Sea Scorpion

From publisher burb:

One of the many horrors of the shallow brackish and salt waterways is the sea scorpion, ancient ancestor of the surfaces modern crayfish and land scorpions. These creatures can be massive, ranging 6 to 12 feet in length, but never being more then 1 foot in thickness. It has a hard segmented shell, allowing for good protection and mobility. It’s powerful forward claws have a reach of 1/2 it’s total body length (thus a 6 foot would have a 3 foot reach, while a 12 foot would have a 6 foot reach). A wide, flat tail like a Crayfish allows for a powerful escape.