One of the most venomous fish in the ocean also has switchblade eyes
Stonefish are found throughout the Indian and Pacific Oceans. They range in size from 4 to 5 centimetres up to 50 centimetres. Prior to the startling discovery that was made recently by Dr. Leo Smith from the Department of Ecology and Evolutionary Biology at the University of Kansas, stonefish were known as one of the ocean's most dangerous fish because they are extremely venomous. Their venom is released by breaking or rupturing one or more of their dorsal spines.
The stonefish has what is called a 'lachrymal sabre' on each side of its head, more specifically on its cheeks just below the eyes. These bony sabres act like switchblade knives. They are normally flush against the side of the head, but when the stonefish is threatened, the blades 'deploy' then 'lock; in place. The switchblades stick out to the side, making the stonefish look a little like it has a moustache, albeit an extremely nasty and dangerous one. There are several theories about how the switchblade actually works. One idea is that it is a visual deterrent; another is that is makes eating this fish physically impossible and very painful; and the third idea is that is can also be used in combat with other stonefish.
Mail cheek fishes
This defensive specialization does not exist in any other fish outside of the stonefish group. In fact, genetic analysis of those that also have the lachrymal sabre, including flatheads, sea robins, scorpionfish and many others, reveals that the larger group known as mail cheek fishes, is more closely related than previously thought. They all have the same unusually large number of muscles and ligaments devoted to making the switchblade mechanism work so well.