Of all the animals on this planet of ours, the horseshoe crab is one of my favorites. There is something special about a creature that has managed to remain unchanged for hundreds of millions of years, surviving mass extinctions caused by meteors and climate change that at times wiped out up to ninety percent of life on earth, yet still persists to this day.
I’ll get into the details of this animal in subsequent diaries, since this entry could conceivably be novel-length otherwise. If you are wondering about specifics of the reproductive cycle, its evolutionary and natural history, exploitation and conservation or the weird blue blood now used by pharmaceutical companies to test the purity of their medicines, all will be addressed in future essays.
Take a mind trip back 550 million years ago. One of the most common groups of animals that inhabited this ancient world included bottom-dwelling marine trilobites. These animals were wildly successful, as witnessed by the vast number of fossils still found throughout the world, probably second only those of prehistoric dinosaurs. Although thousands of species existed, all but one genus, Limulus, was wiped out during the great Permian extinction. This era marked the end of the trilobite reign and the beginning of the rise of sharks and other vertebrates. Only the horseshoe crab survives to this day.
Above is a fossilized horseshoe crab (Limulus polyphemus), virtually indistinguishable from modern specimens. This animal has simply not evolved. Now, don’t take this as a negative. Evolution isn’t a process with some kind of endpoint (hubristically taken at times to mean working towards mankind), but simply a method used by nature to determine the success or failure of a phenotype as the world around it changes. Adapt or die based on a cosmic dice roll. Or, like in the horseshoe crab’s case, remain the same and weather the storm.
The horseshoe crab is often referred to as a "living fossil", a term I really don’t care for. Technically a fossil is simply the remains, or "footprint", of an organism that has been replaced by minerals over time. The term living fossil (an oxymoron) is usually applied to animals that have remained unchanged for such a long period of time that fossils millions of years old still resemble their modern ancestors. Don’t confuse this with Lazarus species, such as the Coelacanth, which are animals that were once thought to be extinct but have been found still living today.
Each spring, at the highest tide of the year, hundreds of thousands of horseshoe crabs all along the east coast emerge from the water to spawn on coastal beaches. Weeks before this event the males, using special hook-shaped appendages on their front pair of legs, stake out a female by attaching to the rear end of the exoskeleton. When the female hauls herself out of the water she pulls the male (or males; sometimes they form a chain based on the availability of females in the area) up onto the beach. Here she digs a pit in the sand and lays a few thousand tiny yellow eggs. The attached males fertilize them and the hole is covered over.
This yearly ritual is an important event for other species, especially migratory seabirds such as plovers and sandpipers, who depend on the eggs as a source of energy to get them through their south to north yearly trek to their breeding grounds. It is also the horseshoe crab’s likely downfall, absent conservation measures, due to the demand for this animal by eel and whelk fishermen who use chopped-up adults as bait. The spawning date can be found on any calendar that shows the lunar cycle, and truckloads of animals can easily be collected in a single night, numbers which most likely represent nearly every mature adult crab within miles. Without proper protection an animal that has survived unbelievable odds over millions of years is in danger of being eliminated by eel fisherman. And what are the eels used for? To serve as bait for striped bass. This ancient animal is being decimated because it is used as bait for bait. But I digress.
Two weeks after they are laid the eggs hatch during the evening of the next very high tide. The tailless young, known as trilobite larvae (above) make their way to the water’s edge and swim away. After a few molts (and the appearance of the tail) the young settle to the bottom and spend their first few years in estuaries and salt marshes. The juveniles spend most of their time buried beneath the mud but come out twice a day, during the rising high tide, to feed on worms, detritus and tiny crustaceans.
The young are often hard to find since they spend most of the time just beneath the surface of the sand. The best way to find them is to watch for the unique, winding tracks they leave behind as they search for food. The young remain in the marshes for up to twelve years before reaching sexual maturity. They then, year after year, make their way to the same sandy shores used by their parents and grandparents.
I’ll stop here and go into more specifics later.
Other diaries in this series can be found here.