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.

Since it was first observed in Israel in the 1970s, and due to its subsequent negative impact on human activities, the nomad jellyfish Rhopilema nomadica has earned itself a spot on the list of the 100 Worst Invasive Alien Species in the Mediterranean. It was assumed to originate in the Red Sea, or in the Indo-Pacific region, but in the absence of additional reports of live specimens outside the Mediterranean, its origins have remained a mystery. Here, via molecular analysis, we present the first verified results of the existence of R. nomadica in the Western Indian Ocean. Moreover, using additional evidence from Cassiopea andromeda and R. nomadica, we propose that the construction of the Aswan High Dam may have led to the proliferation of R. nomadica in the Levantine Basin.

Aeolid nudibranchs are known to prey on various cnidarians and incorporate nematocysts from their prey into their cerata (dorsal appendages) for self-defense. Nematocyst incorporation is selective, and is based on species, diet, prey availability, and predation pressure. This study investigated the interactions between two nudibranch species, Caloria militaris and Flabellina affinis , and scyphozoan polyps from common eastern Mediterranean jellyfish species, including Rhopilema nomadica , Cassiopea andromeda , Phyllorhiza punctata , and Aurelia sp. Laboratory experiments were used to assess whether these nudibranchs feed on scyphozoan polyps, and if they incorporate the prey nematocysts into the cerata. Results indicate that while F. affinis avoids scyphozoan polyps, C. militaris consumes all tested species, and can subsist on them for extended durations (up to 255 days). Predation rates varied among prey species tested, ranging from a mean of 8 to 43.3 polyps day ^-1 . However, C. militaris does not store scyphozoan nematocysts. These findings contribute to understanding nudibranch feeding ecology and the potential roles these predators play in regulating jellyfish blooms. Identification of scyphozoan nematocysts in the cerata of predator aeolid nudibranchs is discussed as a means to locate cryptic scyphozoan polyps in the marine environment.

We thank Galil and Rinkevich for their interest in our recent publication

Since it was first observed in Israel in the 1970s, the ‘nomad jellyfish’ Rhopilema nomadica has established a reputation as one of the worst invasive species in the Mediterranean Sea. It was assumed to originate in the Red Sea, or in the Indo-Pacific region, but in the absence of additional reports of live specimens outside the Mediterranean, its origins remained a mystery. Here, via molecular analysis, we present the first verified results of the existence of R. nomadica in the Western Indian Ocean. Moreover, using additional evidence from Cassiopea andromeda and R. nomadica , we propose that the construction of the Aswan High Dam may have led to the proliferation of R. nomadica in the Levantine Basin.