cuda_smallBarracuda (Sphyraena is the genus) are well known on the reef because of their fierce looking teeth and habit of opening and closing their mouth while looking keenly at you!  No surprise that they are predators and in fact are the top predator on coral reefs, eating snappers, grunts, cowfish & wrasses to name only some of their prey.  They seem rely on seeing their prey so these fish are active only during the daytime although they feed more frequently near dawn and dusk (this is called crepuscular feeding).  One can often see them swimming lazily nearby (sometimes they follow immediately behind you!) and then they disappear in a flash.  The same flashing speed is used to attack prey and swallow small fish whole or remove a major chunk of a bigger fish with razorsharp teeth. 

Fortunately barracuda attacks are rare and usually are the result of a diver spearfishing and having "his" prey taken by a passing barracuda.  Certainly many divers have noted that a flash of light from something shiny does attract their attention so wearing of shiny jewelry is not encouraged! 

You can find out more about barracuda and especially the giant barracuda at lots of info from a graduate student who is studying them.