This post originally referred to an online site that was selling Horseshoe Crabs that was misleading. They advertised a small one and suggested their size would remain small. That is their size when young.

Read closely what the online sources (or your LFS) say about this life form. They don't claim theirs won't outgrow the tank, nor do they claim the ones they sell will remain that size. What are you told they eat? You're not told the whole truth. So the best thing to do before you acquire ones of these is to beware that they are NOT to be bought!

Get the truth and facts:
Horseshoe Crab, Limulus polyphemus at MarineBio.org

From the above:
1. They grow large;
2. They are not tropical -- they prefer cooler waters of the Atlantic northern coast;
3. They take so long to reach sexual maturity that their numbers must be kept high or they will easily become extinct. Their eggs supply a lot of other marine life a means of sustenance.

1. Not suitable for a home aquarium;
2. In a tropical marine aquarium, they slowly cook to death -- water temperature should be around 70 or less.
3. Harvesting/collecting them should be crime -- it most likely will be soon.