The new analysis, published in the journal Subterranean Biology, compiled 291 measurements of oxygen consumption rates from 37 species spanning five taxonomic classes, from tiny copepods weighing less than a microgram to the olm, a blind amphibian that can outweigh a small mammal. The dataset drew on 23 studies published between 1971 and 2023, with research effort accelerating sharply after the year 2000, and covered groundwater fauna from both the Nearctic and Palearctic regions, including sites in Canada, the United States, Mexico, France, Portugal, Spain, Italy and Slovenia. The findings reveal that groundwater species obey the same universal metabolic scaling rules that govern life everywhere else, yet respond to temperature in a way that sets them apart from their surface-dwelling relatives.

Metabolic rate, the energy an organism burns over time, is central to virtually every aspect of animal ecology. It determines how much food an animal needs, how fast it grows, how quickly it reproduces and how much heat it produces as an inevitable byproduct of the biochemical machinery of life. For ectotherms, animals whose body temperature follows that of their surroundings, metabolism is also profoundly shaped by the environment. Within permissive thermal ranges, a rise of 10 degrees Celsius often doubles or even triples metabolic rates, an effect quantified by the familiar Q10 coefficient or by the Arrhenius activation energy, which typically falls between 0.5 and 0.8 electron volts for most ectotherms, equivalent to a 7 to 12 percent increase in metabolism per degree Celsius.

Metabolic rate also scales with body size according to the allometric law, in which standard metabolic rate equals a taxon-specific constant multiplied by body mass raised to a scaling exponent. When Kleiber first described this relationship in 1932, he found an exponent close to three-quarters, and many invertebrate taxa cluster between 0.66 and 0.80, a shared characteristic whose underlying reason remains one of biology’s enduring puzzles. Exponents below one indicate non-isometric scaling, meaning metabolic rates do not double when body mass doubles. Proposed explanations range from heat exchange with the environment, since larger animals have proportionally less surface area relative to their volume, to the nonlinear scaling of circulatory and respiratory systems and to cellular and mitochondrial constraints.

To test whether groundwater species follow this universal pattern, the researchers systematically searched the Web of Science and grey literature, screening more than 2,200 papers to isolate studies that quantified oxygen consumption in groundwater metazoans. For the scaling analysis, they assembled data on 10 stygobitic species across 64 measurements, including two copepods, three isopods, four amphipods of the genus Niphargus and the cave-dwelling amphibian Proteus anguinus. Only measurements from resting, fasted adults, unexposed to stressors and kept within temperatures matching their native habitats, were included, and one value per species per study was retained to prevent statistical bias. The result was striking: body mass explained 90.8 percent of the linear variation in oxygen consumption, with a slope of 0.80, squarely within the range seen in crustaceans and other surface taxa. Groundwater life, it seems, adheres to the universal allometric law without deviation.

Yet the picture changes when temperature enters the equation. The team compiled 288 measurements from 19 stygobitic species, including copepods, isopods, amphipods, crayfish, the olm and the cavefish Astyanax mexicanus, and compared them with data from 12 non-stygobitic species drawn from the same literature. Because aquatic subterranean organisms naturally cope with periodic oxygen deficiency, shifting from normoxia above 3 milligrams per liter to hypoxia below 0.3, the analysis included measurements taken under both oxygen regimes. Oxygen consumption rates were converted to a standard unit of microliters of oxygen per milligram of dry weight per hour, and the thermodynamic response across the range from minus 2 to 30 degrees Celsius was calculated from Arrhenius plots, with the slope estimated separately for low and high temperature ranges.

The results followed a classic bell-shaped thermal profile, with metabolic rates rising up to roughly 16 to 18 degrees Celsius and declining thereafter. In the low temperature range, the activation energy for stygobitic species was 94.7 kilojoules per mole, well below the 132.2 kilojoules per mole calculated for non-stygobitic species. This means that at low to moderate temperatures, the metabolisms of groundwater species are less responsive to warming than those of surface species, a pattern consistent with the stable, cold conditions of their habitats and possibly with reduced costs for mitochondrial biosynthesis and maintenance. In the energy-poor subterranean realm, where every joule counts, a metabolism that resists temperature-driven acceleration may be an adaptation for conserving scarce resources.

But above the thermal break, the relationship reverses dramatically. In the high temperature range from 19 to 30 degrees Celsius, the activation energy for stygobitic species plunged to minus 232.7 kilojoules per mole, compared with minus 83.6 for non-stygobitic species, indicating that metabolic rates in groundwater species decline far faster with warming than in surface taxa. The authors interpret this as evidence that hypogean species possess a less efficient enzyme system at elevated temperatures, with respiratory enzymes becoming inactivated outside their usual thermal range and leading to an irreversible loss of function. Surface species, which live in environments with large temperature fluctuations, may carry a more robust molecular defense against heat stress. Comparative genomics of terrestrial subterranean and surface species supports this idea, showing that surface taxa express a notably larger number of genes in response to thermal stress, suggesting a stronger heat shock response capability, although this has not yet been demonstrated for aquatic groundwater fauna.

The activation energies reported in the study, roughly 0.98 and 1.3 electron volts for the low range and minus 2.41 and minus 0.86 electron volts for the high range, exceed the average of 0.48 electron volts calculated across 314 aquatic and terrestrial ectothermic species within their permissive temperature range. This suggests that the temperature ranges investigated may not entirely fall within the permissive thermal limits of the species analyzed, and that even modest global warming scenarios could impose disproportionate metabolic impairments on groundwater species compared with surface organisms. The finding aligns with a recent meta-analysis on the effects of climate change on subterranean ecosystems and adds a quantitative physiological foundation to growing conservation concerns.

The study also uncovered remarkable metabolic variability among closely related groundwater groups. Stygobitic cyclopoid copepods showed oxygen consumption rates roughly six times higher than those of stygobitic harpacticoids, despite both belonging to the same class, indicating that factors beyond temperature and body mass shape metabolic rates. In some stygobitic species, including the copepod Diacyclops belgicus, the isopod Proasellus lusitanicus and the amphipod Gammarus acherondytes, oxygen consumption appears mass-independent altogether, a pattern regarded as an adaptive trait to energy-limited habitats where individuals may grow larger without increasing their metabolic rates. Weight gain in these animals is attributed to metabolically inactive reserves, and stygobitic species possess larger fat stores than their surface relatives, allowing them to endure prolonged fasting. However, low metabolic variability within a species may heighten population vulnerability to anthropogenic disturbances, adding a further layer of fragility.

The authors acknowledge limitations inherent in any synthesis of published data. Inconsistent reporting of body size forced calculations and estimations, including the assumption that dry weight equals 15 percent of wet mass for amphipods, and pooling species into broad ecological groups may mask species-specific variation. Certain species are overrepresented in particular temperature classes, and the dataset contains no hyporheic species, the groundwater fauna living in the saturated sediments beneath rivers, despite their integral connection to surface ecosystems. Future research, the authors argue, should expand into lesser-studied habitats, employ experimental approaches to directly assess the impact of environmental stressors, and integrate energy budgets, food web dynamics and ecosystem functioning into metabolic studies.

For conservation biologists, the implications are potentially far-reaching. Understanding the metabolic responses of stygobitic species to temperature change can inform strategies to protect these animals in regions predicted to experience significant warming, and the identification of a physiological thermal break around 18 degrees Celsius offers a concrete threshold for managers of cave and aquifer systems. Groundwater species, with poor dispersal abilities and high habitat specialization, will depend almost entirely on their physiological capacity to cope with environmental change. As the planet warms, the slow, stable metabolisms that have sustained life in the dark for millions of years may prove to be the very trait that determines whether it can persist.

Subject of Research: groundwater ecology

Article Title: Metabolic rates of groundwater species as a function of body mass and temperature

Article References: Di Lorenzo, T., Mori, N., & Simčič, T. (2024). Metabolic rates of groundwater species as a function of body mass and temperature. Subterranean Biology, 49, 53-74. https://doi.org/10.3897/subtbiol.49.125131

Image Credits: AI Generated

DOI: 10.3897/subtbiol.49.125131

Keywords: stygobitic species, groundwater, metabolic rate, oxygen consumption, allometric scaling, Arrhenius activation energy, temperature, climate change, Subterranean Biology, ectotherms, cave fauna, thermal tolerance