Latitudinal Range Shift of the Blue Button Jelly (Porpita porpita) in the Bay of Bengal: A Signal of Ecological Imbalance in Coastal Bangladesh

Abstract

Porpita porpita (Linnaeus, 1758), commonly known as the blue button jellyfish, is a pleustonic hydrozoan distributed in tropical and subtropical oceans and occasionally observed in mass strandings. On April 10, 2024, the first mainland beach stranding of this species in southeastern Bangladesh was recorded at Mundar-Deil beach along the Teknaf coast, during a national heatwave. This represents only the second confirmed occurrence of P. porpita in Bangladesh, ~23 km north of the first sighting at St. Martin's Island in 2014. Environmental parameters, including salinity, wind, currents and sea surface temperature were recorded on-site. Morphological and morphometric analyses of 65 collected specimens revealed that the total diameter was significantly correlated (p < 0.05) with the disc diameter (r = 0.981), mantle length (r = 0.895), and the average tentacle length (r = 0.751). Additionally, disc coloration and float structure aided in species verification and ecological interpretation. The occurrence during peak pre-monsoon heat and elevated salinity highlights the species' possible northward latitudinal range shift in response to climate change. This event may be driven by synergistic effects of global warming, monsoonal currents, strong pre-monsoon onshore winds, and declining predation pressure particularly from sea turtles, whose nesting has decreased by over 80% due to entanglement, habitat loss, and predation by feral species. These findings underscore the ecological indicator potential of P. porpita and the need for continued surveillance of gelatinous zooplankton to understand ecosystem responses under warming scenarios in the Bay of Bengal. The presence of P. porpita signals a potential ecological imbalance and suggests that this hydrozoan may serve as an emerging bioindicator of environmental shifts in the Bay of Bengal region.