About

Professor Gretchen Hofmann is a faculty member in the Ecology, Evolution, and Marine Biology department at UC Santa Barbara, where she serves as the Principal Investigator of the Ocean Global Change Biology Lab. Her research centers on organism-environment interactions, with a particular focus on marine invertebrates. She is dedicated to making her research relevant to decision makers and managers of marine ecosystems, emphasizing the practical applications of her scientific findings. Professor Hofmann's work addresses critical environmental challenges such as marine heatwaves and ocean acidification, and she is involved in long-term ecological research at the Santa Barbara Coastal LTER. Her research also extends to Antarctic studies, larval biology, environmental epigenetics, aquaculture, and fisheries. Through her leadership, the lab investigates how marine organisms respond physiologically and ecologically to changing ocean conditions, contributing valuable insights into the impacts of global change on marine ecosystems.

Research topics

• Ecology
• Biology
• Genetics
• Zoology
• Neuroscience
• Evolutionary biology
• Demography
• Botany
• Agronomy

Selected publications

• Rapid Evolution in a Coral Population Following a Mass Mortality Event

  Evolutionary Applications · 2026-02-01

  articleOpen access

  Globally, corals face an increased frequency of mass mortality events (MMEs) as populations experience repeated marine heatwaves which disrupt their obligate algal symbiosis. Despite greater occurrences of MMEs, the relative roles of the environment, host, and symbiont genetic variation in survival, subsequent recovery, and carry-over effects to the next generation remain unresolved. High-resolution temporal and spatial whole genome sequencing of corals before, after, and several years following an MME reveal that host genetics have an impact on bleaching and mortality and that selected alleles important for adaptation persist through the next generation, demonstrating rapid evolution in this coral population. Bleaching resistance and survival following the bleaching event were highly polygenic, and allele frequency shifts show reef habitat specificity, emphasizing the spatial complexity of environmental selection and how it shapes population recovery following an MME. This study reveals how MMEs reshape the genomic landscape and the spatial and temporal distribution of genomic diversity within coral populations facing severe threats from global change.

  PublisherOA PDFDOI

• Heterotrophy of particulate organic matter subsidies contributes to divergent bleaching responses in tropical Scleractinian corals

  Limnology and Oceanography · 2025-05-14 · 6 citations

  articleOpen access

  Abstract Heterotrophic feeding on plankton and particulate organic matter (POM) by tropical Scleractinian corals is known to aid in the resistance and recovery from thermally induced bleaching. However, the relative importance of heterotrophy in promoting bleaching resistance and recovery is likely to vary based on ecological context and the severity of heat stress. In 2019, the Pacific Island of Mo'orea experienced mass coral mortality during a widespread marine heatwave. Many Acropora hyacinthus colonies on the shallow reef slope (5 m) were resistant to bleaching, while colonies on the deeper fore reef (14 m) often bleached and subsequently recovered, despite similar thermal stress. The role of heterotrophy in this divergent bleaching response was investigated using fatty acid (FA), isotopic, and elemental biomarkers. Multiple complementary lines of evidence, including feeding proxies, isotopic niche overlap, and putative POM FA biomarkers, indicated that bleaching resistant colonies were likely consuming more POM than their bleached and recovered counterparts. Additionally, although visually recovered, host energetics in recovered colonies remained compromised and exhibited proportionally less monounsaturated and polyunsaturated FAs and less relative nitrogen than resistant colonies. We show that corals that rely more on heterotrophic nutrient acquisition can better resist thermally induced bleaching. Our results also revealed the long‐term energetic costs of bleaching even after visual recovery. Ultimately, these results underscore the vital role of coral nutrition in shaping coral bleaching response and recovery.

  PublisherOA PDFDOI

• Coastal marine heatwaves in the Santa Barbara Channel: decadal trends and ecological implications

  Frontiers in Marine Science · 2024-12-16 · 4 citations

  articleOpen accessSenior author

  Marine heatwaves (MHWs) are of increasing concern due to the emerging ecological and socioeconomic impacts on coastal ecosystems. Leveraging the data of the Santa Barbara Coastal Long-Term Ecological Research project, we analyzed the MHW event metrics observed in the kelp forest ecosystem and across Santa Barbara Channel, CA, USA. Not only was there a significant positive trend in the number of MHWs recorded, their duration and intensity were also increasing over time. MHWs were detected year-round, suggesting that marine organisms have exposure risks regardless of their phenology. Exposure at one life history stage could have a legacy effect on the subsequent stages, implying little temporal refuge. In contrast, the coastal mooring data revealed that near-surface and bottom events were not necessarily coupled even at less than 15 m. Such spatial variation in MHWs might provide a temporary refuge for mobile species. These observations also highlight the importance of depth-stratified, long-term coastal monitoring to understand spatio-temporal variation in MHW stress on coastal communities.

  PublisherOA PDFDOI

• Kelp-associated variability in seawater chemistry during a marine heatwave event connects to transgenerational effects in the purple urchin Strongylocentrotus purpuratus

  Marine Ecology Progress Series · 2024-01-17 · 2 citations

  articleSenior author

  Giant kelp Macrocystis pyrifera provides the foundation for immense biodiversity on the coast of California, USA. Kelp forests can change seawater retention time, altering water chemistry, including pH and dissolved oxygen (DO), as well as the magnitude and predictability of variability in the same properties. Environmental heterogeneity across space and time could drive organismal performance and processes such as transgenerational plasticity (TGP), where parental experience modifies the offspring phenotype, potentially conferring tolerance to future environmental stress. We monitored environmental variability by deploying temperature, pH, and DO sensors inside and outside a temperate kelp forest in the Santa Barbara Channel (SBC) throughout the gametogenesis period of a key herbivore, the purple urchin Strongylocentrotus purpuratus . Over the 6 mo period, pH and temperature were slightly elevated inside the kelp forest, accompanied by more predictable, low-frequency variability relative to outside. Adult S. purpuratus were conditioned inside and outside the kelp spanning gametogenesis. The urchins were spawned and their larvae were raised under high (1053 µatm) and low p CO 2 (435 µatm) at 15°C in the laboratory to assess their physiological response to the maternal and developmental environments. Larvae raised under high p CO 2 were more susceptible to acute thermal stress; however, within each larval treatment, progeny from outside-conditioned mothers had a 0.4°C higher lethal temperature (LT 50 ). Our results indicate that heterogeneity in abiotic factors associated with kelp can have transgenerational effects in the field, and interactions between factors, including temperature and pH, will impact purple urchins as local variability associated with marine heatwaves and upwelling evolves with climate change.

  PublisherDOI

• Thermal plasticity has higher fitness costs among thermally tolerant genotypes of <i>Tigriopus californicus</i>

  Functional Ecology · 2024-05-02 · 10 citations

  articleOpen accessSenior author

  Abstract Under climate change, ectotherms will likely face pressure to adapt to novel thermal environments by increasing their upper thermal tolerance and its plasticity, a measure of thermal acclimation. Ectotherm populations with high thermal tolerance are often less thermally plastic, a trade‐off hypothesized to result from (i) a phenotypic limit on thermal tolerance above which plasticity cannot further increase the trait, (ii) negative genetic correlation or (iii) fitness trade‐offs between the two traits. Whether each hypothesis causes negative associations between thermal tolerance and plasticity has implications for the evolution of each trait. We empirically tested the limit and trade‐off hypotheses by leveraging the experimental tractability and thermal biology of the intertidal copepod Tigriopus californicus . Using populations from four latitudinally distributed sites in coastal California, six lines per population were reared under a laboratory common garden for two generations. Ninety‐six full sibling replicates ( n = 4–5 per line) from a third generation were developmentally conditioned to 21.5 and 16.5°C until adulthood. We then measured the upper thermal tolerance and fecundity of sibships at each temperature. We detected a significant trade‐off in fecundity, a fitness corollary, between baseline thermal tolerance and its plasticity. Tigriopus californicus populations and genotypes with higher thermal tolerance were less thermally plastic. We detected negative directional selection on thermal plasticity under ambient temperature evidenced by reduced fecundity. These fitness costs of plasticity were significantly higher among thermally tolerant genotypes, consistent with the trade‐off hypothesis. This trade‐off was evident under ambient conditions, but not high temperature. Observed thermal plasticity and fecundity were best explained by a model incorporating both the limit and trade‐off hypotheses rather than models with parameters associated with one hypothesis. Effects of population and family on tolerance and plasticity negatively covaried, suggesting that a negative genetic correlation could not be ruled as contributing to negative associations between the traits. Our study provides a novel empirical test of the fitness trade‐off hypothesis that leverages a strong inference approach. We discuss our results' insights into how thermal adaptation may be constrained by physiological limits, genetic correlations, and fitness trade‐offs between thermal tolerance and its plasticity. Read the free Plain Language Summary for this article on the Journal blog.

  PublisherOA PDFDOI

• Parental stress alters the fitness effects and genetic correlation of offspring performance traits and their gene expression pathways

  bioRxiv (Cold Spring Harbor Laboratory) · 2024-10-11

  preprintOpen accessSenior author

  Abstract Transgenerational effects, where parental environments influence offspring phenotype, facilitate acclimation over ecological timescales. Transgenerational effects may also influence evolution by altering the fitness costs of offspring traits and the expression of genetic variation. We tested the evolutionary role of transgenerational effects in the sea urchin Strongylocentrotus purpuratus , whose populations are exposed to coastal upwelling (periods of low temperature and pH) and exhibit local adaptation, parental effects, and phenotypic plasticity in response to upwelling. Using a quantitative genetic breeding design, we conditioned parents and larvae to upwelling or control conditions and combined RNA-seq with larval phenotyping (body size, biomineralization, survival). Larval upwelling exposure caused widespread differential expression (DE), reduced biomineralization, and reduced size. Survival was linked to biomineralization, body size, and their plasticity under upwelling, but only in offspring of upwelling-conditioned parents -evidence that parental environment affected selection on offspring traits. DE under upwelling was associated with adaptive plasticity in biomineralization and size, but adaptive gene expression changes shared negative genetic correlations. However, genetic correlations in gene expression associated with body size plasticity were significantly more positive in larvae from upwelling parents. Parental conditioning strengthened selection on offspring performance and reduced genetic tradeoffs between performance-associated expression pathways, potentially accelerating adaptation.

  PublisherOA PDFDOI

• Genomic signatures of coral adaptation and recovery following a mass mortality event

  bioRxiv (Cold Spring Harbor Laboratory) · 2024-09-25 · 1 citations

  preprintOpen access

  ABSTRACT Globally, corals face an increased frequency of mass mortality events (MMEs) as populations experience repeated marine heatwaves which disrupt their obligate algal symbiosis. Despite greater occurrences of MMEs, the relative roles of the environment, host, and symbiont genetic variation in survival and recovery remain unresolved. We explore these roles using high-resolution temporal and spatial whole genome sequencing of corals before, after, and several years following a MME. We show that host genetics has an impact on bleaching and mortality and that selected alleles important for adaptation persist through the next generation. We also demonstrate that survival for the bleaching event was highly polygenic and that allele frequency shifts have reef habitat specificity. This study reveals how MMEs reshape the genomic landscape and the spatial and temporal distribution of genomic diversity within coral populations facing severe threats from global change. SIGNIFICANCE Investigations of natural selection in coral genomes following bleaching events have primarily relied on indirect inference, using contemporary populations to explore signatures of past selective pressures, or failed to link these events to the subsequent generation. This has left an open question about the ability of a coral population to adapt from host standing genetic variation in the face of a bleaching event. We demonstrate rapid evolution in a Mo’orean coral population following an MME from a marine heatwave by capturing allele frequency shifts through time. However, the complex polygenic architecture of bleaching survival shows strong habitat specificity, complicating the path towards a genomic prediction of bleaching.

  PublisherDOI

• Bringing heatwaves into the lab: A low‐cost, open‐source, and automated system to simulate realistic warming events in an experimental setting

  Limnology and Oceanography Methods · 2024-12-21 · 2 citations

  articleOpen accessSenior author

  Abstract Aquatic ecosystems face increasing threats from heatwaves driven by anthropogenic climate change, necessitating continued research to understand and manage the ecological consequences. Experimental studies are essential for understanding the impacts of heatwaves in aquatic systems; however, traditional experimental methods often fail to capture real‐world complexity. Here, we present a method for simulating aquatic heatwaves that match the dynamic nature of real‐world heatwave events in an experimental setting. Our method allows researchers to re‐create heatwaves that have happened in the past or produce entirely new heatwave scenarios based on future projections. A Raspberry Pi serves as the foundation of our autonomous, customizable temperature control system, leveraging a low‐cost and open‐source platform for adaptability and accessibility. We demonstrate system functionality for laboratory experiments by first simulating a hypothetical marine heatwave scenario with defined temperature parameters and then replicating a real‐world marine heatwave that occurred in the Santa Barbara Channel, California, in 2015. The average difference between desired and observed temperatures was 0.023°C for the basic heatwave simulation and less than 0.001°C for the real‐world heatwave simulation, with standard deviations of 0.04°C and 0.01°C, respectively. Our novel method facilitates broader access to high‐quality and affordable tools to study extreme climate events. By adopting a more realistic experimental approach, scientists can conduct more informative aquatic heatwaves studies.

  PublisherOA PDFDOI

• Transgenerational plasticity as a mechanism of response to marine heatwaves in the purple sea urchin, Strongylocentrotus purpuratus

  Frontiers in Marine Science · 2023-07-21 · 19 citations

  articleOpen accessSenior author

  Kelp forests of the California Current System have experienced prolonged marine heatwave (MHW) events that overlap in time with the phenology of life history events (e.g., gametogenesis and spawning) of many benthic marine invertebrates. To study the effect of thermal stress from MHWs during gametogenesis in the purple sea urchin ( Strongylocentrotus purpuratus ) and further, whether MHWs might induce transgenerational plasticity (TGP) in thermal tolerance of progeny, adult urchins were acclimated to two conditions in the laboratory – a MHW temperature of 18°C and a non-MHW temperature of 13°C. Following a four-month long acclimation period (October–January), adults were spawned and offspring from each parental condition were reared at MHW (18°C) and non-MHW temperatures (13°C), creating a total of four embryo treatment groups. To assess transgenerational effects for each of the four groups, we measured thermal tolerance of hatched blastula embryos in acute thermal tolerance trials. Embryos from MHW-acclimated females were more thermally tolerant with higher LT 50 values as compared to progeny from non-MHW-acclimated females. Additionally, there was an effect of female acclimation state on offspring body size at two stages of embryonic development - early gastrulae and prism, an early stage echinopluteus larvae. To assess maternal provisioning as means to also alter embryo performance, we assessed gamete traits from the differentially acclimated females, by measuring size and biochemical composition of eggs. MHW-acclimated females had eggs with higher protein concentrations, while egg size and lipid content showed no differences. Our results indicate that TGP plays a role in altering the performance of progeny as a function of the thermal history of the female, especially when thermal stress coincides with gametogenesis. In addition, the data on egg provisioning show that maternal experience can influence embryo traits via egg protein content. Although this is a laboratory-based study, the results suggest that TGP may play a role in the resistance and tolerance of S. purpuratus early stages in the natural kelp forest setting.

  PublisherOA PDFDOI

• Paternal heat exposure affects progeny larval development in green-lipped mussels Perna canaliculus

  Aquaculture Environment Interactions · 2023-11-17 · 4 citations

  articleOpen access

  The green-lipped mussel Perna canaliculus is critically important to the New Zealand aquaculture industry. However, the rise in marine heatwave (MHW) events poses an emerging threat to this industry through summer mortality events. This study investigated the potential for paternally mediated transgenerational plasticity to improve offspring performance under heat stress. We simulated a week-long MHW event, exposing male P. canaliculus broodstock to elevated (22°C) or ambient (17.5°C) temperatures immediately prior to spawning, and evaluated the effects of paternal heat exposure on successful development, size and acute thermal tolerance of their larvae that were also reared under ambient or elevated (20°C) temperatures through to completion of the lecithotrophic trochophore stage. Elevated paternal and larval temperatures both increased incidence of abnormal development, reducing larval yield, while initial D-veliger shell length was predominantly influenced by developmental temperature, with longer shells formed at 20°C. Veligers from heat-exposed fathers raised under 20°C showed a small, but significant, elevation in lethal tolerance 50 (LT 50 ), the temperature at which 50% of the larvae are predicted to die, when exposed to an additional 1 h heat-shock. These results indicate that paternal heat exposure over a relatively short period can influence offspring performance in this species. The paternal exposure investigated showed limited positive effects on offspring thermal tolerance, which may be outweighed by the negative impact on larval development. As MHWs are forecasted to continue accelerating, understanding transgenerational effects of heat stress will be critical for maintaining high-quality hatchery yields through broodstock selection and may inform wild population forecasting models.

  PublisherDOI

Recent grants

• Finding the Genes That Matter: Profiling Gene Expression in Strongylocentrotid Sea Urchins with Different Biogeographic and Temperature Distributions

  NSF · $384k · 2004–2009

• Mechanisms of physiological plasticity in early stage marine invertebrates in response to multiple stressors - epigenomic perspective in a global change context

  NSF · $779k · 2017–2023

• Synergistic Effects of Climate-Related Variables on Larval Sea Urchins: Performance to Gene Expression

  NSF · $605k · 2010–2016

• OCEAN ACIDIFICATION - Category 1: COLLABORATIVE RESEARCH: Acclimation and adaptation to ocean acidification of key ecosystem components in the California Current System

  NSF · $473k · 2010–2014

• OCEAN ACIDIFICATION - COLLABORATIVE RESEARCH: OMEGAS II - Linking ecological and organismal responses to the ocean acidification seascape in the California Current System

  NSF · $320k · 2012–2015

Frequent coauthors

• Richard O. Hynes

  Howard Hughes Medical Institute

  48 shared

• Taku Hibino

  Saitama University

  43 shared

• Antonio Fernández-Guerra

  University of Copenhagen

  43 shared

• Patrick Cormier

  Laboratoire de Biologie Intégrative des Modèles Marins

  43 shared

• Cynthia Messier

  Baylor College of Medicine

  43 shared

• Celina E. Juliano

  University of California, Davis

  43 shared

• Sorin Istrail

  John Brown University

  43 shared

• Odile Mulner‐Lorillon

  Centre National de la Recherche Scientifique

  43 shared

Labs

• Ocean Global Change Biology LabPI

  Ecology, Evolution, and Marine Biology