Meet ‘u‘u the Hawaiian bigscale soldierfish! This common, foot-long fish also has another name, mempachi. This name came from the Japanese who came to Hawaii over 150 years ago, and both the Hawaiians and Japanese consider this schooling fish a delicacy.
But why is ‘u‘u red? When fishermen catch this fish and bring it to the surface it is bright red, but underwater in the caves where it lives, the fish is actually black! Hawaii reefs are laced with many lava tubes that are created by gas in the molten lava when it hardens.
Underwater these lava tubes wind through the reef, making it look like Swiss cheese! Large schools of ‘u‘u flow through these dark caves and they never see the light of day!
The red wavelength from the sun does not penetrate saltwater deeper then about 40 feet, and also does not reflect into the caves. So the soldierfish is actually pure black in its normal cave habitat!
This way it is hard for predators to see it. The red pigment in fish is controlled by a single gene, where as black pigment is controlled by several genes. So it is easier genetically for a fish to be red then black, and in the caves where the sun does not shine ‘u‘u is pure black!
These fish often form in large schools of 50 or more individuals, and they are just stunning to see in a black cave when you turn on the bright dive camera lights!
They glow bright red in the black cave and they dart in every direction as they disappear back under the coral reef. On many of my dives when I venture into the underwater lava tubes I see hundreds of these red fish also mixed with other red cave fish like the aweoweo.
So when you are out fishing off of a boat and catch a deep water or cave fish and it is bright red just remember that the fish has never seen its own red color, and to its neighbors it has always been black!
You can see both ‘u‘u and aweoweo in their stunning colors in the movie “The World’s Guide to Hawaiian Reef Fish” on the underwater educational web page www.underwater2web.com. Children can learn all about these amazing creatures in the nonprofit Marine Science Kids Camp at www.reefguardianshawaii.org.
Terry Lilley, marine biologist, is a Hanalei resident. His websites include underwater2web.com and www.gofundme.com/5urrm4zw.
Source: The Garden Island