Species Spotlight: Atlantic Horseshoe Crab
For this week’s species spotlight let’s discuss a species that’s especially important to the CCE Marine Team around this time each year - the Atlantic horseshoe crab!
Horseshoe crabs are often nicknamed “living fossils” because they’ve existed largely unchanged for at least 445 million years - they even predate dinosaurs!
Did you know horseshoe crabs play a vital role in human health? Their blood is used to create a test (the Limulus Amebocyte Lysate, or LAL, test) that pharmaceutical companies use to test the sterility of their products. These tests ensure that heart stents, pacemakers, joint and cataract replacements, radioactive tracers in PET scans, along with millions of doses of flu vaccines, insulin, and intravenously delivered antibiotics and chemotherapies, are free of endotoxin (a toxin that is dangerous to humans).
Essentially almost everyone has benefited from a horseshoe crab at some point in their life!
So if you spot a horseshoe crab while you’re out enjoying your local beaches this summer, remember you’re looking at a species that’s nearly identical to its ancient relatives, and don’t forget to thank it for all the ways it has helped modern science!
Atlantic horseshoe crab
Limulus polyphemus
Horseshoe crabs are harmless to humans! Their tail is only used to flip themselves over when they get stuck on their back.
The topside of a horseshoe crab.
The underside of a horseshoe crab. Note the “boxing glove claws” of the male.
Horseshoe crabs gather in large aggregations for spawning every year.
Juvenile horseshoe crab molt
Atlantic Horseshoe Crab
Scientific Name: Limulus polyphemus
Identification: The horseshoe crab has a greenish-brown, segmented exoskeleton. These three segments are called the prosoma (head and vital organs), opisthoma (abdomen and gills), and telson (tail). They have ten eyes, ten legs, and can grow up to 2 feet in length. Mature females are larger than mature males. Mature males also have a modified first set of walking legs that resemble boxing gloves.
Diet: Mostly small mollusks, crustaceans, and worms. They’re not picky as long as they can crush it with their claws before passing it into their mouth.
Habitat: Adult horseshoe crabs spend most of their time deeper in the ocean until they return to protected, sandy beaches for spawning each year.
Find this Species: Try looking for horseshoe crabs at high tide during spring and summer along sandy beaches with low wave action. It’s quite a sight to see spawning season in full swing!
Interesting Facts: Despite their name, horseshoe crabs aren’t true crabs! They’re in a class of their very own called Merostomata and are more closely related to spiders and ticks than crabs and lobsters!
Their tail is harmless! It is not poisonous and does not sting, it’s used to flip themselves over when they are stuck on their back. Remember: never pick up a horseshoe crab by its tail, this can injure the animal!
Their blood is an unusual bright blue color! It’s copper-based, not iron-based like ours.
Beachgoers often mistake horseshoe crab molts for dead crabs. Like all arthropods, horseshoe crabs must shed their old exoskeleton to grow. They will continue to molt until they are fully matured - which isn’t until about 10 years old!
Females can deposit up to 100,000 eggs each mating season! A single cluster can be up to 4,000 eggs, and they will lay multiple clusters each season.
Horseshoe crab eggs are a major food source for migrating shorebirds, such as the Red Knot.
They are also an important species to our fisheries. Horseshoe crabs are the preferred bait for both American eel and whelk.
There are only 4 extant species of horseshoe crabs, and only 1 of these 4 can be found in America (Limulus polyphemus).