Heat and Cold Sabotage Coral Larvae Through Opposite Cellular Routes, Study Finds

The larvae of one of the Caribbean’s most iconic reef-building corals are far more vulnerable to temperature swings than their adult colonies might suggest, and new research reveals that the damage unfolds in strikingly different ways depending on whether the water is too hot or too cold. A team of biologists at Boston University and the University of North Carolina Wilmington has shown that both cool and warm conditions reduce survival and settlement in larvae of the mountainous star coral, Orbicella faveolata, but that the two stressors sabotage development through fundamentally distinct molecular mechanisms. The findings, published in BMC Genomics, carry sobering implications for a species already listed as endangered under the US Endangered Species Act, because they suggest there is no single physiological weak point to protect — the larvae are being attacked from multiple directions at once.

Orbicella faveolata, once a dominant framework-builder across the Florida Keys and wider Caribbean, reproduces through mass spawning events in which colonies release eggs and sperm into the water column. The resulting embryos develop into planktonic larvae that must survive for days or weeks without feeding, living entirely off the lipid and protein reserves packed into each egg — a strategy biologists call lecithotrophy. Eventually, a larva must find a suitable surface, respond to chemical and physical settlement cues, and metamorphose into a benthic polyp that will, over centuries, become part of a massive coral colony. Every step of this journey is a potential bottleneck, and the new study demonstrates that temperature stress narrows the bottleneck at both ends: more larvae die, and those that survive are less capable of making the critical transition from swimming plankton to attached settler.

The researchers exposed O. faveolata larvae to three temperature treatments: a cool condition of 20 degrees Celsius, an ambient control of 27 degrees Celsius, and a warm condition of 30 degrees Celsius. These values were chosen to reflect realistic oceanographic extremes. Marine heatwaves now routinely push shallow reef waters several degrees above summer norms, while cold spells — sudden drops in water temperature driven by cold fronts, upwelling, or winter storms — can chill coastal reefs just as abruptly. Both extremes are projected to become more frequent and intense as climate change destabilizes regional weather patterns, yet the biological consequences for coral larvae, the most delicate life stage of the coral life cycle, have remained largely uncharacterized until now.

The phenotypic results were unambiguous. Compared with larvae held at the ambient 27 degrees Celsius, larvae in both the cool and warm treatments suffered significantly decreased survival and decreased settlement. Crucially, the team went a step further than most previous studies by calculating mortality-corrected settlement — a measure of settlement capacity among the individuals that were still alive. Even after accounting for the larvae that had simply died, the survivors in both stressed treatments settled less than their ambient counterparts. That distinction matters enormously for reef persistence. If temperature stress only killed larvae, surviving siblings might still found new colonies. Instead, the study shows that stress leaves a lasting functional impairment in survivors, meaning that even the larvae that make it through a thermal event may be developmentally compromised when it comes time to build the next generation of reef.

The real surprise, however, emerged when the researchers turned to the transcriptome — the complete inventory of genes being actively expressed in the larvae. Given that cool and warm exposure produced superficially similar outcomes, one might have expected similar underlying biology. Instead, the gene expression profiles told two completely different stories, revealing that the two stressors disrupt development through divergent mechanisms. This dissociation between phenotype and molecular mechanism is one of the most compelling aspects of the work, because it demonstrates that similar ecological damage can arise from opposite cellular pathologies — a lesson that applies broadly to developmental biology under environmental change.

Under cool conditions, the expression data pointed to a shutdown of fundamental cellular machinery. Genes involved in cell division were suppressed, as were components of aerobic metabolism, the oxygen-dependent process that powers nearly all energy-intensive life functions. In practical terms, cold-stressed larvae appeared to be slowing down at the cellular level, halting the proliferation and energy generation needed to build tissues and progress through development. This kind of metabolic suppression is a classic response to cold in many marine invertebrates, whose biochemical reaction rates are tightly temperature-dependent. But for a coral larva on a deadline — one that must settle before its finite energy reserves run out — a slowed metabolism translates directly into a shortened window of opportunity and a diminished capacity to complete metamorphosis.

Warm conditions produced the mirror image. Rather than suppressing cell division and aerobic metabolism, the 30-degree treatment appeared to ramp these processes up. On its face, accelerated cellular activity might sound beneficial, but for a non-feeding larva running on a fixed fuel tank, it is anything but. Elevated cell proliferation and heightened metabolic demand likely depleted the limited cellular and metabolic resources available to the lecithotrophic larvae, effectively burning through their reserves faster than they could be replaced. The warm-stressed larvae, in other words, may have been living too fast for their own good — accelerating their developmental engine while starving it of fuel, a scenario consistent with the reduced settlement capacity observed among survivors of the warm treatment.

Amid these divergent responses, one striking pattern was shared. Genes involved in neurotransmission and environmental sensing were downregulated in both the cool and warm treatments relative to ambient conditions. These gene families underpin a larva’s ability to detect and respond to settlement cues — the chemical signatures and surface characteristics that tell a drifting planktonic animal where to attach and transform. Their coordinated suppression in both stress treatments suggests that temperature stress, regardless of direction, degrades the sensory and neural apparatus larvae rely on to navigate their final transition to the benthos. A larva that cannot properly sense its environment, even if it survives and retains energy reserves, may drift past suitable habitat and never settle at all — an invisible failure that would compound the visible mortality documented in the survival assays.

For O. faveolata, a species whose populations have collapsed under the combined pressures of disease, bleaching, and habitat degradation, the implications are stark. Successful recruitment — the arrival of new settlers that will grow into reef-building adults — is the demographic engine of recovery, and this study shows that recruitment is vulnerable at multiple, mechanistically distinct points. Conservation planning that focuses solely on heatwaves would miss the threat posed by cold spells, and vice versa. More broadly, the work underscores that the effects of ocean warming and cooling cannot be inferred from adult coral responses alone; the larval stage operates under its own physiological rules, constrained by non-feeding energetics and a narrow developmental timetable.

The research also highlights the power of transcriptomic approaches to reveal mechanisms that phenotypic measurements alone cannot distinguish. Two treatments that look identical in a survival curve can be doing damage through entirely different cellular routes, and only by profiling gene expression can biologists identify those routes — and, potentially, the biomarkers that signal stress before mortality becomes visible. As marine cold spells and heatwaves intensify in a changing climate, studies of this kind provide the mechanistic foundation for predicting which species, populations, and life stages will be most at risk. For the larvae of the mountainous star coral, the message of this study is clear: whether the ocean turns too hot or too cold, the journey from egg to reef-builder is becoming a gauntlet, and the molecular odds are stacked against the next generation of one of the Caribbean’s great ecosystem engineers.

Subject of Research: Effects of cool and warm temperature stress on survival, settlement, and gene expression in larvae of the reef-building coral Orbicella faveolata

Article Title: Cool and warm conditions disrupt development through divergent mechanisms in larvae of the reef-building coral Orbicella faveolata

Article References: Glass, B. H., Fogarty, N. D., Gilmore, T. D., & Davies, S. W. (2026). Cool and warm conditions disrupt development through divergent mechanisms in larvae of the reef-building coral Orbicella faveolata. BMC Genomics. https://doi.org/10.1186/s12864-026-13438-8

Image Credits: AI Generated

DOI: 10.1186/s12864-026-13438-8

Keywords: coral larvae, Orbicella faveolata, gene expression, settlement, marine heatwave, cold spell, climate change, lecithotrophy, aerobic metabolism, cell division, neurotransmission, coral reefs