Temperature does not seem to affect nematocyst size in the upside-down jellyfish Cassiopea

The effects of ocean warming on jellyfish blooms have garnered considerable attention, but few studies have investigated the effects of rising seawater temperatures on changes in jellyfish physiology, in particular nematocyst development. Our study examines the differences in the sizes (length, width, length/width ratio) and volumes of two types of nematocysts (a-isorhiza and medium-sized birhopaloids) in the polyps of a tropical coastal jellyfish ( Cassiopea sp.) when grown at different temperatures (28 °C, 30 °C and 32 °C) over four weeks. Surprisingly, exposing Cassiopea polyps to higher temperatures did not significantly change the sizes and volumes of both a-isorhiza and birhopaloid nematocysts. Additionally, sizes and volumes of both types of nematocysts also did not exhibit any identifiable trends across time after exposure to their respective treatment temperatures. Interactions between temperature and time were recorded for both a-isorhiza and birhopaloid sizes, while interactions between temperature and time were recorded for a-isorhiza volumes only in the 30 °C treatment. Our findings suggest that rising seawater temperatures pose minimal effects on Cassiopea sp. nematocyst sizes, enabling them to thrive in the face of rising ocean temperatures. Future studies should be carried out at higher treatment temperatures to determine if the limited impact of temperature on nematocyst size and volume will persist beyond 32 °C. • Cassiopea nematocyst sizes and volumes show no clear trends under thermal stress. • Increasing temperatures do not influence a-isorhiza and birhopaloid nematocyst sizes. • Interaction between temperature and time may moderate nematocyst sizes. • Temperature-time interaction influences a-isorhiza volume at a specific temperature.