Medical Use of Horseshoe Crabs

Apparently the class of merostomata don't have phagocytizing immune systems so their only defense is that the blood detects the membrane proteins of bacteria and it forms clots in response

http://www.horseshoecrab.org/med/med.html

Because of this property horseshoe crabs are used to quickly diagnose things like meningitis that can kill really quickly or detect small amounts of bacteria in biomedical supplies to mark contaminated stocks.

I thought you'd like that since your so into physiology and comparative studies
I've heard it said that because the ocean is so unexplored that many of these great things could be waiting to be discovered.