The sun produces two beneficial UV rays and one harmful UV ray. UV (Ultraviolet) light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10nm to 400 nm, and energies from 3 eV to 124 eV(electron volt).The two important UV rays are UVA and UVB. Turtles are believed to see through the UVA spectrum and turtles are able to identify its prey and predators. Due to these reasons, UVA is known to have a physiological effect on a turtle’s well-being. UVA ranges from 320nm to 400nm in the ultraviolet light spectrum. The next beneficial sun ray is UVB, which is from 280nm to 320 nm. UVB is used by reptiles to synthesize Vitamin D3. Vitamin D3 is what helps our turtles have strong healthy bones and shells. UVB is filtered by the ozone layer but UVB from 290nm will get thru to the earth’s surface. The sun also produces harmful UVC, which is UV with wavelengths shorter than 280nm. UVC has the largest range in the UV light spectrum. But all of this is filtered out by the ozone layer; none reaches the earth’s surface. UVC is harmful to living cells. DO NOT USE UVC EMITTING LIGHT SOURCES FOR BASKING TURLTES.
The following information is provided from UV Guide UK (with permission).
“Vitamin D and Ultraviolet Light – a remarkable process”
“The way in which many vertebrates utilize sunlight in the production of vitamin D3 in the skin, and how this process is also regulated by the light itself, is still the subject of much research. Most of the studies relate to human biology, but researchers such as Holick, Ferguson and Gehrmann in the United States are gathering evidence that the process is very similar in reptile skin.
Illustration provided by UV Guide Uk
The pathway begins when a cholesterol, provitamin D, (full name 7 –dehydrocholesterol, or 7DHC) is manufactured by cells in the skin. When exposed to UVB at wavelengths between 290 – 315nm, this provitamin D, held within the cell membrane, is converted very rapidly indeed to previtamin D3. The peak production is at 297 nm. Previtamin D3 is then isomerized (transformed by a re-arrangement of atoms in the molecule) slowly, in warm skin, over several hours, to vitamin D3. Warmth is needed for the reaction to proceed at a normal rate. Reptiles obtain this heat from the sun, as they bask.” (© UV Guide UK 2005, used with permission)
Slider turtles will stretch their limbs out and flatten them to expose as much skin as possible to the sun’s rays and heat. Typically when you see a slider basking, he/she will have their hind legs lifted off the surface of the basking zone rotated in a flat position exposing the skin to the heat and rays. The front legs will be positioned to make an escape if a predator is encountered. The head is stretched out and held up in the air to search for predators. The turtles are on high alert when basking, due to the fact they are highly vulnerable to predators when out of the water. This position is known by slider keepers as the famous Superman pose. Turtles normally make most of their vitamin D3 in warm skin while they are basking in the sun. UVB rays pass very well through clean clear water; but not through cloudy or dirty water. But the main reason for putting UV and basking lamps close together is that this makes “sunlight” – heat, light and UV all in one place, as in nature. It is important to make sure the rays are directed towards the basking zone. A good rule of thumb for many lamps is 12 inches from the basking zone but look for manufacturer’s instructions. For MVB (Mercury Vapor Bulb) light sources this measurement is highly dictated by the heat on the basking surface. When using a MVB these two factors have to be considered, the heat of the basking zone and the UVB measurement of the basking zone.
“The Vitamin D3 is released from the skin cell membranes and is taken up by a “vitamin D-binding protein” into the plasma. It is thus carried in the bloodstream from the skin to the liver, where it is hydroxylated to calcediol, 25-hydroxy-vitamin D3. This is the substance which is tested for in blood samples taken to assess the reptile’s vitamin D status. Calcediol is then circulated in the bloodstream all around the body. In the kidneys, some is converted to the active hormone calcetriol. This plays a major part in the calcium metabolism, governing the levels of calcium in the blood by controlling absorption of calcium from the gut and also from the bones, should dietary levels be inadequate for the body’s needs. Calcediol has also, in recent years, been found to play a vital part in the normal functioning of other organs. It is taken up by cells throughout the body, and converted intracellularly to calcetriol. This local action has beneficial effects upon the immune system, the cardiovascular system, and in preventing cells in many organs from becoming cancerous by controlling cell division. There is also new evidence that skin cells in the sunlight can actually complete the entire pathway from provitamin D to calcediol intracellularly, which may increase the skin’s resistance to cancer. Calcediol, in humans, has a half-life of about two weeks in the bloodstream. In some reptiles, this circulating calcediol may act as the body’s main store of vitamin D. Vitamin D3 does not remain in high concentrations in the bloodstream. In humans, that which is not hydroxylated to calcediol in the liver is taken up into body fat, where it is apparently stored but we are not aware of any studies ascertaining whether such storage takes place in reptiles, or if it does, how long such a store might last.” (© UV Guide UK 2005, used with permission)
The vitamin D process is dependent on a source of heat and UVB. Some have said you can use vitamin D3 supplements. I don’t recommend this due to the natural diet of slider turtles. Due to sliders being largely omnivorous, the bone consumption in their natural habitat is not large enough to sustain them without photo-synthesis of Vitamin D3 to stay strong, healthy and active. The radiant heat from a heat bulb builds the metabolism and entices the appetite of our turtles.
As you can see UV rays and heat play multiple roles to keep your turtles healthy and active. Slider turtles get their name from basking and sliding into the water to evade danger. Basking is a very important and way of life and survival for slider turtles. It is our responsibility as the primary care giver to provide the elements required for our turtles to thrive in captivity. I would like to thank Frances Baines for letting me provide this information.
Baines, Frances. UV Lighting for Reptiles: Vitamin D Synthesis in Ultraviolet Light. UV Guide UK, 2005. accessed July 2012. <http://www.uvguide.co.uk/vitdpathway.htm>.
