Strobilation of three scyphozoans (Aurelia coelurea, Nemopilema nomurai, and Rhopilema esculentum) in the field at Jiaozhou Bay, China

Abstract

Studies of strobilation of scyphozoan polyp populations in the field are essential for determination of the natural seasonal period of ephyra release and for predicting the size of medusa populations, but they have rarely been conducted. We set up experimental polyp colonies of 3 scyphozoans (Aurelia coerulea, Nemopilema nomurai, Rhopilema esculentum) on settling plates in Jiaozhou Bay, China, and monitored them at 2-7 d intervals for 8 mo (December 2012 to August 2013). Polyps of A. coerulea survived, even proliferating by budding during the entire period, while those of N. nomurai and R. esculentum experienced high mortality and died out after strobilation. Strobilation in all 3 species occurred from late winter to early summer, but the exact timing, duration, frequency, strobilation percentage, and disc numbers strobila(-1) differed by species. A. coerulea started strobilation earliest, on 25 February (temperature: 3.9 degrees C), and persisted for 105 d until 9 June (18.1 degrees C). N. nomurai strobilated from 28 March to 9 June (6.4-18.1 degrees C), with some performing a second, less conspicuous strobilation from 6 May to 9 June (12.0-18.1 degrees C). R. esculentum strobilated last, from 15 April to 8 July (9.0-21.5 degrees C). Greater polyp population resilience in A. coerulea over N. nomurai and R. esculentum can be attributed, at least in part, to persistent propagation by budding, which may facilitate the annually recurring medusa blooms of A. coerulea observed in the region. The latter 2 species may give rise to medusa blooms only when their vulnerable polyps have survived well. Due to the later strobilation, R. esculentum ephyrae may be susceptible to competition for food with and predation by post-ephyra stages of A. coerulea and N. nomurai.