Our study reveals a hitherto overlooked ecological threat of climate change. Studies of warming events in the ocean have typically focused on the events’ maximum temperature and duration as the cause of devastating disturbances in coral reefs, kelp forests, and rocky shores. In this study, however, we found that the rate of onset (R_onset), rather than the peak, was the likely trigger of mass mortality of coral reef fishes in the Red Sea. Following a steep rise in water temperature (4.2 °C in 2.5 d), thermally stressed fish belonging to dozens of species became fatally infected by Streptococcus iniae. Piscivores and benthivores were disproportionately impacted whereas zooplanktivores were spared. Mortality rates peaked 2 wk later, coinciding with a second warming event with extreme R_onset. The epizootic lasted ∼2 mo, extending beyond the warming events through the consumption of pathogen-laden carcasses by uninfected fish. The warming was widespread, with an evident decline in wind speed, barometric pressure, and latent heat flux. A reassessment of past reports suggests that steep R_onset was also the probable trigger of mass mortalities of wild fish elsewhere. If the ongoing increase in the frequency and intensity of marine heat waves is associated with a corresponding increase in the frequency of extreme R_onset, calamities inflicted on coral reefs by the warming oceans may extend far beyond coral bleaching. Significance Our study reveals a hitherto overlooked effect of warming on coral reefs. Traditionally, ecological studies of warming events focused on maximum temperatures and duration, rather than the rate of warming at the onset. Here, we show that onsets can trigger widespread mortality of reef fish. Hundreds of thermally stressed fish, belonging to dozens of species, became fatally infected with a common pathogen in the Red Sea. Differential susceptibility led to selective mortality, with disproportional death among predators and benthic feeders. A reassessment of past reports suggests that extreme onset might have been an overlooked trigger of fish kills elsewhere. Warming tropical and subtropical reefs may face an increasing frequency of extreme onsets, eliciting calamities far beyond coral bleaching.

Copepods are among the most abundant and diverse groups of mesozooplankton in the world's oceans. Each species has a certain depth range within which different individuals (of the same life stage and sex) are found. Lipids are accumulated in many calanoid copepods for energy storage and reproduction. Lipid content in some species increases with depth, however studies so far focused mostly on temperate and high-latitude seasonal vertically migrating copepods and compared lipid contents among individuals either from coarse layers or between diapausing, deep-dwelling copepods and individuals found in the photic, near-surface layer. Here we examined whether lipid contents of individual calanoid copepods of the same species, life stage/sex differ between finer depth layers within the upper water column of subtropical and Arctic seas. A total of 6 calanoid species were collected from samples taken at precise depths within the photic layer in both cold eutrophic and warm oligotrophic environments using SCUBA diving, MOCNESS and Multinet. Measurements of lipid content were obtained from digitized photographs of the collected individuals. The results revealed significant differences in lipid content across depth differences as small as 12–15 meters for Mecynocera clausi C5 and Ctenocalanus vanus C5 (Red Sea), Clausocalanus furcatus males and two clausocalanid C5s (Mediterranean Sea), and Calanus glacialis C5 (Arctic). We suggest two possible explanations for the differences in lipid content with depth on such a fine scale: predator avoidance and buoyancy.

Tracking the movement of all individual group members in their natural environment remains a challenging task. Using advances in computer vision and Deep Learning, we developed and tested a semi‐automated in situ tracking system to reconstruct simultaneous three‐dimensional trajectories of marked individuals in social groups of a coral‐reef fish. Our system has a temporal resolution of 10s of milliseconds, allowing for multiple 30‐min tracking sessions that have been repeated over weeks to months. We present the technique and illustrate its application for Dascyllus marginatus, a planktivorous damselfish that lives in social groups associated with branching corals. Our technique identified all individuals 85–100% of the time, with a mean spatial error of ~ 1.3 cm. It provides a cost‐effective semi‐automated tool for in situ research on movements and foraging of individuals within small site‐attached groups of animals in their natural environment.

A major challenge faced by sessile animals that feed in the flow is to maintain effective feeding postures while enduring hydrodynamic forces. Garden eels exhibit an exceptional lifestyle: feeding on drifting zooplankton while being “anchored” in a burrow they dig in the sand. Using underwater observations, sampling and 3-D video recording, we measured the feeding rates and characterized feeding postures of garden eels under a wide range of current speeds. We show that the eels behaviorally resolve the tradeoff between adverse biomechanical forces and beneficial fluxes of food by modulating their body postures according to current speeds. In doing so, the eels substantially reduce drag forces when currents are strong, yet keep their head well above bottom in order to effectively feed under conditions of high prey fluxes. Those abilities allowed garden eels to become one of the rare oceanic fishes that live in sandy, predation-rich habitats and feed on zooplankton while being attached to the bottom.

Back‐to‐back marine heatwaves in 2016 and 2017 resulted in severe coral bleaching and mortality across the Great Barrier Reef (GBR). Encouragingly, some corals that survived these events exhibit increased bleaching resistance and may represent thermally tolerant populations that can better cope with ocean warming. Using the GBR as a natural laboratory, we investigated whether a history of minimal (Heron Island) or severe (Lizard Island) coral bleaching in 2016 and 2017 equates to stress tolerance in a successive heatwave (2020). We examined the genetic diversity, physiological performance, and trophic plasticity of juvenile (25 cm) corals of two common genera (PocilloporaandStylophora). Despite enduring greater cumulative heat stress (6.3°C week⁻¹vs. 5.6°C week⁻¹), corals that experienced the third marine heatwave in 5 years (Lizard) exhibited twice as high survival and visual bleaching thresholds compared to corals that had not experienced significant bleaching in >10 years (Heron). Surprisingly, only one shared host–Symbiodiniaceae association was uncovered between locations (Stylophora pistillata–Cladocopium“C8 group”) and there was no genetic overlap inPocillopora–Cladocopiumpartnerships, suggesting turnover in species composition from recent marine heatwaves. Corals within the species complexPocilloporathat survived the 2016 and 2017 marine heatwaves at Lizard Island were the most resilient, exhibiting three times greater calcification rates than conspecifics at Heron Island. Further, surviving corals (Lizard) had distinct isotopic niches, lower host carbon, and greater host protein, while conspecifics that had not experienced recent bleaching (Heron) had two times greater symbiont carbon content, suggesting divergent trophic strategies that influenced survival (i.e., greater reliance on heterotrophy vs. symbiont autotrophy, respectively). Ultimately, while corals may experience less bleaching and survive repeated thermal stress events, species‐specific trade‐offs do occur, leaving open many questions related to the long‐term health and recovery of coral reef ecosystems in the face of intensifying marine heatwaves.

Quantifying the energy costs of various activities is critical to understand key aspects of animal behavior and ecology. Currently, calorimetry is the most widely used method to measure those costs in laboratory studies, whereas field studies use the doubly labeled water method, heart rate and dynamic body acceleration (DBA). However, these methods are limited or even biased because of restricted space for movement, low temporal resolution and/or the need for logger attachment or implantation. Measuring energy costs of behaviors is difficult, especially in small, highly mobile animals. Here, using a damselfish, Chromis viridis, we demonstrate that DBA, obtained from marker-less, automatic video tracking and 3D reconstruction, can effectively estimate oxygen consumption rate. We show that our video-based DBA method can be used to estimate metabolic costs of various activities, such as locomotion and feeding, on an individual basis.