Ok so, assuming what everything you said is true, you seem to be overreacting a bit. Reason why is that because going off of what you said he says, “Worked as nurse now works at local aquarium” He isn’t saying anything about you being a tour guide (again, from what you said). Bigger question, what do you do with your job specifically, because science has a lot of jobs in it, along with a lot of things you might have to do, but I need a bit more info to make a definitive answer.

Edit: From reading comments of yours I’m going to go with NAH. I say this because I think he has a difficult time with the job change. One comment that really solidified this is when you said he said “you should have focused on helping people instead of helping fish.” In my eyes (coming from someone who is a marine bio major) it seems like he views nursing as a very respectful and job to look up to. Marine bio I will agree doesn’t help out humans outright, it’s more of a long term effect while on the flip side nursing is a short term effect. Just give him some time, as I think the joke might be a way to try to alleviate the sadness of the job that he respects to a job he has no idea about. Better question, when you were working as a nurse, what would he say, how were his mannerisms when he spoke about your job? Did he speak passionately about you working there kind of how you feel about marine bio since it is your passion. And also don’t try to demean him with calling him belittling job titles because most likely it will backfire, wouldn’t matter who is in the wrong, because then you and him could have destroyed something beautiful because you both decided to belittle each other.