Abstract
Temperature is a crucial factor determining biology and ecology of poikilothermic animals. It often constitutes an important barrier for invasive species originating from different climate zones but, on the other hand, may facilitate the invasion process of animals with wide thermal preferences and high resistance to extreme temperatures. In our experimental study, we investigated the thermal behaviour of two Ponto-Caspian amphipod crustaceans—Dikerogammarus villosus and Dikerogammarus haemobaphes. Both species are known to live under a wide range of thermal conditions which may promote their invasion. Moreover, both these amphipods are hosts for microsporidian parasites which co-evolved with them within the Ponto-Caspian region and spread in European waters. As the presence of a parasite may influence the thermal preferences of its host, we expected to observe behavioural changes in infected individuals of the studied amphipods leading to (1) behavioural fever (selecting a warmer habitat) or (2) anapyrexia (selecting a colder habitat). The experiment (N = 20) was carried out for 30 min in a 100 cm. 20 cm from boths sides were not avaliable for amphipods long thermal gradient (0–40 °C), using 30 randomly selected adult amphipod individuals of one species. At the end of each trial, we checked the position of amphipods along the gradient and determined their sex and infection status (uninfected or infected by one of microsporidium species). D. villosus was infected with Cucumispora dikerogammari whereas D. haemobaphes was a host for C. dikerogammari, Dictyocoela muelleri or D. berillonum. Thermal preferences of amphipods depended on their species and sex. Females of D. villosus preferred warmer microhabitats (often much above 30 °C) than conspecific males and females of D. haemobaphes, whereas no significant differences were found among males of both species and both sexes of D. haemobaphes. Moreover, infected males of D. villosus stayed in warmer water more often than uninfected males of this species, selecting temperatures higher than 30 °C, which may be explained either as a behavioural fever constituting a defence mechanism of a host against the infection, or as a parasite manipulation of the host behaviour increasing the parasite fitness. On the other hand, none of the parasite species affected the thermal preferences of D. haemobaphes, including also C. dikerogammari, changing the behaviour of D. villosus. Our research presents the complexity of the thermal behaviour of studied amphipods and the evidence that microsporidia may trigger a change in temperature preferendum of their host species and those observations may be the result of different host-parasite coevolution time which may vary for the two host species (Poulin, 2010).
Highlights
Temperature is the main variable determining multiple aspects of the functioning of poikilothermic organisms
All the statistical tests were carried out with IBM R SPSS R 24.0 (Chicago, IL, USA). Both Dikerogammarus species were infected with microsporidia; the prevalence of microsporidiosis was higher in D. villosus (52%, N = 600) than in D. haemobaphes (27.4%; N = 583)
The gammarid hosts differed in the parasite species composition: D. villosus was infected only with Cucumispora dikerogammari, whereas D. haemobaphes was infected by C. dikerogammari, Dictyocoela berillonum and Dictyocoela muelleri
Summary
Temperature is the main variable determining multiple aspects of the functioning of poikilothermic organisms It shapes their metabolic rate and their responses to other environmental factors, feeding, oxygen demands, growth rate and reproduction (Liu & Walford, 1972; Newell, 1966; Cox & Rutherford, 2000; Sardiña et al, 2017). Daily and seasonal changes in temperature take place in water bodies (e.g., Irons III et al, 1994; Moore et al, 1997; Schindler, 1997) This gives aquatic organisms the possibility of adjustment and optimization of their microhabitat selection with regard to temperature conditions by behavioural thermoregulation (Macan, 1961; Cox & Rutherford, 2000). Crustaceans belong to the richest groups in terms of species number (Gherardi, 2007) and their recent spread can be associated with global warming (Stachowicz et al, 2002; Maazouzi et al, 2011; Hulme, 2017)
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