They were tigers. Very attractive and interesting to watch, but aggressive when mated. The female was about 2.5", and the male was a bit smaller. They would kill gobies at night that were sleeping on the sand bed. I don't know if it was opportunistic hunting, or just protecting their burrow, but I lost a lovely tangaroa goby (their partner) and two neon gobies. That was enough for me. I currently have a fairly larger pistol shrimp, but it's not the kind that takes a symbiotic partner. I have two tiny flaming prawn gobies in the tank with him along with other passive fish, and he doesn't seem to bother them at all.