{"id":411,"date":"2010-09-06T17:37:30","date_gmt":"2010-09-06T07:37:30","guid":{"rendered":"https:\/\/scienceillustrated.com.au\/blog\/?p=411"},"modified":"2010-09-07T10:42:51","modified_gmt":"2010-09-07T00:42:51","slug":"living-fossils-long-live-the-horseshoe-crab","status":"publish","type":"post","link":"https:\/\/scienceillustrated.com.au\/blog\/features\/living-fossils-long-live-the-horseshoe-crab\/","title":{"rendered":"Living fossils: Long live the Horseshoe Crab"},"content":{"rendered":"
\"\"<\/p>\n

Image: Fred Bavendam\/Minden Pictures<\/p>\n<\/div>\n

These arthropods are living fossils, practically unchanged in 445 million years. Born survivors, they have endured countless major changes on Earth, but we’re only just beginning to understand them.<\/strong><\/p>\n

Between late spring and midsummer at new and full moons, a tide of horseshoe crabs emerges from the waves to mate and lay eggs on land. The females carry the greater burden in this yearly ritual. Not only are they somewhat heavier than their mates, but after emerging from the surf, a male will fasten himself solidly to his bride’s back, where he will fertilise the eggs that she deposits in the sand. As many as six suitors also hoping to have access to her eggs surround each female. It’s an exhausting process, and it happens in incredible numbers. Once, over the course of a single night in 2008, volunteers and staff from the Nature Conservancy estimated that about 350,000 horseshoe crabs came ashore on the beaches of Delaware Bay, south of New Jersey in the US. The waters just off the coast of the bay may be home to as many as two million of these ancient animals.<\/p>\n

Horseshoe crabs are one of nature’s greatest success stories. Hundreds of millions of years ago, horseshoe crabs developed into a prototype that, with very few modifications, has been viable ever since. The animal’s anatomy and way of living have made it so hardy that it has survived many violent changes on the planet, changes that led to the extermination of countless other species. The oldest known fossil of a horseshoe crab, found in Manitoba, is 445 million years old and is astonishingly similar to today’s specimens. Both ancient and contemporary horseshoe crabs have bodies with a combined head and thorax, known as a cephalothorax, covered with a hard, horseshoe-shaped carapace, or shield, an abdomen and a dagger-shaped tail.<\/p>\n

Despite their long history on Earth, these animals have never been the kind of dominant species that takes over large areas. Their strategy is quiet endurance. Even when they were at their most diverse and populous during the Carboniferous period between 300 and 360 million years ago, there were only eight or nine known genera, and soon after, many of those went extinct amid massive changes in global climate, ocean chemistry and sea level. The horseshoe crab achieved something of a comeback during the Triassic period, when the first dinosaurs were establishing their dominance. Since then, palaeontologists have found only a few fossils in a limited number of locations, and they believe that horseshoe crabs during the past few million years have consisted of just a few species living in just a few areas in the world’s oceans. The horseshoe crab survived the ecological chaos of the mass extinctions 65 million years ago, an event that wiped out the dinosaurs and changed animal life on our planet forever. Four species remain: Today the American horseshoe crab lives in shallow water off the East Coast and the Yucat\u00c3\u00a1n Peninsula, and the other three species live in the waters of southern and eastern Asia.
\n<\/p>\n

\"\"<\/a><\/p>\n

Image: Piotr Naskrecki\/Minden Pictures<\/p>\n<\/div>\n

Built the old-fashioned way<\/strong>
\nDespite their name, horseshoe crabs are not actually crabs \u2014 they make up their own class of animal, known as Merostomata (meaning “legs attached to the mouth”\u009d), a kind of arthropod. The creatures evolved in the Palaeozoic Era 540 to 248 million years ago with other arthropods that are now extinct, and their nearest living relatives are spiders and scorpions, all of them terrestrial and much smaller.<\/p>\n

Just like spiders and scorpions, horseshoe crabs have six pairs of limbs. Five pairs are used for locomotion, and the foremost pair, called chelicerae, grip food. Horseshoe crabs are predators and carrion eaters. Their hindmost legs dig into the seafloor to find dead animals, worms, molluscs and other bottom-dwellers. The legs are equipped with leaf-like appendages that can sweep bottom sediment away to expose hidden carrion. The ends of each pair of legs are equipped with pincers to bring food toward the mouth. The legs also tear the food, so that the animal’s toothless, jawless mouth can swallow the meal.<\/p>\n

The abdomen has six pairs of appendages, five of which are flat plates that function as gills (and as paddles to propel the horseshoe crab through the water). Each gill consists of about 150 lamellae that resemble the pages of a book. Like other gills, these extract oxygen from seawater. As long as the gills remain moist, the horseshoe crabs can use them on land as well \u2014 during egg-laying, for example \u2014 but only for short periods.<\/p>\n

Because of their large carapaces, horseshoe crabs have a hard time righting themselves if they are turned over on their backs on the beach. A breaking wave on the shore can mean disaster for a horseshoe crab. It can use its tails to partially lift itself in an attempt to roll over and end up on its legs. But every year, about 10 per cent of adult horseshoe crabs become stranded on the beach and die of overexposure or are eaten alive by seabirds while on their backs in the sand. A horseshoe crab standing on its legs is well protected, given its tough carapace, against predators from above, but on its back it is easy prey.<\/p>\n


\nTo survive minor injuries, the animals rely on their blue blood. A horseshoe crab’s blood changes from straw-coloured to blue when oxygenated by the gills or exposed to the air, as from an open wound. The blue colour (as with the blood of squid and other molluscs) results from the presence of the protein haemocyanin, in which two copper atoms bind to an oxygen molecule. In mammals, oxygen binds to the iron atoms in haemoglobin, found in red blood cells, to form the red pigment. In horseshoe crabs, there are no blood cells to transport oxygen inside the body, because the haemocyanin itself circulates in the blood.<\/p>\n

Horseshoe crabs do not have the intricate mammalian circulatory system, with its dense network of capillaries. Instead they have one in which large, blood-filled cavities make direct contact with the living parts of the body. This design creates two potential problems: Even a small injury can mean losing a lot of blood, and bacteria can easily gain access to the organism. Meanwhile, horseshoe crabs have no immune system to help them resist harmful bacteria. But their blue blood does have special cells, amaebocytes, that solve both problems. If an amaebocyte detects endotoxins, a constituent of bacterial cell walls, it secretes a protein that effectively stops blood loss and prevents new bacteria from entering the horseshoe crab’s body. Researchers believe that studying the animal’s blood could lead to new methods of detecting and perhaps treating human afflictions such as cancer.<\/p>\n

All-seeing eyes<\/strong>
\nThe horseshoe crab’s eyes are also unique. Haldan Keffer Hartline, a physiologist at the Rockefeller University in New York, shared the Nobel Prize in 1967 for his work with horseshoe crabs. His discoveries laid the foundations for our present knowledge of the function of the human eye. Hartline studied the two largest of the animal’s 10 eyes, the ones located high on its carapace. Using these eyes, the horseshoe crab can see in all directions. The two large seeing organs are, like those of insects, compound eyes consisting of 1,000 ommatidia, or receptors. Photoreceptors at the base of each ommatidium register visible light and infrared and ultraviolet radiation and send signals to the brain.<\/p>\n

Surprisingly, each ommatidium functions well both at night and in direct sunlight, when it is bombarded by 100 trillion times as many photons as during the weakest illumination that the compound eye can detect.
\nTo carry out the same tasks, the human eye is equipped with cones for seeing in daylight and rods for seeing at night. The ommatidia in horseshoe crabs are about 100 times as large as the rods and cones in the human eye’s retina, making them the largest known photoreceptors in the animal kingdom.
\n<\/p>\n

\"\"<\/a><\/p>\n

Image: Fred Bavendam\/Minden Pictures<\/p>\n<\/div>\n

For many years, no one knew what these arthropods used their sense of sight for. But various experiments have revealed that horseshoe crabs use their compound eyes to find mates during their hectic breeding season.<\/p>\n

In addition to their two compound eyes, horseshoe crabs have eight eyes in various spots on their body. At the front of the carapace, there are three small eyes that detect ultraviolet radiation from the sun and light reflected from the moon, and these may track moon phases for their spawning cycle, which peaks at
\nnew and full moons. Behind each of the two large compound eyes is a rudimentary eye that may provide light information during larval and embryonic stages. Near the mouth on the underside of the body are two ventral eyes, which may help the animals orient themselves while swimming, as horseshoe crabs are often observed swimming upside down. The final “eye”\u009d is a row of light sensors located on the tail. Scientists suspect that these receptors may be used to detect moonlight.<\/p>\n

Despite their longevity, there is critical concern among biologists over the future of the species. Over-harvesting on the East Coast and habitat loss in Japan have greatly decreased their numbers. Current research focuses on finding ways to conserve these most resilient of threatened animals.<\/p>\n

Read more: For the full article, see Science Illustrated magazine, July\/August 2010 Australian edition.<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"

These arthropods are living fossils, practically unchanged in 445 million years. Born survivors, they have endured countless major changes on Earth, but we’re only just beginning to understand them.<\/p>\n","protected":false},"author":13,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[4,11,32,6],"tags":[],"class_list":["post-411","post","type-post","status-publish","format-standard","hentry","category-features","category-in-the-mag","category-marine-biology","category-nature"],"aioseo_notices":[],"_links":{"self":[{"href":"https:\/\/scienceillustrated.com.au\/blog\/wp-json\/wp\/v2\/posts\/411"}],"collection":[{"href":"https:\/\/scienceillustrated.com.au\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/scienceillustrated.com.au\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/scienceillustrated.com.au\/blog\/wp-json\/wp\/v2\/users\/13"}],"replies":[{"embeddable":true,"href":"https:\/\/scienceillustrated.com.au\/blog\/wp-json\/wp\/v2\/comments?post=411"}],"version-history":[{"count":18,"href":"https:\/\/scienceillustrated.com.au\/blog\/wp-json\/wp\/v2\/posts\/411\/revisions"}],"predecessor-version":[{"id":492,"href":"https:\/\/scienceillustrated.com.au\/blog\/wp-json\/wp\/v2\/posts\/411\/revisions\/492"}],"wp:attachment":[{"href":"https:\/\/scienceillustrated.com.au\/blog\/wp-json\/wp\/v2\/media?parent=411"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/scienceillustrated.com.au\/blog\/wp-json\/wp\/v2\/categories?post=411"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/scienceillustrated.com.au\/blog\/wp-json\/wp\/v2\/tags?post=411"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}