Differences in Aurelia coerulea and Nemopilema nomurai colony proliferation and biofouling invasion at various in situ polyp densities in Jiaozhou Bay, China

Scyphozoan polyp colonies are typically patchily distributed on substrate surfaces at varying densities in the natural environment. We investigated the differences in Aurelia coerulea and Nemopilema nomurai colony proliferation and biofouling invasion at high and low in situ polyp densities in Jiaozhou Bay, China, to explore potential polyp density-dependent effects. The study found that in situ A. coerulea polyp colonies on settling plates consistently survived at high initial density (24.4 ± 2.8 ind•cm−2) over a one-year experiment, but 60% of colonies disappeared at low initial density (0.6 ± 0.1 ind•cm−2). Polyp density in these colonies constantly fluctuated with a significant difference between the two treatments, decreasing by 71.5 ± 37.0% relative to the original level at high initial density by the end of the experiment, but increasing by 237.4 ± 662.7% at low initial density. In contrast, all N. nomurai polyps, whether at high or low initial density (1.9 ± 0.6 ind•cm−2 and 0.2 ± 0.0 ind•cm−2), died after approximately six weeks. Strobilation occurred in A. coerulea, but not in N. nomurai owing to the untimely die-off of polyp colonies. No significant differences in strobilation percentage, number of discs strobila−1 and the calyx diameter of polyps were observed between the two density treatments. Macro-fouling organisms (e.g., ascidians and bryozoans) invaded the settling plates one week after the start of the experiment. Their coverage percentage was significantly lower at high initial polyp density in A. coerulea in a year, but there was no significant difference between both treatments in N. nomurai polyps. Nudibranch predators were discovered in the biofouling communities during the summer, with no significant differences in their numbers between the two treatments for each polyp species. The results indicated that density-dependent effects may play an important role in regulating in situ polyp colony proliferation and combatting biofouling invasion in A. coerulea, but not in N. nomurai. High colony survival and low levels of biofouling invasion at high initial polyp densities in A. coerulea might favor their extensive colonization in natural environment over extended periods of time.