An association between a cusk eel (Bassozetus sp.) and a black coral (Schizopathes sp.) in the deep western Indian Ocean

Detailed observations in the deep sea can reveal previously unknown behaviour, species interactions and fine-scale habitat heterogeneity. Here, the first in situ images of the black coral Schizopathes sp. (Anthozoa: Antipatharia) in the deep western Indian Ocean have been obtained from remotely operated vehicle video footage and time-lapse photography. In these images, there appears to be an association with the cusk eel Bassozetus (Family: Ophidiidae). In the primary observation, chance encounters revealed the fish interacted with the anitpatharian on multiple occasions over several days. Subsequent time-lapse camera footage showed the fish remained almost exclusively underneath the antipatharian for the duration of a 30-h deployment. Excursions from the cover of the antipatharian were for less than 2 min. The primary observation is supported by two similar encounters in the same region. Observed reduction in the tail-beat frequency of the fish under the antipatharian suggests reduced energy requirements for the ophidiid in this position. The observations demonstrate the role that even individual coral colonies play as a source of three-dimensional structure, providing habitat heterogeneity in the deep sea.