Small jellyfish, large consequences: The overlooked predatory role of hydromedusae in subtropical estuarine ecosystems

Abstract

Hydromedusae are often overlooked in pelagic trophic ecology compared to large scyphozoans. We investigated the diet, prey selectivity, ingestion rates, digestion times, and predation impacts of the native Liriope tetraphylla and non‐indigenous species Cnidostoma fallax and Moerisia inkermanica in a subtropical estuary of the southwestern Atlantic. While all three species predominantly consumed copepods, L. tetraphylla exhibited the broadest diet and highest number of preys in gut content (up to 13). Digestion time for L. tetraphylla decreased significantly with increasing temperature, from 94 min at 18°C to 64 min at 28°C, with Q 10 values showing that digestion rates roughly double with a 10°C increase. Cnidostoma fallax exhibited longer digestion times (180 min at 23°C). Daily ingestion rations ranged from 16 to 212 copepods day −1 for L. tetraphylla , 7–15 for C. fallax , and up to 48 for M. inkermanica . Predation impact (PI), the proportion of a prey stock consumed by L. tetraphylla population, averaged 38.1% and peaked at 103.5%, while C. fallax and M. inkermanica had lower impacts (~ 5%). Carbon intake analyses revealed that L. tetraphylla consumed daily up to 280 times its own carbon weight, far exceeding the non‐indigenous species. These results highlight the critical role of hydromedusae in estuarine food webs, challenging the prevailing view that only large gelatinous predators exert top‐down control. We demonstrate that small hydrozoans can be key agents shaping prey community structure and carbon flow in subtropical estuarine ecosystems.

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