Even larvae act sick when they are poorly.

Bustling around their reef homes, Red Sea dascyllus damselfish (Dascyllus marginatus) do quite well out of their co-habiting arrangement with hood coral (Stylophora pistillata). ‘The coral provide the fish with shelter from predators and a place for social interactions and egg laying’, says Nur Garcia-Herrera, from the Alfred-Wegener-Institut, Germany. Yet the agreement isn’t entirely one sided. ‘In return, the damselfish remove sediments from the coral surface, protect it from predators such as butterflyfish or crown-ofthorns starfish, and excrete nutrients such as nitrogen and phosphorus that the coral takes up’, Garcia-Herrera explains. But she and her colleagues, Amatzia Genin, from theHebrewUniversity of Jerusalem, Israel, and Sebastian Ferse and Andreas Kunzmann, from the Leibniz Center for Tropical Marine Ecology, Germany, also wondered whether the gentle water currents generated by the fish’s ceaselessly fanning fins may also nurture the coral and enhance coral photosynthesis.