Pinched mantle?

[Netta Fish]

Bought this clam about a month ago. It seems to have developed a small pinch in the mantle, but not like in the picture I have seen. It has been this way for at least 20 days but has not gotten worse. Any help would be appreciated.

 

[Netta Fish]

Pictures

 

[bsagea]

Doesn't really look like pinched mantle IMO. Is it possible that it might have gotten stung by something or that something is irritating it on the shell.