also, horsecrabs are used to make a medicine (of course i cant remember which one) in the beaches of the oceans they are said to live over 40 years, in our tanks they rarely last 1 full year.
they do a great job of churning the sand and or crushed coral, depending on their starting size and how much sand you have they could last a while. One of the people that shops at my LFS had one in his 460g reef tank,,, its was only about 3 inches long total when he got it,, it died after a little more than a year his sand was very pristine white while the crab was in it, but it neve surfaced to die and he had a HUGE ammonia spike when he figured it out and pulled the corpse out of the sand realeasing everything.