Image Source: The Goldfish Bowl: Canterbury Writers
Gills are to fish what lungs are to mammals. This is where gas exchange between the animal and the environment takes place. As water flows across the gills, oxygen is taken up into the blood of the fish and carbon dioxide is removed. In both systems, oxygen is transported throughout the body by binding to hemoglobin molecules within red blood cells.
Image of goldfish gills from: How to take care of a goldfish: Pet goldfish care guide
Researchers Velislava Tzaneva, Shawn Bailey, and Steve F. Perry from the Department of Biology at the University of Ottawa have been studying how the structure of goldfish gills change with varying environmental conditions. They were specifically interested in the build-up of cells on the lamellae termed the interlamellar cell mass (ILCM). Looking at the diagram above, you can imagine how large ILCMs would limit the surface area available for gas and ion exchange across the gills.
Their findings were recently published in The American Journal of Physiology and show that exposure of fish to phenylhydrazine (PHZ), to reduce red blood cell counts, results in loss of the ILCM. Likewise, exposure of fish to carbon monoxide (CO), to prevent oxygen binding to hemoglobin, reduces the ILCM compared to the gills of untreated fish shown in panel A. Both treatments reduced the oxygen content in the blood of these fish (hypoxia) while the environmental oxygen levels were kept constant. What these findings mean is that even though the fish were able to sense normal environmental oxygen levels, blood levels of oxygen could be impacted by changes in the oxygen carrying capacity of blood. By reducing the ILCM, the fish are attempting to increase their uptake of environmental oxygen. These findings are shown in figure 3 from the paper:
Exposing fish to conditions of high oxygen levels (hyperoxia) has the opposite effect and promotes the build-up of the ILCM. This effect appears to be dependent on temperature since fish housed at 7 degrees Celsius develop more ILCM compared to fish housed at 25 degrees, which is roughly room temperature. Since warm water contains less oxygen than cold water, these findings again show that the size of the ILCM depends on the oxygen content of the water. Having a smaller ILCM provides more surface area for gas exchange and the uptake of more oxygen into the blood when environmental levels are low. In contrast, a larger ILCM reduces gas exchange when environmental levels of oxygen are high.
By injecting the buccal cavity of fish with sodium cyanide, which activates oxygen receptors, they were able to determine that the remodeling of the ILCM is driven at least in part by these chemoreceptors.
The findings in this study shed some light on how fish adapt to changing environmental conditions.
Evans DH, Piermarini PM, Choe KP. The Multifunctional Fish Gill: Dominant Site of Gas Exchange, Osmoregulation, Acid-Base Regulation, and Excretion of Nitrogenous Waste. Physiol Rev. vol. 85:97-177, 2005.
Tzaneva V, Bailey S, Perry SF. The interactive effects of hypoxemia, hyperoxia, and temperature on the gill morphology of goldfish (Carassius auratus). Am J Physiol Regul Integr Comp Physiol 300: R1344 -R1351, 2011.