Hey! What's Going On Over There?
A Look at Relationships Underwater
Story and photos by Tom Ryan
The first thing that caught my eye was a colorful tiger grouper cruising along the reef. The grouper was easily the largest fish around and it struck me as curious that none of the smaller grunts or creole wrasse nearby appeared to feel threatened. I know these fishes are on the preferred diets of tiger groupers. But the grouper did not appear to be in a hunting mode at that moment, and it seemed obvious that the potential prey were able to sense that the grouper posed no immediate threat.
I remember thinking that sometimes dogs and cats lie down next to each other under a shade tree, while at other times the same two animals are mortal enemies. As I was wondering if something similar was going on with the grouper and the reef fishes, a trumpetfish that was hovering just above the grouper's back suddenly darted out and grabbed a creole wrasse. Lunch! In the blink of an eye it was all over but the swallowing, and there was my lesson in natural history. Often, when groupers are on the hunt, the prey senses danger and reacts accordingly. When a potential predator appears relaxed, the prey often senses that fact and relaxes as well. In this case, it was a fatal mistake for one creole wrasse. Trumpetfish commonly accompany a variety of larger fishes and use them as moving "hunting blinds." The theory is that the prey of the larger fish senses the nonhunting mode and relax, giving the trumpetfish the perfect chance to catch its prey off guard. Mother Nature is a clever lady indeed.
As I turned to make my way back toward the boat, I looked up the slope and saw another grouper holding its mouth wide open. Even from a distance I recognized that the grouper was being cleaned, rid of ectoparasites, dead tissue, bacteria and fungi on its skin. Inching closer I could see that the cleaners were a variety of gobies that were scampering all over the grouper's body as they searched for food for themselves.
So often when we think about bigger animals and smaller animals we think of the larger as a predator and the smaller as prey. But different species of animals have a wide variety of fascinating relationships, and that of predator and prey is only one. This will provide an overview of the relationships shared by animals that are different species. Gaining some insight into the nature of various relationships and learning how to recognize them can add a lot of pleasure to your diving as it increases your appreciation of the world of nature.
Predator and Prey
Certainly the dynamics of predator and prey is a major consideration when examining interspecific relationships. Most of us are quick to think about predator and prey because the connection is so obvious, and because it is all over our television sets when shows about animals are being aired. Whether it is a pack of lions attacking a zebra or a great white shark feeding on a sea lion, the link of predator and prey gets our attention. Clearly, this relationship is vitally important, and a basic understanding of how energy is passed up food webs is important to understand the natural world.
No matter how they gain access to food, every animal depends upon other plants and animals for food. We often think of creatures like great white sharks, killer whales and barracudas as predators, and species of smaller schooling fishes like grunts, jacks and snappers only as prey. But it is just as important to realize that these prey animals along with other organisms such as the sponges that filter organic matter out of the water column and the snails that eat algae are predators as well. The paths through which energy is transferred from one organism to the next are called food chains, and there are many links, or levels, in a food chain and many food chains in the larger, more complex food web.
Plants form the foundation of all major food chains. In any given chain there can be several species of animals that are the links between the plants and the top end of the chain, the animals that are often called apex, or top-end, predators. In the marine kingdom, the top-end predators are often animals like big sharks, barracudas, killer whales, sperm whales and groupers. In the oceans, tiny animals called zooplankton feed upon small plants known as phytoplankton. Small fishes such as the numerous minnowlike species that are often lumped together and called "baitfishes" and "silversides" feed on the zooplankton. Jacks prey on the baitfishes, groupers prey on the jacks, and eventually Caribbean reef sharks prey upon some of the groupers.
In this rather straightforward, relatively simple example, each group of animals represents a different trophic level in a food chain. The flow of energy in all food chains is from sunlight toward the level of the top-end predators. The transfer of energy from one trophic level to the next is not very efficient with only 6-20 percent of the energy normally being passed upward in the food chain. In rough translation, it takes 6-20 ounces of small animals to produce 1 ounce of a larger predator. This explains why there are so many more small animals than large ones, and why there are many more baitfish than big sharks.
Marine animals share several relationships besides that of predator and prey. Collectively, these are called "symbiotic relationships," and the organisms involved are the symbionts and the hosts. Their classification depends upon their role in the relationship. Many other factors determine whether the symbiotic relationships are specified as commensalism, mutualism, parasitism or phoresis.
In a commensal relationship, members of different species live together, and one member benefits without seriously affecting the other party. As a rule, the animal that benefits is usually the physically smaller of the two. The animal that benefits is known as the commensal or symbiont, while the nonbenefiting species is known as the host. Commensalism is classically exhibited in the relationship between a variety of host jellies and the symbiont fishes that live among the tentacles. The potent tentacles of the jellies provide safe haven for the small fishes without altering the behavior of the jellies. The same is true in the relationship between the hydroid (a close relative of jellies, but not a jelly) known as the Portuguese man-of-war and the symbiont man-of-war fish that live amongst its tentacles. Several species of shrimps and small fishes use the protective spines of sea urchins in a similar way, providing other examples of commensal relationships.
Mutualism is a type of symbiotic relationship in which members of two different species live together. Both benefit and neither is harmed. The relationship shared by cleaner shrimps and cleaner fishes with the fishes they service provide classic examples of mutualism. The cleaners remove irritating ectoparasites, dead tissue, bacteria and fungi from the skin of their hosts, and in this process the cleaners receive food. A list of cleaner species includes a variety of gobies, wrasses, hogfishes, angelfishes and shrimps, while host fishes include groupers, snappers, jacks, chubs, barracudas and sharks. Turtles, dolphins, manta rays and many more species regularly frequent cleaning stations as well in order to maintain their health.
Cleaning often occurs on a regular basis in areas known as cleaning stations. The cleaning stations are often denoted by a large sponge, coral head or other prominent structure or organism on a reef. It is quite common for the cleaners to advertise their presence by swimming up into the water column in ways that are obviously intended to make them seen. Cleaner shrimps "pop up and down" in the water and wave their antennae to make their presence known. These displays essentially say "we are here and ready to go to work!" Often the hosts communicate their request for cleaning services. The display of the hosts varies from fishes that approach cleaning stations and open their mouths as wide as possible to make their desires known, to fishes that swim horizontally through the water column hanging vertically with their head up or down. Some requesting hosts darken their skin to make more lightly hued parasites stand out so they are easier for the cleaners to see. When these host species have "had enough," they often lighten, or blanch, their skin to communicate that message.
Often, when we think about big fishes and little fishes, we conclude that the smaller fishes represent a potential meal for the larger fishes. But the hosts somehow seem to instinctively understand the long-term benefits of the cleanings they receive, and they do not eat the cleaners.
In parasitic relationships, one party, known as the parasite or symbiont, depends on the other, the host, for food that is taken from the body or eggs of the host, or the parasite uses the host for living space. A variety of small crustaceans often parasitize the fishes that visit cleaning stations. Several relationships that first appear to be obvious examples of parasitism are not. For many years, the small anemonelike creatures known as zoanthids that live on a variety of sponges and soft corals were thought to feed on their hosts. While it is true that the zoanthids take up some of the surface area of the hosts, it is believed that the noxious taste of the zoanthids repels fishes that might otherwise prey upon the sponges. In this relationship, the presence of the zoanthids clearly benefits the host and is an example of mutualism, not parasitism.
It is not uncommon to see small gray-to-black fishes attached to the bodies of manta rays, sharks, turtles, groupers, barracudas, parrotfishes and other animals. Upon first glance, many people mistakenly assume that the smaller fishes are parasites of some kind. But they are not. These fishes are commonly known as remoras and sharksuckers. The relationship they share with their hosts is described as phoresis, a type of symbiotic relationship in which a host animal provides transportation for the symbiont.
It can be argued that the presence of a remora alters the swimming efficiency of the host so the host is affected. While this is true, the effect is thought to be so insignificant that it is all but irrelevant and the use of the term phoresis accurately describes the nature of the relationship.
Next time you go diving, see if you can recognize any examples of these symbiotic relationships. You might not encounter any, but depending on where and when you dive, you might see several of these relationships on display. Learning to recognize the various relationships and gaining insight into how and where to find them will not only be educational, but it is certain to add a lot of fun to your diving.
Finding A Cleaning Station
Whenever I see a big fish hovering close to a reef, I know there is a chance the fish is being cleaned. If I see a group of fish hovering over a prominent physical feature such as a sponge or coral head, I look for cleaning activity. Encountering a fish that normally swims horizontally hovering with its head up or down is often a sign that the fish is being cleaned or is requesting cleaning services. In addition, several species of cleaner shrimps such as the Caribbean species, the Pederson's Cleaner Shrimp, often associate with anemones such as the corkscrew anemone. So, when I discover a corkscrew anemone, I look for cleaners.
Cleaning commonly takes place at the same place for days on end. So, if you find cleaning in a given place, the odds are good that you will find cleaning there again the following day, and often on a follow-up dive trip.
If you want to find a cleaning station, but are having a little trouble finding one, do yourself a favor and ask a divemaster or old salt to give you a hand. Once you see one, you will find it much easier to find more. And watching all the cleaning activity will surely add a lot of pleasure to your diving.
How Important Are Cleaning Stations?
For many years scientists debated the importance of cleaning stations, but studies in recent years have demonstrated that cleaning plays a vital role in the health of many reef ecosystems. To determine the value of cleaning, scientists removed every cleaner species from given reefs in several Indo-Pacific study areas. In many cases, within only a matter of days, many reef fishes became infested with parasites, and in some cases the infested species were quick to become seriously ill because of the infestation. When the cleaners were reintroduced, they had to work "overtime," but they quickly restored a natural balance as well as restoring the health of many host fishes.