Follow the Food Chain: Making Sense of the Chaos on a Coral Reef

I’ve never tired of watching student divers return from their first open-water experience. Their enthusiasm is always contagious, and you couldn’t shut them up...

coral reef

I’ve never tired of watching student divers return from their first open-water experience. Their enthusiasm is always contagious, and you couldn’t shut them up if you held a gun to their heads.

If you are a diver, I’m sure you can remember your first time. If you are lucky enough to watch this scene unfold after a dive on a coral reef, then I’ll wager that most of the exuberant discussion involves one subject: fish.

“Did you see that bright blue one? The color was so awesome; I’ll bet it glows in the dark!”

“Did you see that one with all the spines? It was so ugly it was cute!”

“I can’t believe how big that one was with all the spots! And did you see that tiny little fish biting him on the gills?”

“I could have sworn I was looking at a rock, and then all of a sudden, it began swimming!”

“I didn’t know fish ate rocks. Did you see that green one biting the coral? It was so amazing!”

There’s no doubt that their incredible diversity, size, form and color make fish the true superstars of the coral reef. Their beauty and inscrutable behavior is almost always the highlight, if not the very purpose, of most reef dives. For novice divers especially, the riotous scene unfolding before their eyes must seem like chaos in the extreme. I can relate to this in my experience. I was a seasoned Caribbean diver long before I made my first sojourn to a coral reef in Indonesia, and was still awestruck on my first dive off the island of Flores. This region, sometimes known as the “Coral Triangle,” is the literal bull’s-eye of the world’s coral and coral reef fish diversity. By comparison, there are about 500 species of fishes in the Caribbean and tropical Western Atlantic, which most divers assume is an incredible variety. That is, until you realize that the Indo-Pacific boasts close to 3,000. Just imagine entering the water and being confronted with five or six times more species of fishes than you have ever witnessed on any other dive. The only word that comes even close to describing the experience is “mind-boggling.”

Many divers welcome the chaos of color and motion as part of their coral reef experience, and have no particular desire to learn more about the characters performing before them. Others dive for decades, learning the names of only a few of their favorites, yet remaining totally ignorant of their role in the reef community. But what about those who want more than to “enjoy the show”? What if you want a fuller understanding of the story unfolding on a coral reef? Is there any way, aside from a degree in ichthyology, that you can make sense of the chaos? Indeed, there is.

The Energy Puzzle

Some say that if you want to truly understand any species on earth, then study its sex life. While there may be something to that approach, there is one thing in life that trumps sex and that’s eating. After all, without the latter, the former would be impossible. Understanding food, and more particularly, the food chain is also how you begin to see reason in the chaos of the reef. It’s what I call the “follow the food chain” interpretation of coral reef fishes. But before we can see how food can be used to understand fishes, we first need to know a bit more about the nature and purpose of food, itself.

While the details of life on earth are exceedingly (though not irreducibly) complex, the basics are pretty simple. Every living thing on our planet needs a supply of energy to keep from succumbing to the Second Law of Thermodynamics. This law states “all work processes tend toward a greater entropy (disorder/lower energy) over time.” In other words, without a continual input of energy, everything winds down. The biological interpretation is that if you don’t eat, you die. So, the trick is, how do you get the energy of the sun into a form that living things can eat? (For now, we’ll ignore places that don’t depend on the sun, like the chemosynthetic ecosystems of the deep ocean.) Fortunately, that’s where plants (producers) come into the picture because they can take the sun’s energy and convert it into a usable form we animals can use (food) via that amazing process of photosynthesis.

Once the producers have established the first link in the food chain, the rest — as they say — is just details. The next line of critters — the consumers — do just that; they eat (consume) the producers. And those that eat the first level of the food chain are called “primary consumers.” Those that eat the primary consumer are “secondary consumers.” Those that eat the secondary consumers are called “tertiary consumers,” and so on. But, just as we humans eat both salad and steak (making us both primary and secondary consumers), other creatures rarely eat at just one level all of the time. So, that simple food chain is better described as a web. The essential problem is still the same for every living thing: How do I get the sun’s energy into me so that I can use it? The producers use the sun directly; so they’re also called “autotrophs” (self-feeders) and the consumers, who can’t use the sun directly, have to eat something else for energy, so they’re called “heterotrophs” (other feeders).

Now that we know about the true nature of food, it’s time to look at what kind of fish food exists on a coral reef. Well, as we just saw, it has to be either plant or animal matter. Yet to the unfamiliar eye, there doesn’t seem to be many plants on or around a healthy coral reef. However, whether you notice it or not, algae grows at a prodigious rate on all reefs. In fact, every square meter of coral reef can produce from 1 to 5 kilograms (2.2 to 11 pounds) of algae by dry weight per year. On a healthy reef, we never notice the algae because it’s eaten almost immediately by plant-eating (herbivorous) fish. (On unhealthy reefs, much of the algae isn’t eaten because it either grows too fast for all of it to be consumed, or the species that grow under what are normally high-nutrient conditions contain chemicals or poisons that make it unpalatable to the fish.)

The other option besides eating plants is, of course, to eat other animals; and there are lots of those on the reef. While there are what seems like countless fish to eat, there are even more meals provided by creatures without backbones, ranging from sponges to crustaceans. Not surprisingly, then, most coral reef fishes are carnivores (meat eaters), some specializing in eating other fish and some specializing in consuming types of invertebrates. But there’s still another option.

As you learned the last time you cleaned out your refrigerator, after something dies, it eventually begins to decay. Besides making a nasty mess if it’s somewhere it doesn’t belong, decay is a process whereby once-living tissue is being rendered back into its original inorganic components. Like the Bible says, “ashes to ashes, dust to dust.” But along the way, this decaying material — as unappetizing as it may sound — is still a source of food. In the ultimate example of one man’s trash becoming another’s treasure, even fecal material is a source of food to some creatures. (What do you think digests the contents of your septic tank?) There’s even a scientific term for consumption of feces. It’s call carpology, and in the open ocean entire planktonic food webs are maintained by this process. But getting back to the decaying process, biologists term all decaying organic matter “detritus,” and detrital food webs are both important and complex, particularly in marine environments. One such detritus-based food web associated with coral reefs can be found in mangrove forests, where the fallen leaves (up to 4,000 pounds [1,800 kg] per year per acre) serve as the primary source of decaying organic matter.

So, like in some aquatic Chinese restaurant, reef fish can order from three columns on the menu: plants (sea grass or algae), another animal (fish or invertebrate) or detritus (a plant or animal that died and has decayed). While relatively few reef fish feed directly on detritus, it’s a vital food source for invertebrates. Still, it’s all food, so the choice comes down to what you, as a reef fish, can catch and eat.

Of course, for a fish it’s not as much a matter of choice as it is evolution. Over the hundreds of millions of years that fish have occupied reefs, they’ve pretty much figured out how they can make a living, and their bodies and lifestyles have adapted accordingly.

Steak or Salad?

An interesting fact about coral reef fishes is that, by species, only about 15 percent are herbivores, and most of these are in three families — parrotfishes, surgeonfishes and damselfishes. What’s amazing is that in some cases, the bulk of the fish biomass on a reef can be composed of herbivores. In other words, while there aren’t that many types of herbivores on the reefs, there sure are lots of them (or there should be if the reef is to be healthy). For an ecosystem to need this many “lawn mowers,” it shows you just how much plant matter (algae) must be present. Not surprisingly, this preponderance of herbivores occurs most noticeably in shallow water, where there’s an abundance of sunlight for photosynthesis. In fact, as a very general rule, herbivores tend to make up the bulk of fish biomass above 33 feet (10 m) while carnivores make up the bulk below that depth.

As on land, the choice of steak or salad, to a very large extent, determines much of a creature’s behavior and lifestyle. The reason is because of the contrasting nutritional value of meat and vegetables. Meat has a higher nutritional value than plants, thus carnivores can have shorter digestive tracts and don’t have to feed as often as herbivores. By contrast, plants have relatively poor nutritional value, compared with meat, and are difficult to break down. Thus, herbivores have long digestive tracks to break down the food, and must feed almost continuously because their food is relatively low in nutrition. Think about the feeding habits of, say, lions versus gazelles, or cougars versus cows. The meat eaters spend much of their time lying around, and eat only occasionally. The plant eaters must graze continuously. On the reef, think of the groupers versus the parrotfishes, and you see that the same idea applies underwater. A diver rarely, if ever, sees a grouper feed, but when he does, it’s a whole fish, which can last him for days. But if you watch parrotfish, you’ll notice that they’re eating constantly, biting the reef thousands of times per day to get the necessary nutritional requirements from the algae. And, like everything in life, there’s an upside and a downside. The upside for herbivores is that, because there are lots of plants to eat, there can be lots of them. The downside is that they have to feed constantly, so they’re also easy prey for meat eaters. Moreover, some scientists believe that the incredible amount of predation on coral reefs has had a significant evolutionary effect. The large number of carnivores, some say, is one reason most invertebrates are so cryptic, and why those that aren’t are often highly toxic.

However, there is one point worth making in the discussion of food preference. Ichthyologists once thought that food preference in fishes was as much a rule as a turkey dinner on Thanksgiving; predators only eat meat, and herbivores only eat plants. But as scientists began spending more time underwater, they soon saw that the feeding line sometime blurs. Coral reef fishes are actually much more opportunistic than was once believed. Indeed, as most experienced reef divers can tell you, sometimes carnivores eat plants and herbivores eat fishes. Their food preference is just that, a preference, not a law of nature. What really dictates what a fish eats is what its body — mouth, in particular — is designed to catch.

It’s Not What’s in a Name, It’s What’s in a Body

With the thousands of coral reef fishes in the world, being able to identify them all is virtually impossible. Most of us are lucky if we can commit a few dozen to memory, and even the most experienced naturalists are lucky to know more than a few hundred. But knowing names really isn’t an essential task to understanding the chaos of a coral reef. Fishes give us valuable clues about their lives in the way they look. So, if we know what to look for, their body features can tell us as much as any textbook.

Generally, the relationship of what a fish looks like and its way of making a living is pretty logical. First, look at its basic body plan. Is it streamlined or not? A cigar shape — what scientists call “fusiform” — is a good clue that it’s a fast swimmer. Fast swimmers are most often hunters that can chase down their prey, like a barracuda. On the other hand, an angelfish may appear “streamlined,” in the sense that it’s skinny, but it really doesn’t have a very efficient hydrodynamic design. But it doesn’t have to be fast because it doesn’t chase anything. Its main food source — sponges — isn’t going anywhere. The reason for its skinny (“laterally compressed”) body is so that it can maneuver in and out of the cracks and crevices of the reef, a handy ability when you’re trying to avoid being eaten yourself. Of course, the antithesis of streamline design is seen in the various pufferfishes. But then again, if you can inflate your body and erect sharp spines, you really don’t have to worry about too many other fish bothering you. There’s not much need to invest the evolutionary energy in becoming a faster swimmer.

Tail design is also another — ahem — tell-tail giveaway about swimming ability. Basically, think of the fastest swimmers in the sea: tuna and billfish. They all have sturdy, cycle-like or “lunate” tails. This is the ultimate design for speed. The rounder the tail becomes, the less speed-efficient it is. However, what’s lost in speed is gained in maneuverability. Look at the angelfish, again. Notice that full, blunt tail design. No speedster for sure, but a great acrobat.

The next issue in understanding the lifestyle of a reef fish is determining what it’s designed to eat. Humans are omnivores; we can eat almost anything. We have the dentition (teeth) to cut, rip and grind, so whether it’s meat, vegetables or pizza, we can handle it. Most animals aren’t as versatile. (Take a look at your dog’s or cat’s teeth, for example; notice that its back teeth, as in all carnivorous mammals, are narrow and sharp, designed to cut, not grind, as in humans.)

Of course teeth are a good clue, but in fish it’s not just their teeth, but also the entire design of the mouth. Big, wide-opening mouths are designed to accommodate a variety of prey. This is the mouth of a predator. Diminutive mouths with comb-like teeth, as seen on butterflyfishes, are designed for grasping tiny things like coral polyps. The crescent-shaped mouth of angelfishes is perfect for taking bites out of sponges.

OK, then, here’s a quiz. What are the grinding jaws and teeth of fishes like puffers used for? Time’s up. They’re crushers, and thus this type of mouth is found on fishes that eat hard-shelled invertebrates, many of which have hard shells. (Ichthyologists actually call these types of feeders “benthivores”; literally bottom feeders.) If you eat invertebrates, then there’s no need to swim fast, but you do need precise maneuverability, good close vision, and a specialized mouth capable of plucking and crushing prey. And if you can’t swim very fast, you’d better have some excellent defense mechanism like the ability to quickly hide, to erect spines, or have a hard, protective skin that doesn’t taste very good. Common families of benthivores are, as mentioned, angelfishes, butterflyfishes, filefishes, boxfishes and pufferfishes.

Now, how about fishes with expandable (what scientists call “protrusive”) jaws, like blue chromis (Chromis cyanea)? The plucking ability of protrusive jaws makes them ideal for capturing zooplankton from the water column. These are termed “planktivores.” Another feature of the chromis is a deep, forked tail; a tail built for speed. Why is this so if they don’t have to chase zooplankton? It’s for protection. The best place to catch zooplankton is up in the water column, away from the reef on the seaward side. This leaves them exposed to hungry carnivores, so they’d better be able to swim fast to escape.

By contrast, sergeant majors (Abudefduf saxatilis) are also planktivores, but because they hide in or near the reef, they need maneuverability, not speed. Thus, their blunt tail design is very different from the chromis. Still other planktivores, like the jawfishes and fairy basslets (Gramma loreto), manage a successful planktivorus life by remaining very close to the reef for shelter.

While the varying lifestyles of herbivores, benthivores and planktivores are intriguing, it’s the fish eaters (picivores) that make up the bulk of the fish population on a reef. In fact, because there are so many fish to eat on coral reefs, 50 percent to 70 percent of reef fishes are picivores, and this is a very opportunistic bunch. They’ll eat just about anything that swims, and some things that don’t. Fish eaters are so numerous on the reef that they have even evolved different hunting strategies. Pursuit hunters like sharks, jacks, mackerel and adult yellowtail snapper, begin with a moving start, from long range. By contrast, stalking hunters like trumpetfishes and barracuda, use stealth and attack from close range. The ambush hunters, such as scorpionfishes, lizardfishes, sea basses and groupers attack quickly with no preliminary maneuvering.

The Shift Change

One reason that there are so many ways of making a living (niches) on a coral reef is that, like a factory, it operate 24 hours a day. There are essentially two shifts that operate: a day shift and night shift. At night, daytime (diurnal) fishes seek shelter and are replaced by a smaller number of specialized nocturnal species, which are mainly inconspicuous during the day. Nocturnal feeders rely more on touch, taste, smell and motion than diurnal feeders. Although they often have large eyes to gather light, it’s mainly to see motion rather than detail. With the other senses being so adept, good visual acuity isn’t important.

Unlike the specialized mouths of most diurnal fishes, the mouths of nocturnal fishes, most of which are predators, are adapted to quickly suck prey in via powerful vacuum action, rather than delicately capture prey. Some of the nocturnal species have similar ecological niches as certain diurnal species. For example, at night the planktivorus role of damselfishes is replaced by cardinalfishes. By exchanging roles between night and day, a greater number of species can exist on the reef without direct competition.

Some predators have evolved an interesting way to take advantage of chinks in the armor of their prey. Obviously, diurnal fishes have adapted to see best during the day, and nocturnal fishes to see best at night. But what about in between — at twilight? At this time of day neither diurnal nor nocturnal species can see very well. But, some predators can see exceedingly well during the dim and shadowless hours of twilight. This group, most notably but not only the sharks, is termed “crepuscular” feeders, as they have eyes especially adapted to this low light level. Once again on the reef, no matter how seemingly well suited to the environment, nothing is immune to the relentless mouths looking for their next meal.

So the next time you’re on a reef dive and confronted with the bedlam before you, remember: You don’t have to know the names of the fish to understand what’s going on. Just “follow the food chain” and the links will soon make sense.

Ichthyologists once thought that food preference in fishes was as much a rule as a turkey dinner on Thanksgiving; predators only eat meat, and herbivores only eat plants. But as scientists began spending more time underwater, they soon saw that the feeding line sometime blurs.

‘Fish’ vs. ‘Fishes’

“Fishes” is the proper English plural form of “fish” that biologists use when speaking about two or more fish species, as in “There are over 25,000 fishes in the world” (meaning that there are over 25,000 fish species in the world). When speaking of two or more individual fish organisms, then the word “fish” is used, as in “There are several million fish of the species Gadus morhua” (meaning that G. morhua — Atlantic Cod — comprises several million individuals). To see both in action, consider the statement “There are 12 fish in this aquarium, representing five fishes” (meaning that the aquarium contains 12 individuals, some of the same species and some of different species, for a total of five species). The usage of the two words is similar to that of the words “people” and “peoples.”

Source: Wikipedia

By Alex Brylske