Body size reduction under starvation, and the point of no return, in ephyrae of the moon jellyfish Aurelia aurita

摘要

Scyphozoan ephyrae need to start feeding before their endogenous nutritional reserves run out, and the success of feeding and growth is crucial to their recruitment into the medusa population. To evaluate starvation resistance in first-feeding ephyrae of the moon jellyfish Aurelia aurita s.L, we determined their point of no return (PNR_(50)), i.e. days of starvation after which 50% of ephyrae die even if they then feed. PNR_(50) values were 33.8, 38.4 and 58.6 d at 15, 12 and 9℃, respectively. Before reaching PNR_(50), the ephyrae showed significant body size reduction: ca. 30 and 50 % decrease in disc diameter and carbon content, respectively. These PNR_(50) values are nearly 1 order of magnitude longer than those of larval marine molluscs, crustaceans and fishes, which is attributable to the ephyra's extremely low metabolic (i.e. respiration) rate relative to its copious carbon reserves. Such a strong endurance under prolonged starvation is likely an adaptive strategy for A. aurita ephyrae, the release of which is programmed to occur during the annual period of lowest temperatures, allowing them to cope with the concomitant seasonal food scarcity. Future studies are needed to identify causes of death for wild ephyrae, which are prone to starvation and/or predation, to forecast outbreaks prior to the season of medusa blooms.