Chondrichthyes

      Cartilaginous Fishes

      The cartilaginous (or chondrichthyan) fishes are a major group that includes 14 orders and more than 1,200 living species. These fishes are distinguished by a skeleton made entirely of cartilage, and a neurocranium with no sutures. Their teeth are derived from placoid scales and are replaced serially. Their fin rays, termed ceratotrichia, are usually soft and somewhat flexible, are always unsegmented, and are developmentally epidermal. They have a well-developed electroreceptive sense, with numerous pores of the ampullae of Lorenzeni often evident, especially surrounding the mouth. Males have pelvic-fin claspers for use in mating, as all species of chondrichthyan fishes have internal fertilization. Some groups are oviparous, releasing protective keratinized egg cases in which embryos develop. Others retain developing embryos within the body of the female, where they develop solely from nutrition supplied in the egg (yolk-sac viviparity). Still others supplement the nutrition of the embyos in an astounding variety of ways (Musick, 2011), including mucous and lipid secretions from the uterine lining (mucoid and lipid histotrophy), additional eggs released by the mother that are ingested by the embryos (oophagy), and transfer of nutrients from the mother to the embryo via a placenta (placental viviparity; Hamlett et al., 2005). Interestingly, Musick and Ellis (2005) concluded that the primitive condition in chondrichthyans is yolk-sac viviparity, from which all other forms, including oviparity, evolved. Most chondrichthyan species are marine, although some can enter freshwater and a very small number are restricted to freshwater. Cartilaginous fishes usually have a high concentration of urea in their blood relative to bony fishes, in order to maintain osmotic balance with seawater. The biology of sharks and rays has been summarized by several researchers including Carrier et al. (2012), Hamlett (2005), and Klimley (2013).

      This monophyletic group is sister to all other living jawed vertebrates, the Osteichthyes, a group that includes the ray-finned fishes and lobe-finned fishes. While comprising only approximately 3% of fish species diversity, this group includes more than 15% of fish orders, implying a high level of fundamental differences in morphology among relatively few species. There are two distinct evolutionary lines of chondrichthyan fishes, the Elasmobranchii (sharks, skates, and rays) and the Holocephali (chimaeras or ratfishes). Among these fishes, the elasmobranchs include 96% of the diversity, while the Holocephali comprises only 4%. The Batoidea (skates and rays) account for 54% of the total chondrichthyan diversity, leaving 42% to the Selachii (shark-like species).

      Hypothesized phylogenetic relationships of the major lineages of the Chondrichthyes (after Aschliman et al., 2012).

      HOLOCEPHALI—Chimaeras

      This chondrichthyan lineage includes one extant order, described below, that is the sister group to the Elasmobranchii (Lund and Grogan, 1997).

      CHIMAERIFORMES—Chimaeras

      The Chimaeriformes includes three families, six genera, and approximately 50 species of generally deep-sea predators characterized by a single gill opening and an upper jaw fused to the neurocranium (holostylic jaw suspension). The plownose chimaeras (Callorhinchidae) are restricted to the Southern Hemisphere, while the longnose chimaeras (Rhinochimaeridae) and the shortnose chimaeras (Chimaeridae) are more widespread. As their common names imply, the shortnose chimaeres have a blunt snout, the longnose chimaeras have long, pointed snouts, and the plownose chimaeras have long, hook-shaped snouts. One member of the latter family (Callorhinchus milli) has become a model organism for comparative genomics because of its relatively compact genome (Venkatesh et al., 2007; Tan et al., 2012). The shortnose chimaeras, detailed below, are the most speciose lineage in this group. Many of the characteristics described for them apply to the other families as well.

      REFERENCES: Grogan and Lund, 2004; Grogan et al., 1999; Lund and Grogan, 1997; Patterson, 1965; Tan et al., 2012; Venkatesh et al., 2007.

      CHIMAERIFORMES : CHIMAERIDAE—Ratfishes, Shortnose Chimaeras

      DIVERSITY: 2 genera, 38 species

      REPRESENTATIVE GENERA: Chimaera, Hydrolagus

      DISTRIBUTION: Atlantic, Indian, and Pacific oceans

      HABITAT: Marine; tropical to temperate; lower continental shelf to bathyal, demersal usually over soft substrates

      REMARKS: Ratfishes, one of two major groups of chondrichthyan fishes, are characterized by a lack of a stomach and the presence of separate anal and urogenital openings. They are deep-sea predators with tooth plates for crushing hard-bodied prey such as benthic mollusks and crustaceans. Ratfishes are oviparous and produce keratinoid egg cases with a long, pointed end and small hooks that anchor them to the substrate.

      REFERENCES: Compagno, in Carpenter and Niem, 1998; Didier, in Carpenter, 2003; Didier, 2004; Lund and Grogan, 1997; Patterson, 1965.

      CHIMAERID CHARACTERISTICS:

      1) body elongate with a whip-like tail, body usually naked

      2) one external gill opening, anterior to pectoral fin

      3) pectoral fins broad and wing-like

      4) two dorsal fins: the first high with an erectile spine, the second low with a long base

      5) mouth inferior

      6) conspicuous lateral-line canals on snout

      7) males with a club-like clasper on top of head

      8) pelvic claspers bi-lobed

      9) spiracles absent

      ILLUSTRATED SPECIMENS:

      A) Hydrolagus colliei, SIO 49–121,134 mm TL (tip of tail missing)

      B) Hydrolagus colliei, SIO 85–73, 448 mm TL (lateral view of head)

      C) Hydrolagus colliei, SIO 85–73, 448 mm TL (frontal view of head)

      ELASMOBRANCHII—Sharks and Rays

      The Elasmobranchii, with 12 extant orders of sharks and rays, is the sister group to the Holocephali. In contrast to that group, elasmobranchs have five to seven separate gill openings, and the upper jaw is not fused to the neurocranium (amphistylic or hyostylic jaw suspension). Additionally, males of this group lack a cephalic clasper organ. The phylogenetic relationships of elasmobranchs, although intensively studied by numerous researchers using a variety of data sets including morphology, molecular data, and the fossil record, have been controversial (e.g., Maisey, 2012; Naylor et al., 2005; Shirai, 1996). Among the main contentious issues have been questions about the monophyly of the sharks (the Selachii) as a group, and the monophyly of the rays (the Batoidea) as a group. Naylor et al. (2005) and more recently Aschliman et al. (2012) concluded that these groups are reciprocally monophyletic, that is, each is monophyletic and they are sister groups, sharing a unique common ancestor. Naylor et al. (2012) recently summarized information on the valid species of elasmobranchs.

      SELACHII—Sharks

      The Selachii includes all species of sharks and is characterized by lateral gill openings and pectoral fins separate from the head (Nelson, 2006). This group includes 518 species, classified in eight orders, 35 families, and over 100 genera (Compagno, 1984a, 1984b, 2005; Compagno et al., 2005; Eschmeyer and Fong, 2013; Naylor et al., 2012). Phylogenetic relationships of sharks have been hypothesized by several researchers (e.g., de Carvalho, 1996; Naylor et al., 2005; Shirai, 1996; Vélez-Zuazo and Agnarsson, 2011), including Maisey et al. (2004), who recognized two major lineages, the Squalomorphii and the Galeomorphii.

      Hypothesized phylogenetic relationships of the Selachii (sharks) after Maisey et al. (2004) and

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