Aeolid nudibranch. A nudibranch is a sea slug and a marine mollusc. The aeolid nudibranch breathes through the spiky structures on its back, which are called cerata. The cerata also serve to digest food. The orange sensory tentacles are called rhinophores, which are used to sense chemicals in the water to help detect food. An aeolid also uses its cerata to store stinging cells taken from its prey as a form of defence. Photographed in Lembeh Strait, Sulawesi, Indonesia.