Spectral sensitivity of vision and bioluminescence in the midwater shrimp Sergestes similis

In the oceanic midwater environment, many fish, squid, and shrimp use lumin escent countershading to remain cryptic to silhouette-scanning predators. T he midwater penaeid shrimp, Sergestes similis Hansen, responds to downward- directed light with a dim bioluminescence that dynamically matches the spec tral radiance of oceanic downwelling light at depth. Although the sensory b asis of luminescent countershading behavior is visual, the relationship bet ween visual and behavioral sensitivity is poorly under stood. In this study , visual spectral sensitivity, based on microspectrophotometry and electrop hysiological measurements of photoreceptor response, is directly compared t o the behavioral spectral efficiency of luminescent countershading. Microsp ectrophotometric measurements on single photoreceptors revealed only a sing le visual pigment with peak absorbance at 495 nm in the blue-green region o f the spectrum. The peak electrophysiological spectral sensitivity of dark- adapted eyes was centered at about 500 nm. The spectral efficiency of lumin escent countershading showed a broad peak from 480 to 520 nm. Both electrop hysiological and behavioral data closely matched the normalized spectral ab sorptance curve of a rhodopsin with lambda(max) = 495 nm, when rhabdom leng th and photopigment specific absorbance were considered. The close coupling between visual spectral sensitivity and the spectral efficiency of lumines cent countershading attests to the importance of bioluminescence as a camou flage strategy in this species.