This is a guest post by PhD student and science writer Jake Buehler. He blogs over at Sh*t You Didn’t Know About Biology, which is full of his “unrepentantly celebratory insights into life on Earth’s under-appreciated, under-acknowledged, and utterly amazing stories.”
Nature is a rough place. If you live your life skirting along the more vulnerable, fragile, tasty threads of the food web, the world is just a whirlwind of claws and fangs flung about by cruel scaffolds of bone and muscle. But just as predators have evolved an assemblage of tools useful in catching, killing, and dismembering prey, their quarries have evolved a staggering number of defenses aimed at keeping themselves from being shoveled down another animal’s maw. There are many ways to protect yourself. You can be face-meltingly fast, like a pronghorn or a gazelle. You can live in huge herds, and rely on communal vigilance and statistics to keep you safe. You can also be as unappealing a meal as possible, and the variation in approaches to this evolutionary strategy that results in nature is astounding. Porcupines and their grapple-deterrent spines. Turtles and their notably unbiteable shells. Skunks and their odoriferous chemical weaponry. And, if you want to get gross with it, there’s always going horned lizard-style and turning your own eyes into a blood Super Soaker, or, like the young of the Eurasian roller, vomiting all over yourself like you took ten shots of tequila as the preamble to a ride on the Tilt-a-Whirl.
These “anti-predator” adaptations abound in our oceans as well, employed by everything from urchins to hagfish, and perhaps no other sea critter provides a more iconic predation avoidance mechanism than the pufferfish (a member of a diverse group of armored/toxic/weird fish called Tetraodontiformes, which I can gush at length about, but I might be a little biased, considering they are the subject of my research). When confronted with a predator, these fish famously gulp down large quantities of water into their elastic stomachs, bloating their svelte, oval bodies into living balloons. In many species, the turgidity also raises small prickles embedded in their hide, and this, combined with their newfound voluminousness (which can leave them several times larger than their normal, deflated selves), makes them somewhat of a chore to choke down a big fish’s gob. The predatory beastie realizes a lunch made of pokey exercise ball isn’t going to be feasible (and an attempt might actually be fatal, as in this sea turtle), so it swims off. The pufferfish stops being “all about that bass” and slims down, surviving another day and (most importantly) potentially going on to breed.
It’s an effective defense, but to work, the pufferfish needs to stay puffed long enough to convince the predator that better options should be sought elsewhere. The question of the energetic sustainability of the, er, “inflational fortitude” of these fish is made more complicated by observations in the scientific literature that suggested that when inflated, pufferfish held their “breath”; it appeared as though water wasn’t actually passing over the gills. To make up for this inevitable loss of oxygen uptake, it was also thought that the fish were increasing absorption through their skin in their rotund form. If indeed pufferfish were “holding their breath” during inflation, one would think that their defense only lasts as long as they can stand going without oxygen. No one had ever actually directly tested whether or not pufferfish stopped respiring when puffed, and that’s precisely what a team of Australian scientists, made up of Georgia Evelyn McGee at James Cook University and Timothy Darren Clark at the Australian Institute of Marine Science, set out to do. Their findings, published this week in the journal Biology Letters, illustrate that not only do pufferfish breathe while inflated, oxygen uptake is boosted fourfold during the act, returning to normal levels following deflation. The researchers also showed that the oxygen is being drawn in through the gills, not the skin.
The puffer of choice for this study was Canthigaster valentini, the black-saddled sharpnose puffer. Sharpnose puffers are a group of diminutive, reef-inhabiting puffers, and the black-saddled variety is common throughout much of the Indo-Pacific. The Canthigaster clan have a fan in me, as they include a unique, endemic form found only in my home of Hawai’i (Canthigaster jactator), and often have interesting ecological stories; for example, the very species in this study, the toxic, black-saddled puffer, is closely mimicked by a species of non-toxic filefish for protection from predators. The fish are also commonly referred to as “tobies”, which is a name I far more instinctively associate with that kid in The Cider House Rules than with any fish.
Black-saddled pufferfish were collected by SCUBA from the Cairns and Lizard Island areas of the Great Barrier Reef, and then once housed and settled in tanks in the lab, they were ready for the inflation experiment. The fish were set up in customized respirometer tanks, hooked up with the capacity to measure the rate of oxygen consumption by the puffer before inflation, during inflation, and post-inflation. To “persuade” the little pufferfish to make like a basketball and get spherical, the scientists introduced a suction tube into the respirometer. This was then used to gently tug and suck onto the fish, imitating harassment from a predator. The pufferfish were not amused by this, and responded in the standard, unhappy pufferfish way.
Oxygen consumption rates collected throughout this process of puffing, deflating, and recovery showed a pronounced spike in respiration, up to four times the pre-inflation level, right as the pufferfish began to bloat (Figure 1, below). This was followed by a decline back down to near pre-inflation consumption rates throughout the duration of inflation, and subsequent further decline and slow return to “normal” oxygen use. So, the conventional idea that pufferfish hold their breaths during expansion couldn’t be more wrong; they actually manage to breathe very well when they puff, well enough to send their oxygen consumption off the charts. This breathing continues throughout their globose tantrums, and does so solely through their gills, confirmed by the research team by 1) checking for an oxygen level dip when water was expelled (which would indicate oxygen absorption inside the stomach), and 2) using a custom-built respirometer against the skin of the fish to measure if anything was getting absorbed through the skin during inflation. No noticeable dip in oxygen levels was found upon deflation, and respiration through the skin was negligible, accounting for some 0.001% of total fish respiration.
While these fish don’t appear to be turning their faces purple and fighting blacking out when engaging in this dramatic behavior, their extended recovery time, which was on average more than five hours long, hints that the process itself is energetically expensive, the authors note in the paper. This energy expenditure appears to be largely driven by the initial phase of inflation, responsible for that early precipitous rise in oxygen consumption, which involves the forceful and rapid swallowing of water. The acquisition and retention of such relatively gargantuan volumes of water in the stomach requires strong muscular sphincters to keep everything contained. The authors contend that the strain of pinching off the ends of the pufferfish’s gut to turn its digestive system into the freaking Hoover Dam is likely to be the source of the exhaustion. There are also accounts of inflation fatigue occurring in other pufferfish, wherein the ability to inflate is lost after several consecutive inflations. The authors argue that these instances support the idea of puffing being an intense, athletic maneuver with major impacts on available energy reserves. The process appears to be an extreme burden on the fish, and works well as a buffer from intermittent attacks, but cannot be sustained in repeated, consecutive use. The pufferfish are engaging in what is the equivalent of a human sprinting for dear life from a charging bear and scrambling up a tree. Yeah, afterwards, your legs are rubbery with lactic acid abuse, your breath tastes like blood, and you’ve lost all control over your bladder, but hey, you’re alive. But if you had to hop down from the tree, book it another mile, and haul yourself into the first pine you see? You might not be as fast that time. Or a third or fourth time. Inevitably, one of these recurrent, desperate efforts for survival will end with your teeth turning up in Yogi’s droppings. The relevance of puffing fatigue to predator-prey interactions for these fish becomes clear when you involve worn down, post-puffed tobies being pursued prematurely by a snaggletoothed would-be diner. The “bear food” scenario might very well play out similarly in the undersea ecological theater.
I really enjoyed this paper because I have a particularly strong attachment to pufferfish and their circus-freak biology partially because of my close interactions with Wiggles, a captive long-spine porcupinefish (Diodon holocanthus) owned by this blog’s usual author, Christie Wilcox. Porcupinefish reside in a different taxonomic family than true pufferfish, but are quite closely related, and share in the Angry Fish Blimp-method of staying undigested…although porcupinefish, like their name suggests, are basically inflatable cactus with fins. Wiggles is delightfully charming as far as fish go, and is clever enough to associate the opening of a refrigerator door with feeding time, so much so that he rushes the glass of the tank if anyone so much as makes a motion to get a soda.
Wiggles has a rapacious appetite, and will occasionally request treats in the form of hard-shelled crustaceans or mollusks by rising to the surface of the tank and spitting at the hand that feeds him.
— Christie Wilcox (@NerdyChristie) July 4, 2013
Wiggles lives a relatively stress-free life, considering he shares a tank with benign fish that are all a fraction of his size, so he doesn’t feel the need to puff up much…but it does happen rarely. On occasion, Wiggles will bloat silently in the tank, seemingly unprovoked, only to immediately deflate, almost as if he is stretching. He’s also puffed with air during a very unappreciated transfer for an ich treatment, causing him to be temporarily wedged (with spines tangled) in a net, body too swollen to be freed, and too buoyant with his stomach of air to sink. All that one could do was keep his face and gills submerged and wait for him to relax and deflate. Luckily, Wiggles came to his senses and returned to his normal state, and now, knowing the oxygen-suck that inflation is for these fish, I’m certainly glad my finned friend was positioned thus so he could easily breathe in that laborious situation.
Source: McGee G.E. & Clark T.D. (2014). All puffed out: do pufferfish hold their breath while inflated? Biology Letters, 10 (12) DOI: 10.1098/rsbl.2014.0823