Onuxodon fowler

Jeffrey T. Williams / Smithsonian Institution, National Museum of Natural History, Department of Vertebrate Zoology, Division of Fishes

A new study has revealed how marine pearlfish communicate with each other from the confines of their very safe and comfy homes inside oysters - they use the internal structure of the shell to amplify their strange, pulsing noises to the ocean outside.

When we’re kids, we learn really early on how to imitate the sounds that animals make. Well, more specifically, we learn how to imitate the sounds we’re told animals make. Cows go ‘moo’, cats go ‘meow’, roosters go ‘cock a doodle doo’, apparently. Fish go? Cue slightly bewildered child making soft popping noises by opening and closing their mouth between two puffed-up cheeks. Cue other child loling because “fish don’t make sounds!”

If only we’d taught them properly. Then when we ask them what sound a fish makes, they could draw from an expansive list of irresistible onomatopoeia, including ‘chirp’, ‘pop’, ‘moan’ and ‘hoot’. As Emily Anthes put it so beautifully in The New Yorker recently:

"We may think of them as silent, but fish make many sounds that are rarely appreciated by the human ear. Clownfish chirp and pop by gnashing their teeth together. Oyster toadfish hum and blare like foghorns by quickly contracting muscles attached to their swim bladders. Croaking gourami make their signature noise by snapping the tendons of their pectoral fins.

Altogether, more than 800 fish species are known to hoot, moan, grunt, groan, thump, bark, or otherwise vocalise. Carol Johnston, an ecologist at Auburn University, is partial to the sounds made by lollipop darters, small fish native to Alabama and Tennessee. 'They sound like whales,' she told me."

The fact that fish are noisy as hell is one of the most unintentionally best kept secrets in marine science. According to those who know how to listen to the yammering of the ocean, its residents never really shut up - sounds play just as important a role in their courtship behaviours as it does for species up here on the land. Coral reefs in particular harbour the chattiest fish folks, they say.

One such unapologetic loudmouth is the Fowler’s pearlfish (Onuxodon fowler), which makes its home in the reef environments of the Indo-Pacific region, stretching all the way from South Africa to Hawaii. This incredibly slender, transparent and scaleless wisp of a creature has figured out that the only way it can survive a day in the ocean is to find an excellent home and stay there.

Rather famously, pearlfish (family: Carapidae) species from the the genera Carapus and Encheliophis make their homes in the living bodies of invertebrate hosts, including sea cucumbers and starfish. Once inside, some of the creepier species even feed off their host's genitals. But how, exactly, do they get in? Either head-first, propelling themselves forward with a few vigorous tail-thrusts, or tail-first, coordinating their inwards slides with the host’s next ‘breath’.

“Oh”, I hear you say, “they go in through the mouth?” Well, not quite. They go in through the cloaca, which is in all intents and purposes, an anus, through which sea cucumbers and starfish breathe. Once inside, a pearlfish will hang out in a unique breathing organ called the 'respiratory tree' all day, very occasionally poking their own anuses outside to relieve themselves into the open ocean. Pearlfish only leave their hosts at night to feed, when their ribbon-thin bodies can hide from predators under the cover of darkness.

Here's what it looks like:

While pearlfish usually prefer to live alone, sometimes a ‘housing crisis’ can occur in a certain area, forcing multiple individuals to pile into the same host. One particularly unfortunate host was a sea cucumber discovered in 1977 by New Zealand biologist, Victor Benno Meyer-Rochow, to have 15 pearlfish living and squirming inside it.

The Fowler’s pearlfish, on the other hand, makes its home exclusively in the shells of black-lip pearl oysters, perched on the rocky floor of a reef. Often alone, sometimes with others. And, according to a new study led by marine biologist Loic Kéver from the Université de Liège in Belgium, it likes to use its outdoor voice inside. Publishing in the Journal of Experimental Biology, Kéver and his colleagues have documented for the first time the sounds made by a Fowler’s pearlfish.

We won’t be asking our children to try and imitate them any time soon though - Kéver describes the sounds they make using the very un-onomatopoeic word “pulse”. I’m not exactly sure what a pulse is supposed to sound like, but the team describes wild and captive Fowler's pearlfish as vibrating certain parts of their bodies to produce "Single-pulsed sounds and multiple-pulsed sounds that sometimes last more than three seconds”.

Whatever these pulses sound like, they need to be loud, as Kéver explains in the paper:

"Acoustic communication has been reported in dozens of coral reef species, and those sounds constitute the dominant component of low-frequency biotic sounds in seas. In this context, coral reef fish sounds need to be conspicuous and species-specific to have a high communicative value, which is especially true for species active in the dark, where acoustic cues cannot be reinforced with visual signals.”

This is also true for the Fowler’s pearlfish, which has to somehow communicate with its conspecifics from inside its oyster shell. To figure out how, Kéver’s team travelled to the remote and pristine Makemo Island atoll in French Polynesia, where 70 percent of the oyster shells placed house to pearlfish lodgers. They collected a number of pearlfish-filled oysters and transferred them to special tanks wired to record sounds. They found that their pearlfish communicated with each other using seconds-long sounds that formed chains of up to 40 pulses that were dominated by three frequencies - 212 Hz, 520 Hz and 787 Hz.

When the oyster shell’s acoustics were tested by one of the team, Marco Lugli from the Department of Neuroscience at Italy’s Universitá di Parma, they found that two frequency bands - 250 Hz and 500 Hz - were actually being amplified inside the shells, perhaps so the pearlfish can communicate to other residents inside their shell. They also found that another frequency - 1000 Hz - was amplified both inside and outside the shells. So it turns out their thick-shelled, clamped-shut homes are actually helping, not hindering, their communication. "Amplification probably improves the efficiency of communication by increasing the propagation distance of the sounds,” Kéver said in a journal press release.

The team opened up some of their pearlfish to see how they were making their unique pulse noises. Using CT scans, they found a ‘mineralised structure’ on the front end of the pearlfish’s swim bladder, called a rocker bone. This bone pushes down on what they refer to as the fish’s "primary sonic muscles", and around the area, several of its vertebrae have been modified to allow for this. Kéver suspects that the rocker bone acts like an anchor for the sonic muscles to attach to so they can vibrate vigorously against the swim bladder. “It is quite exceptional to see that soft tissue can be hardened when subject to certain constraints,” he says.

The team also found significant differences between the rocker bones of the male and female Fowler’s pearlfish, which suggests that they’re able to make different types of sounds to each other as a way of identifying potential mates from inside their shells.

So these guys have basically perfected homebody life and I’m super jealous.

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