Effect of a temperature gradient on the behaviour of an endangered Mexican topminnow and an invasive freshwater fish

Time using the rock as refuge

Temperature had an effect in the refuge usage of both species when analysed together (lme.zig: F_3,192 = 7.97, p = 0.0001; Fig. 1A). However, species behaved differently (lme.zig: F_1,192 = 14.79, p = 0.0004; Fig. 1A). As hypothesised, there was an interaction between temperature and species (lme.zig: F_3,192 = 11.90, p < 0.0001, Fig. 1A), while twoline skiffias decreased their refuge usage as temperature increased (lme.zig: F_3,96 = 7.26, p = 0.0003, Fig. 1A), guppies showed no change in their behaviour (lme.zig: F_3,96 = 0.64, p = 0.594, Fig. 1A).

There were differences between sexes (lme.zig: F_1,192 = 5.70, p = 0.021, Fig. 1A) in the refuge usage and an interaction with temperature (lme.zig: F_3,192 = 7.14, p = 0.0002, Fig. 1A) of both species when analysed together, however, males and females of the two species behaved differently (lme.zig: F_1,192 = 3.96, p = 0.053, Fig. 1A). Female twoline skiffias used the rock as a refuge more than males (lme.zig: F_1,96 = 3.98, p = 0.059, Fig. 1A) and twoline skiffia, as species, used less the refuge when temperature increased (lme.zig: F_3,96 = 2.82, p = 0.046, Fig. 1A), whereas guppies showed no differences between sexes (lme.zig: F_1,96 = 1.41, p = 0.249; Fig. 1A), and there was no interaction with temperature (lme.zig: F_1,96 = 0.93, p = 0.432; Fig. 1A).

Size had no effect in the time spent using the refuge (lme.zig: F_1,192 = 2.16, p = 0.15) neither for twoline skiffias (lme.zig: F_1,96 = 1.13, p = 0.301) nor for guppies (lme.zig: F_1,96 = 0.99, p = 0.332). However, for guppies, there was an interaction between temperature and size (lme.zig: F_3,96 = 2.76, p = 0.049, Fig. 2), suggesting that at 30 °C they hardly used the refuge, and bigger fish used the refuge more than smaller fish.

Time exploring the rock

We found that temperature did not had an effect in the time both fish explored the rock when analysed together (lme: F_3,192 = 1.09, p = 0.345, Fig. 1B). However, species behaved differently (lme: F_1,96 = 23.41, p < 0.0001, Fig. 1B), twoline skiffias interacted more with the rock than guppies. This was confirmed when species were analysed separately, temperature did not affect the behaviour of twoline skiffias (lme: F_3,96 = 1.03, p = 0.387, Fig. 1B) or guppies (lme: F_3,96 = 0.88, p = 0.455, Fig. 1B) and there was no interaction between the temperatures and species (lme: F_3,192 = 0.90, p = 0.445, Fig. 1B).

In the analysis including both species, sex was not a significant variable (lme: F_1,192 = 1.92, p = 0.173, Fig. 1B) for the exploring behaviour and there were no interactions between temperature (lme: F_3,192 = 2.37, p = 0.073, Fig. 1B) and species (lme: F_3,192 = 1.20, p = 0.280, Fig. 1B). Still, in the analysis for each species we found that female and male guppies behaved differently (lme: F_1,96 = 6.60, p = 0.018, Fig. 1B), females explored more the rock than males. When twoline skiffias were analysed solely, we did not find any significant interactions (lme: F_1,96 > 0.30, p > 0.14, Fig. 1B).

Size had an effect in the time exploring the rock (lme: F_1,192 = 6.91, p = 0.012, Fig. 3) when species were analysed together, but there was no interaction with temperatures (lme: F_3,192 = 0.42, p = 0.74, Fig. 3). We found that the interaction between species and size was close to be significant (lme: F_1,192 = 3.62, p = 0.064, Fig. 3), implying that possibly smaller fish spent more time exploring the rock than bigger fish. However, when analysed separately, we did not find an effect of size in the exploring behaviour neither for twoline skiffias (lme: F_1,96 = 2.99, p = 0.099, Fig. 3) nor for guppies (lme: F_1,96 = 0.33, p = 0.569, Fig. 3).

Time spent swimming

Temperature had an effect in the time spent swimming for both species when analysed together (lme: F_3,192 = 23.48, p < 0.0001, Fig. 1C), and species behaved similarly (lme: F_3,192 = 0.61, p = 0.440, Fig. 1C). We found a significant interaction between temperature and species (lme: F_3,192 = 4.03, p = 0.009, Fig. 1C), results of analysis for each species showed that twoline skiffias spent more time swimming around the aquarium when temperature increased (lme: F_1,96 = 24.74, p < 0.0001, Fig. 1C), and guppies behaved similarly but the effect seemed to be weaker (lme: F_1,96 = 4.68, p = 0.005, Fig. 1C).

We found no differences between sexes (lme: F_1,192 = 3.33, p = 0.075, Fig. 1C) in the time spent swimming for both species when considered together. However, when analysed separately, males of twoline skiffias spent more time swimming around the aquarium than females (lme: F_1,96 = 5.22, p = 0.033, Fig. 1C), while guppy males and females spent a similar proportion of time swimming (lme: F_1,96 = 2.90, p = 0.103, Fig. 1C). There was no interaction with temperature (lme: F_3,96 = 1.49, p = 0.219, Fig. 1C) neither for twoline skiffias (lme: F_3,96 = 2.35, p = 0.081, Fig. 1C) nor for guppies (lme: F_3,96 = 0.56, p = 0.640, Fig. 1C).

Size had an effect in the time spent swimming around the aquarium (lme: F_1,192 = 4.19, p = 0.047, Fig. 4) when species were analysed together, suggesting that smaller fish spent more time swimming than bigger ones. But when analysed separately, this variable had no effect in the time spent swimming neither for twoline skiffias (lme: F_1,96 = 0.11, p = 0.748, Fig. 4) nor for guppies (lme: F_1,96 = 0.23, p = 0.637, Fig. 4).

Source: Ecology - nature.com