Overwinterende moerasschildpadden in met ijs bedekt water

Gepubliceerd in jaargang 76, nummer 1 van Lacerta
Borg, J.P. ter

Overwinterende moerasschildpadden in met ijs bedekt water - Hoe doen ze dat?

Intro

Hibernating turtles in ice-covered water. How do they do it?

Part of the research of Jackson (2011) is discussed, because of the impact of the presented results on our knowledge and understanding of this intriguing matter.
Many turtles in temperate climates, for instance in North America, are forced to hibernate, but do so in ice-covered water. For many months they don’t have access to air, but they still can survive these hardships.
There are two strategies for solving this problem.
Some species, like the Map turtle (Graptemys geographica), the Musk turtle Sternotherus odoratus) and the Softshell (Apalone spinifera) are quite good in extracting oxygen from the water, especially in cold conditions. However, they rely on sufficiently oxygen-rich water. This forces them to hibernate in water that stays oxygenated during the winter. The Map turtle for instance chooses rivers to hibernate for this reason.
Other species, for instance Chrysemys picta, are capable of hibernating with little or no oxygen available. The only way they can do this is by using anaerobic (oxygen-less) metabolism, instead of the normal aerobic metabolism.
This poses some urgent problems. In the first place, although anaerobic metabolism can use glucose or glycogen as energy-source, the fat-reserves can’t be metabolized without oxygen. So only part of the energy-reserves can be used under anaerobic conditions.
Second, anaerobic metabolism is very inefficient, producing only two ATP per molecule glucose with lactic acid as waste product, in contrast to aerobic metabolism, which produces (in theory) 38 ATP per molecule glucose, with CO2 (carbon dioxide) and water remaining.
Both limitations can result in a prematurely draining of the available energy-sources in the winter, although their metabolism is on a very low level due to the low temperatures.
Third, lactic acid as waste product increases the acidity of the blood (acidosis), which can be fatal. The higher the metabolic rate, the higher the production of lactic acid during anaerobic metabolism. Luckily this metabolic rate is low due to the low temperatures. Moreover the turtles manage to decrease the metabolic rate even further, up to 80-90% of the normal rate under these conditions. This is called metabolic depression.
Although the lactate levels in the blood of anoxic turtles can increase dramatically, the increase of acidity falls behind notably. At the same time calcium levels in the blood increases strongly. It was found that calcium carbonate from the shell was used to buffer the excess hydrogen ions, resulting in the production of carbon dioxide (which diffuses into the surrounding water), the increase of calcium ions in the blood and preventing a life-threatening acidosis. Moreover it was found that much lactate accumulated in the skeleton, especially in the shell, probably in the form of calcium lactate. So the shell is of utmost importance for the turtles to survive anoxic conditions during hibernation.
An additional test was performed with tree tropical turtle species (Elseya novaeguineae, Emydura subglobosa and Pelomedusa subrufa) from the southern hemisphere, which were kept for six hours at 20°C under anoxic conditions. All these animals survived, suggesting this tolerance for anoxic conditions is widespread under turtles. Another point of interest is that these last three species belong to the suborder Pleurodira, in contrast to the former species discussed, which all belong to the suborder Cryptodira. These two groups separated from each other some 200 million years ago. So this tolerance for anoxic conditions is a very old feature, which probably co-evolved with the turtle’s shell.