Feeling the heat
Scientists capture icefish for series of experiments on thermal tolerance
Posted July 31, 2009
The fish that swim in the Southern Ocean around Antarctica have evolved over millions of years to survive and thrive in salty ocean water that hovers around minus 1.8 degrees centigrade at its coldest.
These notothenioids have a variety of unusual characteristics. All lack a swim bladder, dwelling almost exclusively on the seafloor, though some possess different strategies for changing buoyancy. Many have antifreeze proteins that help them survive the frigid waters. And one oddball family of the eight that make up Notothenioidei lack hemoglobin, the protein that carries oxygen to the body’s cells.
This last family, called Channichthyidae, and its possible vulnerabilities to withstand changes in temperature compared to its red-blooded cousins, drew a group of scientists to the Antarctic Peninsula during this past austral autumn.
The team, led by principal investigators Kristin O’Brien from the University of Alaska , and Bruce Sidell from the University of Maine , spent more than a month away from home. The scientists and their students fished for various species of the white- and red-blooded fish aboard the ARSV Laurence M. Gould and conducted experiments on their specimens at Palmer Station , a research facility of the U.S. Antarctic Program .
One of the main goals of the project is to determine whether or not there are “thermal tolerance differences” between the white-blooded icefish — some of which also lack myoglobin, another oxygen-carrying protein — and other notothenioids, according to O’Brien. In other words, are icefishes more sensitive to elevations in temperature compared to red-blooded fishes?
A follow-on question, she said, is if there are indeed significant differences in heat tolerance between the two types of Antarctic fish, are they related to the adequate delivery of oxygen? Or some other mechanism? Or both?
The cold waters of the Southern Ocean are rich in oxygen, a boon for a fish lacking hemoglobin. But an increase in temperature would affect the oxygen content of the water, and perhaps the icefish’s ability to supply oxygen to tissues. So the question of heat tolerance becomes an important one as ocean temperatures begin to rise from climate change.
The so-called “bloodless” icefish — a species already living on the edge — could be one of the first affected by global warming in the Southern Ocean.
Sidell his graduate student Jody Beers tested the heat tolerance differences by putting both white- and red-blooded Antarctic fish into a specially designed tank and slowly increasing the water temperature.
At a certain temperature threshold — well above what would happen in nature — the icefish experience what Sidell called a “loss of righting reflex,” a neural failure in its ability to right itself in the tank. The bloodless icefish reach that threshold far sooner than their hemoglobin-packing cousins, he said.
“Those animals that contain hemoglobin and red cells are able to withstand a higher temperature than the white-blooded icefishes, which is pretty much what we anticipated to begin with based on anecdotal evidence,” Sidell said.
“The icefishes are probably going to be more susceptible to a longer term elevation in temperature than the red-blooded animals,” he said, referring to projections of global warming.
The question then becomes: What is the underlying mechanism for this significant difference in thermal tolerance?
O’Brien thought part of the answer might involve the marked differences between the mitochondrial structures of the icefish and other notothenioids. Mitochondria are specialized units within a cell, surrounded by a membrane, that serve as a sort of cellular power plant that uses oxygen to generate most of the cell’s supply of adenosine triphosphate (ATP), which it uses as a source of chemical energy.
In her experiments at Palmer, O’Brien and her student Irina Müller isolated mitochondria from heart tissue of both types of fish to determine how efficiently each uses oxygen to produce ATP.
“It seems that the differences in structure don’t significantly impact function,” O’Brien reported. “Even though they have strikingly different morphologies, their ability to produce ATP and their efficiency are fairly similar.”
The results of their Palmer-based studies seem to point back toward oxygen uptake as the most likely weak link in the icefish’s vulnerability to temperature change.
While the scientists only experimented with three red-blooded species that they could catch around Palmer Station, the early results indicate that the amount of blood volume occupied by red blood cells (called hematocrit) correlates with an ability to withstand changes in temperature.
“The higher the red blood cell content, the better ability to withstand elevation in temperature,” Sidell explained. He said it suggests that the reservoir of oxygen-binding hemoglobin in the animals may be related to what kind of temperature they can survive.
This year’s field team for the project also included scientist Elizabeth Crockett and her graduate student, Jeffrey Grim, from Ohio University. The project is scheduled for another expedition to the peninsula in 2011. Sidell said future experiments at that time would include elevating oxygen content in tanks while raising temperatures.
If the icefish are able to withstand higher temperatures like their red-blooded cousins thanks to the enriched O2 water, then the researchers can say with more confidence that higher water temperatures affect the fish’s ability to supply enough oxygen to tissues.
While the research of O’Brien, Sidell and their colleagues focuses solely on the physiology of icefish and its thermal tolerances, their work and that of other polar scientists, also have implications for medical science. [See previous article: Bloodless icefish.]
For example, the icefish exhibit an elaborate, extensive vascular system. Its prolific ability to create new blood vessels is of interest to biomedical research, Sidell said. If those scientists can figure out what controls the growth and proliferation of blood vessels, they may also be able to figure out how to flip the switch off when it comes to cancer tumors, which create their own vascular network to supply blood for their growth.
“Personally, that’s not what motivates me when I do this work. I just want to figure out how these critters work,” Sidell said, referring to the icefish. “These are pretty unique animals, and we get a chance to ask some questions with these that you can’t ask with other animals on the planet.”