Planktonic ciliates as food for the scyphozoan Aurelia aurita (s.l.): Effects on asexual reproduction of the polyp stage

Abstract

The value of planktonic ciliates as prey for the asexual reproduction of the polyp stage of the moon jellyfish (Aurelia aurita) was investigated by monitoring somatic growth (calyx diameter), bud production and prey consumption of polyps on a diet of the tintinnid ciliate Favella taraikaensis labeled with a stable nitrogen isotope (15N). These results were compared with those for polyps on a diet of metazoan larvae (Artemia spp.). In addition, nitrogen and carbon specific gross growth efficiencies on the ciliate diet were estimated from increased 15N content of polyps and consumption of ciliate 15N, and from the increase of polyp dry-weight (somatic growth and bud production), a weight:carbon factor and consumption of ciliate carbon, respectively. The calyx diameter of polyps increased with incubation time during the first 7–10days with ciliate prey and during the first 11–15days with Artemia prey. Bud production started after a lag period of 6–7days in all prey treatments, and the cumulative bud number increased with incubation time. The mean bud production rates after the initial lag period were higher with larger amounts of prey and estimated at 0.08–0.38indpolyp−1d−1 with ciliate prey and 0.12–0.35ind.polyp−1d−1 with Artemia prey. The mean relative change in calyx diameter with Artemia prey between day 10 and day 15 was significantly higher than that with ciliates at the same prey carbon supply, but bud production rate on a diet of Artemia consumed at 5μg-Cpolyp−1d−1 was significantly lower than that on a diet of ciliates consumed at the same rate. Promotion of bud production of polyps on a diet of ciliates rather than the somatic growth may be related to fragility characteristic of the ciliate prey, which are easily digested and absorbed. The nitrogen specific gross growth efficiency of A. aurita polyps on a diet of ciliates ranged from 59% to 78% (mean 68%). The relatively high values were supported by high carbon specific gross growth efficiencies ranging from 42% to 64% (mean 54%). This characteristic of polyp may be due to the small size and the low metabolic loss of assimilated energy source. These results indicate that planktonic ciliates, which are readily available to polyps in nature, can serve as a sufficient diet for asexual production of A. aurita polyps.