Chemical communication between copepods: Finding the mate in a fluid environment

Abstract

Copepods are small heterosexual aquatic microcrustaceans (1-10 mm, moving at ∼1-10 bodylengths/s) that must mate to reproduce. Living in a low Reynolds regime (Re<1 to >1,000), copepods in some families exhibit an unusual mate-seeking behavior using a guidance system not found elsewhere in the animal kingdom. When copepods move through water, they leave a hydrodynamic wake whose structure varies with swimming style. Pheromones, produced specifically by the female or derived generally as a byproduct of metabolic transformations of body contents, are packaged within this hydrodynamic envelope, resulting in a concentration gradient. Chemosensitive male copepods respond to coherent chemical trails with dramatic acceleration along the trail accompanied by precise and accurate trail following, showing little error in staying on the path taken by the female, accompanied by an uncanny ability to retrace their mistaken ways to find and capture his mate. Viscosity-induced attenuation of mixing enables the persistence of small-scale chemical signals and thus, precise mate finding strategies may be a key adaptation for pelagic copepods. By leaving a coherent pheromonal trail, the female increases her signal size by up to 100 times her body size or more, thus multiplicatively improving the probability of encounter with her mate, thereby enhancing reproductive success. Evidence that copepods can follow trails in all directions indicates that guidance of trail-following copepods is solely and uniquely reliant on chemical cues without collimation by other cues, in contrast to so many other organisms that require collimation by fluid flow or gravity to guide them to the source. Thus copepods may provide novel insights into chemically-mediated guidance mechanisms. Small-scale physical gradients in the open ocean may give more definition to the niche of pelagic plankton. The large number of copepod species and the variations in their habitats provide a natural laboratory to examine the significance and specificity of chemical signals in the aquatic environment at the small-scale, and the adaptations microscopic animals have taken to live in the open ocean.