Friends, foes and followers of fishes

Summary Larger fish species are often seen accompanied by smaller species — anything from helpful cleaners, via unobtrusive fellow travellers, through to flesh-eating parasites. Some of these ecological relationships have been studied in detail, but many, especially the more dynamic ones, remain poorly understood. For our understanding of the fate of marine species in a changing oceanic environment, these connections may be of vital importance. Michael Gross reports.