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Despite the name, horseshoe crabs are not crabs at all. They belong with spiders, scorpions and ticks, and animals very like them were already living in the sea about 445 million years ago.

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Horseshoe crabs are chelicerates, the group of arthropods (animals with jointed legs and a hard outer skeleton) that also includes spiders, scorpions, mites and ticks. True crabs are crustaceans, a separate branch. Some genetic studies have even placed horseshoe crabs inside the arachnids, the spiders' own group, though a more recent analysis disagrees. Only four species survive: one along the Atlantic coast of North America and three in Asia. Their blood is blue, because it carries oxygen with a copper-based protein instead of iron-based haemoglobin, and an extract of it is used to test medicines for bacterial contamination. In spring and summer, Atlantic horseshoe crabs come ashore to spawn, usually at high tide around the full and new moons. On Delaware Bay in the eastern United States, migrating shorebirds called red knots refuel on their eggs, so catching the crabs there is restricted to protect the birds.

Türkçe oku
Adları yanıltmasın: Nal yengeçleri aslında yengeç değildir. Örümceklerin, akreplerin ve kenelerin akrabasıdırlar; onlara çok benzeyen hayvanlar yaklaşık 445 milyon yıl önce de denizlerde yaşıyordu.

Nal yengeçleri keliserliler grubundandır. Eklembacaklıların, yani eklemli bacakları ve sert bir dış iskeleti olan hayvanların bu kolunda örümcekler, akrepler, akarlar ve keneler de yer alır. Gerçek yengeçler ise ayrı bir koldan, kabuklulardandır. Bazı genetik çalışmalar nal yengeçlerini, örümceklerin de dâhil olduğu örümceğimsiler grubunun içine bile yerleştirdi; ancak daha yeni bir analiz bu görüşe katılmıyor. Bugün yalnızca dört türü yaşıyor: Biri Kuzey Amerika'nın Atlantik kıyılarında, üçü Asya'da. Kanları mavidir, çünkü oksijeni demir içeren hemoglobinle değil, bakır içeren bir proteinle taşırlar; bu kandan elde edilen bir özüt, ilaçlarda bakteri kaynaklı kirlilik olup olmadığını sınamakta kullanılır. Atlantik nal yengeçleri ilkbahar ve yaz aylarında, çoğunlukla dolunay ve yeni ay günlerinde, sular kabardığında yumurtlamak için kumsallara çıkar. ABD'nin doğusundaki Delaware Körfezi'nde göç eden büyük kum kuşları bu yumurtalarla güç toplar; bu yüzden orada kuşları korumak için nal yengeci avına sınır konmuştur.

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Background reading: Horseshoe crab
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