Limulidae (Pfeilschwanzkrebse)

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Horseshoe crabs are not Crustaceans. They are actually closer related to spiders and scorpions than crabs.
I have placed them under Crustacean because many people think of them as crabs.

Horseshoe crabs requires a large aquarium with a deep sandbed.
In most aquariums they will starve. Live rock formations may damage horseshoe crabs.