Growth processes – both general processes associated with length and mass increase but also in terms of changing body proportions – help explain many life history, behavioral, ecological, and physiological phenomena.
Ontogenetic differences within species
Throughout the above discussions, we have emphasized anatomical, behavioral, and ecological differences among size classes of a species. Additional examples of ontogenetic differences are probably not necessary. The major point here is that larvae have to be adapted to larval life, juveniles to juvenile life, and adults to adult life. These different stages often differ in habitat and ecology and must function both during definable stages as well as during transitional periods.
Adaptations appropriate to one stage may therefore create constraints for other stages. Young fish may be constrained by structures and proportions that are primarily adaptive in later life. For example, small juvenile Largemouth Bass are morphologically miniature adults. Instead of feeding on fishes, for which their morphology would be best suited, they eat relatively small zooplankton. This puts them in direct competition with juvenile and adult Bluegill Sunfish, which are constructed to feed on zooplankton throughout their lives and hence have a competitive advantage over juvenile Largemouths (Werner & Gilliam 1984; Population dynamics and regulation). Conversely, later stages may retain characteristics of early ontogeny that may constrain them (see below). Regardless, the differing selection pressures on larval, juvenile, and adult fish within a species help clarify the general occurrence of differently appearing and behaving individuals.
An additional conflict exists during ontogeny, brought about by the need for each stage to be immediately functional at a variety of tasks, including feeding, locomotion, and predator avoidance. All tasks are important, but the balance shifts as a fish ages. Hence predator avoidance may take precedence over feeding effi ciency among younger, smaller fishes that are more vulnerable to predators. Such a trade-off has been shown in a range of fish species (e.g., salmonids, sculpins, cichlids) with respect to muscular and skeletal development and action. Juveniles exhibit relatively high levels of performance of locomotory and other defensive traits (e.g., fast-start escape responses) relative to their feeding and foraging abilities. The opposite applies to adults of the same species, in which feeding performance is maximized (Herrel & Gibb 2006).
Although the life history of a fish appears as a continuum of events from birth through maturation to death, with each phase preparing the fish for the next, some evidence exists to suggest that adaptation to one phase actually inhibits progression into the next. For example, smolting and maturation in salmonids appear to be conflicting processes. Atlantic Salmon that smolt rapidly at 1 year of age may mature much later than fish that bypass the smolt stage and remain behind in fresh water. Administration of male hormones to young male Masu Salmon, Oncorhynchus masou, inhibits smoltifi cation but causes maturation; castration of older fish causes them to undergo many of the transformations of smolting. The complexity, timing, and changes in habitat that occur during an animal’s life cycle may function not only to prepare an individual for later phases but to also overcome inhibitory or conflicting influences of previous phases (Thorpe 1978).