Morphological differentiation within the Daphnia longispina group

Although morphological evolution is assumed to be slow within Daphnia species complexes, discontinuities in morphological space can be detected. Here, morphological data derived from females of genetically-defined clones (cf. Gießler et al., 1999) are presented, in order to estimate the genetic component of phenotypic variance under standardised laboratory conditions. Animals originated from clonal assemblages of pre-alpine lakes and ponds, and a remote lake in western Germany, covering a wide range of morphotypes known from the traditional species D. cucullata, D. galeata, D. hyalina, D. rosea, and a variety of interspecific hybrids. Phenotypic analyses were based on quantitative and qualitative morphological characters of females in the first and fifth instars. Morphological divergence between clones was analysed using discriminant analysis or multidimensional scaling and the significance of the morphological groupings was estimated using neighbour-joining trees and bootstrapping. All analyses confirmed that (a) phenotypic similarities among taxa change with instar, (b) in contrast to low genetic divergence, pronounced morphological divergence exists between animals separated on the lake/pond level favouring speciation by the habitat shift hypothesis.     

Scaling of sensorimotor control in terrestrial mammals

Sensorimotor control is greatly affected by two factors—the time it takes for an animal to sense and respond to stimuli (responsiveness), and the ability of an animal to distinguish between sensory stimuli and generate graded muscle forces (resolution). Here, we demonstrate that anatomical limitations force a necessary trade-off between responsiveness and resolution with increases in animal size. To determine whether responsiveness is prioritized over resolution, or resolution over responsiveness, we studied how size influences the physiological mechanisms underlying sensorimotor control. Using both new electrophysiological experiments and existing data, we determined the maximum axonal conduction velocity (CV) in animals ranging in size from shrews to elephants. Over the 100-fold increase in leg length, CV was nearly constant, increasing proportionally with mass to the 0.04 power. As a consequence, larger animals are burdened with relatively long physiological delays, which may have broad implications for their behaviour, ecology and evolution, including constraining agility and requiring prediction to help control movements.     

Allometric scaling of maximum population density: a common rule for marine phytoplankton and terrestrial plants

A primary goal of macroecology is to identify principles that apply across varied ecosystems and taxonomic groups. Here we show that the allometric relationship observed between maximum abundance and body size for terrestrial plants can be extended to predict maximum population densities of marine phytoplankton. These results imply that the abundance of primary producers is similarly constrained in terrestrial and marine systems by rates of energy supply as dictated by a common allometric scaling law. They also highlight the existence of general mechanisms linking rates of individual metabolism to emergent properties of ecosystems.