JOHNSON, AS*; ELLERS, O; WINDECKER, L; Bowdoin College; Bowdoin College; Bowdoin College: Power Law Scaling of Growth in Sea Urchins

In contrast to field populations in which growth rates are described by Tanaka functions, under conditions where food is plentiful, sea urchins (Strongylocentrotus droebachiensis), that are smaller than 20 mm, grow at rates that are a function of weight to the 3/4 power. These urchins develop gonads after they reach 20 mm diameter; thus the observed scaling of growth represents the scaling of energy intake and metabolic rates. After gonads start developing, growth rate increases as a function of weight to the 0.4+/-0.2 power. This change in the scaling of growth rate may be attributable to the investment in gonads; the weight of the gonads is proportional to weight. These patterns of scaling are similar to those described in vertebrates. Such similarities of scaling laws across taxa imply either universal constraints on intake and metabolic rate scaling, or universal solutions to the trade-off between growth to functionally determined size versus allocation to reproduction. This power law scaling of growth in well-fed laboratory-raised urchins contrasts with the observed diameter specific growth rates seen in field populations. In field populations, growth rates are normally distributed with a maximum growth rate occurring at about 30 mm. The difference between field and lab-raised urchins implies environmental diameter specific food limitations, allowing the detection of scaling effects versus environmental effects. Furthermore, growth rates achieved in the laboratory are much higher than those observed in the field across all diameters. If laboratory rates were sustained, urchins would grow to legal size in 2 rather than 10 years and would reach maximum legal size in 6 rather than 100 years. This plasticity of growth rates has implications for aquaculture and fisheries management.