Jodie is based at the ARC Centre of Excellence for Coral Reef Research, in Townsville. In addition to being a volunteer for Danielle Dixson (see previous blog, Goby Gardening), Jodie and Danielle are collaborating on a project linking fish ecology and physiology.
From previous work undertaken with the gobies Gobiodon histrio and Gobiodon erythrospilus, Jodie knows that when G. histrio consumes the toxic alga Turtle Grass (Chlorodesmis fastigiata), the goby itself becomes more toxic. What she doesn’t yet know is if there is a metabolic cost to the goby involved in digesting the alga and becoming more toxic.
The first step is to find the gobies and catch them. For this, Jodie and Danielle don SCUBA gear and carry hand nets, plastic bags and bottles of a clove oil/ethanol mix underwater with them.
Although the gobies hide within their coral homes, a spray of the clove oil mixture slows their movements enough for Jodie and Danielle to ‘sweep’ them out, by hand movements in the water. Once caught in a net, the fish are transferred carefully to a plastic bag and thence to the boat and on to the lab.
Gobies, like humans, use oxygen for metabolic processes. The maximum metabolic rate (VO2max in ‘science speak’) for a human is generally described as the body operating at maximum oxygen uptake rate and maximum carbon dioxide exhalation rate.
Scientists regularly measure this in elite athletes to determine their VO2max with the aim of enhancing performance.
For ‘water breathers’ such as gobies, the equivalent measurement is known as the maximum metabolic oxygen consumption rate (MO2max). At the other end of the spectrum, the minimum metabolic rate, occurring when the body is at rest, is MO2 min.
Jodie’s work with the gobies in the lab involves measuring the MO2 min of gobies from two different environments: coral homes with and without Turtle Grass. If this alga, which kills the corals on contact, is present on their coral hosts, the gobies trim it (eating the prunings) regularly to keep the coral safe. In the process, the gobies become more toxic which may make them less appealing food for potential predators. It is possible the gobies use a lot of energy to process the toxins in the Turtle Grass.
Jodie and Danielle collected gobies that had algae to eat for three days and gobies that did not have access to algae. In the lab, after a 24-hour period of rest and fasting, the MO2min of the gobies is measured using a respirometer (“goby garage”).
The goby garages are part of a sealed system in which water passes a fibre-optic sensor, which takes readings of the oxygen concentration of the water every two seconds. After six minutes of readings the sensors stop collecting data, the system is flushed with fresh (oxygenated) seawater, and the readings commence again. This cycle is repeated for 24 hours.
Data are sent directly to Jodie’s computer from the respirometer, giving her information about the fish’s resting rate of oxygen consumption.
If there is a metabolic cost to the gobies as a result of eating the Turtle Grass, the data will probably indicate the resting oxygen consumption rates of the gobies that had been pruning alga prior to capture, to be higher than for gobies that were not consuming Turtle Grass.
Jodie is also using a respirometer system with Fiveline Cardinalfish (Cheilodipterus quinquelineatus), looking at the impacts of ocean acidification (due to increased carbon dioxide levels in the atmosphere) on the metabolism of these fish.
In this experiment, Jodie exercises the cardinal fish for two minutes, in water where CO2 has been added to simulate what the oceans are predicted to be like by the end of the century. After weighing the exhausted fish, Jodie places each of them into a respirometry chamber, where it is allowed to recover.
The recovery from exercise uses a lot of oxygen, which can be a good indicator of the fishes’ maximum metabolic rate. Jodie allows the fish to recover for 24h so that they also achieve a resting state.
Having both the maximum and resting oxygen consumption values can tell Jodie about the total scope the fish have for all of the activities they have to do in life, such as swimming, finding food, reproduction, etc. and whether environmental conditions have an impact on that total scope.
Because this particular species is nocturnal, Jodie has undertaken her work both in the morning (when the fish are probably ‘tired and sleepy’) and in the evening (when they are probably most alert).
Once she returns to her lab in Townsville, Jodie will analyse the results from the ‘goby garages’ and the cardinal fish exercise experiments to see if she can draw any conclusions or if she needs to investigate further, the physiology of these common little fish.
Southern Cross University