Substrate preferences and the effect of temperature on planulae settlement of the scyphozoan Rhopilema nomadica

Rhopilema nomadica is the most prominent jellyfish species in the eastern Mediterranean, forming huge annual summer swarms. Although planula settlement success is essential in the formation of jellyfish outbreaks, quantitative studies to examine its response to environmental variables have not been performed thus far due to the difficulty in collecting planulae. We concentrated large numbers of R. nomadica planulae to study the effect of substrate type and seawater temperature on planula settlement. In the substrate preferences experiment, two natural substrates (bivalve shells and kurkar rock) and two artificial substrates (concrete and plastic (PVC)) were offered for planula settlement. Settlement success was not different between concrete, PVC, and kurkar substrates, but was significantly lower on the shells. In the temperature experiment, planulae were able to settle and metamorphose into polyps at all five temperatures (16—32 °C) tested, on glass slides, and most settled within three days. Higher temperatures were not only associated with accelerated settlement but also increased settlement success. The anticipated increase in marine infrastructures (potential settlement substrates), and rising seawater temperatures may favor planula settlement success and further enhance outbreaks of R. nomadica in the Mediterranean.