LIGHT RESPONSES AND DIEL MIGRATION OF THE SCYPHOMEDUSA-CHRYSAORA QUINQUECIRRHA IN MESOCOSMS

We present evidence of a strong response to light by Chrysaora quinque cirrha, the common sea nettle, based on studies in large enclosures (m esocosms). This result is somewhat unexpected due to the reported abse nce of light-sensing ocelli in marginal sense organs of this scyphomed usan jellyfish. We used 5-m-deep mesocosms to document light responses and vertical migration of C.quinquecirrha in both natural and manipul ated light fields. Chrysaora quinquecirrha appear to be negatively pho totactic; a large fraction of the population always rose towards the s urface within 10-15 min after the mesocosms were covered to exclude Li ght, and descended just as quickly when they were uncovered. When natu ral daylight entering the mesocosms was unaltered, C.quinquecirrha exh ibited a consistent pattern of diel vertical migration during 1-3 week experiments. Medusae remained at depth during the day and rose to the surface (top 1 m) of the mesocosms during late afternoon Medusae were abundant at the surface during most of the night. When at the surface , the medusae characteristically aggregated in very dense swarms of 30 -50 individuals. In continuous artificial darkness, the medusae gradua lly increased their numbers at the surface throughout the daytime hour s, indicating an endogenous component to the migration. We did not att empt to document diel vertical migration for the medusae in the field, but the presence of this behavior in the mesocosms clearly indicates the ability of C.quinquecirrha to perceive and respond to light.