Black dot underneath the surface of shell?


New Member
Jul 26, 2020
Location (City and/or State)
Boston, MA
Hi everyone,

As I was examining my marginated tortoise Houdini's plastron this morning after his soak I noticed a black dot beneath the surface of his shell at his left hind leg area. Attaching some pics here for reference.

IMG 2752
IMG 2753
IMG 2754

Sorry in advance that the pics came out a bit blurry; I had a hard time making my phone camera focus on the area. As shown clearer in the third pic, most of the black dot is beneath the surface, with a small opening on the left side of the dot.

Is this shell rot? Or is the black dot dried blood that remained after the shell got chipped (causing the small opening)? The opening does not smell, nor produce any discharge. Houdini is still a happy fella that can walk properly and doesn't seem bothered when I touch the opening. Though when I flipped him over to examine the area, his nail got caught once in the opening when he was moving his left hind leg.

Should I be concerned, and should I apply any ointment or cream to the opening? I'm using coco coir mixed with a little bit of orchid bark as the substrate, and the substrate is damp but not soaking wet. Should I intervene, or just let nature do its magic to heal?