Blood may be thicker than water but it isn’t always red

By LJ Evans

Kristin O’Brien grew up fishing with her dad near their home in Saratoga Springs, N.Y. Even as a 10-year-old kid, her interest was a harbinger of the future.

“I liked cutting out the guts better than I liked fishing,” she says with a chuckle.

GillsO’Brien still examines fish innards, but in a completely different setting. In laboratories at UAF and in Antarctica she studies the peculiar icefish — family Channichthyidae, suborder Notothenioidei — the only vertebrates in the world whose blood is milky white.

“Icefish are a wondrous physiological phenomenon,” O’Brien says. Biologists have been fascinated with icefish since early British whalers discovered them in the Antarctic Ocean in the late 1800s; Norwegian scientist Johan Ruud first described them in the scientific literature more than 50 years ago.

Icefish occupy the coldest marine environment on earth, constantly near the freezing temperature of seawater. Icefish and their relatives are by far the most plentiful fish in the waters surrounding Antarctica.

In most animals, hemoglobin in the blood transports oxygen from the lungs (or in fish, from the gills) to tissues in the body. Muscles and organs need oxygen to convert energy stored in food into the chemical energy they can use. Not having hemoglobin means that the total oxygen-carrying capacity of icefish blood is only about 10 percent that of red-blooded fishes. Icefish adapted by developing special physiological processes to survive without hemoglobin.

Icefish print
A gyotaku, or Japanese fish print, made from a species of icefish, Chaenocephalus aceratus, by Kristin O’Brien.

It turns out that living in icy cold water helps make that possible. The amount of oxygen that can dissolve in water goes up as the temperature goes down. The waters of the Antarctic Ocean, which hover around freezing year-round, contain considerably more dissolved oxygen than seawater in warmer climes. This means there is more oxygen available for icefish gills to extract and transfer into their blood. It also means their blood plasma — composed mostly of water — is able to transport more oxygen. In addition, icefish have adapted to the lack of hemoglobin by having larger hearts, more blood and bigger blood vessels than similar, red-blooded species. In organs that need a lot of oxygen, such as the retina of the eye, they have more blood vessels per square inch.

Another peculiarity of icefish is that some species have no myoglobin in their heart muscle. Myoglobin, a protein with some similarities to hemoglobin, binds and stores oxygen within muscle tissue. Myoglobin also contains iron and is the molecule responsible for making muscles red.

A juvenile icefish.
A juvenile icefish. Photo by Uwe Kils.

Hemoglobin and myoglobin both bind an atom of iron at the core of the protein. Iron is a highly reactive element that, if too abundant and running around loose, promotes formation of free radicals. In living organisms free radicals can be associated with cell destruction, as is seen in Alzheimer’s and Parkinson’s disease. O’Brien and her co-principal investigator, Lisa Crockett of Ohio University, are comparing icefish with fish that are closely related but have red blood.

“We are building knowledge concerning a group of animals that have evolved in an extreme environment,” Lisa Crockett said in a phone interview. “We want to know if there is an advantage in not having hemoglobin as far as being a protective mechanism against oxidative stress [the effects of free radicals].”

How do icefish get away with these radical biochemical adaptations? O’Brien and her colleagues will continue to study icefish for information that might shed more light on how these systems work in other animals.

It may be a long shot, but basic research has led to major discoveries in medicine in the past, Crockett points out.

“They weren’t looking for an antibiotic when they discovered penicillin,” she says.