Some years ago I was freediving around an open ocean kelp paddy with Mike Johnson. We were photographing a few blue sharks that we had attracted to the paddy with a small bucket of chum. Two small yellowtail jacks were flanking one of the blue sharks, swimming closely beside it and repeatedly brushing along he shark.
I believe the yellowtail were bumping the shark in order to remove parasites against the rough skin of the shark. The shark seemed bothered by this but there was nothing the shark could do — the yellowtail are so much quicker and more agile that the blue shark had little hope of actually biting them or chasing them off. This behaviour persisted for about 10 minutes, at which point the blue shark probably became exasperated at the yellowtail that were pestering him and left. Open ocean about 10 miles offshore of San Diego.