Meeting Abstract
Transparency allows animals to match any background, but it is unknown how complex tissues are modified to achieve whole-body transparency. We used transmission electron microscopy to investigate how shrimp muscle is modified to minimize light scattering. We investigated muscle ultrastructure in a transparent species, Ancylomenes pedersoni, in comparison to a similarly sized opaque species, Lysmata wurdemanni, and found that the myofibrils of the transparent species were twice the diameter of those of the opaque species (2.2μm vs.1.0μm mean). Over a given distance of muscle, light will pass through fewer myofibrils due to their larger diameter, and there will be fewer opportunities for light to be scattered at the interfaces between the high-index myofibrillar lattice and the surrounding lower-index sarcoplasmic reticulum (SR). A similar result was found in a distantly related vertebrate group – silurid catfish – with transparent catfish having larger myofibrils than opaque catfish (Johnsen and Kier, unpublished data previously reported at SICB). This suggests that multiple taxa have arrived at the same solution of minimizing scattering interfaces in muscle tissue. Additionally, because transparency is not always a static trait and can sometimes be disrupted after exercise or physiological stress, we compared the ultrastructure of muscle in transparent A. pedersoni with the ultrastructure of muscle in A. pedersoni that had temporarily turned opaque after exercise. We found that in the opacified tissue, the SR around myofibrils had an increased thickness of 360nm as compared to a normal thickness of less than 20nm. We modeled the light scattering across a range of SR thicknesses and possible refractive indices to show that this observed increase in SR thickness dramatically reduces transparency.
