Transparent anemone shrimp (Ancylomenes pedersoni) become opaque after exercise and physiological stress in correlation with increased hemolymph perfusion.

Whole-body transparency, an effective camouflage strategy in many aquatic species, can be disrupted by environmental and/or physiological stressors. We found that tail-flip escape responses temporarily disrupt the transparency of the anemone shrimp Ancylomenes pedersoni After as few as three tail flips, the previously transparent abdominal muscle became cloudy. Eliciting additional tail flips to the point of exhaustion (16±1 s.e.m.; n=23) resulted in complete opacity, though the original transparency returned after 20-60 min of inactivity. We hypothesized that an exercise-induced increase in blood volume between muscle fibers creates regions of low refractive index fluid between high refractive index muscles, thereby increasing light scattering. We documented pre- and post-contraction perfusion by injecting Alexa Fluor 594 wheat germ agglutinin that labeled sarcolemmal surfaces and endothelial cells in contact with hemolymph and found more hemolymph perfused through the abdominal tissue post-exercise, presumably owing to more capillaries opening. In addition, we altered salinity (to 55‰ and 8‰), perforated the abdomen and injected a vasodilator. All three treatments increased both perfusion and opacity, lending further support to our hypothesis that increased hemolymph perfusion to the abdomen is one mechanism that can disrupt a shrimp's transparency. The fact that transparent shrimp at rest have little to no evidence of perfusion to their abdominal musculature (unlike the opaque shrimp Lysmata pederseni, which had more perfusion even at rest) indicates that they may experience significant physiological trade-offs in order to maintain their transparency; specifically, limiting blood flow and thereby reducing oxygen delivery may result in reduced performance.