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The world of larval plainfin midshipman fish may look alien, but it could be as close as the cobbles beneath your feet, if you walk the rocky shores found along much of the North American West Coast. Adults of this species swim each spring from the ocean depths—up to 1,200 feet beneath the surface—to the intertidal zone to spawn in the shallows, where males excavate nests beneath large rocks. There, they set about trying to attract females and, if successful, rear young like those pictured in this award-winning image by Shane Gross.
Perhaps because this species of toadfish is “grotesque in appearance,” as one 1948 description put it, with muddy coloring and vampiric fangs, most males rely on their voices to summon potential mates. They like to croon at night, rapidly contracting muscles along their swimbladders to produce monotonous tones resembling the low notes of a trombone. On occasion, they will make this call for more than an hour at a stretch. Multiple males singing at once creates a drone that’s audible through the bottom of a boat, and reportedly loud enough to disrupt conversation or wake someone who is fast asleep. And no wonder, since at least one observer compared the sound to “a huge hive of bees or a group of motorboats.”
Female midshipman fish find it irresistible, however. Those drawn to a particular male’s croon will collectively lay hundreds of eggs on the stone ceiling of his watery nest, where he can fertilize them. Unless someone else does, that is. Smaller “sneaker” males often lurk nearby and sometimes inseminate eggs before the guarding male can fend them off. Sneakers don’t sing or build nests, but what they lack in vocal skills and architectural acumen they make up for in cunning—and in testes, which can be as much as seven times larger relative to their body size than those of the crooning “guarder” males.
Multiple males singing at once creates a drone that’s loud enough to wake someone who is fast asleep.
When this courtship chaos is over, so too are the parts played by the sneaker males and the brood’s moms. Thus abandoned, the singing male braves up to four months of high and low tides to tend those eggs, even though many of them may not hold his offspring. And yet, plainfin midshipman—so named because the pattern of bioluminescent photophores on their bodies resemble the buttons on a naval uniform—are stalwart attendants. They rely on special physiological adaptations to stay put through wild temperature swings and occasional exposure to air. And all the while, they ward away predators and use their fins to clean debris from and oxygenate the developing larvae.
This is hungry, even starving, work. With so many hundreds of eggs and babies available in each nest, researchers have shown that some 69 percent of guarding males snack on them, even when other food is present. The practice seems primarily aimed at making way for newer broods in which paternity may be more certain, given that it primarily occurs at the start of the egg-guarding season, when competition for females is most fierce. Fortunately, there are enough babies in each brood to ensure that most survive this dubious aspect of their guardians’ tending. They eventually absorb all of the nutrients in the golden yolk sacs anchoring them to their nest rocks and thrash free. Then they return to the deep, where they will begin this bizarre and fascinating cycle all over again. 
This story originally appeared in bioGraphic, an independent magazine about nature and regeneration powered by the California Academy of Sciences.
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