You never know who is watching you from the reef but a cluster of empty shells around a rocky crevice is a dead giveaway that you've stumbled on the lair of an octopus.
Flabellina albomarginata usually have orange cerata. The brown cerata on this specimen are likely to arise from the nudibranch eating a different hydroid food.
When the phytoplankton blooms in Spring in New Zealand, it provides salps with the food source they need for rapid reproduction. Colonial species can form chains twenty metres long.