a bulky brine sifter  

Once I was over by the Ruined Shrine of Prexus and freaked when I went by the small entrance on the west side located at about 14, -16 and was attacked by a group of 4 of these that came out of that small enclosure, yes they indeed we're aggro (red around the name) and I haven't ever seen these before or since, so they might be very rare spawns. That was scary because not only do the big crabs scare me, every other one of these have been non aggro.

What's really strange is there wasn't a named in the group which really scares me. :P