Triggerfish are a warm water species, which only occasionally swim in the North Sea. This unusual fish is named after the appearance of the front spine on its back, which looks like a pistol trigger. This spine can be erected and held in place by the adjacent spine, which is then held in place by the third spine. In that way, triggerfish use their dorsal fin when in danger to clamp themselves in a crevice between rocks on the sea floor. It's almost impossible for the attacker to get the fish out of the crevice.