CHEMORECEPTION, ODOR LANDSCAPES, AND FORAGING DURING THE EDIACARAN-CAMBRIAN

The detection of chemicals, their discrimination, and the behavioral reactions they evoke may well be the most primitive of all activities of living organisms. Studies in diverse metazoan taxa have identified striking similarities in the cellular and neurobiological mechanisms of odor detection and transduction. At the level of the organism, scale is clearly important. Microscopic organisms are in a physical realm where chemical detection is controlled by diffusion. They react to changes in concentration gradients experienced over time and move their entire bodies to detect these changes, using a combination of “runs” and tumbles” to move within gradients. In contrast, larger organisms are in a physical realm where chemical detection is largely controlled by convection and thus by spatially and temporally complex odor plumes. The resulting “odor landscape” is both complex and dynamic. These organisms react to changes in concentration over space, using bilaterally located sensory organs that are often moved independently. During the Ediacaran-Cambrian interval, the spatial complexity of the marine odor landscape should have increased due to the “agrarian revolution” (Seilacher and Pflüger, 1994), which greatly increased the heterogeneity of the sea floor. Complexity was further increased by the advent of larger metazoans, which produced higher quality resource patches comprised of spatially discrete carcasses and fecal pellets. The increase in spatial heterogeneity made it necessary for foragers to detect and navigate among these discrete resource patches. In addition, it became necessary to chemically detect the presence of potential predators. Increases in organism size would have led to a transition from diffusion to convection dominated chemoreception, which may be responsible to morphologic innovations such as arthropod antennae. In sum, the critical innovations that would have led to optimal foraging by early mobile marine organisms in an environment of increasing patchiness are directly related to their abilities to obtain, process, and retain information about the spatial properties of their environment.