Temperature, oxygen, and the evolution of giant Antarctic sea spiders
Over a century ago scientists identified a biogeographic pattern now known as “polar gigantism,” in which many marine invertebrate taxa grow to unusually large size in polar waters. Polar gigantism occurs in diverse taxa including glass sponges, echinoderms, foraminiferans, polychaete annelids, crustaceans, and pycnogonids (sea spiders). Despite being a common and dramatic phenomenon, polar gigantism remains a physiological and evolutionary mystery; how and why do polar taxa grow to such large sizes? Using Antarctic sea spiders (pycnogonids), we tested one of the leading physiological hypotheses about polar gigantism—the oxygen hypothesis, proposed by Chapelle and Peck in 1999, which states that polar giants are ‘permitted’ in very cold water by high levels of oxygen supply coupled to low metabolic demand for that oxygen. We combined physiological, morphometric, microscopic, biomechanical and behavioral experiments with field work in the Antarctic and mathematical modeling to understand the oxygen dynamics of sea spiders and how they are affected by body size and temperature. So far, our data suggest that sea spider cuticles become increasingly porous and show increased oxygen diffusivity as body size increases; nevertheless, internal PO2 drops with body size, suggesting that the largest spiders may be running up against a body size ceiling and as such may be vulnerable to warming ocean temperatures.