Millions of years of exposure to similar selective pressures have resulted in convergent evolution between modern coleoid cephalopods and modern chondrichthian and teleost fishes
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چکیده
Millions of years of exposure to similar selective pressures have resulted in convergent evolution between modern coleoid cephalopods and modern chondrichthian and teleost fishes (Packard, 1972; O’Dor and Webber, 1986; Hanlon and Messenger, 1996). The cephalopod circulatory system was shaped by this convergent evolution (Packard, 1972; O’Dor and Webber, 1991; Wells, 1994; Hanlon and Messenger, 1996). Unlike other molluscs, which have open circulatory systems (Brusca and Brusca, 1990), modern coleoids (e.g. octopods, cuttlefish and squid) have high pressure (Wells, 1979; Bourne, 1982), high output (Shadwick et al., 1990), closed (Williams, 1909; Tompsett, 1939; Wells, 1978; Schipp, 1987a) circulatory systems resembling those of fishes (Farrell and Jones, 1992). Furthermore, coleoids have two separate circulations (Tompsett, 1939), one for the gills, powered by the single-chambered branchial hearts at the base of the each gill, and one for the systemic circulation, powered by the ventricle and its two auricles (Fig.·1). In this regard, the coleoid circulatory system appears more avian or even mammalian than fish-like (Packard, 1972). Despite the many similarities between coleoid and vertebrate circulatory systems, coleoid systems are based on the molluscan Bauplan, and therefore differ in important ways from vertebrate systems. For example, some of the valves guarding the entrances and exits to coleoid hearts are muscular and innervated (Smith and Boyle, 1983). This is more reminiscent of crustacean (Wilkens, 1997; Davidson et al., 1998) than mammalian cardiovascular valves (Berne and Levy, 1997). Unlike vertebrates and crustaceans, many coleoid venous vessels contract in vitro (Smith and Boyle, 1983; Schipp, 1987a). Perhaps one of the most important factors affecting basic circulation in cephalopods is their oxygen transport protein, hemocyanin. It is not contained in blood cells, but instead is dispersed freely in the blood (Mangum, The Journal of Experimental Biology 208, 2071-2082 Published by The Company of Biologists 2005 doi:10.1242/jeb.01575
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