# Osmoregulation and hypoxia tolerance in the cenote isopod Creaseriella anops: insights into its distribution in karst subterranean estuaries

## TL;DR

This study explores how a cave-dwelling isopod adapts to changing salinity and oxygen levels in Yucatan's groundwater, revealing its strong physiological resilience.

## Contribution

The study provides new insights into the osmoregulatory and hypoxia tolerance mechanisms of Creaseriella anops in anchialine ecosystems.

## Key findings

- C. anops is isosmotic at 17.7‰ and shows strong osmoregulatory capacity.
- It has a critical temperature range of 19.0 to 33.6°C and maintains routine oxygen consumption despite salinity shifts.
- Antioxidant enzyme activity and oxidative stress markers increase during salinity changes.

## Abstract

Groundwater systems of the Yucatan Peninsula form one of the world’s most intricate aquifer systems, supporting a unique and ecologically essential subterranean fauna. The physiological capacities of resident species, and their tolerance or ability to cope with changing environmental challenges is unknown for most species. Considering the vertical stratification of anchialine ecosystems, accelerated climate change and anthropogenic pressures, we sought to investigate the physiological characteristics of a key groundwater-restricted species (stygobionts) that is broadly distributed in the Yucatan Peninsula and has been observed in fresh- and marine groundwater. Thus, we (i) characterized the respiratory metabolism, osmoregulatory capacity and thermal tolerance of the cave isopod Creaseriella anops and (ii) evaluated how variations in salinity and oxygen concentration impact its physiological performance and antioxidant defence system. Our findings indicate that C. anops is isosmotic at 17.7‰ (580.8 mOsm/kg) and possesses a strong osmoregulatory capacity. When acclimated to freshwater (0‰) at 26 ± 1°C, C. anops demonstrated a maximum critical temperature of 33.6 ± 1.3°C and a minimum critical temperature of 19.0 ± 2.0°C, with an aerobic scope of 0.053 mg O₂/g/h. Dissolved oxygen levels (15 to 100% saturation) did not influence routine oxygen consumption rates. Acute shifts in salinity (from 0‰ to 8, 14 or 35‰) initially elevated oxygen consumption rates, which returned to routine levels within three hours across all salinity conditions. This metabolic response was associated with a slight activity increase in antioxidant enzymes and elevated protein carbonylation and lipid peroxidation. In summary, C. anops showed outstanding osmoregulatory, metabolic and antioxidant capacities that likely contribute to its wide distribution within the YP’s anchialine ecosystems, providing insights into how this species may respond to future environmental shifts.