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Although both sexes may develop breeding tubercles, they typically appear only on males. Breeding tubercles may occur along the upper surface of the pectoral fins, the ridges of the pectoral-fin rays, on the first dorsal-fin ray, and over the dorsal surface of the caudal peduncle (Opuszynski and Shireman, 1995). Similar Species The Grass Carp can be distinguished from native cyprinids by the position of its anal fin, which is set far back on the body (fig. 19) and the deep lateral grooves in its pharyngeal teeth (fig. 20). Native cyprinids have an anal fin that is more anterior than that of the Grass Carp, and have pharyngeal teeth that lack deep lateral grooves. The Grass Carp closely resembles Black Carp, but can be distinguished by its pharyngeal teeth and (in most cases) body color. The Grass Carp has long, serrated pharyngeal teeth (sometimes with hooks) whereas those of adult Black Carp are smooth and molariform. Adult Grass Carp are lighter in color than Black Carp, espe- cially the fins. For additional characteristics useful in distinguishing Black Carp from Grass Carp, refer to the Black Carp species account. Variation Concern over ecological impacts in natural systems has resulted in widespread use of triploid Grass Carp that are presumably sterile (Clugston and Shireman, 1987). Morphologically, triploids are indistinguishable from (fertile) diploids (Bonar and others, 1988). Triploid Grass Carp may be produced using heat, pressure, or chemical shocking of the fertilized eggs (Clugston and Shireman, 1987; Opuszynski and Shireman, 1995). However, because most treatments used to create triploid Grass Carp are not 100 percent effective, the ploidy of each fish must be verified (that is, either diploid, 2n or triploid, 3n). Ploidy is typically determined by analysis of blood taken from live or freshly killed specimens. The preferred technique uses a particle sizer (for example, Coulter Counter~) with a channelyzer to estimate ploidy by analyzing the distribution of red blood cell nuclear volumes (Wattendorf, 1986). Before widespread use of triploid Grass Carp became the main alternative to releasing reproductively viable (diploid) individuals, there was limited production and release of a sterile hybrid formed by crossing female Grass Carp with male Bighead Carp (Fuller and others, 1999; Nico, 2005). The resulting offspring of this cross had closely spaced gill rakers and an abdominal keel like the paternal Bighead Carp, as well as pharyngeal Grass Carp 33 teeth and an elongated body resembling the maternal Grass Carp (Berry and Low, 1970; Veri gin and others, 1975; Kilambi and Zdinak, 1981). Although hybrids can allegedly feed on either plankton or macrophytes, Kilambi and Zdinak (1982) reported a preference for zooplankton. Karyology of the hybrid was given in Marian and Krasznai (1978) and Beck and others (1980). Kilambi and Zdinak (1981) described the hybrid larvae. Meristics for Grass Carp X Bighead hybrids are given in appendix B. Grass Carp have been artificially hybridized with Common Carp (Makeyeva and Veri gin, 1974a; Stanley and Jones, 1976; Avault and Merowsky, 1978), Bighead Carp (Andriasheva, 1968; Beck and others, 1980), Silver Carp (Andriasheva, 1968) and Black Carp (Makeyeva and Veri gin, 1993). Reproduction Sexual maturity is reached at an average age of 2-5 years in subtropical/tropical areas and 4-7 years in temperate regions (Alikunhi and Sukumaran, 1964; Bardach and others, 1972). Males generally mature a year earlier than females (Opuszynski and Shireman, 1995). Shireman and Smith (1983) reported that Grass Carp may mature earlier than these averages. For example, mature 1-year old males and 2-year old females were documented in tropical India and Malaysia (Hickling, 1967). Alternately, maturation can take 9-10 years in cold climates (Makeeva, 1963, in Hickling, 1967). The condition of gonads through the stages of development was reviewed by Opuszynski and Shireman (1995). Fecundity has been found to range widely, from 255,000-2,000,000 eggs (Vinogradov and others, 1966; Gorbach, 1972; Shireman and Smith, 1983; Opuszynski and Shireman, 1995), and fecundity reportedly increased with age and mass (Gorbach, 1972). Egg and larval development were described by Bailey and Boyd (1970) and Nakamura (1969). Although Grass Carp spawn- ing commonly occurs in large rivers, Tang (1960a,b) reported an unusual instance of a few spawning events in a Taiwan reservoir. The success of those particular spawning events is uncertain (Nico and others, 2005). Krykhtin and Gorbach (1981), who studied the down- stream drift of Grass Carp eggs in the Amur Basin, determined that spawning occurred at water tempera- tures between 17- 26 oC, with peak activity at 21-26 oc. Stanley and others (1978) reviewed literature from Asia and Europe on requirements for spawning, including temperature, water level fluctuation, and spawning site characteristics.