|Five hundred and 25 million years ago, the first major animal groups developed in the mild climate of a world barren of life on the land. Shellfish, trilobytes, mollusks, primitive fish and our friends the horseshoe crab crawled the shallow seas between small separated continents. They left records of their existence in the rocks and sediments of a long ago age. Half were consumed in mass extinction by what is thought to be a glaciation brought on by cooling of the earth (or perhaps a gamma ray burst from colliding stars). Trilobytes died out and the horseshoe crabs survived and continued to crawl along the shores in a world where the only plants were algae and sponges.
Fifty million years later the world had warmed again. Life in the sea exploded and plants began to cover the land. Corals developed, a symbiotic combination of a plant and an animal. Cephalopods (squid, octopus) and gastropods (snails, abalone, limpets) appeared. But the continents drifted south and the land was covered in thick sheets of ice. The sea level fell. The environment was hostile to many of the new life forms. Sixty per cent of the marine invertebrates disappeared and 25% of all the families of living things disappeared. the horseshoe crabs crawled on.
Another 40 million years pass. The earth has warmed again. The ice has melted and the seas have reclaimed much of the land. Temperature fluctuations are small. The corals build great reefs of limestone on the margins of the land. The land now has plants that can suck up water, so plants taller than mosses begin to appear. Fishes develop jaws and move into fresh water. The horseshoe crab has not changed.
By 380 million years ago, the plants have developed into great tree ferns and trees with scales. They cover a great continent that has formed from the smaller continents of earlier periods. The first amphibians are crawling out of the seas. insects are walking, not flying, around on the ground. Sharks prowl the seas preying on the many new species of fish. The horseshoe crab crawls on.
Sixty million years later, things are really getting hot. There is a large continent straddling the tropics. Carbon dioxide levels are very high and it is hot. Hugh scale bearing trees 130 feet tall and 6 feet in diameter populate extensive swamps whose decaying vegetation creates huge coal beds. Reptiles (snakes lizards) appear. Insects begin to fly and reach sizes we find remarkable today. Dragonflies have 29 inch wingspans and cockroaches appear and reach lengths of 4 inches. The horseshoe crab ignores it all.
Two Hundred fifty million years ago the continents had all coalesced into a single land mass. The collision created great mountain ranges, the Appalachian mountains being the greatest of these. The biomass of giant tree ferns had fixed the carbon dioxide into the ground in the great coal beds. The world was now rich in oxygen. The large mass of land so far from the oceans become dry and experience large ranges of temperatures. A new type of plant emerges - seed bearing plants (conifers - cypress). Many new bug and beetle species appear. This period comes to an end when most of the life in the seas dies.
And then the reptiles take over. They start out as turtles and lizards of huge dimensions swimming in the seas. Big flying lizards soar above. At about 200 million years ago, a third of the marine reptiles go extinct. but not he horseshoe crab.
Reptiles dominate the land and the sea. And they get bigger. We call them dinosaurs. For 60 million years they dominate the land. Ferns, cyads and gingkoes and conifers cover the land. Fish lizards, huge crocodiles, rays and sharks swim the seas. The continents begin to break apart. the horseshoe crab crawls on.
The earth as very warm and even the polar regions are tropical. There is no ice anywhere in the world. the seas cover large parts of the land. North America has a large shallow sea in the middle of it. Trees begin to flower (magnolias, sycamores, laurels and palms) and attract the new species of insects, bees. Grasshoppers, ants and flies start crawling about. Hiding in the nooks and crannies of the swamps, the first mammals appear. Small cat like life forms scurrying around trying to keep from being eaten or stepped on. The world belongs to the big lizards. Then at 60 million years ago something massive happens - perhaps an asteroid, perhaps a concentrated period of volcanic eruption. But the world goes dark and the plants die off. The big reptiles that strode the earth for so many years can't cope. Half the worlds marine invertebrates - things without backbones that live in the sea - disappear.
The world begins to look much as it does now. The continents are dispersed and there are tropical sub-tropical and temperate zones. Ice appears again and the sea level drops. Plants on land are mostly flowering plants. Their fruits and seeds encourage the development of the small mammals into a series of larger and larger mammal species. Birds diversify, becoming smaller and with more feathers. By 30 million years ago the broadleaf trees have arrived. Big mammals roam open prairies of grassland. Mammals dominate the animal life on the land. Horses, dogs, monkeys and bears appear. Horseshoe crabs can't be bothered with the new comers.
Just three million years ago, a small ape like creature appear in the dry rift valleys of Africa. Man's ancestors have arrived. From 2 million years ago to 13000 years ago, the earth goes through a number of wide temperature swings. There are over 25 glacial advances when large sheets of ice advance from the poles coming as far south as central Pennsylvania. These sheets of ice thousands of feet thick, lower sea level as much as 500 feet. Areas like the Delaware Bay and the Chesapeake Bay are dry with rivers at the bottom of two hundred foot deep gorges and they deliver melting water from the glaciers to the ocean at the continental borders which now are a hundred miles out to sea. The horseshoe crabs crawls on.
Since the last ice age, the earth has remained relatively mild with small fluctuation in temperature compared to previous periods. The horseshoe crab continues its remarkable journey, surviving new predators and new challenges to its survival. Hopefully it will continue to survive. However they face a new threat - us. They are caught and chopped up as bait for American eels and fed to conchs in conch farms. We also take much of their habitat for development. And we kill 20 to 30 thousand of them for their blood used in the biomedical industry.
As they take a decade to reach sexual maturity, we need to monitor their numbers closely as it will take a long time to regenerate their populations even if we stop any overfishing. We use them for our purposes, but many species of birds need them to survive. If we greedily exploit this fantastic creature, the ramification to a world that has known horseshoe crabs for 500 million years may be extensive. And do we really want to be the ones who finally cause the horseshoe crab to be become extinct? A 550 million year old creature killed off by a 3 million year old one? Hardly seems right does it?
Department of Marine Resources State of Maine
Each May and June of the full moon and new moon high tides, horseshoe crabs crawl into the shallows of the Delaware Bay where each season females lay 90,000 eggs. Their smaller male companions scramble to ride their backs and provide the sperm that washes over the eggs. The little eggs develop rapidly and in just four days the first legs appear. In just 14 days and a number of internal molts, the egg hatches a tailless horseshoe crab to begin its 10 year journey to adulthood.
4-5 day old horseshoe crab
About 10% of the horseshoe crab population is killed by sea gulls who have learned to flip them over and eat their lungs and a few other soft parts under their protecting helmets. But we kill nearly as many with our harvesting operations. The spring concentrations of these animals in their mating and egg laying migrations make them easy prey for the harvesters. So if you see a horseshoe crab struggling up side down on the beach, give it a hand. Flip it back by the edge of its shell. Do not pick it up or turn it over by its tail which is easily injured. Don't worry. this ancient creature does not bite or sting.
Did You Know That a Horseshoe Crab?
has blue blood?
their blood detects bacterial endotoxins?
has ten eyes?
sees visible and ultraviolet light?
is named Limulus Polyphemus? Polyphemus was the one eyed son of Poseidon.
are related to spiders, ticks and scorpions not crabs?
a compound in their shells is used for dressings for burn victims and on sutures?
their tails were used as spear tips?
farmers used them for fertilizer?
between 2 and 5 million crabs live in Delaware, Maryland and Virginia?
birds fly 6,000 miles to feast on their eggs in spring?
can go a year without eating?
can swim in the open ocean on their back?