Priest, T., Craig, Franklin. 2002. Effect of Water Temperature and Oxygen Levels on the Diving Behavior of Two Freshwater Turtles: Rheodytes leukops and Emydura macquarii. Journal of Herpetology. Vol. 36, pp. 555–561
Ok, so basically some turtles can obtain oxygen directly from the water they are swimming around in. Some turtles, the ones that withdraw their head straight back into their shell, do this by pumping water back and forth along the vascular lining of their throat. Other turtles, the ones that withdraw their head by swinging their neck sideways, do this by pumping water through their cloaca which is vascularized. R leukops has developed this ability to such an extent that they actually have sacks within their urogenital tract which are highly vascularized and have little filaments which increase surface area. Essentially, they have primitive gills inside their rectums from which they can derive a whopping 41% of the oxygen they need during a long dive.Unlike the crypodires that obtain aquatic oxygen partly via the buccopharynx, pleurodires can obtain aquatic oxygen via enlarged cloacal bursae (King and Heatwole, 1994a). Cloacal respiration has reached its pinnacle in R. leukops, a chelid from northeast Australia. The cloacal bursae of this turtle are greatly enlarged and are lined with highly vascularized, multibranched papillae (Legler and Cann, 1980; Legler and Georges, 1993). By rhythmic ventilation of these structures, R. leukops is able to extract, on average, 41% of its oxygen requirements from the water (Priest, 1997; C. E. Franklin, M. Gordos, and T. Priest, unpubl. data). Emydura macquarii does not have extensive cloacal modifications and has a limited capacity for aquatic respiration, extracting approximately 11% of its total oxygen consumption from the water (Legler and Georges, 1993; Priest, 1997).
We hypothesize that diving time of both R. leukops and E. macquarii will be dependent on water temperature and that only R. leukops will respond to changes in aquatic oxygen level and that the degree of the response will be dependent on temperature.
Materials and Methods Return to TOC
The investigators set out to determine if this actually has an effect on dive length.
It doesSix R. leukops (mean mass: 1325 g ± 193.9 SE; range: 595–1810 g) and five E. macquarii (mean mass: 1595 g ± 104.9 SE; range: 1200–1800 g) were used in this study. Emydura macquarii were collected from the Albert River, Brisbane and R. leukops from the Fitzroy R., Rockhampton, Queensland. Turtles were housed in two 2000-liter covered outdoor holding tanks at 23 ± 2°C with water depth 250–300 mm. A full spectrum light was positioned above each tank, and a basking platform was provided, although R. leukops were never observed basking. Turtles were fed twice weekly with chopped meat and a variety of fruit and vegetables. The water was continually filtered and was changed after every feeding.
Experiments were conducted at 15°C, 23°C, and 30°C and at three aquatic PO2s: anoxia (0 mmHg), hypoxia (80 mmHg), and normoxia (155 mmHg), for each temperature. The temperatures chosen approximate the water temperatures over a year in the turtles' natural home range. Turtles were placed in a rectangular tank (500 mm × 1000 mm) that was filled to a depth of 250 mm with tap water and housed in a controlled temperature room. The water was continually circulated and filtered and the feeding regimen maintained. Rheodytes leukops were placed in the tank in pairs and E. macquarii in one pair and one group of three. Because the experimental tanks were relatively large and no interindividual aggression was observed, any effect of studying the turtles in groups rather than individually should be minimal. Turtles were marked with colored paint to allow individual recognition. Each group was allowed one week at 23°C to become accustomed to the tank. Preliminary videotaping demonstrated that average dive time reached a plateau after this time. The temperature was then changed to the experimental temperature and the first PO2 level set. Temperatures were randomly selected, and then within each temperature the PO2 was again randomly selected. To minimize possible thermal stress to the animals, the test temperature was maintained until all three PO2 trials for that temperature had been completed. Maintaining the test temperature until all PO2 treatments were completed may have allowed some thermal acclimation to occur; however, randomizing PO2 treatments and having multiple groups that were therefore presented with both temperature and PO2 in different order should remove any bias caused by possible thermal acclimation.
To maintain the oxygen level a TPS dissolved O2 electrode (ED500) was suspended in the tank and connected to a TPS oxygen analyzer, model 2052A. The O2 analyzer was connected to a Mann Industries UTC/R Universal temperature alarm (thermocouple inputs) that was wired so that when the input from the oxygen analyzer moved beyond a preset level the alarm was tripped and a solenoid valve opened. The solenoid controlled the flow of either N2 or O2 through an airstone into the tank. The oxygen electrode was suspended at the outflow of the filter system to ensure a good flow of water over the electrode.
The controlled temperature room was maintained on a 12:12 h light:dark photic regime. The duration of the trials was varied depending on the experimental temperature, being 24, 18, and 6 h for 15, 23 and 30°C, respectively. The turtles had access to room air for the entire duration of the experiment, and basking platforms were not supplied.
Experiments were videotaped during daylight hours only with a National F10 video camera and National AGG010 timelapse videocassette recorder. All tapes were later viewed and the emergence and submergence time of every dive during the experimental period was recorded.
The ecological implications and evolutionary questions are also rather interesting.Dive Duration. There was a significant difference in the way the species responded to changes in aquatic PO2 (F = 6.51, P < 0.0025). Average dive time for R. leukops more than doubled from 22.4 ± 7.65 min to 49.8 ± 19.29 min when PO2 was increased from 0 to 155 mmHg (Fig. 1). In contrast, aquatic oxygen level had no effect on dive duration of E. macquarii (Fig. 1). Rheodytes leukops had on average significantly longer dives than E. macquarii in normoxic water at all of the experimental temperatures (Figs. 1–2). In hypoxic water, dives for R. leukops were significantly longer at 15 and 23°C only. Under aquatic anoxia average dive duration of R. leukops and E. macquarii was not significantly different (Fig. 1)
Decreasing temperature resulted in significantly longer dive durations for both species, but the magnitude of the response differed (F = 10.03, P < 0.0001). Decreasing the temperature from 30 to 15°C resulted in a sevenfold increase in dive time from 10.1 ± 2.7 to 70.5 ± 15.0 min in R. leukops and a fivefold increase in average dive duration from 6.7 ± 1.2 to 31.3 ± 8.3 min for E. macquarii. Average dive times of E. macquarii at 23 and 30°C were not significantly different.
The response to changes in PO2 shown by R. leukops was not dependent on temperature (F = 1.87, P < 0.1256). Dive times under normoxia were approximately twice the length of dives under anoxia irrespective of temperature.
Frequency of Dive Durations. The longest dives recorded for R. leukops were at 15°C and in normoxic water; the longest single dive recorded was 538 min. Although R. leukops was capable of extremely long dives, 41% of dives under the aforementioned conditions were less than 30 min (Fig. 3). For E. macquarii under equivalent conditions, the longest dive was 166 min, and 61.5% of dives were less than 30 min (Fig. 4). The extreme right skew of the dive time histograms was evident for both species and under all temperatures and aquatic oxygen levels (Fig. 3).
Surfacing. Surface times for both species were not significantly related to either water temperature or oxygen level (F = 1.95, P > 0.15; Fig. 5). Rheodytes leukops, however, spent significantly less time at the surface after each dive than did E. macquarii, spending on average 42 ± 2 sec at the surface between dives compared with 107 ± 20 sec for E. macquarii (F = 29.48, P < 0.0001).
One question I have for this particular paper though is that one would expect significantly increased dive times with 11% of needed oxygen coming from cloacal respiration. Might there be something else going on that limits this turtles ability to utilize the oxygen? Or is the amount just not high enough to register a significant difference from dive length in anoxic conditions, say there is some sort of increasing marginal benefit with higher efficiency? Hmm...Ecological Implications. Rheodytes leukops is commonly found in the riffle zones of its riverine habitat. These riffles are areas of relatively shallow, flowing water with a high level of saturated oxygen. Rheodytes leukops is a bottom dwelling turtle that forages on and among the rocks and debris of the substrate for aquatic invertebrates and algae (Legler and Cann, 1980; Cann, 1998). Cloacal respiration would therefore serve to greatly increase the amount of time the species can spend foraging and thus reduce the time and energy spent traveling to the surface. Emydura macquarii, in contrast, is a generalist and can be found in impoundments and large waterholes as well as riverine habitats. The benthic zone of deep waters are frequently anoxic, a habitat where cloacal respiration would be of no advantage.
Also: wouldn't an aquatic generalist benefit from cloacal respiration as well? The turtle may not be a bottom dweller, and basks more often but in general increased dive times are a good thing.
I would also like to draw your attention to the near 10 hour dive. Sperm whales, eat your heart out...
