Before reading anyone's post my first thought was those are bites. Doesn't look like burns or any other degenerative issue with discs.
On a side note, not to derail thread or anything, but I am noticing that while rays have pretty decent healing power I feel like they are prone to scarring. Anyone else agree/disagree? I know with bites that get actually taken out of their discs sometimes they don't grow back fully to their natural circle, sometimes they grow a bit more and protrude a little bit. I am noticing the love bites that happened a year ago I can clearly see to this day, not in the sense of damage but discolouration and sometimes small pimple like bumps in the healed areas. Most noticeably 3 months ago I tried to divide my female and she ended up with a scratch about 4 inches across her back trying to sneak under it last minute. Slime coat restored quick and it repaired itself well with no issues, but I can still clearly see the scratch in its entirety because it is a much lighter black almost brown compared to her regular black base. Can't help but think maybe if I added salt or stress coat or something that could have reduced it, but I wonder if that will ever go away (it's minor and you would probably only notice if I told you, but I am a bit OCD/perfectionist so I see it lol).