BODY SIZE IN ANTARCTIC AND TEMPERATE SEA SPIDERS: THE ROLE OF TEMPERATURE AND OXYGEN

Abstract

Body size is an important part of an organism’s physiology and is perhaps one of the most important traits determining an individual’s performance and life history. In Antarctica, a number of marine species reach unusually large body sizes compared to relatives in temperate or tropical regions, a phenomenon known as polar gigantism. While there are many potential explanations for why these animals reach such large sizes, the hypothesis that has received the most attention is the oxygen-temperature hypothesis (OTH). This hypothesis posits that polar gigantism arose from a combination of cold-driven low metabolic rates and high oxygen availability in the polar oceans. If the oxygen-temperature hypothesis indeed underlies polar gigantism, then polar giants may be particularly susceptible to warming temperatures. In this dissertation, I explore the effects of temperature on the physiology of Antarctic and temperate pycnogonids (sea spiders) as a way of testing an underlying principle of the OTH that large bodied animals face stricter limits to aerobic performance as temperatures warm. First, I tested the effects of temperature on performance using two genera of giant Antarctic sea spiders (Pycnogonida), Colossendeis and Ammothea, across a range of body sizes. I found no support for the oxygen-temperature hypothesis but discovered differences in thermal responses between species. I found that the porosity of the animals’ cuticle increased with body size, which may allow these animals to compensate for the increasing metabolic demand from elevated temperatures and longer diffusion distances of larger animals by facilitating diffusive oxygen supply. I also tested whether temperature induced oxygen limitation in two species of temperate sea spiders by measuring oxygen consumption at a range of temperatures. Here, I found that the aerobic metabolism of temperate pycnogonids does not appear to be oxygen limited at elevated temperatures, suggesting that the generally small size of sea spiders does not reflect constraints on oxygen supply to larger bodies in warmer environments. Finally, I measured the thermal sensitivity of the Antarctic pycnogonid, Ammothea glacialis, over ontogeny. Antarctic organisms are thought to be highly stenothermal meaning they can only function within a narrow temperature range, but little work has tested if this is true over ontogeny. I tested this idea by measuring the oxygen consumption of larvae, juveniles, and adults of A. glacialis at a range of temperatures. I found temperature sensitivity at all stages but particularly in adults. Together, this work shows that pycnogonids, both temperate and Antarctic are affected by elevated temperatures, but these effects are stronger in some taxa than others. While elevated temperature from ocean warming will undoubtedly have profound effects on the physiology of pycnogonids, giant pycnogonids appear to have found a way around these oxygen-temperature related constraints by increasing cuticle porosity. However, this work is just a piece of the larger puzzle on how climate change will affect the Antarctic benthos and emphasizes our need for a better understanding about the physiology of Antarctic ectotherms.