ZOOPLANKTIVOROUS PREDATION BY LARGE SCYPHOMEDUSAE PHYLLORHIZA PUNCTATA (CNIDARIA, SCYPHOZOA) IN LAGUNA JOYUDA

This study examined the influence of predation by a population of the scyphomedusa Phyllorhiza punctata von Lendelfeld on temporal and spati al variations in abundance of planktonic copepods in Laguna Joyuda. Ex perimentally determined clearance rates were independent of prey densi ties (Type I response) and increased linearly with medusa bell diamete r, within the range of prey concentrations and size of individuals tes ted. Variations in size of medusae (range 3-24 cm bell diameter) expla ined 81% of the variability in clearance rates on copepod nauplii, 63% on post-naupliar copepods, and 83% on a combined copepod assemblage f rom Laguna Joyuda. Population clearance rates were highest during July and August, representing a filtering capacity of approximate to 22 an d 35%.day(-1) of the lagoon's volume. Relatively large medusae (7-21 c m) were the principal predatory size group during July and August, Cle arance rates of P. punctata remained constant for prey densities above 200 prey.1(-1) on the mixed copepod assemblage. These prey densities are higher than average (even patch) densities in Laguna Joyuda, which imply that ingestion rates were constantly varying in the field. Vari ations in estimated field ingestion rates on average and patch prey de nsities explained 79 and 84%, respectively, of the variability in obse rved growth rates of Phyllorhiza in Laguna Joyuda. The estimated (pote ntial) field ingestion rates represent mean daily ratios [as % medusa (ash-free dry weight (AFDW)] of 3.1% (9.1% max.) on mean prey densitie s, and 6.4% (16.3% max.) on patch prey densities. These daily rations on planktonic copepods appear low in relation to the observed growth r ates of Phyllorhiza a in Laguna Joyuda, suggesting that algal endosymb ionts and/or other nutritional sources play an important role in its b ioenergetics.