crocea with mantel sucked all the way in the shell

[falling ice]

i have had this crocea clam for about 3-4 months and 2 days ago the mantel was sucked all the way into the shell!!!
i checked the clam closely and it didn't have any pyrmid snails on it since i have had them before.. there is nothing visible to tell me what is wrong. the foot of the clam is okay and it doesn't appear to be eaten on the inside going out....
is there anything i should try or any recommendations or should i toss it in fear of it being a virus or somthing that could effect my other clams

[Peabody]

Wnat kind of lights do you have? What are your current chemistry readings (and how are they measured)? And What were you feeding it?

[falling ice]

the clam is currently in a 33g w/ 440watts of vho
the salinty is 1.024 temp 77 ca 410 dkh 8 and i really do not feed the clam i have added phyto. once in a while
nitrates 0-5

[falling ice]

the clam is 6inch size