When Fish Bite
by Frank Stephenson

Fish have been chomping things for 400 million years. How they do it today is a fascinating study in the vagaries of evolution.

They've been catching and eating things longer than the swiftest, most terrifying predator that ever roamed the earth.

Two hundred million years before the first dinosaur clamored about in search of a meal, the seas teemed with them, and their appetites were every bit as ravenous and as varied as their saurian cousins' would ever be.

Fish are such commonplace animals-wherever there's water you'll likely find them-that it's easy to understand why they're often taken for granted even by serious students of wildlife. But to scientists with a passion to learn how Earth's incredible web of life came into being, few organisms that have ever existed come close to offering as rich a field of evolutionary evidence as fish.

FSU biologist Dr. Peter Wainwright (Ph.D. Chicago) is one such scientist whose fascination with the processes of natural selection has led him into a research career that focuses almost entirely on the evolutionary history of fishes.

"There's just no end to what you can find by looking at fish," he says. "Looking at the range of shapes and sizes in the animal world, I'd stack them up against anything except maybe insects."

If anything stands out about the 430-million-year-old history of fish, Wainwright says, it's that the forces of evolution have been stupifyingly creative. Scientists figure there are at least 24,000 species alive today, a figure which easily makes them the most common vertebrate (backboned) animal on earth. In terms of diversity among vertebrates, their closest rival is birds (with 8,600 species) followed by mammals, most of whose 8,000 species are bats and mice.

And among themselves, fish are so confoundingly varied-there are more than 2,000 species of gobies alone-that fish taxonomists (scientists who try to figure out what's what) are to be pitied. To complicate matters even more, consider nature's bizarre packaging: there are fish with and without jaws; with and without bony skeletons; fish whose entire bodies are encased in rock-hard "boxes;" fish with no teeth and others with fangs that rival anything that ever carved prey on land. There are fish that look like snakes (eels); like doormats (flounders, skates and rays); like boxcars with fins (whale sharks); like visitors from a distant planet (seahorses).

Perhaps not surprisingly, such gaudily diverse morphology (bodily shapes and sizes) begets a wide range of behavior. Like most wild animals, fish really don't have much to do in their lives but elude predators, breed and eat. But in practicing such basic survival habits, fish demonstrate an astonishing latitude in skill and finicky behavior that may surpass anything in the animal kingdom, particularly when it comes to meal-time. What fish eat has been a matter of supreme interest to humans ever since the invention of the fish hook by Stone Age man. Given that three quarters of Earth's surface is drowned by their domain, fish have a menu before them like no other living creature. For just about any object-animate or otherwise-found either on or in the world's rivers, lakes, streams, ponds, bays, seas or oceans, there's probably a fish that eats it.

For example, there are vegetarian fish that eat floating nuts, seeds and berries; fish that plow bottom sediments for worms and such; gigantic fish that eat only microscopic plants and animals; fish that can eat fish bigger than they are; parasitic fish who live off their hosts' good fortune; fish that eat nothing but sponges, others that dine solely on sea urchins; even fish that literally eat rocks-e.g. parrotfish.

A motley bunch of trenchermen to be sure, the finny tribe. But how and why fish eat the things they do have been far more interesting questions among scientists who've ever taken a serious look at the way fish feed.

Judging by the collection of skeletalized fish heads in his office and lab, Wainwright easily figures to be among this group. As it turns out, he's a specialist in piecing together the evolutionary history of feeding behavior in fishes. As arcane as that may sound, such work falls into a classic line of research dating to Darwin, whose theories on the origins of species sprang from his observations on the eating habits of Galapagos finches more than 150 years ago.

Just as Darwin marvelled at how the size and strength of birds' beaks neatly match the size and hardness of what they feed on, so have modern fish biologists noted with unending fascination the exquisite symmetry inherent in fishes' feeding patterns.

"People have always had this intuition that you can figure out what a fish does for its living just by looking at its mouth," says Wainwright. "It may not be quite as obvious as in birds, but that's true. And if you take a closer look, you find out that it's even more true than you might ever have imagined."

It's little wonder that fishes' heads are by far the most interesting part of their anatomy-they have little option but to use their mouths to do what most animals can do easily with claws, paws or hands. Seemingly, the animals have tried to make up for being limbless by evolving what clearly are the most complex mouth and feeding assemblages found in the vertebrate world. The task has been complicated by the fact that fish live in a medium-water-that is 900 times as dense as air and 80 times as viscous. The result is a morphological tour de force: the head of an average fish contains as many as two dozen separate bones, and up to three times that many muscles and ligaments, says Wainwright.

"Compare that to a typical mammal, such as man, with a head that has only one moving part, the jaw. In contrast, fishes' heads are positively amazing. When they feed, nearly all of these dozens of parts move in concert."

Most fish that prey on highly mobile food-such as other fish-rely on speed, cunning or both. Most ingest their food with sucking action that can be downright explosive-with prey literally vanishing in the blink of an eye. Fish accomplish such feats by creating powerful vaccuums inside their mouths, springing them open like hydraulic traps in the presence of food, Wainwright explained.

Since his days as a graduate student in biology, Wainwright has been keenly interested in how the forces of evolution have shaped the way fish eat. His doctoral dissertation focused on how hogfish-a member of the wrasse family-use a powerful, secondary set of jaws (called pharyngeal jaws) to crush and grind up clams and other shellfish, the animal's favorite food.

One of his first papers as a full-fledged scientist was published on the remarkable prey-snaring capability of one of the hogfish's cousins, the sling-jaw wrasse, Epibulus insidiator, a reef-dweller of the tropical Pacific. The paper gave the first detailed account of just how well-suited the fish is to its common name. As it turns out, Epibulus is capable of extending its jaw farther than any other fish known-a distance equaling 65 percent of its head length. This Aliens-like bite is made possible, Wainwright and his collaborator found, by a drastically enhanced bone-and-ligament apparatus that makes up the lower jaw. Small wonder Wainwright says the fish also wears the moniker "face-chucker."

His work since has led him into investigations of several other species, including members of the large freshwater family Centrar-chidae, home to largemouth bass, bluegill and a host of other panfish including the redear sunfish-popularly known among the Southeast's cane-pole crowd as the "shellcracker."

But since 1993, Wainwright's National Science Foundation-supported research has centered on an odd group of spiny-finned saltwater fishes belonging to the order Tetraodontiformes. Descended from a line of coral-dwelling species that arose 40 million years ago, modern "tetradonts" have no close relatives among living fish, says Wainwright. Examples include triggerfish, cowfish, puffers, filefish and surely one of the oddest-looking animals ever to swim, the ocean sunfish (Mola mola).

"If ever there was a group of related animals where it's obvious there's been a lot of evolutionary changes, it's the tetradonts," Wainwright said. "This group includes some of the strangest fish you'll ever see." Even a glance at photos of tetradont specimens reveals sharp contrasts in the fishes' overall looks. Triggerfish, noted for dagger-like dorsal spines and tough, leathery hides, bear considerable likeness to their filefish cousins, but hardly any to cowfish, which get their name from two "horns" protruding from their bony foreheads. Cowfish, and their cousin the trunkfish, are akin to swimming rocks, with skins quite literally made of solid bone.

Puffers, and their close cousins, the porcupinefish and the burrfish-both of whom bristle with gristle-like spines-are improbable relatives, too. With perhaps the lone exception of the swell shark, these fish are the only fish capable of expanding the size of their bodies, surely among the most creative self-defense mechanisms in all of nature.

And then there is the sunfish, best represented by the genus Mola. Looking for all the world like earless, swimming heads, these tail-less wonders spend their entire lives wandering the tropical seas, often basking their ponderous bodies-which can weigh up to a ton-at the surface. Mid-ocean sailors reportedly have mistaken large specimens for submerged life rafts. Such profound diversity in body shapes within any single group of related animals is extraordinary, says Wainwright. "When it comes to investigating evolution's role in functional morphology (the relation of function to form), this is nothing less than a goldmine."

The highly variegated tetradont family tree is rooted by two striking characteristics common to all members, Wainwright says. First, all of them are missing gill covers, flaps of flesh and bone that flank the heads of most fish, practically a standard-issue item in fishdom for protecting the animals' delicate breathing organs. Tetradonts' gills on the other hand are almost entirely concealed by skin or bone, with only a slit or small hole appearing where rows of gills should be.

But it's the second distinction that intrigues Wainwright. It's the way tetradonts use their mouths when foraging for food, for eating, and-in the bizarre case of the puffers-for blowing themselves up. When frightened, puffers madly gulp water to the point where they could pass for softballs-even basketballs-with fins. Out of the water, the fish can do the same trick with air, inflating themselves to comical proportions in an instant.

Almost all fish are noted for their talents at "spitting out" undesirable items (e.g. fish hooks), a behavior Wainwright says is more accurately described as "coughing." Snail- and other mollusc-munching species are experts at ejecting showers of shell fragments, for example.

Tetradonts are superb "coughers," says Wainwright, but where some of them really shine is in their abilities to blow water, a specialty which he believes may be unique to the group whose members all have rather small, flute-like mouths eminently suited to the task. Triggerfish, for example, can fire jets of water powerful enough to overturn large sand dollars and even small rocks, he said.

Using an evolutionary history of the tetradont family worked out by others in the 1970s (such a study is called a phylogeny), Wainwright noted that while all the family members "cough," as do most fish, only the more advanced forms can do much else. For example, "blowing" behavior shows up in the triggerfish, a species which appeared sometime after the early triplespines (see chart, page 8). The strange ability to inflate shows up only in the puffers, among the latest tetradont arrivals.

The phylogeny clearly suggests a link between all three behaviors, but what physiological evidence was there to prove it?

First, Wainwright had to establish whether there was anything unusual about how the mouths or heads of various tetradonts are constructed that allow for such remarkable lattitude in behavior. After a detailed comparison of skeletal and muscular tissue collected from the fishes' skulls and jaws, he found that in the main, tetradonts share the same skull bones, muscles and ligaments of most bony fish.

But between species he discovered striking differences in how these same parts looked, and often how they were linked together. Wainwright not only found wide variation in length, thickness and definition of muscles, for example, but also in where some of the same muscles tied together bones in the head and mouth.

The study turned up no different parts-just different sizes and shapes of the same parts which were often connected to each other in different ways. Perhaps, then, the fish were using their modified muscles in different ways to take advantage of modified skeletons to produce different "mouth action"-coughing, blowing, inflation.

Perhaps. To answer the question, Wainwright collected electrical impulses recorded directly from the living, muscle-bound heads of the various species while they did all three things. Such delicate work involves implanting fine-wire electrodes in the fishes' heads and jaws. (Unlike other vertebrates, fish don't seem to mind this procedure in the least, another reason biologists like to use them as research models, says Wainwright.)

Analysis of the data revealed a surprising find: all of the fish were using basically the same patterns of muscle contraction, whether they were coughing, blowing, or blowing themselves up. The finding was consistent with what Wainwright had seen in earlier recordings he'd made on largemouth bass and bluegill, but these were freshwater species whose feeding behaviors are far more limited than the tetradonts.

Interestingly, despite having highly diversified capabilities, with exquisite control of their varied muscles, Wainwright realized that the tetradonts were using a pattern of muscle contraction that apparently governed not only their own rather curious feeding behavior but that of other, entirely unrelated species as well.

"Here we were looking at animals possessing great freedom of movement in their heads, perhaps greater than most fish, and yet they were relying on the same, primitive motor patterns to feed. This pretty much tells the story of evolution's role in complex feeding behavior."

Triggerfish didn't acquire their spewing talents by evolving different ways to use their jaw muscles-they evolved differently shaped bones and muscles instead. The signal to "blow" sent by the triggerfish's brain to muscles in its mouth is the same signal, in other words, that might prompt a puffer to inhale water or inflate.

"When you apply the same muscle contraction patterns to different sets of mouth parts, you get different responses," Wainwright said. An analogy might be two car engines, both of which run off the same fuel, applied the same way, but with internal parts configured differently. Pressing the accelerator effects both engines, but performance can-and most likely will be-quite different between the two.

Wainwright had to conclude that for some reason, through the eons the forces of evolution left the basic motor functions that dictate how fish eat comparatively untouched. Instead of changing the way feeding muscles are used, evolution has instead had a field day changing how such muscles look, as well as the bones and ligaments associated with them.

"During evolution of feeding biology in fish, evolution has clearly tinkered with the morphology-the shape and size of the skeletal structures of the head-and left the neuro-muscular patterns pretty much alone," Wainwright said. "Although most fish have a broad range of muscle patterns available for use, they tend to choose the same ones over and over again for the same task, relying on their different anatomies to get different things done, yet with the same neurological information."

Apparently, in trying to make a fish a better feeder or a more successful predator, by tweaking the way its nerves stimulate its muscles, evolution hit a dead end, with the neuromuscular system finally reaching a point where it became as efficient as it was ever going to get. The path to advances in feeding capability thus lay in radically changing the size and shape of the mouth. From there, further advances lay in reshaping the entire body and, among fish-eaters, improving the ability to swim.

Wainwright says such a finding is surprising, since there's no apparent physiological or biochemical reason why evolution shouldn't be able to crank out brand new motor patterns-neurological codes so fundamentally stamped into the brain that they amount to involuntary reflexes-to drive new or greatly remodeled bones and muscles. After all, a general trend in evolution is that neurological systems become more complex the farther up the ladder an organism gets.

"You might think that evolution would play on the most flexible systems, such as neural patterns. An individual, for example, can control and alter its motor patterns infinitely easier than it can change its anatomy." So it would seem that the way to go would be to evolve new contraction patterns to facilitate the evolution of radically new behaviors-such as puffers' ability to inflate.

"But that's just not the way it works," says Wainwright. "Though there are exceptions, it's basically a story of new behaviors arising from an ancient set of motor patterns."

The redear sunfish (shellcracker) is one of the few fish known to have evolved a totally new contraction pattern, which it uses to crack snails, he said. Other snail-eaters, such as the sheepshead, make do with the motor patterns of old.

This exception aside, what Wainwright found in the tetradonts gives new meaning to what he'd found earlier in bass and bluegill. A largemouth bass will use the same motor patterns to get its cavernous mouth around a crayfish, cricket or minnow that a bluegill or speckled perch (another cousin) will. The only difference is the equipment each uses to snare the meal. A fish's ability to feed itself, then, is far more a function of its form than of canny ways of using it.

"This is an underlying theme we're seeing which we believe applies to all fish. These animals have evolved mainly by changing their bones and muscles, and much less so by changing the motor patterns that drive them." Information gained from recording the tetradonts' nerve impulses during feeding also produced conclusive evidence that "coughing" gave rise to blowing behavior, which in turn gave rise to puffers' extraordinary ability to inflate-just as the animals' phylogeny suggests, said Wainwright. He discovered only a single muscle pattern difference between blowing and inflating behavior, albeit four major anatomical differences in the puffer family are required for inflation.

Could it be that this is how evolution works in other vertebrates-birds and mammals in particular? Although it's much harder to collect muscular impulse recordings in mammals and birds, Wainwright said, what work has been done suggests that's a possibility. For example, considerable evidence comes from studies of how ungulates-hoofed mammals such as cows-chew their food, he said. Such routine feeding behavior appears to be driven by deeply embedded neural codes called "pattern generators" that have controlled cud-chewing for millions of years.

Whatever the case, Wainwright's research offers yet another commentary on the fundamental curiosities of evolution. As a dynamic system that literally feeds on change, on occasion evolution finds it prudent to quit fidgeting with things and leave them as they are-as though following the handyman's dictum: "if it ain't broke, don't fix it."

Wainwright muses the thought.

"You could easily spend a lifetime speculating on just why that is."

The Puff Factor

Mystery solved: Now we know how these funny fish blow themselves up.

Consider the lowly pufferfish. If nothing else, the animal serves as a constant source of amusement among saltwater anglers, both young and old, who often find it dangling in distended animation at the end of their lines. Hands down, the fish's amazing ability to turn itself into a scaly spheroid in a snap surely rates as one of nature's all-time showstoppers. For at least a century, fish scientists (ichthyologists) have marvelled, too, at how puffers puff, a behavior certifiably among the rarest in all of animaldom. Until now, their studies of puffers' anatomy, and those of puffer cousins such as porcupinefish and burrfish, have amounted to little more than collective guesswork.

In studying the feeding behavior of the Tetraodontiformes, the group of fishes to which the puffer belongs, Dr. Peter Wainwright stumbled upon the secret to the fish's bizarre attribute-a special oral valve attached to the floor of its mouth.

Scientists who had previously noted the organ's presence had speculated that it played some part in inflation, but couldn't pin down what that might be. Wainwright has been able to explain the organ's vital role and also to show how the whole process works. After filling its mouth with water, the fish flexes a large muscle at the base of the oral valve which then catapults forward against the entire front of the mouth, forming a tight seal against the back of the front teeth. This prevents the water from escaping while a "plunger" type of apparatus -a mechanism driven by a highly modified gill arch called a branchiostegal ray-mounted at the base of the throat forces the water upward where it shoots down the fish's esophagus and into its stomach. Using a series of electrodes embedded in the puffer's head muscles, Wainwright learned precisely which muscles are involved and how they fire in rapid sequence to accomplish the task. The trick doesn't stop there, of course. Scientists have long known that puffers have stomachs and skin of unparalleled elasticity. Unimpeded by ribs (puffers don't have them), the fish's water-(or air-) filled stomachs are thus free to balloon, making their owners a difficult mouthful indeed for any passing predator.

Which is, after all, the whole point. Woefully shortchanged by evolution on speed genes-divers can catch them bare-handed-puffers, with more than 300 known species, obviously have found all the means of self-defense they need, even the ability to entertain the granddaddy predator of them all.

Source: Florida State University, Research in Review, Winter 1995 issue

Photo #2: HEAD COUNT: As these skulls show, although highly varied in size and shape, members of the tetradont family share many things in common, a powerful set of dentures being one of them.

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