They remind me of a spider. Not black sand. Its like rocks. It was mixed in with my live sand. I don't like it much. My next tank will have different sand.
They tend to get very aggresive as they get larger. I had one that I ended up having to evict because it kept trying to catch my yellow watchman goby a few years ago. Great hair algae grazers, but too damn mean.
yeah, I only plan on keeping him a while. I had an emarald crab that was way meaner. He killed off anything I had. He went back to the lfs.
I also got a Queen Conch. Totally cool. But no pictures came out.