The same species, not the same invader: Metabolic responses of genetically distinct invasive populations of Dikerogammarus villosus and their intraspecific hybrid to environmental stresses

Climate change has a profound impact on ectotherms, which suffer
suboptimum thermal conditions in their native areas or spread to
previously unavailable locations. However, it is often neglected that
responses to environmental changes are likely at a population level,
rather than species-specific phenomenon. Two groups (Western and Eastern)
of the invasive Ponto-Caspian amphipod Dikerogammarus villosus
independently spread in Europe from two geographically isolated and
genetically distinct source populations living in the Danube and Dnieper
deltas, respectively. They are expected to meet in the near future in
Polish inland waters. This makes this species a perfect model to
investigate the intraspecific variability in responses to environmental
changes, as well as effects of hybridization between different genetic
units on invasiveness. We scaled maximal metabolic rate (MMR), standard
metabolic rate (SMR), and aerobic scope (AS, a measure of physiological
performance) of D. villosus at different temperatures (15 and 25 oC) and
oxygen levels (9.5 or 7.5 mg/l at a temperature of 15 and 25 oC,
respectively and 5.5 mg/l at both temperatures) to test for differences in
responses to environmental changes among the distinct invasive groups and
their hybrid. Large Western Group individuals showed a lower AS at a high
temperature, compared to low temperature and to the Eastern group
individuals, whereas small animals exhibited similar AS irrespective of
temperature and group identity. Moreover, large individuals from both
groups experienced a reduction in MMR and AS under low oxygen conditions,
whereas small animals did not change these parameters in response to
oxygen reduction. Although parental groups reduced their AS in response to
oxygen reduction, AS of the hybrid remained unchanged. In conclusion,
organism responses to climate change depend on intraspecific variation,
which should be considered in studies related to this topic. Furthermore,
D. villosus appears sensitive to oxygen deficits and temperature changes
(especially the Western group and large individuals), but intraspecific
hybridisation can enhance the environmental tolerance of invaders.