This Knife-Wielding Mini-Monster Wants You to Back Off

They are ugly, and they are poisonous. And now scientists have discovered that they carry switchblades.

Stonefish, a group that includes many species, have a previously unknown defensive weapon: a “lachrymal saber” in each cheek that can be drawn and retracted as needed. A description of the novel appendage appeared recently in the journal Copeia.

As if their bony spines, irregular shapes and gaping mouths did not make them frightening enough, they can stick out the blade and keep it in a locked position, like a ratchet with slanting teeth that allows movement in one direction only. (Think of those plastic ties used to permanently close garbage bags.)

Stonefish can release the blade by flexing their jaws, pulling it back once they no longer feel so unsettled.

This novel weapon may mean rearranging the classification of the animals. Stonefish with hidden swords may be more closely related than the ones without, the researchers said.

The lead author of the study, W. Leo Smith, an evolutionary biologist at the University of Kansas, has kept stonefish in aquaria. He says that they stick the knife out when you approach with a net, so it appears to be used for defense.

It also could be used in mating rituals — no one has ever seen a stonefish mate — but since both males and females wield the blade, this seems unlikely.

In any case, “it’s a complex system,” Smith said. “It involves ligament, bone, muscle, plus a behavioral recognition that it has a use. It’s hard to imagine that it evolved purely by chance. This has to have some survival value.”

The discovery has practical value for humans. “Stonefish are the most venomous fishes in the world,” Smith said. “It’s handy to have a characteristic to identify them.”