Chelicerata Horseshoe Crabs Sea Spiders and Marine Mites

There are about 94,000 described chelicerates, those arthropods that lack antennae, possess chelicerae (feeding appendages), and have the body divided into a cephalothorax and abdomen. Most chelicerates are arachnids (spiders and their kin; approximately 93,000 species), but only about 2,000 chelicerate species are found in purely marine habitats. The majority of these marine taxa fall into two groups: the sea spiders (Pycnogonida) and the mites (Acari). In contrast to the sea spiders, which are exclusively marine (with more than 1,000 species), only about 1,000 of the 45,000 described mites are known from truly marine habitats. Many mites have adapted to life in mildly halophilic (salt-loving) habitats, such as beaches and salt marshes, but relatively few species have secondarily adapted to a truly marine environment, making them the only arachnids to do so. Most marine mites are shallowly sub-tidal, but some species have been taken from abyssal depths as deep as 7,000 m. Mites are difficult to define morphologically as a group, and the synapomorphies (that is, shared evolutionary characters) for uniting them are unclear. In fact, the term mite is used for three distinct orders of chelicerate arthropods: Opi-loacariformes, Parasitiformes, and Acari-formes; the last two orders have only marine representatives. Marine mites contain both free-living species and parasites of both marine reptiles and invertebrates.

Unlike mites, the pycnogonids all share a fairly homogeneous body plan. They are perhaps the most poorly known arthropods, both in terms of biodiversity and biology. As a group they are defined by the presence of a piercing proboscis and a pair of parapalps behind the chelicerae. Many species are known only from single specimens, and the distributions and biology of most taxa are unknown. Most pyc-nogonids are small and subtidal, but some are large (for example, Dodecolopoda, with a legspan of up to 60 cm); a few species are known from depths down to 6,800 m. All are free-living as adults, but some species develop within hydroids and are thus parasitic in part of their lifecycle.

The horseshoe crabs are the best known marine chelicerates, despite the fact that there are only four extant species in three genera (Limulus, Carcinoscorpius, and Tachy-pleus). These animals have fused thoracic segments and reduced/fused abdominal segments. They resemble the primitive che-licerates of the Cambrian and are close relatives of the extinct sea scorpions. Three of the four extant species live in the Indo-Pacific ocean, while the fourth occurs in the western Atlantic, from Nova Scotia to the Yucatan Peninsula; all are free-living. All of these species have been heavily impacted by over-fishing for use as bait (for example, for lobster and whelk fisheries in the United States), and all have received some measure of protection in at least part of their ranges, especially in Japan and the mid-Atlantic United States. They are unique among marine arthropods in coming ashore on sandy beaches to breed; their eggs, as a food source, are intimately involved in the lifecycles of numerous species of terrestrial animals, principally migratory birds.

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