4.1 Life history heat response between heat-tolerant and heat-sensitive Brachionus species
Rotifers living in temporally variable habitats are exposed to frequent changes in their environment that may impact their life history. In both species we found a profound effect of temperature on survival; in both species increasing temperature led to reduced life span. Our results corroborate studies in other rotifer taxa that showed a profound effect of temperature on survival (Kauler & Enesco, 2011; Li et al., 2010; Ma, Xi, Zhang, Wen, & Xiang, 2010; Xiang, Jiang, Tao, Chen, & Xi, 2016; Xiang, Xi, Zhang, Ma, & Wen, 2010). Due to their ectothermic nature, rotifer body temperature increases with increasing ambient temperature, which accelerates metabolic rates. As a result, juvenile and egg development are accelerated up to a certain critical temperature limit, given unlimited food resources (Stelzer, 2017). Fecundity, on the other hand did not follow the same pattern. Fecundity was significantly reduced only at high heat exposure (B. calyciflorus s.s., 32oC; B. fernandoi, 26 oC), suggesting that fecundity is maintained across variable temperatures in both species, up to a limit above which conditions are too stressful.
Population growth rate (r ) is considered a proxy to evaluate environmental specializations and stress response, representing the ability of rotifers to grow in a particular environment (Stelzer, 2005; Lowe, Kemp, Díaz-Avalos, & Montagnes, 2007; Kauler & Enesco, 2011; Weisse, Laufenstein, & Weithoff, 2013). Previous work in the rotifer complex of Brachionus plicatilis used population growth as a proxy to evaluate the salinity constrains between sibling species and found that - while species tolerated a wide range of salinities - their population growth rates respond differentially (Lowe et al. 2007; Gabaldón, Montero-Pau, Serra, & Carmona, 2013; Walczynska and Serra 2014). Based on this, it was suggested that growth rate variation implies specializations among species, which facilitate their dominance in different periods of the year and makes sympatric co-occurrence possible (Serra & Fontaneto, 2017). In rotifers, broad temperature tolerance has been found (Zhang et al., 2017; Li et al., 2010) which might reflect an adaptation to temperature fluctuations occurring in aquatic habitats. Both B. calyciflorus s.s. and B. fernandoi experience temperature fluctuations in their natural habitats and can survive a broad range of temperatures, however, their densities vary considerably relative to ambient temperature (Li et al., 2010; Wen et al., 2016; Zhang et al., 2017). High densities of B. calyciflorus s.s. have been reported during summer up to 32oC, while high densities of B. fernandoi have been reported during spring and winter even down to 4oC (Yang et al., 2017; Zhang et al., 2017). We have shown that although both species can survive a range of temperatures, both their population growth rate and expression of representative heat-stress genes is different. Our results further corroborates that these species are specialized in temperature tolerance, which might also translate into habitat specializations.
Differences in life histories between sibling species of the B. calyciflorus species complex have been found in response to competition or/and predation risk (Wang et al., 2014). A recent study of Zhang, Lemmen, Zhou, Papakostas, and Declerck (2019) performed under stable food and temperature conditions (24 oC), showed differences in life history traits such as egg and juvenile developmental times, and egg production between these species. As a corollary, they demonstrated that observed differences are consistent across tested clones within species, i.e., they really represent differentiation between the species. According to our fitness results, our two species have evolved different strategies to respond to increased temperature, with B. calyciflorus adopting a life strategy of high population growth and low survival, as opposed toB. fernandoi with a strategy of low population growth and high survival. All above findings indicate that life histories of these two sibling species are differentially adapted to multiple stressors, biotic and abiotic, supporting the idea that the species are ecologically diverged and specialized for different environmental conditions, in particular with regard to temperature.