Horseshoe Crab (Limulus Polyphemus)

Also Known As: Atlantic Horseshoe Crab

Compatibility and Behavior
Here is a fun bit of trivia to lead with: the Horseshoe Crab (Limulus polyphemus) isn't actually a crab at all. It is a distant relative of spiders and scorpions, and the lineage is so ancient it predates the dinosaurs, which is exactly why it brings such a genuinely prehistoric look to a tank. The hard, dome-shaped carapace, long pointed tail spine, and rows of walking legs give it an armored-rover quality as it glides across the sand, frequently plowing right under it with only part of the shell left exposed. It spends nearly all its time doing one thing: sifting and bulldozing through the substrate in search of worms, small inverts, and leftover food. It does not bother fish or corals directly and is peaceful by nature, but its appetite and digging are the real planning considerations, which is why this one is reef safe with caution. A horseshoe crab can quickly strip a smaller tank of the beneficial sand fauna it relies on, and it will happily bulldoze unsecured frags and rockwork as it goes. It is a much better fit for large, spacious systems or species-specific setups than for a typical packed mixed reef.

Aquarium Requirements
Everything about housing this animal comes back to sand and space. It needs a deep, soft sand bed and plenty of open bottom to roam, which makes it a poor match for small or heavily rock-packed reefs. Even small individuals in the 1 to 3.5 inch range belong in larger, mature systems with wide footprints, extensive sandy areas, and a well-established population of worms and microfauna to support natural feeding. Any rockwork should be sturdy and well braced to survive the constant plowing, and loose frags need to be secured so they don't get overturned. Two things are worth being honest with yourself about before taking one on: they can outgrow home aquariums over time, and they can deplete their own food sources in a closed system, so plan on supplemental meaty foods like chopped seafood and sinking carnivore pellets, and go in understanding this is an animal built for a large, dedicated display rather than a standard reef.

Specifications
Approximate Size: Sm/Md: 1–2.5", Lg: 2.5–3.5" (can grow larger over time)
Diet: Carnivorous scavenger; worms, small invertebrates, detritus, leftover meaty foods, and sinking pellets
Origin: Western Atlantic and Gulf of Mexico
Care Level: Advanced
Reef Safe: With caution
Temperament: Peaceful
Special Considerations: Powerful digger that can bulldoze rockwork and loose frags and deplete beneficial sand fauna; needs a deep sand bed and a large, open-bottom system