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.