Just when you think you've heard it all, researchers have discovered groupers and coral trout using their head to signal the location of hidden prey. The abstract for the original research article can be seen here.
In a nutshell, "the researchers found that when a prey fish escaped its hunting party, a grouper occasionally moved over the place where the fugitive prey was hiding. The grouper would then rotate its body so that its head faced downward, and it would shake its head back and forth in the direction of the potential meal, in what researchers call a 'headstand' signal. Coral trout make a similar sign, the researchers found."
The researchers also note that the headstand qualified as a referential gesture because "it was directed toward an object, not useful for any immediate mechanical purpose, aimed at a recipient, seemingly intentional and followed by a voluntary response from the fish’s partner."