In this experiment factors of dissolved oxygen as well as depth were manipulated to examine their effects on both opercula beats per minute as well as breaths taken by the test specimen Corydoras aeneus per hour. The oxygen concentration was decreased by bubbled nitrogen gas through the water until it became more abundant that the oxygen gas. The initial dissolved oxygen concentration was found to be 5.74 mg/L, where the opercula beats/minute was calculated to be 163 beats/minute.
The dissolved oxygen concentration was then manipulated to concentrations of 4.46mg/L, 3.98mg/L, 2.61mg/L and lastly, 1.70mg/L, whereby the opercula beats showed a steady increase going from 163 beats/minute to 193 beats/minute. The test specimen only took 20 breaths/hour at the dissolved oxygen concentration of 1.70mg/L. The steady increase of opercula beats per minute, suggests that the catfish increases its ventilation to extraction as much oxygen as it can in the decreasing oxygen water. Therefore, as dissolved oxygen decrease, ventilation increases, quicker aquatic respiration extracts more oxygen for the fish’s bodily needs. Notably, at 1.73mg/L the fish was able to take 20 breaths/hour at the water’s surface and this is solely due to the fact that the water has become so deprived of dissolved oxygen that the Corydoras aeneus then utilized aerial respiration. Thus, it’s at 1.73mg/L where the catfish switches from aquatic respiration to aerial respiration.
With respect to increasing depth and its effect on opercula beats/minute, there was a steady increase of opercula beats/minute from 249 beats/minute at 8cm to 273 beats/minute at 25cm. Even though the test specimen was in low oxygenated water, it’s ventilation was quickened, as to extraction as much oxygen as possible. Though, theoretically, it would’ve been more energetically efficient to utilize aerial respiration, in this case there is a trade-off with the energy consumed to actually swim to the surface and utilize aerial respiration versus conserving as much energy as possible and even decreasing ventilation. Essentially, it becomes more energetically efficient for the catfish to remain at the bottom and converse its energy, rather than expending that same energy to swim to the surface to utilize aerial respiration and swim back to the bottom. Additionally, staying at the bottom helps to avoid predators. The graph shows initial increase and a decrease, this pattern continued, and this maybe the catfish’s method of first extracting as much dissolved oxygen, and trying to decrease ventilation to converse energy and then again increasing ventilation to extract as much dissolved oxygen, thus continuing the pattern.
Aerial respiration has quite a few advantages and some disadvantages over aquatic respiration. Physiological advantages of utilizing aerial respiration over aquatic respiration is that air contains at least 30 times more oxygen per unit volume than water, but this varies due the effect of temperature. Additionally, air is less dense than water by 100 times and 30 times less viscous than water. Thus, air as a respiratory medium is physically easier to extract oxygen whilst passing it over a respiratory surface, such as lungs or gills, thus being energetically less cost. Ideally, a fish of 100 g naturally has to move approximately 30 to 65 g of water per minute through its gills; that is 1/3 to 2/3 of its body weight, consequently making it a physically exhausting process (Reebs 2007). Aquatic respiration has disadvantages such that the oxygen content in water is much lower than that of air. Additionally, the rate of diffusion is greatly slower, however, aquatic organisms benefit since their respiratory surfaces are always kept moist and gives them their natural buoyance as their plasma and water density is approximately equal. Fishes that live in commonly hypoxic habitats may have more haemoglobin in their red blood cells, and more of those cells in their blood, and therefore a higher blood capacity to take up and transport oxygen (Reebs 2007). Corydoras aeneus possess several adaptations that make it ideal for aerial respiration even though being an aquatic organism.
This catfish posterior intestine is modified so that it can function as an air breathing organ by being air filled, thin-walled and highly vascularized. This area is the longest region in the gut (Persaud, Ramnarine and Agard 2000). Aerial respiration happens in a rapid dash to the surface, taking in air during the 0.06-0.07s that the mouth is exposed and expiration occurs via the anus while going back down (Kramer and McClure 1984-1991 ).
Fish increase their efficiency in oxygen uptake in several ways but mainly through their gills, which are located one set on either side of the body and near the back of the head they are open to the gullet at the front, and open to the external environment behind. Thus, water can flow constantly through the mouth and passing through the gills. Some gills especially in bony fish have a plate, an operculum protecting and regulating the opening and closing of the gill. This ability to have water continually passing over the gills is one of the major factors making gills more efficient than lungs. With lungs the air comes in, fills the space, and then must be expelled before any more oxygen rich air can be brought in.
Since the gills are open, no energy is spent on expelling old air or reversing direction of flow. Gills have numerous folds that give them very large surface area. The rows of gill filaments have many protrusions called gill lamellae. The folds are kept supported and moist by the water that is repeatedly pumped through the mouth and over the gills. These lamellae have an efficient transport system that maintains the concentration gradient across the lamellae. As water is flowing past the gills in the opposite direction referred to as counter current flow. In counter current flow blood low in oxygen passing through the lamella comes into close contact with water that is fully saturated with oxygen, thus a concentration gradient is formed. The oxygen from the water diffuses into the blood moving along the lamella, this continues until both water and blood are saturated, eliminating the gradient. However, since blood flows in the opposite direction it always flows next to water high in oxygen, thus the blood can absorb as much oxygen as possible. In comparison to mammals that lose their gradients over distances, counter current flow always has a sustained gradient.
