We are delighted to speak with Dr. Christian Damsgaard who is currently an Assistant Professor at the Aarhus Institute of Advanced Studies & Section for Zoophysiology, Aarhus University, Denmark. Dr. Damsgaard is a member of the Comparative and Evolutionary Physiology section of the American Physiological Society and he presented his research “Active Blood Acidification Greatly Enhances Oxygen Supply to the Teleost Retina” at the 2021 Experimental Biology conference today.
What initially interested you in studying ray-finned fishes such as teleosts?
It has been known for half a century that ray-finned fishes can generate high oxygen pressures within their eyes. These oxygen partial pressures can exceed one atmosphere, and therefore, this mechanism has been termed oxygen secretion. This unique ability allows these animals to maintain a steep oxygen diffusion gradient through the retina, which lacks the typical network of oxygen-supplying capillaries. However, the physiological mechanism that can generate these high oxygen pressures in the fish eye has remained highly elusive. This is what we sought to identify.
I have never heard of active blood acidification. Can you explain what that means?
Fishes’ oxygen-binding protein in the blood, hemoglobin, has special mutations that render it much more sensitive to changes in pH. Here, acidification of the blood releases a large part of hemoglobin’s oxygen into the surrounding tissues. We were interested in identifying the biochemical mechanisms that support acidification of the blood within the eye. To do this, we examined the proteins expressed in the eye’s blood supply and identified a set of enzymes that use metabolic energy to pump protons into the blood plasma and into the red blood cell, where hemoglobin resides. These findings suggest that the fish eye acts as a gas-gland that actively acidifies the blood to boost the oxygen supply to the retina.
How does this help with visual processing and detecting prey?
We too were interested in determining how this superior mode of oxygen supply affects fishes’ vision. To do this, we specifically inhibited the enzymes involved in blood acidification while measuring the retina’s electrophysiological responses to light stimulation in anesthetized rainbow trout. Here we showed that blocking the oxygen secretion mechanism impaired the specific area within the retina involved in the signal processing of light impulses. We also compared the retinal morphology in 32 fish species that either possess or lack oxygen secretion. This analysis showed us that the signal processing areas were particularly enlarged in species with oxygen secretion. These findings strongly suggest that the evolution of the specialized vasculature in the fish eye conferred highly improved visual capacities. This mechanism may have supported improved prey detection, novel feeding strategies, and the invasion of new environments, which in turn may have fueled the adaptive radiation of the ray-finned fishes.
Are there other animals that use a similar mechanism too?
The oxygen secretion mechanism appears to have evolved only once in a common ancestor of the teleost fishes more than 200 million years ago. Therefore, oxygen secretion is only found among members of the teleost clade, and it has not been identified in any other vertebrate group. However, in addition to generating high oxygen pressures in the eye, the O2 secretion mechanism also works in expanding the teleost swim bladder to remain buoyant at depth.
What are you studying right now?
I currently study oxygen diffusion in the eyes of birds. Birds have the highest visual acuity in the animal kingdom, which is supported by a photoreceptor-packed retina. The bird’s retina also lacks internal capillaries, so oxygen must diffuse 50 times longer than in the human brain. I am therefore studying the physiological and biochemical adaptations that support improved oxygen supply in the bird retina.
For more information:
Here is the link to the final publication: https://doi.org/10.7554/eLife.58995