Good shells are very hard to find. I hate ordering things online that I want to see first but good shells are important to the crab's health and what I find locally are usually chipped, painted or too thin. Mine prefer the Turbo style because the shell is a medium to heavy density, which provides good protection from attack and holds inner humidity for their bodies. Crabs are amazingly strong and can lug around even the heavy Murex type shells fairly easily (though the spikes on the Murex's often get snagged on things). I avoid the Cherry and Fairyland shells because they are too thin and don't breathe well and our crabs just don't like them. Another thing to watch for in shell buying is the shape of the opening. E. Rugosa's (also known as Ecuadorians or Ruggies) tend to prefer rounder oval openings that fit their bodies better but I have had some that go for the more flat ovals but can't figure out how they can be comfortable squeezing in there. All our Purple and Reds, prefered a round non-oval opening as their bodies are rounder and legs tend to be longer so they can carry them better. Painted shells also don't breathe and if given a choice, most crabs will choose a natural shell. Be sure to sterilize any shells in boiling water before introducing to your crabitat to kill any parasites and molds. Watching them switch shells is a bit unnerving because that isn't the most attractive part of their bodies
but it is fun watching them pick over and peek into shells as they try to decide which one to take. Aside from molting time, it is also the most likely time for an aggressive crab to attack so its a good idea to put the clean empty shells up on a higher shelf or in a dish so they can swap without fear.
Humidity is difficult to control but I found that by keeping a pool at soil level and one up on the shelves with the top closed, the air stayed humid without promoting mold as long as the food was changed every to every other day. One pool is fresh water and the other is salt water btw as they need the salinity and one must be deep enough for them to flush out their shells in. If the sand/moss mixture (3 parts sand to 1 part crumbled compressed coconut fiber block) in the bottom is less than 2" deep, it dries out and they can't dig into it to molt or hide because it will collapse. I have a low side where the pool (a ceramic ash tray actually) is located but the deeper side is mounded to about 5-7 inches deep and I occasionally sprinkle that side with water to moisten it. They dig tunnels and caves in it and it's fun to watch when they clear a cave up along the glass and peer out at you.
The Ruggies are the most vocal between the two species. Strawberry crabs are also very vocal but can be aggressive and are known for rearranging their crabitat so we have had only one. She would sit up on a rock and when ever anyone challenged her "perch" she'd chirp like crazy - a definite warning to back off. One of my purples got caught up inside a tower that I'd put in and one night I heard this frantic chirping as a bigger Ruggie cornered him and wouldn't let him out. I ended up taking the tower out and he happily crawled out. The one Ruggie we have left now will sit on an upper shelf where we put their food and chirps if I put grapes or raw shrimp with shells (his favorite treats) in for him. The sound is like a cross between a cricket chirp and a stone rhythmically scraping on glass.