We gathered all available published and unpublished long-term
time-series on jellyfish abundance across the oceans (Table S1).
For simplicity, the term “jellyfish” is used synonymously with gelatinous
zooplankton, and includes medusae, ctenophores, and
pelagic tunicates. The datasets encompassed multiple metrics of
abundance across different sampling frequencies (Table S1);
therefore, each dataset was standardized to an annual mean of
0 and SD of 1 to create a jellyfish index, allowing comparisons
among diverse metrics (21). Because previous long-term studies of
jellyfish abundance have indicated decadal-scale variability (18–
20), we only included observational series exceeding a decade.