TankBud

Horseshoe crab

Limulus polyphemus

Also known as Atlantic horseshoe crab, American horseshoe crab

SaltwaterInvertebrate

Horseshoe crab (Limulus polyphemus)
Hans Hillewaert · CC BY-SA 4.0

Care at a glance

Temperature
68–79°F
20–26°C
pH
8.1–8.4
Adult size
23.6 in
60 cm
Minimum tank
185 gal
700 L
Temperament
Peaceful
Social behavior
Solitary
Tankmates
With caution
Swim level
Bottom
Diet
Carnivore
Difficulty
Difficult
Breeding
Hard
Rarity
Common
Reef safe
Not reef safe
Family
Limulidae
Origin
Atlantic coast of North America, Maine to Florida, the Gulf of Mexico, and the Yucatan

Overview

The horseshoe crab is a peaceful sand dweller best suited to a public aquarium exhibit. It is a difficult animal, for experienced keepers only.

Did you know

Horseshoe crab blood is blue, because it carries oxygen with copper-based hemocyanin instead of iron-based hemoglobin. The blood clots the instant it touches bacterial toxins. Drug companies harvest it to make Limulus amebocyte lysate, the standard test for endotoxin contamination in vaccines, injectable drugs and implants.

Caring for the horseshoe crab

A horseshoe crab usually starves in a home tank. The ones in the trade are small juveniles of 3 to 10 cm, collected on the US coast. A juvenile soon strips a sand bed of worms and does not compete well for sinking food. Nearly all of them waste away over a few months.

If you keep one anyway, give it a tank of at least 185 gal (700 L) with a deep, wide, mature sand bed. Use almost no rock, since rockwork blocks the animal and scrapes it up. Keep the water at 68 to 79°F (20 to 26°C) and a pH of 8.1 to 8.4.

It plows through the sand looking for worms, clams and other small animals. It has no jaws, so it grinds food with the bases of its legs. Target feed it pieces of squid, shrimp and clam every day.

A horseshoe crab grows too large for home tanks. Females reach about 30 cm across the shell, and they molt around 17 times over roughly nine years before maturing. Never pick one up by the tail.

It is peaceful, but choose its tank mates with caution. It is not reef safe, since it bulldozes sand, topples rock, and eats the clams and worms a reef relies on. Breeding it is hard, and wild adults come ashore in spring to spawn on sheltered beaches.

Good to know

Limulus polyphemus is not a crab. It is a chelicerate in the family Limulidae, closer to spiders and scorpions than to any crustacean. It is also the only horseshoe crab in the Americas. Wild animals live 20 years or more. The species is listed as Vulnerable by the IUCN, because of harvest for bait and biomedical bleeding, plus the loss of spawning beaches. Bled crabs are returned to the sea, but an estimated 10 to 30 percent die afterward.

In the wild

Horseshoe crabs live on sand and mud bottoms along the Atlantic coast of North America. Their range runs from Maine to Florida and around the Gulf of Mexico to the Yucatan. They live from tidal flats out to the edge of the continental shelf. In the wild, the species handles much colder water.

Common questions

What size tank does a horseshoe crab need?

A horseshoe crab needs at least 185 gal (700 L). It spends most of its time in the bottom.

What temperature and pH does a horseshoe crab need?

A horseshoe crab is comfortable at 68–79°F (20–26°C) with a pH of 8.1–8.4.

What does a horseshoe crab eat?

A horseshoe crab plows through the sand for worms, clams and other small animals. In a tank, target feed it pieces of squid, shrimp and clam every day.

How big does a horseshoe crab get?

An adult horseshoe crab reaches about 23.6 in (60 cm).

How does a horseshoe crab breed?

Horseshoe crabs come ashore in spring to spawn on sheltered beaches. Breeding one in a tank is hard.

Related species