Allometric scaling in small colonies of the scleractinian coral Siderastrea siderea (Ellis and Solander)

Although most physiological traits scale allometrically in unitary organism s, it has been hypothesized that modularity allows for isometric scaling in colonial modular taxa. Isometry would allow increases in size without func tional constraints, and is thought to be of central importance to the succe ss of a modular design. Yet, despite its potential importance, scaling in t hese organisms has received little attention. To determine whether scleract inian corals are free of allometric constraints, we quantified metabolic sc aling, measured as aerobic respiration, in small colonies (less than or equ al to 40 mm in diam.) of the scleractinian Siderastrea siderea. We also qua ntified the scaring of colony surface area with biomass, since the proposed isometry is contingent upon maintaining a constant ratio of surface area t o biomass (or volume) with size. Contrary to the predicted isometry, aerobi c respiration scaled allometrically on biomass with a slope (b) of 0.176, a nd colony surface area scaled allometrically on biomass with a slope of 0.7 30. These findings indicate that small colonies of S. siderea have dispropo rtionately high metabolic rates and SA:B ratios compared to their larger co unterparts. The most probable explanations for the allometric scaling of ae robic respiration are (1) a decline in the SA:B ratio with size such that m ore surface area is available per unit of biomass for mass transfer in the smallest colonies, and (2) the small size, young age, and disproportionatel y high growth rates of the corals examined. This allometric scaling also de monstrates that modularity, alone, does not allow small colonies of S. side rea to overcome allometric constraints. Further studies are required to det ermine whether allometric scaling is characteristic of the full size range of colonies of S. siderea.