Everywhere I looked I saw sharks — big hammerhead sharks — some close to 11 feet/3 m long. I don’t know how many sharks were within view, but there had to be at least 200. Most were high up in the water column, but a smaller group of six or eight sharks was swimming just over the reef as they made their way toward me.
I tucked in behind a mound of boulders and waited with great anticipation. Peering out from behind the rocks, I saw a school of bright-yellow barberfish and several king angelfish rise high off the reef and begin fluttering around in the water column. You might think it strange that all the fish on the reef wouldn’t flee for their lives into the nearest crevice when big sharks showed up, but it was quite obvious that these small reef fishes were advertising their presence as they danced above a prominent outcropping.
As the hammerheads neared, the lead shark turned sideways and began to quiver slightly as it maneuvered through the water directly into the gathering of reef fishes. The shark’s orientation and swimming motion were clearly different than the norm, different from what I’d witnessed just moments before. Within seconds a dozen hand-sized barberfish boldly swarmed the shark and began nibbling away at its skin. A juvenile hogfish and an angelfish joined in, pecking away at the shark — within inches of its mouth. A second, third and fourth shark swam over the outcropping, and each time these delicate-looking reef fishes greeted the sharks in the same manner.
If you’re a new diver, you might wonder why reef fish wouldn’t immediately shy away from a large predator like a shark. If you’re a marine biologist or an experienced diver, you would probably not be surprised by the scene in front of you. In fact, the barberfish, hogfish and angelfish were actually doing the hammerheads a “favor” — ridding them of parasites, dead skin, bacteria and fungi. Cleaning is a daily activity in many reef communities. The more you discover about the natural world, the more you will learn that it’s filled with surprises, including a fascinating variety of relationships involving animals that live together. In fact, in many cases it is the very nature of these relationships that enables these species to survive.
Obviously, cleaning is an example of a nonpredator/prey relationship. Cleaning is a type of interspecies relationship known as “mutualism,” an interaction between two different species of plants or animals in which both species benefit and neither is harmed. In this case, the cleaner fish gain by getting a meal. The hammerheads benefit by being cleaned, and the sharks do not prey upon the cleaners. It is a win/win relationship.
Cleaners include many species of shrimps as well as fishes, while the beneficiaries, commonly referred to as hosts, vary from other fishes to turtles, dolphins and more. Knowing what to look for and where to find this cleaning activity among marine creatures can add a lot of pleasure to your diving.
The key to observing cleaning behavior or any other animal relationship is understanding enough about the relationships and the behaviors involved to recognize what is taking place. Many relationships can be observed on a regular basis once you are armed with some knowledge.
In this article and a follow-up piece that will appear in the next issue of Dive Training, we will examine several of the more important relationships divers have a chance to see during typical ocean dives. We will start with the relationship of predator and prey, and move on through a number of other relationships, from cleaning to parasitism to schooling and more. As various relationships are revealed, you will gain insight into the inner workings of marine ecosystems. As cliché as it might sound, a marvelous aspect of exploring the underwater world as a sport diver is that the more you learn, the more you realize there is to see, and the more reasons there are to continue diving.
Predator and Prey
When discussing the animal kingdom, the relationship of predator and prey immediately comes to mind for many of us. If asked to name some top-end, or apex, predators that live in the sea, most elementary school students could quickly rattle off a list that included great white sharks, tiger sharks, sperm whales, killer whales and giant squid. What they, and we, often fail to realize is how many smaller organisms it takes along the way to support a single top-end predator.
Every animal species in the sea preys either on plants, some other animal or both. After all, an animal has to eat to live. The paths through which energy is transferred in nature are called food chains. In fact, one of the fundamental ways in which scientists describe animals is according to their trophic relationships, or how they relate to one another in various food chains. Understanding trophic relationships involves the analysis of “what plants and animals eat, and what eats them.” For example, herbivores are animals that eat plants, while carnivores prey upon other animals. The terms herbivore and carnivore describe trophic relationships.
Plants form the foundation of all major food chains. Plants get all the energy they need for the process of photosynthesis from the sun. In any given chain there are typically several layers of animals linked between the plants and the chain’s top-end predators.
For example, in the sea tiny animals called zooplankton feed upon small plants called phytoplankton, and in turn, comparatively small fishes commonly referred to as minnows feed on zooplankton. Slightly larger fishes such as silversides prey on the minnows. Groupers feed on silversides and Caribbean reef sharks prey upon groupers. The complete link from phytoplankton to reef sharks is known as a food chain, and each step along the way is known as a trophic level.
The flow of energy in all food chains is from sunlight toward the top-end predators. A fundamental fact in nature is that the transfer of energy from one trophic level to the next is rather inefficient, usually ranging between 6 and 20 percent. The practical translation of this statistic is that, at a minimum, it takes 6 to 10 pounds/3 to 4.5 kg of plants to produce 1 pound/.5 kg of the animal that eats the plants, and 6 to 10 pounds of the animal that eats the plants to support 1 pound of the predator that preys upon them.
Continue up the food chain and it is pretty easy to see that it takes many pounds of plankton to produce a pound of great white shark. As a result, the laws of nature dictate that top-end predators cannot be nearly as abundant as creatures that are positioned on a lower level of the same food chain.
In current-day jargon, the term food web is used to describe many interlinked food chains. In the natural world, there are very few linear food chains, as a number of chains link together at different levels to form a larger food web. In some food chains a given animal might represent one trophic level, while in other chains the same animal might represent entirely different levels.
Blue sharks are a good example. A blue shark that feeds on anchovies might represent the fourth level in a chain in which phytoplankton is eaten by zooplankton, which in turn is eaten by a minnow, which is eaten by an anchovy, which is eaten by the blue shark. But if the shark is feeding on a yellowtail that ate a mackerel that ate the anchovy, the shark is on the sixth trophic level. And if that blue shark were to be eaten by a bigger mako shark, the mako shark would represent still another trophic level.
If you look around in a typical reef community, you will probably see some fishes that you can easily identify as mid-level predators. For example, if you think about the design and behavior of fishes such as jacks, grunts and groupers, it is fairly easy to envision them in their roles as mid-level predators — animals that feed upon other fishes, shrimps, lobsters, squids and more. And it is relatively easy to understand that, in turn, these mid-level predators are preyed upon by jewfish, sharks and other larger predators.
As divers, we often overlook many of the tiny plants and animals toward the lower end of food chains. But you will often see fishes such as creolefish, jawfish, blennies and mackerel feeding on many of these smaller organisms. You might not recognize the behavior as feeding because it just looks like these and many other fishes are picking at the water. Referred to as “mid-water pickers,” these species are in fact mid-level predators that feed on abundant supplies of organisms that are so small they are almost impossible for human eyes to detect.
In a California kelp forest, you might also see señorita fish picking hydroids and bryozoans off the blades of a kelp plant. At first glance, it might look like the señoritas are eating the kelp, but close inspection will reveal concentrations of tiny “fuzzy-looking” animals attached to the kelp.
On a typical Caribbean reef, you are likely to see parrotfishes feeding on corals (primarily in an effort to gain access to algae in the corals), surgeonfish grazing on algae, or angelfish, butterflyfish and filefish feeding on sponges. Sea urchins and sea cucumbers typically graze on algae or detritus, the decay or waste products of other organisms.
Sea stars and brittle stars sometimes scavenge, sometimes capture live prey and sometimes feed on detritus. Octopi readily prey on crabs, small lobster and shrimps, and in turn, octopi are a favorite food source for many fishes, including moray eels. You might see a school of jacks, or a tarpon, barracuda or other closer-to-the-top-end predator rush a school of silversides. Or, if you are lucky and observant, you might see a camouflage artist such as a peacock flounder or lizardfish laying in wait for some unsuspecting fish to get within striking distance.
Just because one species is very large doesn’t mean that it feeds on creatures that are only slightly smaller. In fact, most of the ocean’s larger creatures feed upon the smallest organisms. The great filter-feeding whales, a grouping that includes the largest creatures on earth — blue whales, humpback whales, right whales and more — feed upon tiny planktonic creatures. Of course, the whales devour them in uncountable numbers. The same is true of manta rays, and it is quite popular in waters of Kona, Hawaii, as well as some other parts of the world for divers to watch these graceful giants feed on concentrations of plankton at night.
Who Eats Whom?
When you consider how certain animals behave, their size and shape, when they are most active and their numbers relative to other species, you can figure out a lot about how different animals fit into various predator/prey relationships. It is not always possible to accurately categorize every animal you encounter, but with a bit of observation, you will begin to see how the community exists. In turn, this information will help you learn where and when to look to increase your chances of finding different animals.
Cycles of Change
If you regularly dive in the same area over a long period of time, you will likely notice that the ratio of predators to prey seems to change. Indeed, the ratio does change. The natural world is in a constant state of flux, as all animals are competing for resources such as space and access to sunlight, nutrient-rich water and food.
California kelp forests provide an excellent case in point. The forest ecosystem centers around the presence and health of the giant kelp plants. The plants provide shelter, food and habitat for more than 800 species. When the kelp is healthy, many species flourish, but if water temperatures rise above 68˚F/20˚C, the demise of kelp forests quickly follows.
When that happens, animals such as sea urchins that feed upon the constantly shedding kelp stipes run low on food. The urchins leave the safe confines of the cracks and crevices of the reef, and begin to prey upon the kelp holdfasts, the rootlike structures that anchor kelp plants to the sea floor. When the urchins feed on the holdfasts, the kelp plant is further weakened and the plants are torn loose from the bottom. Soon the sea floor will be absent of kelp and covered with hungry urchins. The weakened and exposed urchins become “easy pickings” for fishes such as sheephead that are equipped to cope with urchin spines.
Other factors enter in as well. In central California, sea otters actively prey upon sea urchins. When urchins are plentiful, the otters feast, but later in the cycle, as the urchin population diminishes, otters turn their efforts toward crabs, lobster, abalone and other resources. Many of these species also depend upon the constant debris of kelp lying on the bottom of a healthy kelp forest. These populations are constantly expanding and shrinking depending on the amount of food available, as well as other factors such as seasonality, water temperature, their reproductive cycle, and the health and well-being of their primary food sources and their major predators.
Another example of the state of flux you will witness in the natural world involves a relationship between mussels, barnacles, chitons, limpets and some species of algae. This cyclical relationship can be observed by examining life cycles in California tide pools.
In its simplest form, the relationship works as follows: In a densely populated intertidal community, these organisms are limited by the amount of available dwelling space on rocks, and on the dissolved nutrients and other food suspended in the water column. The organisms compete for the available resources, and all attempt to dominate the others.
Available space is a critical resource, but even in the intertidal zones it is almost never completely utilized. Open space is routinely created by a combination of biological and physical factors. Mussels and barnacles are preyed upon by sea stars and snails. Algae density varies with the seasons. Pounding from heavy surf and foreign objects such as trash or driftwood also create some cleared patches.
Pure chance, the time of year, reproductive cycles and other factors create greater opportunity for some species and less for others. In a common scenario, barnacle larvae and algae spores settle on the same rocks and compete for space. As a rule, the barnacles win out and dislodge the algae.
But just because the barnacles “win” the first leg of the competition does not mean they are guaranteed long-term, unchallenged success. The barnacles are heavily preyed upon by carnivorous snails and sea stars. Even herbivorous limpets sometimes dislodge barnacles in the competition for space.
In their larval stage, mussels do not require barren rock for attachment. They will settle on algae, on barnacles and in patches of adult mussels. If left undisturbed, mussels eventually attach to the substrate and overrun barnacles and algae. Enter the sea stars. They love to prey upon mussels. Some larger mussels manage to survive onslaughts by sea stars, as well as predation by snails, but many mussels are devoured. Their demise gives algae and barnacles a chance, and the cycle begins anew. This ever-fluctuating cycle helps guarantee the diversity of species.
Of course, not all the relationships we encounter are that of predator and prey. If you have made even a handful of ocean dives, you have probably stopped to marvel at the synchronous movement of a school of fish. Schooling is a different type of relationship, one that exists between members of the same species. Fishes that live in schools vary from big hammerhead sharks and tuna to tiny baitfish. Dive a typical reef community in the Bahamas or Caribbean and you are likely to see a colorful school of grunt, snapper, jacks and flashing silversides. In California waters, the school might consist of blacksmith fish or jack mackerel, but they could also be barracuda, yellowtail or bonito. Many fish species school — big fish, small fish, colorful fish, drab fish, faster swimmers, slow swimmers, open-water species and bottom dwellers.
Studies reveal that schooling is an adaptation utilized by approximately 80 percent of all fish species as juveniles and roughly 20 percent of all species as adults. Exactly why fishes school is a question that has intrigued scientists for years. There are a number of scientific theories regarding why some fish school, and there is probably an element of truth in all of them. Of course, the most difficult aspect in answering the question of why fish school is that we can ask, but the fish can’t answer. So experts observe. And guess.
Safety in Numbers
The concept of safety in numbers is believed to play a fundamental role in schooling behavior. There is a lot of truth in the old adage that “big fish eat little fish.” To overcome the problem of their diminutive size, many small fishes gather in schools. It is believed that in some cases the schools appear as a single, large shape, and at least some potential predators tend to leave large things alone.
Another part of the safety-in-numbers concept is the fact that even if a predator does attack a school, the odds are small that any particular fish will be the one the predator chooses to pursue. In other words, it is safer hiding within the school than it is to roam the waters alone. In addition, within a school there are a lot of eyes and lateral-line sensory systems on the lookout for potential predators.
When a predator does attack, the movement of all the individuals within the school can confuse the predator. And that momentary confusion might give the potential prey a chance to escape. In the wild, it simply doesn’t work for a predator like a tarpon or jack to swim, mouth open, through a school of small baitfish, like silversides, just hoping to get lucky and end up with a meal. Small as they are, each silverside is thinking and reacting, so the predator must select a single victim and be able to pursue it with all its cunning.
To combat this confusion, some predators, such as a variety of jacks and tuna, cooperate by hunting in schools of their own. They frighten their prey into smaller groups, making it easier to target a single victim as the predators “take turns” rushing the tightly packed prey.
Other Advantages to “Staying in School”
Reproduction is another advantage of schooling. Close proximity makes it easier to reproduce more offspring than potential predators can consume, thus helping to ensure the survival of an individual fish’s genetic code, and of the species.
Schooling can also help overcome the defenses of territorial fishes in an effort to gain access to food. For example, some damselfishes are highly territorial, and they will bite or ram into any fish that invades the algae patches they are cultivating. But a large school of surgeonfishes can easily overcome the damsel. As a result, the surgeons are able to consume considerable quantities of algae despite the damsel’s spirited efforts. (See this month’s “What’s That” column for more information on damselfish behavior.)
Schooling is also believed to make swimming easier and conserve the fishes’ energy by reducing water resistance. Just as cyclists “draft” the bike in front of them, staying close behind in order to reduce wind resistance while getting sucked along by the bike in front, schooling fishes position themselves in a similar manner.
While some fishes certainly benefit from schooling, there is a cost as well. It requires a lot more food to feed a school of fish than it does to feed a single fish. Any diver who has ever watched a school of fish feed is well aware that sharing is not a concept fishes abide by. Even schooling fishes are competitive feeders, but this cost does not outweigh the advantages for many species.
We have examined predator/prey relationships and schooling, and taken a brief look at some cleaning relationships. In the next issue of Dive Training, we will continue with a closer examination of cleaning as well as other relationships, such as parasitism and a fascinating type of relationship known as phoresis, in which one animal serves as another’s mode of transportation. We will also investigate the compelling relationship between anemonefishes and sea anemones, and touch on a number of relationships between animals of which scientists are aware but don’t fully understand. In the meantime, enjoy some great dives and keep an eye on the animals and organisms around you. You might see something unexpected.