Atlantic Horseshoe Crab (Limulus polyphemus)

The Atlantic Horseshoe Crab is one of the most extraordinary animals available in the marine hobby, and one of the most important animals on Earth. Despite its name, it is not a true crab or even a crustacean; it is more closely related to spiders and scorpions, representing a highly divergent lineage with a deep evolutionary history stretching back over 400 million years. It has remained virtually unchanged across that vast span of time, earning it the title of living fossil. Its domed, helmet-shaped carapace, multiple jointed legs, and long spike-like telson tail give it a silhouette unlike anything else in the hobby, and for the marine enthusiast with the right setup, keeping one is a genuinely humbling experience. This listing comes with full transparency: the Atlantic Horseshoe Crab is a magnificent and long-lived animal that demands a very specific commitment from the hobbyist, and it is not suitable for most home aquarium setups at adult size. Juveniles, however, can be maintained responsibly in appropriately planned systems by experienced keepers.

Why we love it

• One of the oldest surviving animal lineages on the planet; a living connection to the Ordovician period kept in your own home
• Utterly unique silhouette and movement style, unlike anything else in the marine hobby; an instant conversation piece
• A tireless substrate worker; adults burrow through sediments to feed on polychaetes, bivalves, and other benthic fauna, constantly aerating and turning the sandbed
• Fully peaceful toward all fish and most larger invertebrates; poses no threat to anything it cannot physically bulldoze or eat
• A fascinating and scientifically significant animal that carries immense ecological and medical importance in the natural world

Suggested parameters

• Temperature: 18 to 24°C (65 to 75°F); this is a temperate to subtropical species that is an ecological generalist capable of tolerating a broad temperature and salinity range, but sustained tropical reef temperatures at the higher end of reef norms are not ideal long-term
• Salinity: 1.023 to 1.025 SG
• pH: 8.1 to 8.4
• Alkalinity: 8 to 12 dKH
• Nitrate: Under 20 ppm; clean, well-oxygenated water with stable parameters is essential
• Stable salinity and dissolved oxygen levels are particularly important; Horseshoe Crabs are sensitive to rapid environmental fluctuations
• Copper-based medications are toxic to all invertebrates and must never be used

Care and temperament

• Difficulty: Advanced; not a casual addition, long-term planning around adult size is mandatory
• Temperament: Peaceful; does not attack other tank inhabitants but will unintentionally disturb corals, dislodge rockwork, and overturn invertebrates on the substrate while bulldozing and burrowing during feeding
• Reef safe: No; incompatible with reef aquariums due to its burrowing, bulldozing behaviour and adult size
• Minimum tank size: 180 gallons (680 litres) is considered the minimum for a single adult; the footprint of the tank matters as much as the volume, as this is a benthic animal that needs extensive floor space to move and forage
• A deep, fine sandy substrate is non-negotiable; this species burrows to feed and rest and needs several inches of soft substrate to engage in natural behaviour
• Juveniles can adapt to home tanks but grow quickly; a long-term upgrade plan must be in place before purchase
• Females grow significantly larger than males; females can reach up to 60 cm including the telson, while males are roughly one third that size
• Pump intakes and other suction hazards must be fully screened; this animal will investigate everything on the substrate

Diet

In the wild, Horseshoe Crabs feed as omnivorous scavengers and substrate hunters, consuming mollusks, polychaete worms, and other sand-dwelling organisms; in captivity they will eat almost any food offered to them.

• Frozen meaty marine foods including mysis, krill, chopped clam, mussel, squid, and shrimp
• Quality sinking carnivore pellets
• Will scavenge uneaten fish food, detritus, and any organic matter that reaches the substrate
• Live or frozen worms are particularly relished and stimulate natural foraging behaviour
• Feed every two to three days; target feed by placing food directly onto the substrate in front of the animal
• A mature sandbed with a healthy population of microfauna provides important supplemental nutrition between feedings

Compatibility and tankmates

• Compatible with most larger, robust fish that will not harass or nip at it
• Not suitable for reef systems with corals, secured aquascaping, or a carefully maintained sandbed community
• Avoid aggressive crustacean-targeting species such as large triggers and puffers
• Larger peaceful fish including tangs, angelfish, and groupers are generally suitable companions in a large FOWLR system
• Do not house with small, delicate bottom-dwelling invertebrates that may be crushed or consumed during foraging

Health and acclimation notes

Horseshoe Crabs are sensitive to rapid parameter changes, particularly in salinity and dissolved oxygen; slow drip acclimation over at least one hour is essential. The telson tail, which looks alarming, is not a stinger; it is used only for righting itself if it becomes flipped onto its back. If you find one upside down, right it gently by supporting the shell from the sides rather than grabbing the telson. Specimens may occasionally go through prolonged inactive periods that can be mistaken for illness; this is normal behaviour, particularly in cooler periods. A lifespan of 20 or more years is possible in the wild; captive lifespans vary considerably based on the quality of the setup and long-term planning around the animal's size.

What you are buying

• You will receive: 1 Atlantic Horseshoe Crab (Limulus polyphemus)
• Carapace colour is greenish brown to tan; the telson and leg undersides are pale; colouration varies between individuals
• Size at time of sale is juvenile; long-term growth must be planned for before purchase

Drip acclimation of at least one hour is strongly recommended. A deep sandy substrate and spacious tank with fully screened pump intakes must be in place before introduction. Copper-based medications must never be used in any system housing this animal. This species is best suited to experienced hobbyists with very large, purpose-planned marine systems.