Is the Deresa sat on anything other than just the sand bed?
I’ve found deresa like to burrow themselves into the sand Atleast a bit. Also, that mantle looks to be slightly more out on the right side, is this due to a shadow causing the left to retract?
The other thing that worries me about the mantle is that spotted line seems to stop as it gets towards that left side. The spotted line should ring the whole mantle. This is my deresa from top down, that blue ring is the spotted line and my deresa lost this ring as it wasn’t getting enough lighting in my previous nano. Now it’s under I believe 350PAR - possibly 300, it’s been a while since I checked PAR however I haven’t changed my rock scape or lighting situation since I last checked.
And here’s what I mean by they like to burrow themselves into the sand (Also with this you can see the amount of new growth that I got in the past 100 days);
Another thing you may not already know is, as Deresa’s get larger they use their weight to keep themselves balanced so they eventually release their byssal threads.