does anyone know if horseshoe crabs, if bought small and put in a 55 gallon tank if they will stay small or outgrow the tank. i have only found one place on the net that sells them, but cant get an answer either way.
...plus they have a tendency to hide for long periods of time...and then they might die and you'll never know. Not a good idea - especially in addition to what the others have said..
Do avoid them. There's some indication that they are threatened in their natural habitat, they're a temperate water animal, they dig, they grow very large (as noted, over a foot in width, if they survive that long at all) and they eat sand bed fauna. Very much best left in the ocean.