A copy and paste
The ability of corals to voluntarily decalcify and/or release from their skeleton is very interesting. It is also a very topical matter, IMO. Here is what we have in fact:
Fungia and some other fungiids can decalcify part of the skeleton in the formation of anthocauli
Most corals (soft and stony), if locally covered long term in deposited material or mechanical obstruction, can " divide" their colony, not simply grow over/around it.
Polyp expulsion can occur in the corals Favia fragum and Oculina patagonica where the polyp and its corallite are released in whole to drift and settle, negatvely buoyed by skeletal material.
Polyp bail out occurs in Pocilloporidae. Here, the polyp only is released from from its corallite. Two small filaments are found at the base of the polyp that are like "quick attachment" anchors.
Stony corals, regularly release small areas of their tissue, lifiting it upward in the calcification process, before skeletogenesis. These areas, first described i 1972 by Barnes, are called lappets, and are usually formed during dicrete periods of the day.
Oulophyllia cripa is reported to voluntarily bail out of its skeleton - a Favidae member.
Thus, it becomes obvious that these guys are, if not directly shown yet, capable to some minimal to highly significant amount of voluntary tissue or skeletal release. From the list above, we see that it is frequently in response to stress, except for the calcification release. Also occurs in quite diverse species. In the non-stress case (calcification), by using blockers of CA (carbonic anhydrase), the lappet formation is stopped. There has also been a work that shows that during such times, microorganisms can enter the skeletal/tissue gap and participate in necrosis/further tissue degeneration. I find this particularly interesting...more in a sec.
Here are anecdotes and suppositions:
Several corals, including Trachyphyllia and Catalaphyllia have been reported numerous times in the hobby to release from theoir skeleton. I have witnessed a "drifting" Catalaphyllia before. While, to my knowledge, they have never been actually documented and/or studied, it is hard to debate the credibility of the observations, nor is it hard to imagine that it is entirely likely. The pocilloporid bails and the decalcification of Fungia are the best studied (Hidaka and Yamazato for Fungia, if I recall correctly), but to my knowledge, no exact mechanism is yet known, although some are suggested. Once again, CA activity seems to be the common thread here.
I have seen many cases of "RTN" in our work with this "disease" where polyps are fully expanded and in feeding mode mere mm's away from the tissue release band. This is not the rule, but it does happen. I know this coral is about to die from sloughing, and yet the coral seems to be acting normally. Is tissue sloughing somehow related to this behavior/ability gone awry? Or are invading microorganisms also involved as in the aforementioned work? I don't know, but again, it is certainly not that much of a stretch to see the possibilities.
Julian Sprung showed some remarkable photos of various asexual buds at the conference in Dallas. These were obviously valid and highly significant findings, but are not "officially reported" - IMO, they should be! For a lot of "buds" gravity tends to sever the attachment of daughter from parent. However, this process, for many corals, seems to be "helped along," and brief suggestions of this have been made in various papers, although none, to my knowledge, have directly addressed this issue. Certainly some of the photos Julian showed (e.g. the Cynarina, in particular) seem to depend on "parental help." The buds on this colony formed from the margins of the septa, if I recall, and if water currents and gravity were the only factors, it seems as though the buds would/could fall directly onto the coral disk of the coral. Perhaps more interesting is that both Catalaphyllia and Trachyphyllia are corals which (not coincidentally?) are attached when juveniles, releasing as they grow...has always been thought that this was a gravity thing...but is it??? Another coinkeedent-they commonly form buds!
Some older papers that are not directly related to corals, but are to cnidaria, suggest that Magnesium is important in basal ahesion of certain anemones. Some mention has been made in older calcification papers, but never continued in relation to my thoughts here, to my knowledge, that at least some corals may form a high magnesium calcite initially, before converting to aragonite in skeletogenesis. This is, IMO, a stretch, but you can see the relationship/direction I am hypothesizing.
Here is a candy cane of mine that did the same thing as you can see I got it to finally attach to a oyster shell and it been that way for 2 months now coming back to life. As you can see in the background there is another im taking care of doing very well so far.
