CRITTER: Light plays tricks on deep water red fish

Most fishermen here in Hawai‘i have caught red fish like the aweoweo, mempachi and kumu that live in caves near shore, or deep water fish like the onaga and ehu. Have you ever wondered why all of these fish are bright red?

Scuba divers understand that white light does not penetrate seawater very well. When sunshine hits the sea surface, the white light breaks into different wavelengths much like white light produces a rainbow when it shines through the rain.

Each wavelength of light penetrates the sea surface down to different depths. The green and blue wavelengths go down the deepest, and the red and yellow wavelengths only penetrate the sea surf down to about 60 feet. When you scuba dive at 80 feet deep, all of the fish look black, green or blue because the red wavelength is being filtered out by the sea water.

The first time I went scuba diving at Tunnels reef in Kaua‘i and went into the dark lava tubes, I could see a bunch of fish and they all looked black, but when I turned on my bright dive lights the fish were bright red.

I wondered why almost all of the cave fish were red when the sunshine does not get back into the caves, so it seemed logical that fish would be black so they would better blend into the shadows in the dark cave.

While doing DNA studies on cold blooded animals like fish and reptiles, we discovered that the red color is controlled by a single gene where as the black color is controlled by several genes. A cave or deep water fish wants to be black to better blend into their surroundings and developing a red color is genetically easier then developing a black color. The fish that live in the caves and in water deeper then 60 feet are not in the zone where the red wavelength reaches, so their red color is actually black!

The deep water and cave fish don’t know that they are red, as where they live they are actually black because the red wavelength does not exist. Nature developed a way for the deep water and cave fish to look black by making their color bright red.

•••

Terry Lilley is a marine biologist living in Hanalei Kaua‘i and co-founder of Reef Guardians Hawai‘i, a nonprofit on a mission to provide education and resources to protect the coral reef. To donate to Reef Guardians Hawaii go to www.reefguardianshawaii.org.