MEPS 591:155-166 (2018)  -  DOI: https://doi.org/10.3354/meps12387

ABSTRACT: Benthic polyps are important in jellyfish outbreaks, which seem to have become more intense and are causing increasing problems for humans. We studied annual and seasonal changes of the moon jellyfish, Aurelia aurita s.l., polyp population dynamics in situ. Over 3 yr, we took monthly photographs to quantify the abundance and occurrence of asexual reproduction of polyps attached underneath oysters on pillars in the Port of Koper (Slovenia) and simultaneously collected data on seawater temperature, salinity, light radiation and pH to test the effects of abiotic factors. The 3 yr in situ study enabled us to establish a clear trend in seasonal dynamics in density and asexual reproduction of polyps. Polyp density averaged ~21 ± 6 polyps cm^-2, differed among seasons and increased with increasing temperature and decreasing salinity. Several hypotheses were tested. Asexual reproduction (buds and stolons) was highest when temperatures were above 25°C, but was significantly lower at high polyp density (≥30 polyps cm^–2). Most strobilae, which asexually produce medusae, were formed when temperature was <15°C. The in situ data enabled us to estimate the average monthly growth rate r (1.16 mo^-1) and carrying capacity K (37.4 polyps cm^-2) using a (mechanistically derived) non-autonomous logistic model with seasonality in asexual reproduction rate (λ). This model enables calculation of average life-history parameters of all polyp populations in situ with seasonal forcing and permits comparisons among populations to understand environmental and anthropogenic effects on polyp populations and the resulting intensity and frequency of medusa outbreaks.

KEY WORDS: Jellyfish bloom · Polyp density · Asexual reproduction · Scyphozoa · Population model · Carrying capacity · Growth rate · Northern Adriatic