Abstract
Environmental temperatures and oxygen availability are important for the balance between oxygen supply and demand. Terrestrial organisms are generally perceived to be less limited by access to oxygen than their aquatic counterparts. Nevertheless, even terrestrial environments can be deficient in oxygen, especially for organisms occurring in soil, litter, wood, rotten fruit or at high elevations. While isopods are the best adapted to a terrestrial lifestyle among crustaceans, many species, including woodlice, occupy environmental gradients of temperature and oxygen. To investigate whether mismatches between oxygen supply and demand can result in a loss of performance in a terrestrial organism, we studied the effects of atmospheric oxygen concentration on the thermal performance of the common rough woodlouse (Porcellio scaber). We compared the thermal preference, thermal sensitivity of running speed, and tolerance to extreme temperatures of woodlice exposed to one of two oxygen concentrations (21% - normoxia, 7% - hypoxia). Under hypoxia, P. scaber preferred microhabitats with temperatures that were on average 3°C lower than those preferred under normoxia. The running speed tended to reach its maximum at a lower temperature under hypoxia than under normoxia (25.13°C vs 28.87°C, respectively, although p was equal to 0.09), and normoxic woodlice ran approximately 1.5-fold faster than hypoxic woodlice at the point of maximum speed. Heat tolerance was significantly lower under hypoxia (38.9°C) than under normoxia (40.7°C), but there was no difference in cold tolerance (5.81°C under normoxia and 5.44°C under hypoxia). Overall, our results indicate that environmental gradients of temperature and oxygen may shape the physiological performance of terrestrial ectotherms, likely via their effects on the balance between oxygen supply and demand, which may have fitness consequences for these organisms in nature.
Highlights
Isopods appear to be the most successful land colonizers of all crustaceans [1], and it is estimated that their terrestrial lifestyle evolved independently at least two times [2]
Hypoxia resulted in an increased proportion of animals that did not survive the performance tests at temperatures above 37.59 ̊C (χ2 = 6.13, p = 0.01), indicating that hypoxic conditions lowered the tolerance of woodlice to high temperatures
In the analysis of critical temperatures, we found that animals exposed to hypoxia had a lower CTmax (38.90 ̊C) than animals exposed to normoxia (40.66 ̊C) (F1,16 = 6.41, p = 0.02, Fig 4)
Summary
Isopods appear to be the most successful land colonizers of all crustaceans [1], and it is estimated that their terrestrial lifestyle evolved independently at least two times [2]. They evolved number of land adaptations, such as pleopodal lungs, a water-conducting system and aggregation and conglobation behaviours [1]. It remains unclear which selective factors drove this. Thermal biology of the common rough woodlice under hypoxia funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript
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