Intra- and inter-specific variation can reveal ecological mechanisms and evolutionary steps leading to the evolution of new species. We tested the hypothesis that egg size and fecundity would vary across an altitude gradient (i) within the widely distributed Galaxias vulgaris and (ii) between G. vulgaris and two other closely related species.
In Galaxias vulgaris, egg size increased and fecundity decreased with increasing altitude. Inter-specific egg size and fecundity of the three species also varied across an altitude gradient, with egg size increasing and fecundity decreasing as altitude increased. Galaxias paucispondylus, a high altitude species compared to the other two species, had the largest mean egg size and lowest fecundity. In contrast, mean egg size and fecundity of lower altitude G. brevipinnis was an order of magnitude higher and lower respectively than G. paucispondylus. Mean egg size and fecundity of G. vulgaris was intermediate relative to the other species, reflecting a distribution that broadly spans the altitudinal gap between the G. paucispondylus and G. brevipinnis.
Our results suggest that egg/ fecundity trait variation is driven by differences in the productivity of larval rearing habitat. High altitude (low productivity) environments require large larvae (and therefore eggs), but at the cost of maternal fecundity. At lower altitudes (higher productivity), maternal fitness is maximised by increasing fecundity by reducing egg size to the detriment of larval fitness. Our results highlight how egg size-fecundity trade-offs could facilitate the evolution of species at either end of the egg size-fecundity life history spectrum.