McKinney, M. L. & Sumrall, C. D. 2010: Ambulacral growth allometry in edrioasteroids: functional surface-volume change in ontogeny and phylogeny. Lethaia, Vol. 44, pp. 102–108.
Most organisms do not maintain geometric similarity as they grow, in large part because of surface-volume interactions. Because respiratory and food-acquiring organs are dependent on surface area, which increases more slowly than volume, organisms have evolved many strategies to increase the efficiency of, and/or the functional surface areas of these organs. Here, we report some preliminary results comparing area of the feeding apparatus (ambulacra) versus the volume of the theca for a suite of isorophid edrioasteroid species at various ontogenetic sizes. Regression of log (ambulacral area) on log (thecal volume) indicates a strongly constrained allometric pattern whereby the ontogenies of all measured species fall on or near the same line (r2 = 0.962, n = 55), with a slope (k) = 0.654 (± 0.018). This slope and associated 0.05 standard error (0.618–0.690) is within the bounds of that expected from increasing surface area to keep pace with the metabolic demands of increasing thecal size. This allometric value is also comparable to the size scaling of metabolism and respiratory and feeding surfaces in many living and a few fossil invertebrates (k ∼ 0.5–0.9). The edrioasteroid species analysed show a very wide variety of changes in ambulacral length, width and sinuosity, documenting many different patterns for achieving the same goal of increasing surface area to keep pace with volume increase. There is no evidence of increasing feeding efficiency. □Edrioasteroid, allometry, ambulacrum, surface-volume, echinoderm.