iNaturalist data suggests a recent northward expansion of the upside-down jellyfish, Cassiopea

Tropicalization, the poleward expansion of tropical species into warming subtropical and temperate latitudes, is fundamentally altering marine community structure. Predicting these ecosystem changes requires understanding of how specific species are shifting their ranges, however, tracking such movements in marine environments presents significant challenges. The purpose of this study was to evaluate the effectiveness of community-generated data for monitoring marine range shifts and to document the potential range expansion of the upside-down jellyfish (Cassiopea) in Florida. We analyzed 1276 observations from iNaturalist (2003-2023) and temperature data from 11 coastal monitoring stations, finding that Cassiopea has recently expanded its range northward in Florida, with consistent populations observed above 27°N latitude since 2021, coinciding with reduced frequency of cold events (temperatures below 15 °C). Northern populations exhibited distinct phenotypic differences, including a higher proportion of brown bell coloration (85.3 % in the northeast vs 55.2-62.7 % in other regions) and slender foliaceous vesicles (57.8-67.1 % in the north vs 35.2-37.5 % in the south). Cassiopea was observed across diverse coastal habitats, suggesting broad environmental tolerance. The average water temperature in colonized northern regions increased by 0.12 °C per year, compared to 0.06 °C per year in southern regions. This study demonstrates the utility of community-generated data for tracking marine range shifts, reveals phenotypic patterns potentially associated with range expansion, and highlights the value of public databases for monitoring ecosystem responses to global change.