Rusty:  On treasure coast they're locally common in Indian river. I have found that species of mangrove shells tend to occur only in certain locations in colonies mainly only of their kinds or one more. That appear to be true for mussels at least one species, eastern oyster, mangrove periwinkle to name a few. I still need to figure out why they do this does it have something to do with salinity, current movement, etc. I have seen this species attach to mangrove trees only. I hadn't seen them wash up on regular beaches as Indian River is too weak to push them out to sea. I found several dead specimens along the living ones by moving through the mangroves at fishing locations near fort pierce inlet. It's not for the faint of heart as there's tons of orb webs of at least one species of spider along with the varying odors of dead matter and the occidental dead fish. I do it in feb March and hadn't seen snakes, chiggers or anything dangerous thou I do see a lot of hobo tents.