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<\/p>\nHowdy, BugFans,<\/p>\n
Back in 2011, the BugLady wrote about a spectacular arthropod called the Horseshoe crab that pre-dates insects by maybe 100,000,000 years. Despite the fact that they occur by the millions, horseshoe crabs are facing dire threats to their populations, and the BugLady wondered if anything had changed since the original episode. New text, old text, and pictures of a 1 \u00bd\u201d long, translucent, infant horseshoe crab that washed up on Sanibel long ago, that the BugLady happened to have in her collection. Put your feet up \u2013 this takes some telling.<\/p>\n
The BugLady had taken herself on a little road trip to the Delaware Bay, the home of the horseshoe crab. Because of their cute faces and their long and primitive lineage (they are called \u201cliving fossils\u201d), horseshoe crabs grab our imaginations – they are one of those creatures whose appearance immediately transports us to ancient swamp forests and shallow inland seas. <\/p>\n
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Horseshoe crabs have, in the words of local news-people, a \u201cWisconsin Connection<\/em>.\u201d They are close relatives of our beloved State Fossil called the Trilobite, an ancient inhabitant of those inland seas. <\/p>\n The story of the Atlantic horseshoe crab stretches from the great extinctions of the Silurian Period to the unique threats of the 21st century. The operative question is whether an animal that has survived for almost a half-billion years will join its extinct trilobite ancestor under our watch. <\/p>\n They are shaped like horseshoes, but they\u2019re not crabs. They are under the same big umbrella as crabs – phylum Arthropoda (\u201cjointed limb<\/em>\u201d) – but horseshoe crabs headed out on a completely different limb of the family tree a very long time ago, and they\u2019re more closely related to spiders and scorpions. They\u2019re in the class Merostomata (“legs attached to the mouth<\/em>\u201d), in the order Xiphosura (\u201csword tail<\/em>\u201d), in the family Limulidae. There are four living horseshoe crab species in the world, three of which reside along the edges of Asia. The Atlantic horseshoe Crab (Limulus polyphemus<\/em>) (\u201cLimulus<\/em>\u201d means \u201caskew<\/em>,\u201d and the BugLady trusts that BugFans recall who Polyphemus was) is found in shallow ocean edges with soft, mucky or sandy floors from Maine through the Gulf of Mexico. <\/p>\n The fact that horseshoe crabs can survive dramatic changes in temperature and salinity and can fast for months (up to a year) helps to explain their longevity, and so does their decidedly tank-like physique. A hard, smooth shell makes them difficult for predators to handle, and the shell is a bulldozer that allows them to dig into the sand\/mud for food. From above, they\u2019re divided into three sections – a curved front shell, a trapezoid-shaped middle segment that is hinged to the front shell, and a long, sharp tail that they use as a rudder and as a pry bar when they get flipped over. They can see potential mates from about three feet away with large compound eyes located on the side curve of the front shell, and several centrally located simple eyes register UV light. It\u2019s murky where they hang out, and the tail is equipped with photo-receptors. <\/p>\n Flip one over and your main impression is legs. The first five pairs are for locomotion and for passing food to the mouth, which is, indeed, surrounded by legs. The final pair is adapted for pushing. <\/p>\n Females are larger than males \u2013 some females may measure two feet from stem to stern. When it\u2019s time to breed, horseshoe crabs move into shallow waters. There is some suspicion that females are imprinted on the muck they hatched out of, but it\u2019s not known what \u2013 or how \u2013 they sense there. A female crawls up on shore, attended by piggyback males<\/a> . She lays eggs, a few thousand at a time<\/a> (up to 120,000 total), in a depression\/\u201dnest\u201d in the sand, the males fertilize them externally, and waves cover them . <\/p>\n Eggs hatch in two weeks and the minute\u2019 young (initially tail-less) head for tidal flats to eat and shed and burrow. Their food supply – clams, worms, a few small fish and crabs and other invertebrates – dwells in the mud and sand with them. Rasps on their legs \u201cpre-process\u201d the food, and their sand-filled gizzard finishes the job. They will molt six times, increasing their size by one-third each time, and reaching a half inch by their first birthday. After they blow out the first candle or two, they move to deeper waters, molting once a year and reaching sexual maturity in about ten years. Older horseshoe crab collect quite a load of hitchhikers during their lives \u2013 limpets, sponges, leeches, mussels, crabs, periwinkles, and other snails attach to the shell. <\/p>\n According to Japanese folklore, warriors killed in battle were reborn as horseshoe crabs, eternally roaming the sea floor, celebrated in art and poetry. Eastern tribes of Native Americans harvested horseshoe crabs for food (the muscles in the second section), fertilizer (and they taught the in-coming Europeans this trick), for bowls made from the front part of the shell, and for the sharp tail, used as a tip for fishing spears. The rubbery eggs, roasted \u201cin the shell\u201d are a delicacy in some Asian cuisines, though the roe of one species contains a neurotoxin called TTX by scientists and \u201cZombie Powder<\/em>\u201d by practitioners of Vodou. <\/p>\n Horseshoe crabs are armored but not invincible, and they face some unique modern challenges. They are vulnerable to predation by sea turtles, gulls, fish, and crabs (especially as eggs and newly-hatched young), to habitat degradation caused by shoreline development, to pesticide run-off, oil spills, and to being stranded during spawning. Their intersection with humans has not been a happy one – historically horseshoe crabs have been used for human and poultry food and for fertilizer, and today they\u2019re taken and chopped up as bait for conch and eel fishing. Females with eggs are the most desirable bait, and they have even been harvested off their spawning grounds, and it doesn\u2019t take a Brain Trust to see where that road leads. <\/p>\n Alas for the horseshoe crab, researchers discovered in the 1960\u2019s that components of its blood could be used to screen drugs and intravenous compounds\/devices for the presence of certain bacterial contaminants. In order to harvest this blood chemical, which is extremely important in today\u2019s medicine, horseshoe crab are caught, relieved of up to one-third of their blood over a period of several days, and then released. In addition, the shell can be used to make absorbable sutures, treat certain eye problems, and speed blood clotting time <\/p>\n When she did her original research six years ago, the BugLady read that the \u201cbleeding\u201d process carried a 3% to 15% mortality rate (a figure offered by the industry). More recent studies suggest that mortality from the process alone could be as high as 30%. Add to that the trauma of being commercially collected and the fact they are stressed by being out of water during the bleeding process (and losing one-third of their blood), and it\u2019s not surprising that they are disoriented and debilitated when they are released, and that females\u2019 ability to spawn may be compromised (In response to criticism, an industry spokesperson noted that, \u201cOne of my suppliers built a water slide to put the crabs back into the water. They love it!<\/em>\u201d). Medical use is expected to go up, not down, but there has been talk of farming horseshoe crabs. <\/p>\n The food web to which Atlantic horseshoe crabs belong is complex and their contribution is vital. In uncountable numbers, horseshoe crab eggs have fueled migrating shorebirds, especially Red Knots, which time their spring migration from South American wintering grounds to coincide with the horseshoe crab spawning season (Watch this excellent video<\/a> , and so is the longer one following it) (nice slide show, but cookies<\/a>). They arrive at Atlantic shore staging areas \u2013 irreplaceable way-stations in their 9,000 mile journey – with depleted energy reserves. After two or three weeks of gorging on horseshoe crab eggs, rich in calories, protein and fat (they graze from nests uncovered by wind and waves, leaving plenty of intact nests undiscovered), up to a million shorebirds continue their journey to the Arctic circle having doubled or tripled their body weight. In recent years, as horseshoe crab numbers have fallen, so have shorebird populations. And when an animal doesn\u2019t reproduce until it\u2019s ten, like the horseshoe crab, it takes at least a decade to recover. <\/p>\n![\"\"](\"https:\/\/uwm.edu\/field-station\/wp-content\/uploads\/sites\/380\/2018\/02\/horseshoe-crab113.jpg\")
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