influences animal life-history tradeoffs, and in efforts to use microbes to increase coral growth or disease resistance in-situ. 

One mechanism giving fleshy algae a competitive advantage over corals during reef degradation is algal-induced and microbially-mediated hypoxia (typically less than 69.5 µmol oxygen L⁻¹). During hypoxic conditions oxygen availability becomes insufficient to sustain aerobic respiration in most metazoans. Algae are more tolerant of low oxygen conditions and may outcompete corals weakened by hypoxia. A key question on the ecological importance of this mechanism remains unanswered: How extensive are local hypoxic zones in highly turbulent aquatic environments, continuously flushed by currents and wave surge? To better understand the concert of biological, chemical, and physical factors that determine the abundance and distribution of oxygen in this environment, we combined 3D imagery, flow measurements, macro- and micro-organismal abundance estimates, and experimentally determined biogenic oxygen and carbon fluxes as input values for a 3D bio-physical model. The model was first developed and verified for controlled flume experiments containing coral and algal colonies in direct interaction. We then developed a three-dimensional numerical model of an existing coral reef plot off the coast of Curaçao where oxygen concentrations for comparison were collected in a small-scale grid using fiberoptic oxygen optodes. Oxygen distribution patterns given by the model were a good predictor for in situ concentrations and indicate widespread localized differences exceeding 50 µmol L^-1 over distances less than a decimeter. This suggests that small-scale hypoxic zones can persist for an extended period of time in the turbulent environment of a wave- and surge- exposed coral reef. This work highlights how the combination of three-dimensional imagery, biogenic fluxes, and fluid dynamic modeling can provide a powerful tool to illustrate and predict the distribution of analytes (e.g., oxygen or other bioactive substances) in a highly complex system.

Over the past decades, numerous studies have reported declines in stony corals and, in many cases, phase shifts to fleshy macroalgae. However, long-term studies documenting changes in other benthic reef organisms are scarce. Here, we studied changes in cover of corals, algal turfs, benthic cyanobacterial mats, macroalgae, sponges and crustose coralline algae at four reef sites of the Caribbean islands of Curaçao and Bonaire over a time span of 40 years. Permanent 9 m2 quadrats at 10, 20, 30 and 40 m depth were photographed at 3 to 6-year intervals from 1973 to 2013. The temporal and spatial dynamics in the six dominant benthic groups were assessed based on image point-analysis. Our results show consistent patterns of benthic community change with a decrease in the cover of calcifying organisms across all sites and depths from 32.6% (1973) to 9.2% (2013) for corals and from 6.4% to 1% for crustose coralline algae. Initially, coral cover was replaced by algal turfs increasing from 24.5% (1973) to 38% around the early 1990s. Fleshy macroalgae, still absent in 1973, also proliferated covering 12% of the substratum approximately 20 years later. However, these new dominants largely declined in abundance from 2002 to 2013 (11% and 2%, respectively), marking the rise of benthic cyanobacterial mats. Cyanobacterial mats became the most dominant benthic component increasing from a mere 7.1% (2002) to 22.2% (2013). The observed increase was paralleled by a small but significant increase in sponge cover (0.5% to 2.3%). Strikingly, this pattern of degradation and phase change occurred over the reef slope down to upper-mesophotic depths of 40 m. These findings suggest that reefs dominated by algae may be less stable than previously thought and that the next phase may be the dominance of slimy cyanobacterial mats with some sponges.

Want to know more about this project? Check out the whole thesis.

MSc report

Land based nutrient input from anthropogenic sources is a worldwide issue and a threat to coral reef health. Corals found in high nutrient environments are less resistant to global stressors such as rising temperature and extreme weather events. The absence of sewage treatment on St. Eustatius and its erosion prone coastline suggests poor water quality. Chlorophyll-a concentrations were monitored at 11 locations around St. Eustatius twice a month from May to August 2022 (n=7). Preliminary results on spatial and temporal variation of chlorophyll-a show an overall good water quality. Average concentrations did not exceed the environmental threshold of 0.3 μg/L at any of the sampled locations. Concentrations varied between locations and over time. Highest average chla values were found close to STUCO's brine outlet, the oil terminal and Golden Rock Dive & Nature Resort. Towards the end of the sampling period, chlorophyll-a concentrations were more variable between locations, which coincided with the onset of more frequent rain showers. No heavy rainfall occurred during the sampling period, so it was not possible to study the effects of higher precipitation on chlorophyll-a concentrations. Current results do not show reason for concern. However they should be re-evaluated when data on stable isotopes and nutrient concentrations are available. Long-term monitoring that includes the rainy season will show if the water quality around St. Eustatius is suitable for coral reefs or if action is needed to improve it.

For full report or more information,  please contact erik.meesters@wur.nl or gulsah.dogruer@wur.nl

Abstract: A biodiversity survey on three corallivorous snails (Mollusca: Gastropoda) was performed at 28 sites around the island of Bonaire to assess their distribution patterns and associated host corals. The snails and their hosts were identified and counted in three depth zones: 5–10, 10–20, and 20–30 m. The snails were Coralliophila galea and C. salebrosa (Muricidae: Coralliophilinae), and Cyphoma gibbosum (Ovulidae: Simniinae). All three species were widespread around the island without apparent interspecific geographical variation. Coralliophila galea was found exclusively on scleractinian corals, Coralliophila salebrosa almost exclusively on octocorals, and Cyphoma gibbosum only on octocorals. Coralliophila salebrosa showed more dietary overlap with Cyphoma gibbosum than with Coralliophila galea. Coralliophila galea was the most commonly encountered species with the largest number of host species. Owing to its hosts distribution, this species also showed a greater maximum depth and a wider bathymetrical range than the other two snails. The other two snails were shallower and their depth ranges did not differ significantly. Host-coral size did not seem to have influence on the number of snails per host. Coral damage caused by the snails was visible but appeared to be low, causing no mortality in Bonaire, which suggests that the relation with their hosts is more parasitic than predatory. Because these three corallivores have occasionally been reported to occur as outbreaks in other Caribbean localities and may act as vectors in the dispersal of coral diseases, it is recommended that future studies should focus on their population dynamics.

Abstract

Reef-building corals are ecosystem engineers that compete with other benthic organisms for space and resources. Corals harvest energy through their surface by photosynthesis and heterotrophic feeding, and they divert part of this energy to defend their outer colony perimeter against competitors. Here, we hypothesized that corals with a larger space-filling surface and smaller perimeters increase energy gain while reducing the exposure to competitors. This predicted an association between these two geometric properties of corals and the competitive outcome against other benthic organisms. To test the prediction, fifty coral colonies from the Caribbean island of Curaçao were rendered using digital 3D and 2D reconstructions. The surface areas, perimeters, box-counting dimensions (as a proxy of surface and perimeter space-filling), and other geometric properties were extracted and analyzed with respect to the percentage of the perimeter losing or winning against competitors based on the coral tissue apparent growth or damage. The increase in surface space-filling dimension was the only significant single indicator of coral winning outcomes, but the combination of surface space-filling dimension with perimeter length increased the statistical prediction of coral competition outcomes. Corals with larger surface space-filling dimensions (Ds > 2) and smaller perimeters displayed more winning outcomes, confirming the initial hypothesis. We propose that the space-filling property of coral surfaces complemented with other proxies of coral competitiveness, such as life history traits, will provide a more accurate quantitative characterization of coral competition outcomes on coral reefs. This framework also applies to other organisms or ecological systems that rely on complex surfaces to obtain energy for competition.

Spirobranchus giganteus is a tube-dwelling polychaete more commonly known as the Christmas tree worm. S. giganteus larvae are planktonic, which is followed by benthic settlement and development into a sessile adult. It has been shown that chemical and physical cues produced by live coral and adult S. giganteus attract larvae to settlement sites. In fact, it has been postulated that there may be a mutualistic relationship between S. giganteus and live coral. However, settlement of S. giganteus on coral rubble has been noted in the field. The aim of this study is to investigate whether there is a relationship between size of S. giganteus and substratum settlement type on the coral reefs of Bonaire. The hypothesis under examination in this study states that S. giganteus settled on live coral will be larger than S. giganteus settled on rubble. The measurement chosen for this study was the diameter of the orifice of the calcareous tube, since it has been shown that this is a good estimate for overall size of the polychaete. The diameter of the orifice of S. giganteus was significantly greater for individuals living on rubble (0.510 cm) than individuals living on live coral (0.457 cm). The species of coral that S. giganteus settled on also appeared to affect the orifice diameter. Individuals on Agaricia agaricites were significantly smaller than individuals settled on both Montastrea annularis and Siderastrea siderea. These results may indicate that the interactions between S. giganteus and live coral, such as extracoelenteric digestion and sweeper tentacles, may be disruptive enough to cause a lower size distribution of polychaetes on that substrate when compared to rubble.

This student research was retrieved from Physis: Journal of Marine Science III (Spring 2008)19: 25-30 from CIEE Bonaire.