Tag Archive: Mariner…

Horseshoe Crabs; the Ancient Mariner

Horseshoe Crabs; the Ancient Mariner

Growing up in the Pensacola area, I do not remember seeing many horseshoe crabs around here, but I do remember them.  What I actually remember was how common they were further east in the Panama City and St. Joe area.  These animals are big fans of grass beds, as are sea urchins and scallops, and all are uncommon in our area now. However, there have been local sightings in recent years, so they may be returning.

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

Horseshoe crabs are actually not crabs at all but members of a group of arthropods called chelicerates, which include spiders, mites, and scorpions.  They differ from crabs in the absence of antenna, in which the crabs have two pairs. Their “horseshoe” shaped body is a body design to plow through the sand and mud searching for food.  They are scavengers and feed on mollusk, worms, and other invertebrates – even algae at times.  They do not have the large claws that blue crabs have, so they grab small bits of food where they can.  Horseshoe crabs do possess a crop and gizzard and have tooth-like structures within the gizzard to grind their food.  When digesting, the flesh is swallowed and the shell is regurgitated.  The large spine near at the end of their bodies makes them appear similar to stingrays, which they are often confused, but the spine is actually a telson and non venomous.  It is used to right itself when flipped and to push themselves in a forward direction.

 

They like shallow water and grassbeds are prime habitat for them. Breeding season is in the spring and early summer.  During the full and new moons, both the large females and smaller males approach sandy beaches in protected areas of the bay.  During the evening, they will begin to emerge into the intertidal zone where the female digs a depression and lays her eggs.  The males, usually riding her back attached by a special hook, will then fertilized the eggs before they are buried.  There may be more than one male trying to court the female (known as satellite males) and the numbers of horseshoe crabs on the beaches can be amazing.

 

Shorebirds, fish, and crabs will feed on the eggs and the young. Sea turtles are known to consume adults.  Being members of the phylum Arthropoda, they will have to molt their exoskeletons as they grow.  Many people will find these thin, tan-colored, molts along the shoreline.

 

There is a fishery for them in some parts of the southeast. They are collected for their blood, which is used in many medical processes needed for surgery and injections, and as bait for eels.  The decline of these animals has been problematic for some species of migratory birds, who feed on their eggs during their migration.  The horseshoe crab is also one of those rare animals that have been around longer than the dinosaurs.  It would be sad to lose this animal on “our watch”.  FWC is interested in where they are nesting.  If you are out walking the beaches of the Florida Panhandle and encounter one of them, please contact the Sea Grant Agent at your county extension office; we are particularly interested in where they may be nesting.  FWC has a website where sightings can be logged, http://www.myfwc.com/research/saltwater/crustaceans/horseshoe-crabs/documenting-beaches/.

 

The full and new moon cycles for spring and summer (2017) are:

 

Full Moon                                                                   New Moon

Apr 10                                                                         Apr 26

May 10                                                                        May 25

Jun 9                                                                            Jun 23

Jul 8                                                                            Jul 23

 

They could nest a few days before or after. We hope you get to see one, they are pretty cool!

PG

Author: Rick O’Connor – roc1@ufl.edu

Sea Grant Extension Agent in Escambia County

Rick O’Connor

Permanent link to this article: http://franklin.ifas.ufl.edu/newsletters/2017/04/15/horseshoe-crabs-the-ancient-mariner/

An Ancient Mariner… the horseshoe crab

An Ancient Mariner… the horseshoe crab

Talk about weird and cool at the same time! The horseshoe crab is one of the oldest living species we have in the Gulf of Mexico.  Fossils of this animal date back to almost 500 million years… this is before there was such a thing as fish!  The separating of Pangea, the rise and fall of the dinosaurs, oh what stories these guys could tell!  And they are here today, trudging along in the soft sands of estuaries along the Atlantic and Gulf coasts… but they seem to be on the decline.  After all they have been through… they may be slipping away.

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

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

Actually, horseshoe crabs are not crabs at all. They belong to the same large group of animals the crabs belong to, Arthropods, but differ from true crabs in that they have fewer jointed legs and no antennae.  They are actually more closely related to spiders and scorpions.  There are 4 species remaining on Earth. Limulus polyphemus is the local variety with the other three living in Asia.  They are tolerant of a wide range of environmental conditions.  Huge swings in water temperature and salinity do not bother them.  This is not surprising considering all of the environmental changes that have occurred since the species first appeared on the planet.  They are scavengers, plowing through the soft bottom of estuaries, they feed on worms, mollusk, and whatever else their crop-gizzard system can breakdown.  Their protective shell deters many predators; most horseshoe crabs meet their fate on the beach – where they must go to breed.

 

Breeding occurs all year in Florida. It typically takes place three days before and after the new or full moon.  The smaller males come near shore and patrol for the oncoming females.  As the females are intercepted the males will use their “hook” to hold on and the pair ride onto the beach.  This usually happens at night (though not always) during the peak of a spring high tide.  The female digs a small depression and deposits between 200 and 300 eggs, the male fertilizes them, and the female buries them.  They leave the young on their own for a month, at which time the next spring tide arrives and the larva, which resemble trilobites, emerge.  Many fall prey to shorebirds and many adults actually become stranded on the beach during nesting and die.

 

So why the population decline?

Well, they do tolerate large swings in environmental change, so increase temperatures, rainfall, salinities, do not bother them. Studies have shown that they are actually quite tolerant of many of the pollutants, including oil, we discharge into our bays – though mercury is a problem for the developing trilobite larva.  Along the Atlantic coast the animals are collected for bait and the biomedical industry.  Horseshoe crabs are used in eel traps and there are several medical uses for their blood.  Some biomedical industries collect the crabs, remove some of the blood, and return them – but not all survive this.  A big problem they are facing, and this would be closer to home, is the loss of nesting habitat.  Seawalls, jetties, groins, and coastal development in general have disturbed nesting beaches.

 

That said, they seem to be making a comeback on Pensacola Beach. There have been sightings at both Big and Little Sabine.  We would like to record where they are nesting in the panhandle.  If you would like to help – the full moon for the next few months will occur on March 22, April 22, May 21, and June 20.  The new moon will occur March 8, April 7, May 6, and June 4.  If you do see a horseshoe crab please contact me at (850) 475-5230, or email at roc1@ufl.edu.

PG

Author: Rick O’Connor – roc1@ufl.edu

Sea Grant Extension Agent in Escambia County

Rick O’Connor

Permanent link to this article: http://franklin.ifas.ufl.edu/newsletters/2016/03/11/an-ancient-mariner-the-horseshoe-crab/