Meeting Abstract

S7.6  Tuesday, Jan. 5  Temperature, Oxygen, and Body Size in the Southern Ocean: Why Might They Be Giants? MORAN, A.L.*; WOODS, H.A.; Clemson University; University of Montana, Missoula moran@clemson.edu

In the Southern Ocean around Antarctica, many marine organisms have unusual life histories and physiological traits that are thought to be adaptations to low temperatures and high oxygen levels. One of these is ‘polar gigantism,’ in which some polar invertebrates reach much larger sizes than their temperate or tropical relatives. Using both nudibranchs and pycnogonids, we tested the hypothesis that polar gigantism is facilitated by cold-mediated low metabolic oxygen demands and high rates of diffusive oxygen delivery. We found that egg masses of Antarctic nudibranchs were on average >2X thicker than temperate masses, and contained embryos that were ~35-fold greater in volume. In addition, field and laboratory data showed that embryos in Antarctic masses were less oxygen-limited than those in temperate masses. However, a biophysical model predicted that Antarctic egg masses could reach substantially larger sizes, without greater internal hypoxia, than they in fact do. Using pycnogonids, which contain some of the best-known examples of polar gigantism, we found no effect of adult size on performance under hypoxia. A negative correlation would have demonstrated that larger animals cannot function under low-oxygen conditions, as would have been predicted if maximum body size were limited by oxygen diffusion. Thus, for both nudibranchs and pycnogonids, the oxygen-temperature hypothesis does not explain observed body size ranges; full explanations must be sought in the context of other ecological or evolutionary processes.