A diverse suite of fish and invertebrates form commensal and parasitic relationships with scyphozoan jellyfish. Associations between ophiuroids and medusae, however, are rare. We investigated the relationship between the common semaeostome jellyfish Aurelia aurita and the ophiuroid Ophiocnemis marmorata at Ningaloo Reef in Western Australia. We assessed the prevalence of the association; described the relationship between medusa size and the number of ophiuroids they hosted; and used stable isotope analysis to determine whether O. marmorata were feeding on medusae hosts or planktonic prey. Seventy-nine per cent of the 92 medusae sampled hosted between 1 and 14 ophiuroids that ranged in size from 1 to 6 mm disc diameter. The number of ophiuroids hosted was positively correlated with medusa size and all medusae >135 mm bell diameter hosted at least one ophiuroid. Stable Isotope Analysis in R modelling of δ13C and δ15N of the ophiuroids and their potential food sources indicated that ophiuroids derived their nutrition predominantly from mesozooplankton and seston and not from medusae. Ophiuroids were never observed filter feeding, so we suggest that the ophiuroids may be kleptoparasites that steal planktonic food from their host. These results highlight the important role of medusae in supporting pelagic biodiversity.

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