Why Are Saltwater Fish More Colorful Than Freshwater?

Saltwater fish come in a dazzling array of colors, from the orange and black of a clownfish to the brilliant cobalt of a tang. Many marine species are more colorful than their freshwater counterparts, and scientists continue to research the reasons for this. Scientist Alfred Russell Wallace, a contemporary of Charles Darwin, first theorized that saltwater fish take on the colors of their surroundings to blend in and hide from predators. Because bodies of saltwater are larger and more colorful than bodies of freshwater, marine fish sport a wider and more brilliant variety of hues, he believed.

  1. Theories/Speculation

    • Wallace's theory is only a starting point for modern research, and much about fish coloration remains a mystery to scientists, says Gil Rosenthal, assistant professor of biology at Texas A&M University. Wallace's theory that bright colors help saltwater fish hide in their colorful environment of sponges, coral and anemone was put forth in the 19th century. A second theory came from biologist Konrad Lorenz in the 1950s. Lorenz believed that fish coloration helps them identify their own species in crowded undersea habitats, keeping them safe. Scientists who study marine and freshwater life are expanding on these two theories.

    Coral Reef

    • The coral reef is home to 25 percent of marine life, including the most brightly colored species. Because the ancient reefs are deeper, older and more complex than freshwater environments, life there has had more time to evolve into the colorful form we see today. Rosenthal has long studied the diverse fish in the reef. The key to understanding their coloration, he says, is not in how humans perceive the colors of sea life, but in how fish perceive them. Fish see things quite differently from humans, including the colors of other fish around them. Fish, for example, have more light-sensing retinal cells than humans and can see ultraviolet light, which is invisible to us. Rosenthal's research focuses on the role fish vision plays in color as an adaptation.

    Camouflage

    • Rosenthal is finding support for Wallace's theory that fish coloration functions as camouflage. For example, midwater or pelagic fish have silver or white undersides that mimic the sky and sea above, concealing them from predators below. Flank markings can make a fish look like the plants and rocks with which it resides. Bottom dwellers resemble the dull substrate of the ocean floor. Conversely, coloration can also be a means of attracting a mate or scaring off a predator. Some fish sport fake eye-spots to make them look more imposing and watchful than they are. Some species, such as the damselfish, fade with stress or when they hide from predators.

    Chromatophores

    • Modern bony fish do not get their coloration from their scales, which are transparent. Instead, their coloration comes from cells in their skin called chromatophores. Chromatophores have red, orange, yellow or black pigments that contract and expand to produce a wide variety of colors and configurations. Reef fish have chromatophores called iridophores, which produce blues and iridescent colors. Freshwater fish have leucophores, which produce paler skin. In addition to chromatophores, fish have a layer of guanin, a byproduct of metabolism, in the skin. Light reflecting off guanin can make a fish appear silver, chalky white or even iridescent.

    Aquarists

    • Fish coloration has always fascinated aquarists. The first fishkeepers in China kept unusually colored carp specimens and selectively bred them in tanks in the early 16th century. Their color was gradually enhanced, and by 1900, these specimens reached America as goldfish. Though freshwater aquariums were common in Victorian homes, the first saltwater aquariums were not kept until the 1960s. That's when the development of silicone sealants and modern heating and filtration systems made it possible for aquarists to recreate colorful saltwater habitats, such as the coral reef. Most of the popular saltwater fish used in aquariums today come from the vivid reef environment.