Color Wars: Reef Fish and Aggression

Author: Valerio Zupo

Ever wonder why marine fish are colored so brightly? The author learns about the evolution of color in reef fish while interviewing the great ethologist Konrad Lorenz.

The Beauty of Reef Fishes

Anyone who sees the fantastic color patterns of reef fishes for the first time feels surprise and disbelief, and questions fill the mind. How did all those bright patterns come about? They almost look as if they were drawn by an artist. What evolutionary purpose do these incredible colors have? How has the gaudy dress of reef fishes aided the survival of their species?


Several years ago I pondered these questions in front of my tropical marine aquarium, and no explanation seemed adequate. I decided that I would go to Altenberg, Austria and ask Konrad Lorenz, the father of ethology (the scientific study of behavior). I informed the publisher of the Italian aquarium magazine I worked for about my intention.

I was very young, and my publisher was more practical. He told me it would never work—Konrad Lorenz would not be available to talk to me. But he was also a pragmatist, I guess. He said, “Just in case you get into his house, bring with you a copy of our magazine, and take a photo of his desk with our publication. That would be really a scoop!”

I am sure this was just in jest—he couldn’t believe that I would get to meet the legendary, Nobel Prize–winning Austrian ethologist, let alone discuss my passion for aquarium fish with him. Undeterred, however, I jumped on a train to Vienna, Austria.


I met a good friend when I arrived in Vienna. She said, “You have in mind an impossible mission…but why not go ahead and try?” Early the next morning I took a local train to Altenberg, and in no time I was walking through Lorenz territory—the countryside I had read about in the various books written by my favorite Nobel laureate. This in itself was such a fantastic experience that it made the trip worthwhile.

As I got closer to Lorenz Gasse (Lorenz Lane), my thoughts ran wild. I must be crazy…of course I’d love to meet the author of the books I loved so much…but certainly he had changed…an aged Nobel laureate, he was probably transformed into a businessman with interests in politics and aquaculture, like most professors I knew…

Searching for the Author

Before I knew it, I was at his house! I rang the bell in the garden, and Gretl, his wife, came and opened the gate. I couldn’t believe it! Here she was, in person, the person who secretly sent Konrad’s first book to a publisher.

Gretl was very gracious. When I disclosed my intentions she welcomed me into the garden, showing me her husband’s stone chair and her beautiful flowers. But she was also very resolute—it was impossible for me to meet Konrad. He was too busy and not available to be interviewed.

I insisted, but she continued to show her pets and flowers. I then made a bold decision. While we were touring the garden, I slipped away as stealthily as I could and entered the house through an open door. Following my instinct I turned left and found myself in the living room. Fantastic! I recognized every object from the descriptions of the books: the simple desk in front of the window, the white chair, the natural aquarium without any filter, the fountain on the floor made exclusively for the dogs. I must be dreaming!


Then a door opened to my left, and an old man entered, eyeing me from the other side of the couch. He had penetrating blue eyes and a long white beard. It was him! He was obviously quite surprised to find me in his house but asked quietly, “Who are you and what are you doing here?”

As politely as I could I answered, “I am an Italian student of biology and I am also a young journalist. I would like to speak with you about aquariums and, if at all possible, ask you about the colors of tropical fish!”

A Wonderful Chat

I wound up having a very long, interesting, and instructive talk with Konrad the Great, the finest person I have met. I discovered that he was just like a child, as any passionate person should be. When I showed him my magazine I didn’t even get to lay it on his desk—he was so eager to read it that he took it from my hands. And he was able to read Italian!

Thus, I returned home with several photos of Lorenz walking in his garden with the magazine under his arm and sitting in his chair while reading it. I will never be able to describe the feeling of calm power he was able to transfer with his presence, nor the clever depth of his eyes, never revealed by any photo seen in books or magazines.

Back to Vienna

When I arrived back in Vienna that night, I stepped down from the train, and my friend, who was waiting for me, screamed “No, I can’t believe it, you did it! You actually got to meet him!”

I asked how she could tell, and she replied, “It’s obvious—you’re walking a meter above the ground!”

Learning to Observe

My meeting taught me how to observe my aquarium fish. The basic technique is to watch with an unbiased mind. If you look at a group of marine tropical fish in a large aquarium or on the reef you can immediately detect some clear relationships, and the colors of reef fishes begin to make sense. Lorenz called the brilliant colors “advertising billboards.” If you observe these “advertisements,” you will notice:

• The most colorful fish are generally linked to a given location.

• The more closely they attach themselves to a particular site, the more they show striking colors.

• The more they are colored, the more they are aggressive against conspecifics.

• It is very unlikely you will observe two individuals with the same colors in the same space on the reef.

These points are interesting and important. In fact, when the colors of a territorial species are compared with those of species that roam over the reef, the difference is noticed immediately. The latter are less colorful (often gray or silvery) and live in schools.

Observing Up Close

There are colorful freshwater fishes, some with gorgeous metallic hues. These colors, however, can only be appreciated up close. They are used in displays between two fish, either aggressive or mating displays. Brightly colored reef fishes, however, have coloration and patterning that can be clearly seen from a considerable distance. Why?

We can observe that the aggressiveness of reef fishes is mainly directed toward conspecifics; they are not bothered by other species, but they immediately attack any fish with their same uniform. Their colors are clear-cut, easy to be recognized from afar, and constant.

In fact, unlike other fish, they do not fade even during the night or when they are losing a fight. After losing a battle, a colorful freshwater fish will lose its color, turning either pale or dark. This coloration signals “I give up! Don’t chase me, I’m leaving!” (Of course, in a small aquarium the loser may have nowhere to go, so it will continue to be attacked, but in the wild it would be allowed to leave the victor’s territory and retreat.)

In the case of garishly colored marine tropical fishes, apparently most of them do not have the ability to change their coloration, and in the wild the reef is vast, so escape from a fight is always easy. These fish are notorious for fighting to the death in captivity—their uniforms always elicit aggressive behavior.

In nature, of course, this behavior serves an important function: It prevents conspecifics from entering a fish’s territory, where they would compete for the same resources. On the bustling reef it would be impossible to banish all fishes of any species from a territory, so each species specializes in battling fish with similar uniforms. In your aquarium you will note that fish with similar coloration are also considered enemies.

Why the Reef?

Why is this aggression so much more prevalent on coral reefs than in other environments? Reefs are very peculiar and fertile habitats. A large and diversified community occupies each square centimeter on the reef. There are resources for a huge number of organisms, but a certain minimum space is needed by each in order to exploit a trophic (feeding) and physical niche. In this complex environment an animal typically chooses one of two different trophic strategies: feed on any soft, palatable organism that is neither poisonous nor spiny, or specialize for feeding on peculiar prey.

The first strategy is simple—typical of larger and less colored predators—but it involves a struggle with many competitors. The second choice is more challenging, but it leads to a larger availability of resources in the absence of competitors. For example, parrotfish evolved a very hard beak and are able to crush coral rock to get at nutrition on and inside it. They digest this and excrete pure coral sand!


Some butterflyfish have a pointed snout and work as cleptoparasites of various sessile cnidarians. When they identify an animal caught in an invertebrate’s stinging tentacles, they wave their pectoral fins to produce a stream of water to flatten the tentacles of the polyps and then quickly grab the meal for themselves.

In the aquarium they prefer a prey item taken from the tentacles of a coral over the same item caught when it is in the water column. This seems to still produce a slight discomfort to the fish, highlighted by its "sneeze" after every meal, but according to Lorenz, this could be a pleasant stimulant, like a pinch of snuff in the nose!


Puffers and other fish in the family Tetraodontidae have specialized instead to prey on mollusks, crustaceans, and other invertebrates fitted with solid shells or exoskeletons. The fish split them effortlessly using reinforced jaws and large teeth.

Pomacanthus semicirculatus

Pomacanthus semicirculatus instead specialize in catching the crowns of polychaetes visible in the coral rock. Normally these worms retreat quickly into the rock when a fish appears, but this angelfish approaches stealthily and then moves in one quick instance to snatch the worm. Even in your aquarium you will notice that when this fish feeds, it quickly grabs from the side; they seem unable to feed in any other way, even on very slow or dead prey.

Other Fish

Other fish have specialized to feed on parasites from the body of other fishes, including large predators, or have adapted to life among the tentacles of an anemone.

Since there is such a large number and diversity of fish, the minimum space requirements are respected thanks to the reciprocal repulsion of several individuals. Such an invisible network of relationships can be imagined: An individual is present at each knot, repelling each other in the surrounding space.

The reciprocal repulsion shapes the spacing out of the community, guaranteeing the availability of similar territories for each individual of a species. This is the most important function that intra-specific aggression plays for the conservation of species: ensuring an equitable distribution of resources.

Seen this way, it can be understood that the colors are simple signals allowing for the ordinate spacing of each individual on the reef to ensure an equal distribution of resources. This also means, however, that the extreme aggression in these fish is directed only against conspecifics, not against individuals of other species. Is this rational?


Many people think of aggression in terms of large predators—lions against gazelles, for instance—but that is not aggression. In fact, prey animals rarely arouse aggression in predators. The lion has nothing against the gazelle; it is just hungry!

I do not feel aggressive toward the slice of meat in my refrigerator, but sometimes I think about it with a bit of appetite. In contrast, I feel aggressive toward the man (my conspecific) who parked his car in front of my garage (competition for the same space). This makes me really aggressive!

Learn By Seeing

Aquarists can learn a great deal about fishes simply by observing them carefully. My meeting with Konrad Lorenz and study of his writings helped me learn the basic rules of proper observation. I highly recommend reading the classics of ethology—Lorenz’s On Aggression (Harcourt, Brace and World, 1966) is a very good place to start.

Sometimes we forget that an aquarium is more than the sum of equipment and livestock—it is also a powerful tool that can be used to understand the life around us and ourselves. Perhaps, one day each of us will proudly paraphrase the words of Lorenz: If I were to put on one plate of a balance what I learned from my aquarium and on the second what I learned from books, oh, how light would be the second!

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