Where oh where did my horseshoe crab go?

Blue reef

Reefing newb
We just brought him home and put him in our 55 gallon tank and he was to obnoxious for our critters. So we put him in our 20 gallon and after a couple of hours we can't find him. Could he have buroughed in the sand? We thought he would never calm down.
Welcome to the forum :)

Horseshoe crabs are not well suited to reef tanks and can grow very large. But yes, it has probably burrowed beneath the sand.