Something prehistoric lurks in the St. Johns River with precious blood

Did you know that there are prehistoric animals lurking in the St. Johns River? But don't be too alarmed. They are horseshoe crabs and they are hardly dangerous.

They show up in the fossil record over 445 million years ago during the Ordovician period. These ancient marine arthropods can be referred to as "living fossils" that predate dinosaurs by almost 200 million years. Though they share the word "crab," they are not crustaceans but are more closely linked to arachnids like spiders and scorpions.

These unique creatures are classified as Xiphosura, which can be translated with xiphos meaning "sword" and oura meaning "tail," hence the name "sword tail." This name comes from the long, sharp and pointy tail spine or telson, which is used to help the creature flip itself over if it gets stuck on its back.

The body of a horseshoe crab is divided into three main parts: the cephalothorax, abdomen and telson. The largest of these, the cephalothorax, houses most of the animal's eyes, limbs and internal organs. It is also where the animal gets its name, as its shape somewhat resembles that of a horseshoe. Horseshoe crabs have changed little since they first appeared in the Triassic period around 250 million years ago, and similar-looking fossil xiphosurans extend back to the Ordovician around 445 million years ago.

Most horseshoe crabs are marine and commonly found in brackish estuarine ecosystems such as the Delaware and Chesapeake bays. I recently was vacationing with my family in Cape May, New Jersey, on Delaware Bay. The horseshoe crabs were along the shoreline in abundance. It's mating season and they had crawled up into the intertidal zone to find a partner. This made for some interesting discussions with my grandchildren about the cycle of life.

Horseshoe crabs, like true crabs, also must molt or shed their shells in order to grow and get larger. So, you can encounter what appears to be a dead crab only to discover that it is just an empty molt.

You can encounter horseshoe crabs in the St. Johns River too. According to Florida Fish and Wildlife Commission catch data, they are more common in the St. Marys River, Nassau River and along the Intracoastal Waterway. In the St. Johns, they mostly occur east of the Trout River. These are the more estuarine or brackish water areas in Northeast Florida.

Horseshoe crabs are often caught and even maintained in aquaria for their blood. It has a bright-blue copper-based blood that also contains special amebocyte cells. The horseshoe crab's (and most other crustaceans') blood turns bright blue when exposed to oxygen. That is because it uses copper-rich protein called hemocyanin to transport oxygen. We humans use the iron-rich hemoglobin that turns bright red for the same purpose.

Horseshoe crab blood also contains special amebocyte cells. A unique property of these cells is they instantly clot when they encounter bacterial endotoxins. In nature, when those cells meet invading bacteria, they clot around it and protect the rest of the horseshoe crab's body from toxins. Scientists have used these special cells to develop a quality assurance test to check new vaccines for contamination. This technique has been used extensively since the 1970s and is a valuable use of the blood.

Regretfully, in recent years, horseshoe crabs have experienced a population decline. This is mainly due to coastal habitat destruction and overharvesting. To ensure their continued existence, many areas have enacted regulations on harvesting and protecting these ancient creatures.

Glad you asked River Life

Do horseshoe crabs have any other uses other than harvesting their blood?

Yes, they are important for other biomedical research too. And even though they are primitive, their eye structure and neurophysiology have led to several important discoveries that help us understand human physiology. When I was doing neurophysiology research for my master's degree, the first animal I experimented on was a horseshoe crab and its eye function. I still find the creature fascinating.

River Life runs the first Tuesday of each month in The Florida Times-Union. Email Quinton White, professor emeritus and former executive director of Jacksonville University's Marine Science Research Institute, with questions about our waterways at qwhite@ju.edu. For more on the MSRI, visit ju.edu/msri.

This article originally appeared on Florida Times-Union: Something prehistoric lurks in the St. Johns River with precious blood

Reporting by Quinton White, Special to Jacksonville Florida Times-Union USA TODAY NETWORK / Florida Times-Union

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This story was originally published July 7, 2026 at 5:11 AM.