First report of Oncaeids (Copepoda, Cyclopoida) feeding on pelagic nudibranchs in the Mexican Pacific

Abstract Zooplankton samples collected from coastal-neritic systems in the Mexican Pacific yielded numerous individuals of a pelagic nudibranch. Several of these specimens were carrying copepods attached to their surface. Copepods belong to the cyclopoid family Oncaeidae, generally deemed as free-living planktonic forms that can display raptorial or symbiotic behaviours. They have been found attached to different groups of macrozooplankton, including jellyfish, siphonophores, and chaetognaths, but had not been recorded in association with planktonic nudibranchs. Thus, the discovery of oncaeid copepods anchored to the body surface of the Mexican coastal nudibranchs represents the first observation of an interaction between these two groups in the marine zooplankton. Our observations and previous data suggest that oncaeid copepods may display a raptorial behaviour to approach the nudibranch and attach themselves to its body surface with their maxillipeds thus enabling them to nibble its integument.