About

Karl D. Castillo is a Professor in the Department of Earth, Marine and Environmental Sciences at the University of North Carolina at Chapel Hill. His research integrates ecology, physiology, and environmental change biology to understand the mechanisms governing coral responses to rapidly changing oceans. He combines field observations, controlled laboratory experiments, and advanced ecophysiological approaches to establish quantitative links between environmental conditions and coral performance across temperate and tropical reef systems. His work spans ecological, organismal, cellular, and molecular scales to examine how global change and local human impacts influence coral resilience, growth, and survival. Through interdisciplinary collaborations, he develops science that advances coral reef conservation, restoration, and predictive ecosystem management.

Research topics

• Environmental science
• Oceanography
• Geology
• Ecology
• Computer Science
• Biology
• Chemistry
• Mineralogy
• Mathematics
• Environmental chemistry

Selected publications

• Comment on essd-2025-598

  2026-03-12

  peer-reviewOpen access

  <strong class="journal-contentHeaderColor">Abstract.</strong> As science fields enter the Big Data revolution, open-access repositories are essential for addressing larger-scale questions than are possible for single researchers by making data findable, accessible, interoperable, and reusable (FAIR). Furthermore, transparent data and code are increasingly important for reproducible research, especially for data types that inherently require subjective human interpretations. These ideas are applicable to coral sclerochronology, as the field has long been characterized by individual researchers collecting and analyzing coral skeletal cores from their study sites without substantial data sharing or archiving of the core images to meet FAIR principles. Here, we present CoralCache, a virtual coral core repository that not only archives image datasets (<em>i.e.</em>, digitized X-rays and computed tomography scans), but also observer interpretations of the density banding patterns. CoralCache is linked to a graphical user interface, CoralCT, which together offer a way forward for coral growth rate analysis that is reproducible and collaborative. The data organization systems presented here could also be readily applied to related archives such as tree rings or bivalve shells.

  PublisherDOI

• Meta-Analysis Reveals Reduced Coral Calcification Under Projected Ocean Warming but Not Under Acidification Across the Caribbean Sea

  UNC Libraries · 2025-07-15

  articleOpen accessSenior author

  Ocean acidification and warming are two of the many threats to coral reefs worldwide, and Caribbean reef-building corals are especially vulnerable. However, even within the Caribbean, experimental acidification and warming studies reveal a wide array of coral calcification responses across reef systems and among species, complicating efforts to predict how corals will respond to these global-scale stressors. We conducted a meta-analysis to investigate the calcification responses of Caribbean corals to experimentally induced seawater ocean acidification, ocean warming, and the combination of both stressors. Calcification rates were reduced for corals reared under warming alone, but acidification and the combination of both stressors did not clearly reduce calcification rates. Calcification responses of corals collected from the Florida Keys and Belize were compared for regional differences since a greater number of studies were performed on corals collected from these two regions. Notably, corals from the Florida Keys did not exhibit reduced calcification under acidification, warming, or the combination of both stressors, while corals from Belize exhibited reduced calcification under warming alone. Further investigation of these regional trends suggests that the warming and acidification treatments employed dictated calcification responses, rather than collection region. Results from this meta-analysis are constrained by the very few studies that have been conducted within the Caribbean to assess ocean acidification and warming and the large variation in experimental procedure among studies. This meta-analysis reveals existing gaps in our understanding of how corals will likely respond to projected acidification and warming and highlights ways to improve comparability among experimental studies conducted on corals within the same region to better predict coral calcification response under global change.

  PublisherDOI

• Future of coral bleaching research

  BioScience · 2025-04-25 · 3 citations

  articleOpen access

  Coral bleaching is the largest global threat to coral reef ecosystem persistence this century. Advancing our understanding of coral bleaching and developing solutions to protect corals and the reefs they support are critical. In the present article, we, the US National Science Foundation-funded Coral Bleaching Research Coordination Network, outline future directions for coral bleaching research. Specifically, we address the need for embedded inclusiveness, codevelopment, and capacity building as a foundation for excellence in coral bleaching research and the critical role of coral-bleaching science in shaping policy. We outline a path for research innovation and technology and propose the formation of an international coral bleaching consortium that, in coordination with existing multinational organizations, could be a hub for planning, coordinating, and integrating global-scale coral bleaching research, innovation, and mitigation strategies. This proposed strategy for future coral bleaching research could facilitate a step-function change in how we address the coral bleaching crisis.

  PublisherOA PDFDOI

• Tracking over 30 years of coral reef infrastructure degradation in Barbados

  Scientific Reports · 2025-01-31

  articleOpen accessSenior author

  Coral reefs face escalating threats from global and local stressors, and these challenges are exacerbated in the Caribbean. This study focuses on coral reef structure in Barbados, where a previous study documented reef degradation in the 1990s. As 30 years have passed, we examined the rate of change of reef structure and quantified associated substrate presence along the western reefs of Barbados. Using satellite and geotagged imagery, we analyzed 19 reef structures over the interval 2013 to 2023 along the west coast of Barbados, comparing them to the previous study’s findings in 1950 and 1991. We ground-truthed five sites previously categorized as highly degraded reefs to confirm their structural integrity and substrate-type. Results confirmed ongoing reef structural loss, averaging ~ 137.68 m1 annually across all sites. We identified four primary substrate types: coral, algae-covered substrate, rubble, and sand, with algae-covered substrate predominating and projected to persist. Our results underscore the urgency of monitoring reef health and highlight the potential limitations of satellite assessment. This research enhances understanding of reef dynamics and offers a framework for identifying vulnerable areas, which are crucial for effective conservation efforts.

  PublisherOA PDFDOI

• Cryptic coral diversity is associated with symbioses, physiology, and response to thermal challenge

  UNC Libraries · 2025-01-23

  articleOpen access

  Coral persistence in the Anthropocene depends on interactions among holobiont partners (coral animals and microbial symbionts) and their environment. Cryptic coral lineages-genetically distinct yet morphologically similar groups-are critically important as they often exhibit functional diversity relevant to thermal tolerance. In addition, environmental parameters such as thermal variability may promote tolerance, but how variability interacts with holobiont partners to shape responses to thermal challenge remains unclear. Here, we identified three cryptic lineages of <em>Siderastrea siderea</em> in Bocas del Toro, Panam&aacute; that differ in distributions across inshore and offshore reefs, microbial associations, phenotypic traits of holobiont partners (i.e., phenomes), and skeleton morphologies. A thermal variability experiment failed to increase thermal tolerance, but subsequent thermal challenge and recovery revealed that one lineage maintained elevated energetic reserves, photochemical efficiency, and growth. Last, coral cores highlighted that this lineage also exhibited greater growth historically. Functional variation among cryptic lineages highlights their importance in predicting coral reef responses to climate change.

  PublisherDOI

• Ocean weather, biological rates, and unexplained global ecological patterns

  UNC Libraries · 2025-01-14

  articleOpen access

  As on land, oceans exhibit high temporal and spatial temperature variation. This "ocean weather" contributes to the physiological and ecological processes that ultimately determine the patterns of species distribution and abundance, yet is often unrecognized, especially in tropical oceans. Here, we tested the paradigm of temperature stability in shallow waters (&lt;12.5 m) across different zones of latitude. We collated hundreds of in situ, high temporal-frequency ocean temperature time series globally to produce an intuitive measure of temperature variability, ranging in scale from quarter-diurnal to annual time spans. To estimate organismal sensitivity of ectotherms (i.e. microbes, algae, and animals whose body temperatures depend upon ocean temperature), we computed the corresponding range of biological rates (such as metabolic rate or photosynthesis) for each time span, assuming an exponential relationship. We found that subtropical regions had the broadest temperature ranges at time spans equal to or shorter than a month, while temperate and tropical systems both exhibited narrow (i.e. stable) short-term temperature range estimates. However, temperature-dependent biological rates in tropical regions displayed greater ranges than in temperate systems. Hence, our results suggest that tropical ectotherms may be relatively more sensitive to short-term thermal variability. We also highlight previously unexplained macroecological patterns that may be underpinned by short-term temperature variability.

  PublisherDOI

• Cryptic coral diversity is associated with symbioses, physiology, and response to thermal challenge

  Science Advances · 2025-01-15 · 14 citations

  articleOpen access

  Coral persistence in the Anthropocene depends on interactions among holobiont partners (coral animals and microbial symbionts) and their environment. Cryptic coral lineages—genetically distinct yet morphologically similar groups—are critically important as they often exhibit functional diversity relevant to thermal tolerance. In addition, environmental parameters such as thermal variability may promote tolerance, but how variability interacts with holobiont partners to shape responses to thermal challenge remains unclear. Here, we identified three cryptic lineages of Siderastrea siderea in Bocas del Toro, Panamá that differ in distributions across inshore and offshore reefs, microbial associations, phenotypic traits of holobiont partners (i.e., phenomes), and skeleton morphologies. A thermal variability experiment failed to increase thermal tolerance, but subsequent thermal challenge and recovery revealed that one lineage maintained elevated energetic reserves, photochemical efficiency, and growth. Last, coral cores highlighted that this lineage also exhibited greater growth historically. Functional variation among cryptic lineages highlights their importance in predicting coral reef responses to climate change.

  PublisherDOI

• Gene expression plasticity facilitates acclimatization of a long-lived Caribbean coral across divergent reef environments

  Scientific Reports · 2024-04-03 · 13 citations

  articleOpen access1st authorCorresponding

  Local adaptation can increase fitness under stable environmental conditions. However, in rapidly changing environments, compensatory mechanisms enabled through plasticity may better promote fitness. Climate change is causing devastating impacts on coral reefs globally and understanding the potential for adaptive and plastic responses is critical for reef management. We conducted a four-year, three-way reciprocal transplant of the Caribbean coral Siderastrea siderea across forereef, backreef, and nearshore populations in Belize to investigate the potential for environmental specialization versus plasticity in this species. Corals maintained high survival within forereef and backreef environments, but transplantation to nearshore environments resulted in high mortality, suggesting that nearshore environments present strong environmental selection. Only forereef-sourced corals demonstrated evidence of environmental specialization, exhibiting the highest growth in the forereef. Gene expression profiling 3.5 years post-transplantation revealed that transplanted coral hosts exhibited profiles more similar to other corals in the same reef environment, regardless of their source location, suggesting that transcriptome plasticity facilitates acclimatization to environmental change in S. siderea. In contrast, algal symbiont (Cladocopium goreaui) gene expression showcased functional variation between source locations that was maintained post-transplantation. Our findings suggest limited acclimatory capacity of some S. siderea populations under strong environmental selection and highlight the potential limits of coral physiological plasticity in reef restoration.

  PublisherOA PDFDOI

• Century‐Long Records of Sedimentary Input on a Caribbean Reef From Coral Ba/Ca Ratios

  Paleoceanography and Paleoclimatology · 2024-05-01 · 5 citations

  articleOpen access

  Abstract Coral reef ecosystems are delicately balanced and are thus prone to disruption by stressors such as storms, disease, climate variability and natural disasters. Most tropical coral populations worldwide are now in rapid decline owing to additional anthropogenic pressures, such as global warming, ocean acidification and a variety of local stressors. One such problem is the addition of excess sediment and nutrients flux to reefs from increased soil erosion from land use changes. Here we present century‐long Ba/Ca records from two Siderastrea siderea colonies as a proxy for local riverine discharge and sediment flux to the southern Mesoamerican Barrier Reef System (MBRS). The coral colonies have linear extension trends, which can be seen as a first‐order indicator for coral health and response. The coral colony that exhibits a decline in linear extension rate from the forereef of the MBRS, mainly receives riverine input from Honduras, whilst the coral from the backreef, which does not exhibit a decline in extension rate, primarily receives riverine input from more sparsely populated regions of Belize. Coral Ba/Ca increased (&gt;70%) through time in the forereef colony, while the backreef colony showed little long‐term increase in Ba/Ca over the last century. Our results suggest that increasing sediment supply may have played a role in the decline of forereef skeletal extension in the southernmost MBRS region, likely stemming from increasing land‐use changes in Honduras.

  PublisherOA PDFDOI

• Cryptic diversity shapes coral symbioses, physiology, and response to thermal challenge

  bioRxiv (Cold Spring Harbor Laboratory) · 2024-07-13 · 1 citations

  preprintOpen access

  Abstract Coral persistence in the Anthropocene is shaped by interactions among holobiont partners (coral animals, microbial symbionts) and their environment. Cryptic coral lineages–genetically distinct yet morphologically similar groups–are critically important as they often exhibit functional diversity relevant to thermal tolerance. Additionally, environmental parameters such as thermal variability may promote tolerance, but how variability interacts with holobiont partners to shape responses to thermal challenge remains unclear. Here, we identified three cryptic lineages of Siderastrea siderea in Bocas del Toro, Panamá that differ in distributions across inshore and offshore reefs, microbial associations, holobiont phenomes, and skeleton morphologies. A thermal variability experiment failed to increase thermal tolerance, but subsequent thermal challenge and recovery revealed one lineage maintained elevated energetic reserves, photochemical efficiency, and growth. Lastly, coral cores highlighted that this lineage also exhibited faster growth historically. Functional variation among cryptic lineages highlights their importance in predicting coral reef responses to climate change. Teaser Cryptic host diversity drives coral phenotypes relevant to climate change.

  PublisherOA PDFDOI

Recent grants

• Investigating the influence of thermal history on coral growth response to recent and predicted end-of-century ocean warming across a cascade of ecological scales

  NSF · $871k · 2015–2023

Frequent coauthors

• Sarah W. Davies

  61 shared

• James J. Leichter

  Scripps Institution of Oceanography

  34 shared

• Clare Fieseler

  Smithsonian Institution

  34 shared

• Brian Helmuth

  NOAA National Marine Fisheries Service Northeast Fisheries Science Center

  33 shared

• Francis Choi

  Northeastern University

  32 shared

• Randi Rotjan

  31 shared

• Scott M Jones

  31 shared

• Justin B. Ries

  Northeastern University

  30 shared

Labs

• Earth, Marine and Environmental SciencesPI