The Horseshoe Crab

Florida Fish and Wildlife Conservation Commision is looking for help from beachgoers during the full and new moons to help with research on the spawning of the horseshoe crab on all Florida beaches. Spawning usually happens just before or after a high tide.  Tonight is a full moon; April 26 there will be a new moon.

According to a press release I saw: “Observant beachgoers can report the time, date and location of  horseshoe-crab sightings through one of several convenient options. Go to
http://research. horseshoe_ crab and fill out an online survey;  E-mail findings to horseshoe@MyFWC. com; or call the FWC at 866-252-9326.

Biologists also want to know the number of horseshoe crabs seen by observers and whether the horseshoe crabs are mating. They also want to know the date, time, location and habitat conditions. If possible, specify roughly how many are coupled and how many are juveniles (4 inches wide or smaller).”

Horseshoe crabs spawn at night. That means they come out of the water onto the beach and lay their eggs.  The males, which are smaller than the females, are the first to arrive on the beach. So, the hang out at around the water line and wait for the females.  Once the girls show up, the guys, using their front pincers, grab hold of the female opisthosma (which is the back edge of its mid-section) and follow her up onto the beach.   When she finds a sandy spot she likes, she’ll burrow into the sand to form a nest.  It is not very deep; roughly 8 inches at the most from what I understand.  She will lay her eggs in the nest and as she leaves, the male (I guess still hanging on) will fertilize the eggs as he passes over the nest.

Amazingly the female can set up several nests on one trip, and she can make additional trips on subsequent tides.   But because of this abundance of activity she could dig up a nest that one of her friends just laid, causing an accumulation of exposed eggs.  But that’s ok.  Studies show that one female can lay 88K of these eggs each season.  The eggs left exposed become breakfast for hungry shorebirds, especially Red Knots, either on migration or during their own nesting season during which they require lots of food.  The unexposed eggs left in their nests will hatch.

So now the eggs are laid, and the nests are secure, but where are they and how long until the eggs hatch?  The nests are usually found between the high and low tide marks on the beach.  The eggs will hatch in about a month releasing the larvae to swim around for a few days before settling down on the sea floor.  Surviving larvae take a long time to grow.  Over the next several years they continue to molt. It will take about 10 years for the horseshoe crab to mature and return to the beach to begin spawning.  They can live to be approximately 16 years old.

The molting process makes it fun for beachcombers in areas where the horseshoe crabs nest.  The young will hang out for a couple of years near its place of birth.  So if you know where previous nests were, keep your eyes open for the horseshoe crab’s molted remains.  We have found numerous horseshoe crab shells around Florida; from Singer Island to Gasparilla Island, Siesta Key and St. George Island. 101_2544 We have collected a varitey of sizes: as small as a quarter to 7 inches wide.  Horseshoe crabs can grow to be as big as 2 feet across.  That would be cool to see.  The small ones are paper thin so if you do find one on the beach, be very careful handling it until you get it home. Even after it sits dry for a few days, its still fragile.  We’ve learned this the hard way, thinking we got it home safely in a bucket but finding it crumpled.  When we visited St. George Island in Apalachicola, we found an amazing amount of horseshoe crabs and empty horseshoe-crab shells washed ashore (maybe from the storm the night before).  In order to get a few of the smaller shells home I put them in separate ziplock type bags and kept air in the bag to cushion the shells; the big ones didn’t require a bag but be aware that they can get stinky.

Horseshoe crabs aren’t even really crabs.  They’re more related to spiders, which is pretty creepy to me, but looking at a horseshoe crab underneath, when it’s moving its little legs is pretty creepy, too.  I can tell you why it’s not a crab but can’t find out why its a spider.  I think the deciding factor on this non-relationship to crabs is that horseshoe crabs lack the mandible, antennae, one pair of claws and 4 pairs of legs that “true” crabs are supposed to have.  The horseshoe crab actually has 5 pairs of legs and, what are referred to as pincers rather than claws.

Do you know what the spike on the back is?  Many people think it could be a barb similar to that on a ray and could be dangerous.  But it’s not.  The spike is called a telson and its primary purpose is to enable the creature to turn itself over.  Sometimes, they might get flipped in a major storm and just can’t get the energy (I guess) to flip themselves so the Ecological Research and Development Group started a “Just Flip ‘Em” program to remind people to help the crab turn over and get back to sea.

Here are some more facts about the horseshoe crab:  A horseshoe crab has five pairs of eyes; it has 5 pairs of gills that allow it to breathe on land as long as the gills stay moist;  the horseshoe crab  has blue blood; they are used in the research of the human eye and their shell material is used to make contact lenses, along with other products; horseshoe crab blood enzymes are used by in the International Space Station to test surfaces for unwanted bacteria and fungus; and while some Canadian and American northerners winter in Florida, horseshoe crabs prefer to spend their winter on the continental shelf.

Horseshoe crabs are such an amazing creation of God’s and they’re fun to learn about.  For more information of this creature and conservation efforts go to  There’s a ton of good information and also teacher resources.  Parents, use the teacher resources for your children.  I did that constantly and then decided it was so much fun to teach the boys, I now homeschool them.  For coloring pages and blacklines of horseshoe crab anatomy go to