book gill

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Related to book gill: pedicel, Malpighian tubules

horseshoe crab

horseshoe crab, large, primitive marine arthropod of the family Limulidae, related to the spider and scorpion and sometimes called a king crab (a name also used for the largest of the edible true crabs). The heavy dark brown exoskeleton, or carapace, is domed and shaped like a horseshoe. The body is divided into a broad, flattened, semicircular front part (the prosoma), a tapering middle part (the opisthosoma), and a pointed, spiky taillike part (the telson).

Horseshoe crabs have no jaws, and the mouth is flanked by a pair of pincerlike chelicera that are used to crush worms and other invertebrates taken as food. They have two compound, primary eyes and five simple, secondary eyes on top of the carapace, and two simple eyes near the mouth, under the carapace; additionally, the telson has photoreceptors. Five pairs of walking legs attached to the prosoma enable the animals to swim awkwardly or burrow through the sand or mud. They swim upside down, with the carapace forward and angled upward. The respiratory organs are called book gills and are unique to horseshoe crabs. Each book gill is made of about 100 thin leaves, or plates; these are fitted like pages of a book onto one pair of flaplike appendages on the opisthosoma. Rhythmic movement of the appendages circulates water over the gill surfaces and drives blood into and out of the gill leaves.

Horseshoe crabs first appeared in the Upper Silurian period, and a number of fossil species have been described. Four species still survive; three of these are found along the Pacific coast of Asia. The American species, Limulus polyphemus, is common along the Atlantic coast from Nova Scotia to Florida. It lives in shallow water, preferring soft or sandy bottoms, and reaches a maximum length of nearly 2 ft (61 cm). The shores of the Delaware Bay form the largest spawning ground of the species, and their eggs make the bay a critical feeding stopover for migrating shorebirds.

Horseshoe crabs are economically important for their blood, which contains amebocytes, invertebrate cells that play a role similar to white blood cells in vertebrates. An amebocyte extract is used in medical safety testing to check intravenous drugs and implants for bacterial contamination. The blood, which is blue due to the copper in hemocyanin (which, like hemoglobin, transports oxygen), is bled from harvested crabs, which are then returned to the wild, but the process kills some of the crabs. Horseshoe crabs are also used as bait in some fisheries.

Horseshoe crabs are considered living fossils; they resemble fossil trilobites and eurypterids of the Paleozoic era. They are classified in the phylum Arthropoda, subphylum Chelicerata, class Merostomata, order Xiphosura.

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book gill

[′bu̇k ‚gil]
(invertebrate zoology)
A type of gill in king crabs consisting of folds of membranous tissue arranged like the leaves of a book.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
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