The effects of salinity and temperature on the transparency of the grass shrimp Palaemonetes pugio.

Transparency is an effective form of camouflage, but it must be present throughout the entire volume of an animal to succeed. Certain environmental stressors may cause physiological responses that increase internal light scattering, making tissue less transparent and more conspicuous to predators. We tested this in the transparent grass shrimp, Palaemonetes pugio, which is found in shallow estuaries where both salinity and temperature change rapidly because of tidal cycles, evaporation and runoff. Animals originally kept at a salinity of 15 p.p.t. and a temperature of 20°C were placed into solutions with salinities of 0, 15, 25 or 30 p.p.t. and temperatures of 13, 20 or 27°C for 12 h (N=26 for each of 12 treatments). Under the control conditions of 15 p.p.t. at 20°C, the transparency of grass shrimp tails was 54±3% (mean ± s.e.). At higher salinities and at both higher and lower temperatures, transparency dropped significantly (P<0.001, two-way ANOVA), reaching 0.04±0.01% at 30 p.p.t. at 27°C. Confocal microscopy of P. pugio's tail suggested that the observed loss of transparency was due to the pooling of low refractive index hemolymph between the high index muscle fibers, creating many index boundaries that increased light scattering. Analysis of a year-long salinity and temperature record from a North Carolina estuary showed that changes of the order of those found in this study are relatively common, suggesting that P. pugio may undergo periods of reduced crypsis, potentially leading to increased predation.