I first encountered a male jawfish with a mouth full of eggs in 2010 while diving in Wakatobi National Park, in Indonesia, while doing independent undergraduate research. His giant eyes were almost comical, and his overflowing jaw reminded me of a three-year-old that had just stuffed an entire piece of birthday cake in his mouth. The sight gave me phantom TMJ pain, and I remember loosening my bite on my regulator to do some jaw stretches. I snapped a photo and went on my way, not realizing how special (and rare) the encounter was. I had worked as a divemaster in Zanzibar, so I had a fair amount of experience locating hard-to-find organisms. However, it would be years before I found another mouthbrooding male fish.

Despite that experience, I didn’t know much about mouthbrooding fish until a major snowstorm hit Colorado right before a scientific conference in New Orleans. My lab mate, who studies them, was trapped in several feet of snow with a cancelled flight, unable to make it out. For some inexplicable reason, I volunteered to give her presentation for her. I’d watched her practice talk before I left, but that was the extent of my knowledge on the subject.

I’m a marine biologist, but I study disco clams and don’t specialize in anything that actually has a backbone. The Q&A aside (“again, this is not actually my research…”), the presentation went quite well—and ultimately re-ignited my interest in this unique group of animals.

Searching for mouthbrooding fish is like an underwater treasure hunt—except that you don’t have a map, and there are lots of fish without eggs to distract you along the way. I have gone entire trips without finding a mouthbrooding male; it’s the ultimate letdown. If you’re lucky enough to find one, however, that’s when the real challenge begins. Mouthbrooding males are extremely wary of predators (and by extension, underwater photographers) and employ a sort of “whack-a-mole” defense, retreating quickly into their burrow if they sense incoming danger. Getting the shot requires hovering motionless as close to the substrate as possible until the fish eventually decides that you are not, in fact, there to eat his children.

Mouthbrooding male fish are truly the ultimate stay-at-home dads. “Paternal buccal incubation” may be a mouthful, but so is carrying every single one of your babies inside your mouth. Depending on the species, paternity leave lasts anywhere from a week (jawfish) to a month (cardinalfish), while dad goes on an involuntary hunger strike. As the eggs’ guardian, he must aerate them and remove waste and fouled eggs to keep them healthy. To accomplish this, he periodically spits all the eggs out and then quickly sucks them back in. Imagine throwing your kid in the air and catching them on the way down, except that you’re throwing 400 of them at once.

For some dads, the “honey-do” list doesn’t stop after incubation. When the eggs hatch (which also occurs inside their mouth, thus protecting the eggs from predators), these dads continue to house the newly-hatched fry inside their mouths for another several weeks. All new parents would probably agree that moments of peace and quiet help maintain sanity with a newborn, but in this case, the dad literally cannot escape the kids, even to go to the bathroom. Furthermore, if the fry are unruly, he couldn’t yell at them even if he had a voice because, well—his mouth is full.

Yellowhead jawfish may be the ultimate piscine real estate tycoons. They have his-and-hers burrows, with the female’s no more than a couple feet away so that she can keep an eye on him. They also excavate a third burrow, which acts as a sort of honeymoon suite. The couple become quite aggressive if another fish tries to build too close to their property line. Getting a photo of the eggs is a delicate balance; slowly moving your camera close enough while simultaneously ensuring the male doesn’t think your camera is planning to break ground next door.

Before mouthbrooding males are selected as suitable day care centers, there has to be an open house. To showcase their would-be nursery, male jawfish court females by swimming in a series of swoops, arching their backs and flaring their fins. For the finale, they get very close to her and simply open their mouth as wide as they can. If the female deems the mouth suitably sizable, they become mates. Mick Jagger, for example, would have no trouble courting a female, were he a yellowhead jawfish.

Not to be outdone, Banggai cardinalfish have a fancy dance of their own. In this case the female makes the moves, isolating the male of her choice. Her selection is based on body size: the bigger the male, the bigger the mouth; the bigger the mouth, the better the mate. Once she gets him alone, she does some vigorous trembling, then breaks out her dance moves: the “rush” and the “twitch.” She keeps dancing until the male signals his receptiveness, which as you may have guessed, is done by opening his mouth.

Nature clearly has confidence in the might of Mr. Mom, as male mouthbrooding has evolved independently in several families of fish. Of the eight such families, three consist solely of paternal mouthbrooders: jawfish, cardinalfish, and sea catfish. Snakeheads, arowanas and gouramis include a mixture of male and female mouthbrooders. Cichlids are typically maternal mouthbrooders, with a few paternal mouthbrooders, and some biparental mouthbrooding (which also occurs in some Bagrid catfish).

Mouthbrooding in cichlids has taken a fascinating evolutionary turn. If you think it’s bad raising your own kids inside your mouth, imagine raising the neighbor’s kids, too. The topic of my impromptu substitute conference presentation, this occurs when female cichlids are parasitized by female cuckoo catfish. The catfish lays her own eggs alongside the cichlid’s, and not knowing the difference between the two, the cichlid mouthbroods both sets of eggs. The worst part about raising this neighbor’s eggs is that when her kids hatch first, they eat all of your kids.

Parental care in cichlids is complex; all species show parental care for both eggs and larvae—some until the fry are months old. Some exhibit communal parental care, where multiple monogamous pairs care for mixed groups of fry. Even the teenagers from previous spawns occasionally help protect the newborns. For cichlids, it really does take a village (or rather, a school). 

I recently returned from a dive trip to Cozumel, where my Easter egg hunt for mouthbrooding males came up empty. I did find the endemic “splendid toadfish,” a blue-striped bottom-dweller with a very fancy hipster beard and yellow-tipped fins; it was one of many reminders that there are countless weirdly wonderful creatures to write about in the sea. Every day on the boat, in order to decide which sites to visit, the divemasters would ask us a question: “Do you want to see marine life, or do you want to see healthy coral?”—as if the two are now mutually exclusive. Multiple guides noted how drastically things have changed in the past three years, with sponges, soft corals and high numbers of animals disappearing from the reefs, along with the overgrowth of algae.

Like all marine life, climate change and ocean acidification threaten the health of mouthbrooding fish. Similar to turtles, many fish exhibit temperature-dependent sex determination. Higher temperatures result in skewed sex ratios, which puts stress on the population. Ocean acidification is especially detrimental to larvae, as studies show survival rates of eggs under heightened CO2 levels (1,000 ppm) decrease by an average of 70 percent. Mouthbrooding fish take extraordinary measures to protect their eggs from natural threats. We as humans should take extraordinary measures to lessen their need for protection against man-made ones.

The author would like to acknowledge Julie Byle of the University of Colorado for her feedback on this piece.