Britannica Sources used in this answer

How large can American horseshoe crabs grow?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Horseshoe crab (Limulus polyphemus) with a hard, rounded shell and long pointed tail lying on sandy ground. — horseshoe crab Horseshoe crab (*Limulus polyphemus*).

The American horseshoe crab (Limulus polyphemus) can reach lengths of more than 60 cm (2 feet).1

While they can grow to this size, there is a typical difference in average length between the sexes:
* Males: Average 36.6–38.1 cm (14–15 inches).1
* Females: Average 45.7–48.3 cm (18–19 inches).1