Arrow crabs may kill very small fish if they can grab them. I've seen them carry small damsels on their horn for a while as if it was a ceremony of some ancient tribe. A huge bristle worm, like 8 inches, can sting a goby and cause some trouble, even death. Its not something they do on purpose, just happens in passing or at ngiht when the goby is buried in the sand and the worm comes passing by when the fish is too shallow. Something likely that killed those gobies are the tentacles of an anemone. Some gobies don't have much sense and will back right into the tentacles and get a leathal sting. Once stung bad enough from either bristle worm spines or tentacles, then the bristle worms will clean up the dead goby and you'll never see the body. When they are first netted from the store and introduced to your tank, their mucous membranes are too thin and damaged to withstand much of any kind of sting and survive. Keep an eye on that shark egg. Don't know what the arrow crabs might do to that.