Hypoxia is a condition in which there is a decreased amount or absence of oxygen in the blood. This condition often arises when an organism is placed in an environment in which oxygen levels are much lower than normal. In the case of ocean dwelling creatures oxygen levels are determined by the amount of dissolved oxygen in the water. Most fish are not able to tolerate poorly oxygenated water conditions for any extended period of time. But, a recent study has shown that the coral-dwelling goby, a tropical fish, often endures the harsh conditions of poorly oxygenated water. Amazingly, although in shorter durations, they can also survive periods of complete air exposure.
Other species of fish avoid hypoxia by swimming elsewhere when conditions are not favorable. This is not an option for the goby because, they choose to spend almost their entire lives in a tiny space created by the adjoining of two coral branches. While the coral provides the goby with a habitat safe from predators in which to live and mate in, the water around the coral is often prone to low oxygen levels. Three factors contribute to the low oxygen levels in this setting. The first is the obvious reason of restricted movement of current, not allowing for proper oxygenation.
The second occurs when night falls and phototrophs can no longer perform photosynthesis, stopping their normal production of oxygen as a byproduct. The third factor is less significant but should still be considered; the coral and surrounding organisms continue to go through respiration and consume oxygen for use as the terminal electron acceptor in the electron transport chain. Extremely low spring tides present an even greater problem because they occasionally remove enough water from the reef to leave the coral standing high and dry for up to four hours.
There were several phases that made up the experiment, all of which were conducted at the Lizard Island Research Station on the northern portion of the Great Barrier Reef. The first was the monumental task of acquiring specimens. Performing the experiment on sight gave the researchers a unique opportunity to acquire the test specimens directly from their environment. Gobiodon histrio and Gobiodon histrio erythospilus , two species of goby, were used interchangeably throughout the experiment because there was no detectable variation between the species in the area that was being tested. Samples of the fish's favorite coral, Acropora, were acquired from the reef, often times with the fish themselves still inside. Both the coral and the fish were held in an outdoor tank filled with circulating water strait out of the ocean.
The first experiment was conducted to test the effect coral has on the oxygen levels in and around it. The scientist made their best effort to simulate a still night on the reef. The coral specimens were placed in separate outdoor holding tanks at night and the water remained uncirculated. An oxygen-sensing electrode was placed within the coral to record accurately. This was preformed on several nights with the water being replaced before each night to renew realistic oxygen levels.
The second experiment preformed was to gather information on the respirometry of the fish in oxygen depleted water. Each fish was placed separately into a closed, water filled container, and oxygen levels were steadily decreased and monitored. The ventilation rate of the fish were observed until they began to show signs of disorientation, at which point they were returned to well oxygenated water to recover. Ventilation rate was determined by counting the rise and fall of the fish's operculum, gill slits. The oxygen level at the point the fish became disoriented is known as the critical value. These experiments ranged from six to nine hours in length.
The third experiment was to gain data the reaction of the fish upon extended exposure to air. The fish was places in a container with only .3 mL of water. This water was to keep the animal calm and also an effort to combat mucus production. The oxygen in the air was maintained at normal conditions. Ventilation rate was observed for up to 4.5 hours.
The results of the experiments yielded few surprises. In the first experiment involving coral, oxygen levels fell steadily after nightfall. This was expected, and can be explained by the three previously mentioned factors: lack of water movement, continued use of oxygen for means of respiration, halted photosynthetic activity resulting in a lack of oxygen production. The average minimum oxygen concentration recorded during the night was 20 percent of air saturation. Also as expected, one hour after sunrise the oxygen levels increased to their normal levels as a result of restored photosynthetic activity.
The results of the respiration experiment in water showed that the average critical value of oxygen in the water needed for the goby to maintain equilibrium and healthy body conditions was 18.3 percent. As the level of oxygen fell the ventilation rate steadily increased until the critical point was reached.
The results of the respiration experiment in air showed that the goby was able to maintain its bodily conditions for up to 4.5 hours. In many cases there was very little change ventilation rate up to 3 hours. The scientist did observe the fish circulating the .3mL of water in through its gills and out through its mouth. There was no detectable change in behavior when the fish were returned to properly oxygenated water.
From the data collected the researchers concluded that the coal dwelling goby has a strong tolerance for hypoxia and has adapted methods for surviving while removed from water for an extended period of time. This plays an important role when considered in the big picture. The goby's ability to live in this type of environment allows it to remain in within the protection of the coral, even when conditions are their harshest. The fewer times a goby has to leave the coral the less likely he is to happen upon any harm and therefore there is a greater likelihood that he will reproduce successfully, making hypoxia tolerance an extremely favorable trait to have. This trait may be what sets the goby apart from other coral dwelling fishes as the most successful in the reef.
I fully agree with the scientists' conclusions. The goby is hypoxia tolerant. The average goby can tolerate almost 2 percent less oxygen (18.3%) than the average minimum oxygen concentration in the coral they inhabit(20%). This is obviously an advantageous trait to have, because it increases the reproductive success of the fish that possess it. By means of natural selection the trait is passed down and strengthened throughout the generations.