HP = Detects Low HP; M = Detects Magic; Sc = Follows by Scent; T(S) = True-sight; T(H) = True-hearing
JA = Detects job abilities; WS = Detects weaponskills; Z(D) = Asleep in Daytime; Z(N) = Asleep at Nighttime; A(R) = Aggressive to Reive participants
Decapods or Decapoda, meaning "ten-footed," are an order of crustaceans within the class Malacostraca, including many familiar groups, such as crayfish, crabs, lobsters, prawns and shrimp. Note though that Final Fantasy XI's crabs are quadrupedal, making this name ironic.
From Merriam-Webster: "Any of an order (Decapoda) of crustaceans (as shrimps, lobsters, and crabs) with five pairs of thoracic appendages one or more of which are modified into pincers, with stalked eyes, and with the head and thorax fused into a cephalothorax and covered by a carapace."