Meeting Abstract
Polar gigantism is common among diverse oceanic invertebrates, but the underlying mechanisms responsible for this phenomenon are not well understood. Temperate species are small in size, whereas those in the poles and at great depth are often huge. Sea spiders (Pycnogonida) are particularly interesting as they do not have specialized organs for metabolic gas exchange, so their respiration must occur via diffusion across their cuticle. We hypothesized that gigantism in sea spiders has evolved under the constraint of a trade-off between the need for a thin cuticle to facilitate diffusion and the need for mechanical support of the body. To begin to test this, we undertook the present scaling study in a comparative context (n = 32 species, 5 orders of magnitude in mass). We used micro-computed tomography (µ CT) to measure cross-sectional anatomy of limb segments, and we used standard photography to measure external morphology. Departures from geometric similarity in cross-sectional anatomy were consistent with our hypothesis that gigantism in sea spiders is associated with proportional thinning of the cuticle. For the femur, scaling exponents relative to body mass were: area (0.54), thickness (0.23) and polar moment of area (1.12). In contrast, external morphology (length, diameter) scaled according to predictions of geometric similarity. Large species exhibited apodemes running the length of limb segments that likely aid with structural support. Some large genera (Colossendeis and Pallenopsis) featured long, thin limbs (aspect ratio > 20), whereas others (Decalopoda, Dodecalopoda) shared similar aspect ratios with smaller taxa (< 10). Ongoing field studies around McMurdo Station, Antarctica, should reveal the ecological and behavioral consequences of these differences among giants. NSF PLR- 1341485.
