Meeting Abstract

28.3  Friday, Jan. 4  Polar gigantism in marine invertebrates: a test of the oxygen constraint hypothesis using temperate and Antarctic Tritonia egg masses WOODS, HA*; MORAN, AL; Univ. Montana; Clemson Univ. art.woods@mso.umt.edu

Among marine ectotherms, high-latitude species are often larger-bodied than their low-latitude relatives, a phenomenon known as �polar gigantism.� One mechanistic explanation invokes cold-driven release of Antarctic organisms from constraints on oxygen delivery. Here we examine this hypothesis using (i) a new diffusion-reaction model of oxygen in egg masses and (ii) a detailed physiological dataset on egg masses of an Antarctic nudibranch (Tritonia challengeriana) and a temperate congener (Tritonia diomedea). This model extends the simpler model that we and others have used in the past by predicting transient oxygen concentrations at all radial positions in an egg mass, and it corrects for major sources of error in estimating diffusion coefficients. We used a large dataset of measurements of oxygen gradients in egg masses and embryo metabolic rates to parameterize and test the model. In both species, embryo metabolic rates were very sensitive to temperature. However, temperature-driven change in embryonic oxygen demand did not cause parallel changes in egg mass oxygen profiles–oxygen concentrations were little affected by temperature in T. challengeriana masses but were strongly affected in T. diomedea. The model, in conjunction with morphological measurements, provided a resolution: metabolic density in T. challengeriana egg masses was so low that even large increases in per-embryo oxygen demand resulted in negligible oxygen drawdown. Egg masses of the Antarctic species were about twice as thick as those of the temperate species, consistent with broader patterns of polar gigantism. Our data support the idea that Antarctic egg masses are released from oxygen constraints, but the model suggests that Antarctic masses could be substantially larger still.