The horseshoe crab (Limulus polyphemus) was designated as the official marine animal symbol for Delaware in 2002. Delaware Bay is the world's largest spawning ground for horseshoe crabs. These ancient creatures predate most species on the planet, including flying insects and dinosaurs, and are considered "near- threatened" according to Wikipedia.

Horseshoe crabs are an essential food source for migratory birds on the East coast and are also important to humankind - they are used in vision studies (their 10 eyes have a structure similar to the human eye); their blood protects them from infection and a compound it contains is used to detect bacterial poisons in some human medications; and a natural polymer found in the horseshoe crab’s shell (chitin) is used to make bandages.

Source:

Delaware Facts and Symbols: Official Delaware Website
The Horsehoe Crab - Natural History: Horseshoecrab.org

Links:

State Fish & Waterlife Symbols - 50 States List
State Animals - Mammals: 50 States List