Edited by Sanaya Narula
Recently, I dissected a mackerel fish in order to take a closer look at the structure and characteristics of gills - the respiratory organ used by fishes in order to absorb dissolved oxygen from water. The fascinating findings explain the mechanism of gas exchange in fish, and why they suffocate when removed from water.
Firstly, I used a scalpel to cut out and remove the operculum (figure 1), a bony structure which covers and protects the gills. This left the buccal cavity¹ and gills of the fish exposed, as seen in figure 2. The gills are located immediately behind and to the side of the fish’s buccal cavity in order to allow water to flow into its mouth and straight over the gills. When a fish opens its mouth, the pressure in its buccal cavity is lower than the external pressure, so water rushes into its buccal cavity. This water then passes over the gills and out through the operculum. This passage of water over the gills is the point where gaseous exchange occurs.
Even without a microscope’s magnification, the stacked piles of gill filaments² are visible, swollen in the fish’s blood. This blood needs to be washed off in order to get a clear view under the microscope later on.
After removing the gills from the fish and washing it, I placed it in water, and then out of water, to demonstrate the effect of removing a fish from water. As seen in figure 4, a gill underwater has its filaments spread out with an incredibly high surface area for the diffusion of oxygen to occur. However, when removed from water as seen in figure 5, the filaments collapse and stick together, with a significantly reduced surface area. This can be equated to our hair - when swimming underwater, our hair spreads apart, however as soon as we emerge from the water, our hair strands stick to our head and to each other.
Therefore, when the fish is removed from water, the reduced surface area of its gill filaments provides an insufficient diffusion of oxygen, and the fish suffocates and dies.
Next, to observe the gill lamellae, I placed the washed gills into a petri dish filled with enough water to submerge the gills. I placed this under a light microscope under the magnifications x40 and x100, and these were the results:
Under the x40 lens, the gill lamellae were visible, at right angles to the filaments. These structures increase the surface area of the gills, so that there is a larger area for gaseous exchange to take place. The flow of water over these structures is in the opposite direction to the flow of blood inside the lamellae, called a countercurrent flow. This countercurrent exchange system means that oxygen-rich blood meets oxygen-rich water, while oxygen-poor blood meets oxygen-poor water. This system ensures that a diffusion gradient³ is maintained across the entirety of the gill lamellae, and that equilibrium isn’t reached, so gas exchange occurs throughout the lamellae. As a result of the countercurrent flow, around 80% of the oxygen available in water is absorbed by blood, as compared to only around 50% with parallel flow⁴.
Finally, under the x100 lens, I could observe the densely-packed lamellae more clearly. Interestingly, the lamellae were quite long, just enough to avoid contact with the lamellae of the next filament.
Overall, this was a fascinating dissection which demonstrated just how well the gills of a fish are adapted for their function of gas exchange. If this topic interests you, I would absolutely recommend trying this at home with any fish from your fridge, as the experiment of placing the fish gills in and out of water is a hands-on demonstration of why fish can’t survive out of water and can be performed at home without any special equipment.
Glossary
Buccal cavity: | Term used for the mouth, through which food and air enter the body. |
Gill filaments: | Protein structures that make up the gills. |
Diffusion gradient: | A gradient in the rates of diffusion of multiple groups of molecules through a medium or substrate. |
Parallel flow: | Blood (in the lamellae) and water flow in the same direction rather than opposite directions. |
References:
Britannica, T. Editors of Encyclopaedia (2020, May 13). mouth. Encyclopedia Britannica. https://www.britannica.com/science/mouth-anatomy
Fish gill. (2022, Feb. 18). https://en.wikipedia.org/wiki/Fish_gill?scrlybrkr=05075565
Diffusion gradient. (2021, Oct. 1). https://en.wikipedia.org/wiki/Diffusion_gradient#:~:text=A%20diffusion%20gradient%20is%20a,temperatures%2C%20or%20other%20differentiable%20groupings
Toole, G., & Toole, S. (2015). AQA Biology: A Level. Oxford University Press.
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