Given that the Common Sunstar, Crossaster papposus, actively predates other starfish, it seems strange to see Common Brittlestars, Ophiothrix fagilis, riding on their backs. Yet this is frequently observed and I am not sure what the attraction is. Maybe the rough surface of the Sunstar gives good purchase or it puts the brittlestars slightly higher in the water column for filtering plankton. Certainly they are relatively safe there as the mouth parts of the Sunstar are in the centre underneath the animal.
North Sea - St. Abbs Marine Reserve