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Rhizostoma luteum is a rarely-seen species of scyphozoan jellyfish documented from the Portugal shoreline, through the Straight of Gibraltar into the western Mediterranean and down the northwest coast of Africa. First described in 1827, sightings were occasionally recorded up until 1955 but in the 60 years since then none were seen until two stranding events in 2012 and 2013 washed up approximately 75 individuals, mostly on beaches along the southern coast of the Iberian Penninsula (Guerrero 2012). Both stranding events occurred following water temperature decline (about 2 degrees C) however the reason for the sudden occurrences of these jellyfish is not known (Prieto et al. 2013). Impressive in size, the largest R. luteum specimens were measured at 60 cm (24 inches) across the umbrella. It has a thick frilly manubrium under the umbrella and long trailing oral arms (Kramp 1961, Russell 1953 cited in Prieto et al. 2013). Recent sequencing of mitochondrial markers confirm that R. luteum is clearly distinct from the two other Rhizostoma species, which have overlapping distributions (Prieto et al. 2013).