MacDill Beach Invasions Expected Soon

  • Published
  • By Thomas Farrell
  • Jacksonville District of the U.S. Army Corps of Engineers
MACDILL AIR FORCE BASE, Fla. --  According to informed sources, full-scale beach invasions will take place on the beach of MacDill Air Force Base sometime in April. Base leaders are not concerned, however, and the invasions will not result in a change in the force protection status.

That's because the "invasions" will be merely the start of the mating season for the American horseshoe crab, pointy tailed, ominous-looking creatures that beach walkers encounter during certain high tides.

Horseshoe crab mating migrations take place along the coasts of the Atlantic Ocean and Gulf of Mexico from Maine to the Yucatan Peninsula and are timed to start each spring during the high tides of the full and new moons.

Horseshoe crabs spend most of the year offshore in deeper bay water. Mating, however, takes place right up on the beach face at the high tide line or slightly higher. As a full or new moon approaches during spring and early summer months, adult horseshoe crabs begin to pair up in the nearshore waters of protected bays and lagoons. Each female eventually will be located by a male who will latch to her abdomen with specialized front claws. About an hour or so before peak tide, the march toward the upper beach begins.

When the tide is about at its highest level, females dig depressions about six inches deep in the sand and release three to four thousand eggs that will be externally fertilized by the males. Each female carries about 90,000 eggs. Female crabs can dig other nests during the tidal cycle and can return for other tides until she has exhausted her supply of eggs.

Mating pairs often are surrounded by one to eight or more single males. These single crabs are known as satellite males and provide back-up support for external fertilization.

As the crabs retreat to the water after the peak of high tide, the nests are soon covered with sand. The eggs, which are green to greenish-blue and about 1/16-inch in diameter, will incubate and develop in the warm sediments until horseshoe crab larvae emerge in three to four weeks.

When mating is over, horseshoe crabs retreat to deeper water. There, on the mud-sand bay bottom, the crabs will feed on small worms and clams until the next full or new moon cycle. As spring progresses to summer, the mating cycles dwindle and may cease throughout the summer. At MacDill AFB, the summer heat and seaweed-clogged beaches appear to keep crabs offshore but mating congregations may pick up as conditions improve in the fall.

Horseshoe crabs enjoy an ancient lineage and have been performing mating migrations on protected beaches and coastal bays for more than 300 million years, long before dinosaurs roamed the earth and eons before the first ancestral human ever decided to walk on two legs.

Horseshoe crabs derive their name from the presence of their hard, horseshoe-shaped shell and their assemblage of jointed legs. They look like crabs but are not classified as true crabs. Biologists place horseshoe crabs in a group more closely related to spiders, scorpions, and ticks.

Four species of horseshoe crabs exist worldwide and all reside only along the eastern coasts of North America and Asia. The American horseshoe crab is the only species that lives along the North American coast and the Yucatan Peninsula. The other three species reside along Asian coasts from Japan and Korea south to the East Indies and the Philippine Islands.

Horseshoe crabs play an important role in the ecology of marine ecosystems. Every spring, migratory shore birds flock to shorelines to feast on the abundance of horseshoe crab eggs. Birds such as the red knot, sandpipers, ruddy turnstone, sanderlings, and semipalmated plover have come to rely on the horseshoe crab breeding migrations.
Birds are not the only diners on horseshoe crabs in their early stages. Sand shrimp and many species of fish, such as eels and catfish, prey on horseshoe crab eggs. Blue crabs, spider crabs, and hermit crabs feed on baby horseshoe crabs.

Humans have long benefitted from horseshoe crabs and our use of these animals ranges from historical use as food, bait, and fertilizer to modern-day health applications.

The most important value we receive today involves the use of horseshoe crabs for biomedical applications. In the 1950s, a researcher from Johns Hopkins University discovered that horseshoe crab blood, which is blue, clots immediately in the presence of bacterial toxins, poisons that can rapidly kill a human upon entering the bloodstream. (Horseshoe crab blood is blue because copper forms the base of the oxygen-carrying component whereas the blood of mammals is red because the oxygen carrier is iron based.) The discovery led to the development of an advanced method of testing for the presence of bacterial toxins on medical instruments and other products. Today, with the use of processed horseshoe crab blood, medical workers can rapidly test medical materials for the presence of toxins and can complete in 15 minutes what used to take up to two days.

Horseshoe crabs are docile animals and are harmless to humans. Their tails do not sting and their pincers are too weak to hurt humans. You can take advantage of this by coming to the aid of any crabs you may find that are unable to right themselves after being turned over by waves. In addition, crabs frequently become lodged among rock structures such as seawalls that often line beaches. If you find trapped horseshoe crabs, you can save them by dislodging them from the rocks and returning them to the water. Do not pull the tail, this could harm the crabs.

Because we are experiencing an unusually cold winter, the onset of this year's horseshoe crab breeding cycle may be delayed. Last year, the crabs first appeared on MacDill AFB beaches during the full moon at the middle of March.
Now that you know the timing of horseshoe crab migrations, you can plan a trip to watch them perform their ancient mating rituals. The best place to look for migrating crabs on MacDill AFB is on the beach between the youth center and the large parking lot on Bayshore Boulevard and at the beach next to the base marina on the southern side of the base.

If your timing is right, you will find yourself sharing the beach with creatures whose direct ancestors lived among the giant dinosaurs. How cool is that?

Thomas Farrell is a regulatory biologist with the Jacksonville District of the U.S. Army Corps of Engineers. His office is in Tampa.