Transparency and Visibility of Gelatinous Zooplankton from the Northwestern Atlantic and Gulf of Mexico.

Transparency measurements (at 400 to 700 nm) were made on living specimens of 29 common species of gelatinous zooplankton from the Northwestern Atlantic Ocean and Gulf of Mexico. Percent transparency ranged from 91% for the hydromedusa Sibogota typa to 0.51% for the pteropod Clione limacina. Percent transparency was linearly and positively correlated with wavelength, with slopes of the regression lines (normalized to the percent transparency at 480 nm) ranging from 0.027%/nm for Sibogota typa to 0.51%/nm for the ctenophore Mnemiopsis macrydi (average 0.17 +/- 0.019%/nm). There was no significant correlation between the percent transparency of an animal and its daytime depth distribution. The relationship between percent transparency and sighting distance when viewed from below was modeled and showed that, due to the increase of the minimum contrast threshold for object detection at lower light levels, the usefulness of transparency as camouflage increases dramatically with depth. A preliminary account of these results was presented by the authors at the fourteenth meeting of the Ocean Optics Society in November 1998.