This bright-red, foot-long reef fish is one of the most common fish in Hawai‘i, but most snorkelers never see them.
They live in large schools, but under or inside of the lava reef in deep caves.
It is very common as a scuba diver to peek into a cave the size of a small bedroom and see 50 of these fish zooming all around the cave and into cracks in the reef that connect to other caves.
A Hawaiian lava reef looks much like Swiss cheese. When the lava from a volcano cools, large gas bubbles form lava tubes, and the ‘u‘u live in almost every lava tube in Hawai‘i from 10 feet deep to over 100 feet deep.
Many fishermen call this fish menpachi. This is actually an old Japanese name. Here in Hawai‘i and in Japan the big-scale soldier fish is an important and abundant food source.
They have large eyes for seeing in the dark caves, but they are not to afraid of divers, and often will come out of the caves to check out a diver and get speared for dinner.
Why are the ‘u‘u bright red? Most of the Hawaiian cave-dwelling fish like the aweoweo are red. But, actually, in the cave they are black.
These fish want to be black so they can hide better from predators, but the black color pattern for fish is controlled by a wide set of genes and it is complicated for a fish to evolve into a black color pattern.
But the red color in fish is just controlled by a single gene. The red wave length from the sun does not penetrate sea water very far and also does not get into the caves, so a red fish at 60-feet deep or in a cave is actually black.
We only see the red ‘u‘u when we catch them and bring them into the sunshine or we turn on our bright lights underwater to take their picture.
We are losing some of our ‘u‘u population due to an invasive fish called a peacock grouper or roi. This large predator fish was introduced from Tahiti into Hawai‘i in the 1950s by the state government. This turned out to be a very big mistake because the ‘u‘u do not recognize the roi as a predator fish, as they did not evolve with them.
The peacock grouper just swims into the deep caves and gobbles up all of the ‘u‘u without too much effort. The roi now on Hawaiian reefs have decimated about 50% of the native ‘u‘u populations.
In their native Tahiti the roi cannot catch the ‘u‘u so easily. When the roi goes into the caves the ‘u‘u come out of the caves very quickly and zoom away, avoiding being dinner for the slower-moving roi. In Tahiti the ‘u‘u know that the roi will eat them because they all evolved together, but this is not the case in Hawai‘i. That is why taking one animal species from one part of the world and letting it go in another part of the world usually upsets the balance of nature in a negative way.
You can see the ‘u‘u and roi in action in the underwater educational video “The World’s Guide To Hawaiian Reef Fish” at underwater2web.com. The hope is to soon have all educational marine-life videos posted soon for a major online marine science educational program for schools to teach at home to their students.
Aloha from under the surf.
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Terry Lilley, a marine biologist, lives in Hanalei.
His websites include underwater-2web.com and www.gofundme.com/5urrm4zw.
