When sarcastic fringe heads meet, they can get a little mouthy. These bug-eyed, bright-lipped fish frequently open their massive jaws to display.

National Geographic Explorer and biologist at Chulalongkorn University in Bangkok, Thailand. They can open so wide because of their unique jawbones, extending far past their faces, unlike most fish.

But it's unclear. According to new research published on October 6 in Ecology, these displays may advertise a male's size and fitness and help prevent physical fights that could result in injuries.

Shelter fights
a with their brilliant colours. Both males and females of this fish species gape their mouths. On the other hand, females are smaller and have more muted mouth colors than males. Hongjamrassilp's team wondered if males open wide to impress females or to compete with other males.

Hongjamrassilp and his colleagues discovered sarcastic fringe heads on scuba dives off the coast of California, where the bottom-dwelling fish usually hang out in shells or rock crevices. The fish mostly hid in their shelters, only their heads poking out. When an intruder—other types of fish or octopuses—came too close, the tenacious fish chased them away and occasionally attacked. When another fish of the same species passed by, the resident fish often gaped.

The researchers captured 15 sarcastic fringehead males and returned them to the lab, where they staged fish gatherings. They placed two males in a tank with a single snail shell. The scientists observed how they interacted after one male took command of the body.

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Male sarcastic fringe heads defend these types of shelters, which are critical to how these fish reproduce, according to Philip Hastings,  in La Jolla, Calif., who was involved in this research. Males court females at shelters, and if the female is receptive, she will lay her eggs there. The male fertilises the eggs while continuing to protect them.

"That's why they fight so hard for the shelters," Hastings explains. "If a male does not have one of these sites, he will not reproduce."

When the other fish approached, the male gaped in nearly 70% of the encounters. The interloper would then either flee or fight, swimming agape at each other before engaging in mouth-to-mouth combat.

The lip-smacking battles ended when the victorious fish bit its opponent. Bites to the head were common, and such wounds can become infected, according to Hongjamrassilp. Larger fish typically won these competitions with more enormous jaws.

The team observed no males gaping at females in the wild or the lab. Instead, they appear to court them by shaking their heads. According to the team's findings, 

According to Hundt, it's a compelling idea. He claims that many fish swim around with battle scars from competing for food or other resources.

fighting and simply settling it with the size of your mouth." However, he claims that the number of observations was limited. So the team should collect more comments, especially in the wild, he says, to ensure they aren't missing any aspects of the behaviour.

Previously, the researchers discovered that the mouth membranes of sarcastic fringe heads reflect UV light and that parts of them fluoresce, for example, glowing green when the light of a specific wavelength falls on them.

Hongjamrassilp first noticed this while working with fish in a lab with a coral tank illuminated by a UV light-emitting blacklight.

"I was ecstatic to see that," he says. These characteristics may heighten the visibility of the umbrella-like open mouth. "In this case, you might have something not involved in the research.

The team is now looking into how the gaping behaviour and the large jaws that go with it evolved. Hongjamrassilp is comparing the behaviour of sarcastic fringeheads to that of related Japanese species with less colourful and wider mouths. Mapping the details of how these fish use their mouths onto an evolutionary tree could help to reveal whether having giant mouths helps to drive gaping behaviour or vice versa.

Kohlmann claims that many fish from various groups gape. "However, these things take it to the next level." Kohlmann is curious if other fishes use gaping displays in the same way that sarcastic fringeheads do.

"Who would have thought two males would fight each other by blowing out parts of their skulls?" Kohlmann inquires. "This is just an incredible example of Earth's diversity."
According to Peter Hundt, an ichthyologist at the University of Minnesota in St. Paul, an animal's gape is frequently considered a limiting factor in what it can eat. But that doesn't seem to explain the sarcastic fringehead's outstretched jaws. "This fish does not require such a large mouth to eat what it is eating—something else is going on."

According to the researchers, displaying their large jaws may deter another male from getting physical. The fish's sharp front teeth can injure an opponent's sensory organs or the eponymous head fringe.

"We believe this behaviour allows them to reduce the number of fights," Hongjamrassilp says.