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Except for arachnids occurring as parasites or as freeliving terrestrial forms in coastal wetlands, marine cheliceriform arthropods in the Gulf of Mexico are rather few. They include poorly known halicarcid mites (marine Acari, not treated here), a single but conspicuous member of the Xiphosura belonging to the genus Limulus, and a small but widely distributed group of usually epifaunal species representing the Pycnogonida. The xiphosuran Limulus polyphemus L. is, in fact, also widely distributed in the Gulf of Mexico, contrary to implications of an earlier treatment in Bulletin 89 (Hedgpeth 1954a). Commonly termed the “horseshoe crab,” it is actually neither crab nor crustacean, but rather a primitive-looking relict of a largely extinct group of arthropods known as the Merostomata. The range of this species broadly encompasses the U.S. Atlantic coast and extends around the Florida peninsula into the eastern Gulf to near the Mississippi River (Saunders, Kessler, and Avis 1986, Shuster 1979). With the exception of rare extralimital reports, the range for some reason does not include the northwestern Gulf. Less commonly reported upon is a somewhat isolated population of this species in the southwestern Gulf that ranges along the coast of Mexico from eastern Veracruz to northern Yucatán, with particularly dense breeding accumulations occurring seasonally in shallow subtidal to intertidal waters of Campeche and Tabasco (Gómez 1993, D. Felder, personal communication ). The pycnogonids, commonly called “sea spiders,” are a homogeneous group of approximately 1200 benthic species found in marine habitats from the shoreline to abyssal depths, and are the primary focus of this chapter . They are not related to terrestrial spiders, although both groups have 8 legs and both belong to the legions of arthropods known as Cheliceriformes. The number of legs will, however, easily distinguish pycnogonids from “sea skaters,” unusual long-legged marine insects of the genus Halobates that live on ocean surface-tension layers and are sometimes mistaken for pycnogonids by the untrained eye. The origins of pycnogonids are unknown because they have no universally accepted palaeontological record except for a very few problematical fossils. Their anatomy is quite simple. They have a 2-valve anterior heart; a suctoral proboscis used to suck the soft tissues from sessile animals such as hydroids, corals, tunicates, and others; a 815 43 Pycnogonida and Xiphosura (Cheliceriformes) of the Gulf of Mexico C. Allan Child  Xiphosura. After Pratt 1916. 816 ~ Pycnogonida and Xiphosura (Cheliceriformes) through either a simple pore or pores along the cuticle or via a tube. The trunk segments and proximal leg segments often have dorsal tubercles, spines, setae, swellings, or other forms of decoration, or these may be lacking. Pycnogonids are found in most benthic communities where adequate sessile food is available. A few species of one genus, Pallenopsis, are known to be parasitic on midwater medusae, and there may be other parasitic species that remain unrecognized. Pycnogonida have been collected from as shallow as along shores to as deep as abyssal trenches, the deepest capture being in more than 7000 meters. They are not known from freshwater. The pycnogonid fauna of the Gulf has not received better attention than other minor faunal groups and requires much greater collecting efforts and closer study than to date. Of the approximately 58 species known to live in the Gulf, most are well known from other Western Atlantic and Caribbean localities. Some are even known from Amphi-Atlantic and Pantropical/Temperate habitats. There is no way to be certain that endemic species of pycnogonids occur in Gulf waters, but a number have thus far been collected only there. It is likely that most of those will probably be collected outside the confines of the Gulf in the course of further studies. The Gulf of Mexico appears to be less studied for its flora and fauna than other bodies of water of similar size, but like many things placed so close to the beholder that they are overlooked, the Gulf has suffered accordingly. Perhaps this is related to fewer marine facilities on shores of the Gulf of Mexico over years past, as well as want of funding and other support for work in the region. The main concentration of marine exploration long centered on the Florida coast and islands of the Florida Keys, as reflected in reports of pycnogonids from that region. Beginning in the early 1900s, an active marine laboratory conducted operations in the Dry Tortugas under management by the Carnegie Institution of Washington, gut with long diverticula extending into each leg owing to crowding in the...

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