The black triggerfish is a common reef fish, not shy about showing how it feels. The top photo shows its usual color, a dark body with bright, pale blue lines along the base of its dorsal and anal fins.
When it gets wound up like the one on the left, bright blue lines appear between the eyes. The more agitated it becomes, the more the blue expands. The one below is clearly not at all happy. I know when I’m not wanted.
In my attempts to identify what I see in the water, I use John P. Hoover’s book The Ultimate Guide to Hawaiian Reef Fishes, Sea Turtles, Dolphins, Whales, and Seals. His website is hawaiisfishes.com.