Meeting Abstract

53.2  Monday, Jan. 5  You can hide, but you can’t run: trade-offs between muscle activation and transparency in glass catfish JOHNSEN, S.*; KIER, W.M.; Duke Univ.; Univ. of North Carolina, Chapel Hill sjohnsen@duke.edu

While the camouflage aspects of organismal transparency have been fairly well explored, little is known about its physical basis, particularly in more muscular species. We examined two species of silurid catfish, the transparent Kryptopterus minor and the opaque Silurichthys indragiriensis. Stained electron micrographs of longitudinal and transverse sections of muscle fibers were analyzed using Fourier and autocorrelation methods. The myofibrils of K. minor were wider than those of S. indragiriensis (1.13 vs. 0.805 um; p < 0.0001, t-test). This difference has both optical and physiological consequences. While the myofilament lattice is essentially crystalline and highly transparent, each myofibril is bounded by the sarcoplasmic reticulum (SR), which has a much lower refractive index and scatters light. Because light attenuation is exponential and there are thousands of such interfaces along the light path through the animal, a 30% reduction in the number of interfaces due to larger myofibrils can significantly affect transparency. While exact values are impossible due to lack of knowledge about the width and orientation of the SR, modeling of light transmission using matrix methods showed increases in light transmission of 2 to 5 fold. However, larger myofibrils cannot be activated or deactivated as rapidly because the calcium sequestered in the SR must diffuse over larger distances. Because diffusion time is proportional to the square of distance, the observed 30% increase in myofibril size leads to a doubling of diffusion time. K. minor is indeed less agile than S. indragiriensis, as is true of many transparent species, and it is possible that this difference in myofibril size is a factor in the evolutionary trade-off between transparency and organismal function.