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A Living Fossil

With fossils dating back to nearly 350 million years ago, it’s hard to believe that our favorite shoreline friends were once crawling across the oceans floor the same time dinosaurs were around. These sea creatures have adapted to their ecosystems over the eons and still remained ultimately unchanged.

Their flexible exoskeleton (made of chitin) has acted as an unscathed armor which has resulted in survival where even the mightiest dinosaurs of have been wiped out due to major events causing extinction.

horseshoe-crab-anatomy-usf

The horseshoe crab has a unique body structure– The body is composed of three parts: the prosoma (head), opisthosoma (central area) and telson (tail). The name is derived from the prosoma, resembling the shape of a horse’s shoe. The telson helps the crab to flip itself over if waves on the beach turn it over. As dangerous as the telson may look, it is not venomous or used as a weapon by the crab. Horseshoe crabs have several pairs of eyes. The two main sets are: two large compound eyes on top of the prosoma, and a pair of simple eyes on the forward side of the prosoma. The eyes on top of the prosoma are sensitive to polarized light and can magnify sunlight 10 times. The eyes on the forward side of the prosoma can sense ultraviolet light from the moon.

There are multiple eye spots are located under the prosoma, with more on the underside of the tail (horseshoe crabs occasionally will swim upside down and may once have actually used these eyes more than they do today). Horseshoe crabs use “book” gills to get oxygen from the water– small flaps resembling the pages of a book. If their gills stay moist, horseshoe crabs can stay out of water for up to four days.
Horseshoe crabs have no jaws or teeth. Their mouth is composed of spiny bristles (it feels like a toothbrush!) at the base of five pairs of legs to maneuver food items such as: razor clams, soft-shelled clams, and marine worms into their centrally located mouth.

In late spring and early summer (May and June) horseshoe crabs arrive on the beaches during the high tides of full and new moons to lay their eggs –  this is when the water rises highest on the beach. When the female is ready to lay her eggs, she crawls up to the high water line on the beach with a male attached to her. The male clasps onto the female’s shell with a modified pair of claws and the female drags him around during the spawning process. In addition to the attached male, several other males may also attempt to fertilize the female’s eggs by arranging themselves on and around the spawning couple during the egg- laying process. A female may have five or more males attempting to mate with her in a single egg-laying episode. On the beach, the female crab partially buries herself into the sand while she deposits a cluster of about 4,000 tiny green eggs— in an evening of egg laying, a female crab can lay several egg clusters and may spawn repeatedly over several nights to lay 100,000 or more eggs.

Within two to four weeks the eggs will hatch and the larvae will emerge from the beaches and into the water during a high tide. The larvae look like miniature adult horseshoe crabs without tails. Newly hatched horseshoe crabs spend their first few years of life on the tidal flats. The older they get, the farther from shore they move. Adults spend the winter in deep bay waters and off-shore areas, but as spring approaches, the crabs move toward the beaches to prepare for spawning.

Horseshoe crabs have a hard shell– they must molt to grow. A horseshoe crab will molt at least six times in their first year of life and about 18 times before they reach sexual maturity. Females are generally larger than males and may molt more than males to reach the larger size. Once they are sexually mature (which takes at least nine years) they won’t have to shed their shells ever again. When a male crab completes his final molt, his first set of claws modify into a boxer-glove shape which he uses to clasp onto the female for spawning. Adult crabs may live another eight to 10 years, making the total lifespan of a horseshoe crab as long 20 years.

Fun Fact:

  • Horseshoe crabs are not true crabs and are instead more closely related to spiders and scorpions.

 

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