These colourful fish use their massive mouths to protect their love dens

When male sarcastic fringeheads get into a conflict, the spoils of war tend to go to the fish with the biggest mouth.

Sometimes, the colourful reef-dwellers will fight over territory, smashing their gaping maws together in what looks like a violent kissing contest. 

But most of the time, if one fish's lips are big enough to strike fear into his challenger's heart, that's enough to prevent a physical altercation.

"They tend to live in rocky crevices or sometimes empty shells, and to protect these shelters and to retain them, they will often give this agonistic display by opening their mouth and trying to show the width and size of their mouth," University of California San Diego biologist Zach Skelton told As It Happens host Nil Köksal.

"The point of the display, [as far as] we can determine, is to deter physical combat. So it's basically showing, 'Hey, I'm really big. Don't mess with me.'"

Skelton co-authored a study about the male mouth-offs, which was published this month in the journal Ecology.

WATCH | Sarcastic fringeheads open their large mouths:

Sarcastic fringeheads, or Neoclinus blanchardi, are small and feisty saltwater fish with bluish-purple scales, buggy eyes and bright pink and yellow lips. They're native to the Pacific ocean, and live mostly off the coast of California. 

When they get aggressive with each other — which they often do — they open their unusually wide jaws, revealing two rows of pointy teeth. 

"They're very colourful, very charismatic," Skelton said. "But then they do this pretty creepy, as you might imagine, display — although they kind of look like clowns when they do it, in my opinion."

More than one writer has compared them to the demogorgon, the Dungeons and Dragons monster featured on Netflix's Stranger Things. Fortunately, they're only about 18 centimetres long.

"If sarcastic fringeheads got to be like six feet long, like, nobody would go in the water," University of California marine biologist Milton Love, who was not involved with the research, told CBC.

The way fringeheads use their big mouths isn't news to scientists, but Skelton and his team wanted to get to the bottom of the behaviour. After all, there are a number of reasons the male of a species might put on a big show, including competition for mates.