Horseshoe Crab Looks Like an Alien, If We're Being Honest

The horseshoe crab might look like a creature from another planet, but it’s a vital part of Earth’s marine ecosystems.

Often called living fossils, horseshoe crabs have existed for over 440 million years. They’re not even true crabs; they’re more closely related to spiders and scorpions.

The American horseshoe crab (Limulus polyphemus) thrives along the Atlantic coast, from northern Maine to the Gulf coasts. During spring and summer, they crawl onto sandy beaches in Delaware Bay and other regions to spawn.

Horseshoe Crab Spawning and Eggs

When female horseshoe crabs lay eggs, it sets off a vital food chain. Females lay thousands of horseshoe crab eggs in shallow sand at high tides, often around the full moon and new moon. A female digs a nest, then several satellite males fertilize the eggs.

Within about two to four weeks, these eggs hatch into horseshoe crab larvae, eventually becoming juvenile horseshoe crabs. As those young horseshoe crabs reach adulthood, they migrate to deeper waters to mature.

Prey at All Life Stages

The eggs are a crucial food source for migratory shorebirds like the red knot, and many fish species also feed on these eggs, while adult horseshoe crabs serve as prey for sea turtles such as the loggerhead.

A drop in horseshoe crab abundance directly affects future generations of other animals.

Anatomy and Adaptations

Horseshoe crabs have a hard shell or hard exoskeleton and five pairs of jointed walking legs, including two pusher legs for digging. Their compound eyes, lateral eyes, and median eyes help them navigate the ocean floor.

They breathe using book gills and have a unique circulatory system. Horseshoe crab blood contains a compound sensitive to bacterial toxins, making it essential for detecting contamination in vaccines and IVs. This has made them indispensable to the biomedical industry.

Harvest and Conservation

Horseshoe crabs harvested — Harvested horseshoe crabs on ice. Akaradech Pramoonsin / Getty Images

Over a half million adult horseshoe crabs are harvested annually, mostly along the Atlantic Ocean and Southeast regions.

Their blood is used in medical testing, and in the past, millions of horseshoe crabs were harvested for fertilizer and livestock feed. Today they are taken primarily as bait for American eel and whelk (conch) fisheries.

States like South Carolina and agencies like the Department of Natural Resources and South Carolina Department of Natural Resources monitor the horseshoe crab harvest to balance ecological research with industry needs. Still, overharvesting and habitat loss remain concerns.

Protecting horseshoe crab species isn’t just about the crabs. It supports ecosystems and ensures enough more eggs for migratory birds, turtles, and scientific reports tracking ocean health.

We created this article in conjunction with AI technology, then made sure it was fact-checked and edited by a HowStuffWorks editor.