The evolution of red blood cell shape in a continental radiation of fishes

The size and shape of Red Blood Cells (RBC) can provide key information on life history strategies in vertebrates. However, little is known about how RBC shape evolved in response to environmental factors and the role of phylogenetic relationship. Here, we analyzed RBC morphometrics in a continental radiation of fishes testing the hypothesis that phylogenetic relationship determines species occupation of morphospace. We collected blood samples of five specimens of 15 freshwater fish species from six orders and used basic stereological methods to measure cell and nucleus area, perimeter, and diameter, cell and nucleus volume, nucleus:cytoplasm ratio, and shape factor of 50 cells per specimen. Then, we conducted a phylogenetic Principal Components Analysis using a dated phylogeny and built a phylomorphospace. To test if the phylogenetic relationship predicted the phenotypic similarity of species, we calculated multivariate phylogenetic signal. We also estimated the evolution rate of RBC shape for each node and tip using ridge regression. Finally, we tested if the position in the water column influenced RBC shape using a phylogenetic MANOVA. RBC shape seems to have evolved in a non-stationary way because the distribution pattern of species in the phylomorphospace is independent of the phylogeny. Accordingly, the rate of evolution for shape was highly heterogeneous, with an increase in the genus <i>Pygocentrus</i>. Water column position does not influence RBC shape. In conclusion, RBC shape seem to have evolved in response to multiple selective pressures independent of life history characters.