Habitat selection by a unicellular alga: Is it adaptive?

Background/Question/Methods

Density-dependent habitat selection theory predicts that the distribution of individuals among habitats should maximize mean fitness. My working hypothesis was that a motile organism with a basic level of sensory perception should be able to respond to habitat differences through adaptive movement.  We used replicate populations of the single-celled alga, Chlamydomonas reinhardtii, to quantify fitness in light and shade control habitats.  I then used the density-dependent reduction in fitness observed in the controls to predict the distribution between adjacent pairs of the two habitats that would maximize mean fitness. I covered one half of a Petri dish to create a shaded habitat, then inoculated algae on either the shaded or unshaded (light) half.  I let the algae move between habitats for 12 hours, isolated the two halves of each dish, measured the optical densities in both habitats, and allowed the populations to grow for an additional 36 hours.  I calibrated density estimates with haemocytometer counts and estimated fitness as the change in density from time t to t+1. 

Results/Conclusions

Fitness declined linearly with increasing density in both the light and shade control habitats, but at different rates. Fitness was higher in light than in shade, so all cells should occupy the light habitat at low density. The relative abundance in shade should increase with increasing population size.  When inoculated in light, the distribution of cells approached that predicted by theory.  But when inoculated in the shaded habitat, cells were distributed equally between habitats and were thus unable to achieve the distribution that should maximize fitness. It thus appears that the ability to achieve adaptive movement in many organisms is likely to be contingent on habitat factors that either impinge or enhance dispersal.  Regardless, it also appears that many motile species with simple sensory capabilities are likely to possess some ability for adaptive movement that can alter their spatial and perhaps temporal dynamics.