NATURALIST'S 



NOTEBOOK 



Again, despite the deadly reputation of 

 the arachnids, the horseshoe crab is indeed a 

 gentle creature. Unless you are a mollusk — 

 mollusks are this so-called crab's main food 

 source — horseshoe crabs can be quite inspiring. 

 Experiencing their benign nature may cause one 

 to yearn for an equally calming and harmless 

 nautical existence. 



THE LIVING FOSSIL' 



The lineage of the horseshoe crab is one of 

 the most amazing aspects of the animal. The 

 genus Limulus originated in the Triassic period, 

 which is the first period in the age of the 

 dinosaurs. And its ancestors — trilobites — 

 were dominant Earth creatures 100 million years 

 before dinosaurs even existed. 



Not only has the horseshoe crab evaded 

 extinction over millions of years of evolution, 

 but the genus Limulus has evolved very little 

 throughout history. 



"Horseshoe crabs are ecological 

 generalists: they can withstand a wide range of 

 conditions, natural as well as man-induced." 

 And their ". . .jack-of-all-trades nature. . .holds 

 the clue to why they have remained so stable, so 

 evolutionarily unchangeable for hundreds of 

 millions of years," writes Niles Eldredge in his 

 book Fossils. 



LIFE CYCLES OF THE 

 BROWN AND SCARY 



You'd be lucky to come across live 

 horseshoe crabs on the beach outside of their 

 spring spawning season. They are benthic 

 creatures, preferring a lifestyle adapted to 

 offshore habitats near the continental shelf, as 

 well as shallow estuarine areas. 



The horseshoe crab is generally nocturnal. 

 When active, it may swim upside down or 

 burrow and crawl along the sandy ocean floor in 

 search of invertebrate prey. Upon finding its 

 meal, the horseshoe crab grinds the sea worm or 

 whatnot with the base of its legs, and then 

 pushes the ground food into its mouth with, 

 again, its all-purpose legs. If, by chance, the crab 

 becomes overturned, it can use its telson, or tail- 

 like appendage, as a tool for flipping back to a 

 more comfortable position. 



Of the four remaining species of the 



ancient horseshoe crabs, 

 three inhabit waters along 

 the coasts of India Japan 

 and Indonesia. 



Limulus polyphemus, 

 the fourth and most 

 abundant species, exists 

 along the Atlantic coast of 

 North America. Its range 

 extends from northern 

 Maine to the Yucatan 

 Peninsula with the largest 

 population of Limulus 

 occurring along the 

 Delaware Bay. 



The Delaware Bay 

 is home to the largest 

 population of horseshoe 

 crabs in the world, 

 regardless of species. And 

 is a main breeding site, 

 where thousands of 

 horseshoe crabs travel to 

 shore at high tide during a 

 new and full moon. 



The smaller males 

 attach to the females' 

 carapace and hitchhike onto 

 shore with flooding tides. 

 Once a couple reach their 

 destination, the female digs 

 a nest, in which she can 

 produce up to 88,000 eggs. 

 Several males may crowd to 

 fertilize the nest in the 

 process. 



Since it takes female 

 horseshoe crabs about 10 

 years to reach sexual 

 maturity — males average 

 eight years — the amount of eggs is not 

 excessive. This is especially true if the eggs 

 become exposed to the sandy shore surface and 

 therefore vulnerable to a variety of predators, 

 such as small fish, juveniles of larger fish and 

 shorebirds. 



In the Delaware Bay area alone, close to a 

 million migratory shorebirds have coevolved 

 with horseshoe crabs. "Horseshoe crabs are 

 critical to the welfare of migrating shorebirds 



1 



f i /' .A 



ABOVE: Nancy Targett, a University of Delaware marine biologist, says 

 local watemien are dependent upon liairesting horseslwe crabs as bah for 

 lucrative fisheries. RIGHT: One female can produce up to 88,000 eggs for 

 several males tofeitilize. The eggs are vulnerable since they are fast food' 

 for shorebirds that stop tofiiel up during their migratory journeys. 



that stop along the Delaware Bay each spring to fuel 

 up prior to the flight north to Arctic nesting grounds. 

 Some of these birds double and even triple their 

 weight by feasting on horseshoe crab eggs," says 

 Bill Hall, marine education specialist for the 

 University of Delaware's Sea Grant Marine 

 Advisory Service. 



If an egg actually survives long enough to 

 hatch and grow to adulthood, the horseshoe crab 

 may live out a lengthy lifespan of 20 or more years. 



28 AUTUMN 2003 



