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Like an audience member at a British Christmas pantomime, I wanted to shout, “He’s behind you,” to warn this clownfish of the menacing-looking great barracuda looming behind it off Cebu City in the Philippines. However, appearances can be deceptive, and a great barracuda is unlikely to prey on a clownfish. In part, this is because the barracuda is likely to pursue a more substantial meal, but it is also thanks to a symbiotic relationship between the clownfish and its hiding place.

Often known as “Nemo,” thanks to Disney, the clownfish shelters within the tentacles of an anemone when threatened. Generally, anemones sting marine creatures that get too close, but the clownfish has a thick mucus layer to protect itself. The relationship is mutually beneficial, as the clownfish nourishes the anemone with nitrogen and carbon from its waste and keeps its host well ventilated. The clownfish also takes an active part in the anemone’s defense strategy, aggressively chasing away fish that would feed on the anemone (and curious divers) with a ferocity that belies its size and appearance. By night, the anemone wraps itself into a tight ball, enveloping its resident clownfish in an underwater safety blanket.