HCA: Hermit Crab Association

do hermit crabs need natural sunlight?

Hermit crabs do not need natural "sunlight" per-say. They obviously need a natural day/night cycle, meaning that they need more or less 12 hours of light. The light can just be provided by regular light bulbs, though.

To really answer your question, crabs do not need the UVB light provided by the sun, but it has been known to improve crab health and coloring. Purchasing UVB light bulbs are used by a few select crabbers that have seen results, particularly in strawberries.

I use a regular aquarium fluorescent bulb with 6500 kv, which is close to natural light.

Actually, we don't know if they need UVB for sure. UVB is the beneficial portion of sunlight that produces Vitamin D in mammals and reptiles, and can only be provided with direct exposure to sunlight (not through a window or screen) or with a speciality bulb as regular lighting can't provide it. UVB doesn't seem to be needed for basic PPs, but there is a lot of evidence pointing to it being helpful with exotic species. At this point because of the hassle of using it and the cost it's not considered a necessity for hermit crabs like safe substrate and proper salt mix is, but it's recommended to try it if you can.