Horseshoe Crab

Quick facts

Handling caution: Long telson and sharp book-gill edges can pinch or scrape; handle by carapace edges, never by the tail

Identity

Also known as

American Horseshoe Crab; Atlantic Horseshoe Crab; King Crab

Synonyms

Xiphosura polyphemus

Environment

Saltwater

Native to

United States (Atlantic and Gulf coasts), Mexico (Gulf coast)

Habitat

Estuaries, sandy and muddy intertidal flats, and shallow coastal waters; adults burrow into soft substrate and move to deeper water in winter

Sourcing

Wild-caught

Conservation

Not Evaluated

Care

Care level

Moderate

Temperament

Peaceful

Adult size

2 – 24 in (5.1 – 61 cm)

Lifespan

15 – 20 yrs

Min tank

180 gal (681 L)

Growth rate

Slow

Placement

Bottom

Water

Temperature

68 – 77 °F (20 – 25 °C)

pH

7.5 – 8.8

Salinity

1.020 – 1.026 SG

Flow

Low-Moderate

Behavior & compatibility

Social

Solitary

Aggression to own kind

Low

Active

Night

Reef safe

With caution

Invert safe

With caution

Shrimp safe

With caution

Clean-up role

Detritus, Sand sifting, Leftover food

Population boom risk

Low

Feeding & breeding

Diet

Carnivore

Foods

Frozen, Live, Freeze-dried

Breeding type

Broadcast spawner

Breeding difficulty

Not bred in captivity

Sexing

Males are noticeably smaller than females and have modified clasper-like first walking legs used to grip the female during mating; females lack claspers and grow much larger

Target feeding

Beneficial

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Care guide

Overview

Greenish-brown to tan coloring covers a rounded, helmet-shaped carapace trailing a long, rigid telson. As a juvenile it sifts sand for detritus and leftover food, but growth is slow and steady toward a large, long-lived bottom dweller that eventually outgrows most home systems and needs a deep sand bed plus mature water quality.

Care

A large, long-lived arthropod that needs a very spacious tank with a deep sand bed for burrowing and a stable, mature marine system to support its bioload. Juveniles are hardy and adaptable, but adults require far more space than most home aquariums can provide and are better suited to large systems or public aquaria. Good water quality is essential, since ammonia buildup and poor filtration are common causes of stress and death in captivity.

Feeding

Wild individuals feed on mollusks, worms, crustaceans, and organic detritus picked from the substrate. In captivity offer chopped squid, shrimp, clams, and pieces of fish, along with frozen meaty foods, dropped near the substrate where the crab can locate them with its walking legs. Small juveniles will also graze biofilm and leftover food scattered across the sand bed.

Tank mates

Small juveniles are generally peaceful scavengers that coexist with most reef inhabitants, but as they grow past a few inches they become capable of preying on worms, small clams, snails, and slow-moving fish. They should not be kept with delicate sessile invertebrates or small crustaceans once they reach a larger size. Their constant burrowing can also uproot corals or plants placed directly in the sand.

Breeding

Spawning occurs naturally on sandy beaches during spring and summer high tides tied to new and full moons, conditions essentially impossible to replicate in a home aquarium. Females bury clusters of eggs in the intertidal sand while males clasp onto their carapace to fertilize the eggs externally. Captive breeding is restricted to specialized research and aquaculture facilities using hatching jars and controlled water flow.

Health

Molting is the most vulnerable period, and soft-shelled individuals should be left undisturbed until the new exoskeleton hardens. Watch for signs of ammonia stress, gas supersaturation, or shell damage from sharp decor or aggressive tankmates. Old, non-molting adults naturally accumulate barnacles and dark pitting on the carapace, which is a normal sign of age rather than disease.

Sources

1. FA254: Candidate Species for Florida Aquaculture: American Horseshoe Crab Limulus polyphemus
2. Clinical Evaluation, Common Diseases, and Veterinary Care of the Horseshoe Crab, Limulus polyphemus
3. Atlantic Horseshoe Crab | Online Learning Center | Aquarium of the Pacific
4. SG190: The American Horseshoe Crab (Limulus polyphemus)