LIGHT AND VITAMIN D3
Ultraviolet light (UV) is electromagnetic radiation that exists in three main bands: UVA, UVB, and UVC. Each band is characterized by a specific wavelength. UVA has the longest wavelength at 320–400 nm while UVB has a wavelength of 290–320 nm. Both UVA and UVB have low tissue penetration with most being absorbed at the epidermis. UVA has a wavelength of less than 290 nm, the deepest penetration, and is the most harmful. These wavelengths place ultraviolet somewhere between visible light (369–769 nm) and x‐rays (0.01–10 nm). The ozone layer is responsible for filtering the majority of ultraviolet radiation hence protecting living organisms from its harmful exposure. UVC is considered germicidal and can have significant harmful effects to living animals while both UVA and UVB have therapeutic and physiologic applications. Still, as we know from human medicine, too much sun/ultraviolet exposure, has a slew of negative implications ranging from sunburn to premature aging, and cancer.
UVB light is essential for the synthesis of vitamin D3. Although commonly referred to as a vitamin, D3 is actually a hormone. UVB light from the sun stimulates conversion of 7‐dehydrocholesterol to cholecalciferol (D3) in the skin. Cholecalciferol is then transported to the liver via the blood stream and hydroxylated into 25‐hydroxycholecalciferol (calcidiol), which is then transported to the kidneys for hydroxylation into 1,25‐dihydroxycholecalciferol (calcitriol), the active form of D3. The 25‐hydroxycholecalciferol in the liver is the storage form of D3 and is a natural source of D3 for animals fed whole prey. Plants are a source of ergocalciferol or ergosterol (D2), which is not readily bioavailable to most animals and is therefore not an appropriate source of D3 supplementation. Calcidiol, an animal source of D3, is more bioavailable for most animal species, and is suspected to be the same for amphibians.
There are two main sources of UVB, natural sunlight and UVB bulbs. Exposure to natural sunlight is likely the most efficient way for amphibians to synthesize adequate levels of D3. However, there is a high degree of variability in the lifestyle of amphibians from cryptic species that barely get sunlight exposure to those that may display basking behaviors similar to reptiles. This variability makes it nearly impossible to provide blanket recommendations for UVB supplementation for amphibians. Instead, we must rely on knowledge of the natural behaviors of the species, their geographical location, and expected UVB exposures at that location to make extrapolations about the use of UVB light supplementation in captivity. One thing that most can agree on is the fact that amphibians should be exposed to lower levels of UVB than reptiles.
CHOOSING UVB LIGHTING
UVB bulbs can be divided into linear fluorescent, compact fluorescents, and mercury vapor. In the future, commercial LED UVB lights may also become available. Fluorescent UVB bulbs provide light in the required wavelength for D3 synthesis, but no heat. Mercury vapor bulbs provide both heat and UVB. Mercury vapor bulbs can provide very high levels of UVB exposure and their use should be avoided in amphibians. There are six main aspects to consider when selecting a UVB bulb:
1 Size/type of the enclosure.
2 Type of material between the bulb and the reptile.
3 Type of bulb.
4 Effective distance of the bulb.
5 Bulb longevity.
6 Species of amphibian.
Another consideration is the type of fixture used for the bulb. There are a number of fixtures that offer the advantage of having reflective materials to increase light reflection and decrease scattered light emission (radiation). Reptile/amphibian‐specific light fixtures or those that reflect light away from the bulb and towards the animal are preferred over plastic fixtures or those without reflective properties.
T8 Linear Fluorescent Tubes
The T8 bulbs (25‐mm or 1‐inch diameter) are available in different lengths and fit into standard aquarium or reptile hoods. Some manufacturers further classify their bulbs by a number system ranging from 2.0 to 10.0. This number represents the percentage of light that is emitted in the UVB spectrum. A 5.0 has 5% of the emitted light in the UVB spectrum, a 10.0 has 10% UVB emission, and so on. Arcadia® has simplified the process by simply stating the percentage of UVB emission with their bulbs ranging from 6% to 14%. For amphibians, bulbs emitting less than 5% UVB are likely the best option to avoid the possibility of overexposure.
Being a longer bulb also means that there is more scattered radiation, so the UVB exposure is not as focused. This may actually be of benefit for amphibian enclosures. The use of a shorter bulb, while leaving the rest of the cage shaded, is a preferred setup for amphibians. These bulbs emit UVB light at an average effective distance of 12–20 inches depending on the brand and percent of emitted UVB. It is generally recommended that animals should not have the ability to get any closer than within 3–4 inches of bulb’s surface. They must be replaced every 6–12 months according to manufacturer’s recommendations.
T5 High‐Output Linear Fluorescents
A high‐output light is a T5 bulb (16‐mm or 0.6‐inch diameter) that requires a hood with a T5 fixture. T5 bulbs are more efficient and provide a higher amount of UVB than T8 or compact fluorescent bulbs. In theory, a T5 10.0 bulb produces a higher amount of UVB than a T8 10.0 or compact fluorescent. It is worth mentioning that Zoo Med Laboratories Inc. produces a T8 10.0 bulb labeled high output but their T5 10.0 bulb is actually a much stronger bulb and is considered as a true high output for our purposes. These bulbs emit UVB light at an effective distance of 12–36 inches depending on the brand and percentage of emitted UVB. While it is generally recommended that animals should not be able to get any closer than within 3–4 inches of bulb’s surface, this distance may actually need to be longer for high output bulbs. They must be replaced every 9–12 months, according to manufacturer’s recommendations. T5 bulbs likely produce more UVB than most amphibians should be exposed to, and should be avoided or only used under extreme caution while monitoring UVB exposure levels. As with the T8 bulbs, only those with 5% UVB or less should be used.
Compact Fluorescent
Compact fluorescent bulbs can be fitted into any incandescent fixture that would fit a household bulb but a reflective or reptile‐specific fixture is preferred. The light is focused over a smaller area so there is less scattered radiation. For this reason, it is possible that a 10.0 compact fluorescent may actually emit a higher UVB output than a 10.0 linear fluorescent over the same area. It is usually recommended to place these bulbs next to the heat lamp, so the reptile gets UVB exposure while basking. All other properties are similar to the linear fluorescent bulbs although some may think of these as being less effective and being “weaker” in the UVB spectrum. As with the linear fluorescent bulb, animals should not have the ability to get any closer than within 3–4 inches of bulb’s surface. A 5% or lower should be used for amphibians.
UVB REDUCTION
As previously mentioned, the type of material present between the bulb and the animal is an important consideration when selecting a UVB bulb. Most glass and plastics are known to block or filter out UVB radiation and are not recommended. The ideal scenario is one of an open‐topped enclosure with no material present between the bulb and the animal. While this is a possible application for salamanders and some aquatic species, it is not feasible when housing anurans and other species. A study by Burger et al revealed the UVB attenuating effects of common materials used in reptile enclosures and provides a sobering insight into the use of UVB bulbs (Burger et al, 2007). The type of material, aperture of the openings, thickness, and reflective properties can all influence UVB attenuation and can significantly decrease the UVB exposure of reptiles. Most amphibian
