Out of pure coincidence I just mentioned a similar idea in another thread.
A while back Stephen Colbert was making fun of a senator (?) with a geology degree calling himself a geologist when he did not work in geology. I would think that you could call yourself a X if you have a degree in X, but not a job in X.
My friend the mud scientist (cat litter, oil rig lubricant, and whatever else mud scientists make) has a degree in biology, but a job in chemistry. I would think that he could call himself both a biologist and a chemist.
I really wish he would put "mud scientist" on a business card and hand it out.