In Reply to: A bunch of reasons to love this set~ posted by Brad on November 02, 2015 at 15:41:42:

You're right, Brad. We've often seen GSBs resting underneath a concrete ledge or overhang. It didn't occur to me that such behavior could eventually erode the fin. And it makes sense - this is one of the larger, and I assume older, GSBs that we see.

This may explain the damage to the top edge of the caudal fin of one of the first GSBs that we photographed.

The other thing GSBs do is to turn sideways and squeeze between the sand and one of the slightly elevated concrete pilings. For example, one of them made this maneuver facing me, only a few feet away. Phil has seen them do this, too. Parasite removal tactic?

We thought that what appear to be scratch marks may be the result of shimmying between the sand and the pilings. We might have some evidence that the scratches are temporary, but I'll have to find the photos and let you know.