The Ecology of Indeterminate Growth in Animals

Body size as an ecologically important characteristic has received significant attention in recent years (15, 93, 143, 148). These reviews however, deal primarily with higher vertebrates where determination of adult size has a large genetic component and where little chance for size alteration exists once full adult size has been achieved. Growth rates and final sizes of marine, freshwa­ ter, and certain terrestrial invertebrates are far less constrained. The genetic component is often less important than environmental conditions , and size is subject to radical alteration as these conditions change. Among the terrestrial vertebrates , significant differences in adult size may occur between pop­ ulations of a single species (e.g . small size in island populations, 20), and there are even greater habitat-related differences in adult size among fishes, (10 , 36, 155). These size differences are caused by some amount of genetic differentiation between populations and by plastic ontogenetic responses to local conditions . Therefore , even among vertebrates, a gradient exists from species with very fixed to quite variable adult size. The staggering array of morphologies displayed by invertebrates provides examples of widely divergent patterns of growth . Certain crustaceans and other arthropods are probably as constrained as the higher vertebrates in growth pattern, while the more soft-bodied forms vary greatly in rate of growth, final size, and response to changing conditions . Many such species have the ability to shrink or 'degrow' (143) to a small percent of their former mass as environmental conditions deteriorate and to grow again when con­ ditions improve (80, 89, 115). Coelenterates , echinoderms, annelids, mol­ luscs, and urochordates are examples of invertebrate groups with this set of growth characteristics. In many ways, the least constrained growth patterns