Why is Diaphanosoma (Crustacea: Ctenopoda) so common in the tropics? Influence of temperature and food on the population parameters of Diaphanosoma dubium, and a hypothesis on the nature of tropical cladocerans

Abstract

Diaphanosoma dubium Manuilova from (sub)tropical South China, cultured in the laboratory at 23–29°C, did not show a shortening of its lifespan at increasing temperatures, although its postembryonic development shortened, while individual clutch sizes became bigger. The optimum temperature for the population performance of this species (measured as lifetime fecundity or intrinsic rate of increase) therefore lies above 29°C. Limited literature data suggest that in tropics-adapted species, the shortening of the lifespan with temperature, which follows a quadratic function, reverses at higher temperatures. Thus, the most general descriptor of the complete temperature-lifespan relationship might be a parabola. D. dubium did not show shorter lifespans when offered more food either, and again shared this characteristic with few other tropical cladocerans (and probably copepods as well) studied by other authors. Both properties combined might be typical of tropical species and offer them an advantage (a long, prolific life in warm water rich in algae) over temperate species (like Daphnia) that, if occurring in the tropics at all, live above their temperature optimum there.