Seasonal dynamics of Cassiopea spp. jellyfish species in a Caribbean mangrove system

Upside-down jellyfish of the genus Cassiopea are ecologically significant components of tropical and subtropical coastal ecosystems; however, their population dynamics in natural mangrove systems remains poorly characterized. This study provides the first seasonally explicit assessment of Cassiopea spp. population structure in a Caribbean mangrove environment (Bajos de Santa Ana, Havana, Cuba), revealing pronounced seasonal demographic restructuring. Quadrat-based surveys (n = 190 total quadrats) were conducted during dry (December) and rainy (July) seasons across two consecutive years (2023–2024). We quantified jellyfish density, bell diameter, benthic community composition, and physicochemical parameters. Satellite-derived environmental data (turbidity, precipitation, sea surface temperature) provided temporal context. We documented a marked seasonal shift in population structure: the most pronounced seasonal contrast occurred in 2023, when December populations showed 3.8-fold higher densities (35.64 ± 19.70 vs. 9.28 ± 5.17 ind. m⁻²), and 29% larger mean bell diameter (8.87 vs. 6.94 cm) compared to July; this contrast was reduced in 2024, no significant difference in bell diameter was detected between seasons (p = 0.9997), consistent with a significant season × year interaction. Negative-binomial and Gamma generalized linear models detected significant seasonal and interannual effects, as well as a significant season × year interaction, indicating temporal variability in demographic responses. Principal Component Analysis of benthic cover data showed clear seasonal segregation in community composition, with Cassiopea abundance negatively associated with unvegetated substrate and macroalgal cover. These findings indicate that Cassiopea populations in natural mangroves experience pronounced seasonal variability, coinciding with contrasting seasonal environmental conditions, contrasting with the relative stable conditions reported from anthropogenically modified habitats. Such temporal restructuring of population size and density highlights the dynamic nature of jellyfish demography in natural systems and provides a baseline for evaluating ongoing environmental change.