Cannonball jellyfish (Stomolophus sp.) recorded under near-freshwater conditions in the Mulegé River, Baja California Sur

Abstract

Background. The cannonball jellyfish Stomolophus sp. (Scyphozoa: Rhizostomeae) typically inhabits marine coastal waters with oceanic salinities around 35‰ and is usually associated with estuarine or nearshore environments under strong marine influence. Although sporadic incursions into brackish waters have been reported, occurrences in truly freshwater reaches are considered highly unlikely for scyphozoan jellyfish. Goals. Here we report an unprecedented occurrence of this species in the freshwater section (salinity = 0.199‰) of the Mulegé River, Baja California Sur, Mexico, and discuss its implications for osmotic tolerance, dispersal processes and habitat connectivity. Methods. On 17 June 2025, we observed a total of 22 individuals distributed along a short river segment, all of which showed signs of poor physiological condition (reduced bell turgor, discoloration and limited swimming activity) but were still clearly identifiable as Stomolophus sp. Results.Their presence in almost-freshwater conditions represents an exceptional case of passive upstream transportation, most likely driven by tidal dynamics and river flow, combined with a Short-term tolerance to extreme osmotic stress. This record expands the known environmental tolerance of Stomolophus sp., previously documented at minimum salinities of about 10‰, and suggests that short excursions into very low salinity habitats may occur more frequently than currently recognized. Conclusions. This report enhances current knowledge of the gelatinous fauna of the Gulf of California, highlights the ecological connectivity linking marine, estuarine and riverine environments, and provides a solid baseline for future experimental and field studies on how jellyfish cope with rapid salinity changes and other forms of environmental stress.