Separating the generational effects of temperature and viscosity on the body size of a freshwater Mesocyclops copepod.

Copepods are small crustacean zooplankton that closely follow Bergmann's rule, which states that larger organisms will be found at higher latitudes. While thermally driven metabolic effects and food availability are often stated to be the major drivers behind this trend, temperature affects multiple variables within the copepod and its environment, a key variable to copepod ecology being viscosity. To test the effects of viscosity on copepod body size, two lineages of subtropical, freshwater Mesocyclops sp. copepods were grown for five generations in cultures of differing temperatures, 30°C and 18°C, and viscosities, natural and altered to mimic 18°C while at 30°C. Copepods grown at 30°C were, on average, 13.20% smaller than those grown at 18°C, regardless of viscosity. Copepods grown in 30°C cultures with a viscosity of 18°C had no body size differences when compared to copepods grown at 30°C and a natural viscosity. Copepods reached sexual maturity after 10 days while grown at 30°C and after 13 days while grown at 18°C, with viscosity playing no role in maturation time. As such, this study provides further support for temperature driving copepod body size, with the viscosity of the environment playing no discernible role in the body size of these small organisms.