Chemical cues affecting recruitment and juvenile habitat selection in marine versus freshwater systems

Marine and freshwater ecosystems differ in persistence, size, population connectivity, and the variance in physical and biotic conditions they experience. These differences may select for differing reproductive modes, life histories, dispersal strategies, and chemically cued recruitment behaviors. In marine systems, adults are commonly less mobile, while larvae spend hours to weeks to months dispersing in the plankton and may move over great distances. It is these immature larval stages that must select appropriate recruitment sites in marine environments. In freshwater systems, the fully developed adults more commonly disperse over greater distances, and it is usually adults that determine juvenile recruitment sites via their placement of larvae or fertilized eggs. Thus, in terms of large-scale habitat choices involving chemical cuing, adult stages should be selected to detect and react to habitat cues among most freshwater species, while juveniles should play this role among most marine species. Few studies assess this hypothesis, but adults of freshwater organisms as different as mosquitoes and frogs do key on chemical cues to select sites for depositing eggs or larvae, while chemical cuing of recruitment in marine systems occurs primarily among the larval stages of the numerous fishes and marine invertebrates investigated to date. Cues to general habitat features, to predators or competitors, and to specific prey or hosts have all been shown to affect recruitment. Here, we review chemically mediated recruitment in marine versus freshwater systems, summarizing what is known and suggesting unknowns that may be productive to investigate.