Population dynamics and production of Phyllorhiza punctata (Cnidaria: Scyphozoa) in Laguna Joyuda, Puerto Rico

Abstract

Medusae of Phyllorhiza punctata were sampled in Laguna Joyuda from October 1985 through October 1986. Maximum numerical abundance (143 ind. per 103 m3) and population biomass (530.6 mg ash-free dry wt [AFDW] m-3) were observed during summer Net increments In population biomass were associated with growth of individuals from October to December and with individual growth and subsequent recruitment of small medusae from May to August. More than 75 ' ' , of the total recruitment into the smallest size classes ( 30 cm) during late summer in the following year suggests that P. punctata follows an annual cycle in Laguna Joyuda. Growth rates, calculated from increases In mean individual weight of held-collected specimens, were exponential during autumn (mean g,. = 0.077 d l ) , and fl-om June through August (summer; mean gw = 0.079 d-l). Individual growth rates declined with increasing size durlng both these growth stanzas, but faster growth rates were observed In autumn for medusae of equivalent or larger size. Total population biomass increased exponentially at rates of ca 0 063 and 0.028 d-l during October to December 1985 and May to August 1986 respectively. The slower growth rates in summer coincided with a marked reduction in abundance of non-gelatinous zooplankton, suggesting that medusae could have been food limited at this time. Nevertheless, estimated population production rates were much higher during summer (maximum 14.3 mg AFDW m-3 d.') due to the higher population biomass compared to autumn (maximum 1.6 mg AFDW m-%-').