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 spatial variations in abundance of planktonic copepods in Laguna Joyuda. Experimentally determined clearance rates were independent of prey densities (Type I response) and increased linearly with medusa bell diameter, within the range of prey concentrations and size of individuals tested. Variations in size of medusae (range 3-24 cm bell diameter) explained 81% of the variability in clearance rates on copepod nauplii, 63% on post-naupliar copepods, and 83% on a combined copepod assemblage from Laguna Joyuda. Population clearance rates were highest during July and August, representing a filtering capacity of approximate to 22 and 35%.day(-1) of the lagoon's volume. Relatively large medusae (7-21 cm) were the principal predatory size group during July and August, Clearance 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. Variations in estimated field ingestion rates on average and patch prey densities explained 79 and 84%, respectively, of the variability in observed growth rates of Phyllorhiza in Laguna Joyuda. The estimated (potential) field ingestion rates represent mean daily ratios [as % medusa (ash-free dry weight (AFDW)] of 3.1% (9.1% max.) on mean prey densities, and 6.4% (16.3% max.) on patch prey densities. These daily rations on planktonic copepods appear low in relation to the observed growth rates of Phyllorhiza a in Laguna Joyuda, suggesting that algal endosymbionts and/or other nutritional sources play an important role in its bioenergetics.