Light responses and die1 migration of the scyphomedusa Chrysaora quinquecirrha in mesocosms

Abstract

We present evidence of a strong response to light by Chrysaora quinquecirrha, the common sea nettle, based on studies in large enclosures (mesocosms). This result is somewhat unexpected due to the reported absence of light-sensing ocelli in marginal sense organs of this scyphomedusan jelly- fish. We used 5-m-deep mesocosms to document light responses and vertical migration of Cquin- quecirrha in both natural and manipulated light fields. Chrysaora quinquecirrha appear to be negatively phototactic; a large fraction of the population always rose towards the surface within 10-15 min after the mesocosms were covered to exclude light, and descended just as quickly when they were uncovered. When natural daylight entering the mesocosms was unaltered, C. quinquecirrha exhibited 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 dark- ness, the medusae gradually increased their numbers at the surface throughout the daytime hours, indicating an endogenous component to the migration. We did not attempt 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.