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17 The Thecostraca, which includes the well-known Cirripedia (barnacles), is a large assemblage of diverse crustaceans in which parasitism and adaptation to unusual habitats have resulted in a wide range of unusual morphologies and lifestyles. Larval morphology has always played a critical role in studies of their systematics and evolution, and until the advent of molecular evidence, the affinities among the thecostracan taxa rested exclusively on the morphology of the larvae (Høeg 1992; Høeg and Møller 2006; Høeg et al. 2009a). Thecostracan larvae share some important characteristics. Development typically consists of a series of naupliar instars, followed by a terminal phase that is called the cypridoid larva (Høeg et al. 2004). The cypridoid larva is specialized to initiate the permanently attached phase found in nearly all juvenile and adult Thecostraca (table 17.1). Historically, the classification of the Thecostraca has changed frequently. Today the taxon thought to consist of 3 major subgroups (subclasses): the Facetotecta, Ascothoracida, and Cirripedia. Thecostracan nauplii gradually develop a trunk, but they never develop any post-cephalic (thoracic) appendages. These limbs all appear in one step, at the molt to the cypridoid stage. The latter is called y-cyprid (or cypris y) in the Facetotecta, a-cyprid in the Ascothoracida, and cyprid (or c-cyprid), in the Cirripedia. The body of the cypridoid is wholly or partially covered by a large carapace (head shield), which in a-cyprids and cyprids is drawn down on either side, with the two halves connected by an adductor muscle. The cypridoid carries a pair of prehensile antennules and six pairs of natatory appendages on the segmented thorax, enabling this larva to both swim through the water and attach securely to the substratum. In facetotectans and ascothoracidans, the cypridoid body terminates with a large segmented abdomen and a telson furnished with unsegmented furcal rami. Cirripede cyprids retain the rami, but both the abdomen and telson are either rudimentary or absent. The cypridoid larva is very well equipped with sensory organs, including five pairs of lattice organs on the carapace, a nauplius eye, a pair of compound eyes, conspicuous frontal filaments, and one or two antennular aesthetascs. The Facetotecta is an incompletely understood group (to say the least), but it is nevertheless widespread and can be very abundant locally. For more than 100 years the facetotectans were known only from their larvae (referred to as y-larvae): a series of five y-nauplius (or nauplius y) stages, followed by a single y-cyprid. Recent work (Glenner et al. 2008; also see Scholtz 2008), using a crustacean molting hormone (20-HE) to induce metamorphosis, has revealed that the subsequent stage is a reduced limbless slug-like juvenile, called an ypsigon. The adults, although still unknown, are very likely advanced parasites in still-to-be-identified hosts. Most of the species or “forms” characterized to date are known from the western Pacific Ocean (off Japan), the North Atlantic Ocean, and the Mediterranean Sea, although that distribution might reflect search intensity, and large unstudied collections, such as those from the Indian Ocean, exist. In some locations more than 40 different types of y-nauplii, all representing undescribed species, can be present in the plankton, indicating a rich biodiversity of adults that is yet to be discovered. The name Facetotecta comes from the unique ornamentation of the cephalic shield in both y-nauplii and y-cyprids. Molecular evidence (Pérez-Losada et al. 2009) indicates both the monophyly of the Facetotecta and their sister-group status to the remaining thecostracans (the Ascothoracida and Cirripedia). The Ascothoracida is a small group containing approximately 100 known species, all of which are ecto- or endoparasitic on cnidarians (the Anthozoa) and echinoderms. Today, 2 subgroups are recognized: the Laurida and Dendrogastrida. In many species the nauplii are brooded, with the larvae released as a-cyprids (previously called ascothoracid larvae), but planktonic nauplii are known from representatives of 3 of the 6 families. The Cirripedia is by far the best known of the 3 subclasses and exhibits an extreme diversity in both body forms and Jens T. Høeg Benny K. K. Chan Joel W. Martin Jens T. Høeg, Benny K. K. Chan, and Joel W. Martin Introduction to the Thecostraca 98 Jens T. Høeg, Benny K. K. Chan, and Joel W. Martin lifestyles. The group includes the familiar suspension-feeding stalked and acorn barnacles (the Thoracica), the parasitic Rhizocephala , and the lesser-known Acrothoracica (D. Anderson 1994). Morphologically, the cirripedes are united...

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