The trigger fish on Koh Tao come in two breeds, the slightly more passive yellow margin and the 'angrier' titan.
But why are they angry? Like most species, they are protecting their territory. Acting in a similar way to the damsel fish, they chase away what they perceive to be a threat to their zone, which is a cone shape above it, along with the odd bite if they are really angry. During mating season, they get angrier than normal. Moody sods! Some of them, however, are simply attacking because they are morons!
lf anyone/thing becomes an annoyance, it will flick up it's hidden trigger fin locked down in its spine. That's your first warning. If you get a second warning, it may tilt on its side to get a better look at the problem. Most times, it will swim away but, if not, it will swim at speed to try and chase the threat away. As a diver, it will actually bite your fins if they are a bright colour as that is what they think the threat is, another fish-like looking thing. It's the one piece of equipment where choosing a colour is actually important!
They mainly feed on coral, using their powerful jaws and teeth to break it apart, they stir up the sand which draws in smaller fish looking to feast on whatever it unearths. Often, they can be seen swimming off with a huge piece of coral in their mouths. The theives of the ocean!
But let's not be afraid of them. The clue is in the name, it's a FISH! A fish that looks awesome and can kick arse.