I lost a 2.5'' T. maxima a few weeks ago. Woke up 1 morning and saw its mantle withdrawn as the posted photos show... 30 minutes after that, the cleaner shrimp came over and started picking on it. The day before it was responsive and I gave it a good phytoplankton feeding for 15 minutes [as usual]. The death was as Knopp's "textbook case". By the time I noticed it, it was already too late...