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In Search of Horseshoe Crabs

Back in the spring, I wrote an article about the natural history of this ancient animal. However, Florida Fish and Wildlife Conservation Commission (FWC) is interested in the status of horseshoe crabs and they need to know locations where they are breeding – and Florida Sea Grant is trying to help.

Horseshoe crabs breeding on the beach.
Photo: Florida Sea Grant

If you are not familiar with the horseshoe crab, it is a bizarre looking creature. At first glance, you might mistake it for a stingray.  It has the same basic shape and a long spine for a tail.  But further observation you would realize it is not a stingray at all.

 

So then… What is it?

 

When you find one, most are not comfortable with the idea of picking it up to look closer. The spine is probably dangerous and there are numerous smaller spines on the body.  Actually, the long spine in the tail region is not dangerous.  It is called a telson and is most often used by the animal to push through the environment when needed, as well as righting itself when upside down.  It is on a ball-and-socket joint and if you pick them up, they will swing it around – albeit slowly – but it is of no danger.  Note though, do not pick them up by the telson – this can damage them.

 

If you do try to pick them up with your hands on their sides, you will find they are well armored and have numerous clawed legs on the bottom side. At first, you are thinking it is a crab, and the claws are going to pinch, but again we would be mistaken.  The claws are quite harmless – they even tickle when handled.  I have held them to allow kids to place their hands in there to feel this.  However, when held they will bend their abdomen between 90° and 120°, as if attempting to roll into a ball – which they cannot.  At this point, they become difficult to hold.  Your hands feel they are in the way and the small spines on the side of the abdomen begin to pierce your skin.  So, you flip it on its back.  It begins to try a 90° bend in the other direction and begins to swing the telson around.  This is probably the most comfortable position for you to hold – but I am not sure what the crab thinks about it.

 

So, what do you have?

 

Well, you can see why they call it a crab. It has clawed legs and a hard shell.  The body is very segmented.  You can also see why it is called a “horseshoe”.  But actually, it is not a crab.

 

Crabs are crustaceans. Crustaceans have two body segments – a head and abdomen, no middle thorax as found on insects.  This is the case with the horse crab as well.

 

Crustaceans have 10-segmented legs, though the claw (cheliped) and swimming paddles (swimmerets) of the blue crab count as “segmented legs”. Horseshoe crabs have 10 as well – seems this IS a crab – but wait…

 

Crustaceans have two sets of antenna – two short ones and two long – horseshoe crabs do not have any antenna. Traditionally biologists have divided arthropods into two subphyla – those with antenna and those without – so the horseshoe crab is not a crab.  It is actually more closely related to spiders, ticks, and scorpions.

Blue crabs are one of the few crabs with swimming appendages.
Photo: Molly O’Connor

It is an ancient animal, fossil horseshoe crabs in this form date back over 440 million years – out dating the dinosaurs. There are four different species of today and there probably were more species in the past.  Their range extends from the tropics and temperate coastlines of the planet.  Today three of the remaining four species live in Southeast Asia.  The fourth, Limulus polyphemus, lives along the eastern and Gulf coast of the United States.

 

Unfortunately, this neat and ancient creature is becoming rare in some parts of its range. There is a commercial harvest for them.  Their blood is actually blue and contains properties beneficially in medicine.  Smaller ones are used as bait in the eel fishery, and there is always the classic loss of habitat.  These are estuarine creatures and are often found in seagrass and muddy bottom habitats where they forage on bottom dwelling (benthic) animals.

 

FWC is interested in where horseshoe crabs still breed in our state. Some Sea Grant Agents in the panhandle are assisting by working with locals to report sightings.  Sea Grant also has a citizen scientist tagging program to help assess their status.  Horseshoe crabs typically breed in the spring and fall during the new and full moons.  On those days, they are most likely to lay their eggs along the shoreline during the high tide.  This month the full moon is October 5 and the new moon is October 19.  We ask locals who live along the coast to search for breeding pairs on October 4-6 and October 18-20 during high tides.  If you find breeding pairs, or better yet, animals along the beach laying eggs – please contact your local Sea Grant Agent.  We will conduct these surveys in the spring and fall of 2018 and post best search dates at that time.

 

For more information on the biology of this animal read http://escambia.ifas.ufl.edu/marine/2017/04/10/our-ancient-mariner-the-horseshoe-crab/.

 

 

References

 

Barnes, R.D. 1980. Invertebrate Zoology. Saunders College Publishing. Philadelphia PA. pp 1089.

 

Florida Fish and Wildlife Conservation Commission. Facts About Horseshoe Crabs https://www.sciencedaily.com/releases/2008/02/080207135801.htm

 

Oldest Horseshoe Crab Fossil Found, 445 Million Years Old https://www.sciencedaily.com/releases/2008/02/080207135801.htm

PG

Author: Rick O'Connor - roc1@ufl.edu

Sea Grant Extension Agent in Escambia County