Sunlight is a significant source of vitamin D because the UV rays trigger vitamin D synthesis in the skin. Calcium can only work with the presence of vitamin D, so both are extremely important to bone health and development for all living creatures.
When we take sea turtles out for sun, it is important to maintain a constant water temperature. Hospital staff and interns in the photo below eat lunch outside on a 95 degree day while they consistently monitor Gumby's water temperature, adding chips of ice to cool down when necessary.
Follow-up radiographs reveal an increasing bone density. Take a close look at the comparison photo below, the flipper bones in particular.
Although Gumby has a long way to go before he will be able to be released into the wild, s/he is making great progress!