Persistent occurrence and life cycle of Pelagia noctiluca in the channel of Bizerte (Northern Tunisia)

摘要

A daily monitoring of the scyphozoan Pelagia noctiluca was performed in the channel of Bizerte (Northern Tunisia) from land station during 2007. Jellyfish density, outbreaks and biometric measurements as well their life cycle were analyzed. Preliminary results pointed out a consistently uncommon observation of this species year round, with a seasonal oscillation in abundance. Highest values were reported mainly during winter. The winter distribution pattern of jellyfish related to the bell diameter appeared to be bimodal, with one small peak for individuals around 2 ±0.4 cm, and a majority for individuals around 6.5 ±0.4 cm. The reproduction experiment performed illustrates a relatively high percentage of spawning female reaching the 100% with anegg production rate about 2545 egg d1 during the first 48h of incubation. Planulae appeared during the 24-48 first hours of egg incubation. These planulae settled and metamorphosed into transparent ephyrae fully developed with their typical 8 tentacles.This rapid and direct development of Pelagia leads to the appearance of different cohorts in the same season in this brackish ecosystem. Indeed, the absence of benthic stage may increases their outbreaks frequency.