So if you have gotten comfortable handling the monsters, and you are confident in your stance that crabs - despite their size - have the brain capacity and complexity of a cockroach (they do)....*
You can turn this:
|a bucket o' crab ready for cooking.|
Stand on a dock facing out towards the water. This is best done with grippy gloves.
Holding the crab by the back of the shell with one hand, grasp around 1 set of legs and claws close to the shell with one hand, and then do the same with the other.
Bonk the crab's "face" on the dock to pop off the shell.
Break the crab in half along the edge of the dock.
Clean off the soft stuff** and the dead-man's fingers (gills) under running (fresh) water.
All of this can go straight into the sea. That is where it came from.
Ta Da! Clean crab ready for the pot!
Alternate method: A small hatchet or bone cleaver can be used to chop them in half.
Then remove the large outer shell and clean off the soft, inedible parts as above.
*Living in water is what allows the crabs (and lobsters) to get so big. Water supports their body structure and provides their gills with oxygen more efficiently than dry air on land that feeds the tube system cockroaches use to breathe. OK... I'll stop, maybe you don't want to know more.
** Biology Warning - Possibly more than you want to know:
The "soft stuff" is the crab's digestive system (intestines, liver, etc.) where the crabs food is digested and it's blood cleared of waste. Yes, crabs have blood of a sort.
If crabs die before they are cooked all this stuff starts to disintegrate rapidly and will begin to taint the meat. Thus, crustaceans are cooked live, or killed and then cooked or cleaned instantly.
The liver & fat found under the shell is tasty, and is enjoyable when prepared correctly. We're just not doing that here.