River redhorse (Moxostoma carinatum) assessment and update status report: chapter 5

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Species information

• Name and Classification
• Morphological Description
• Genetic Description

Name and classification

The river redhorse is a large catostomid (sucker) of the genus Moxostoma (Figure 1). Due to similarities in appearance among Moxostoma species that result in identification difficulties and previous confusion in nomenclature, this genus has been considered one of the most troublesome groups of freshwater fishes (Scott and Crossman 1973). It was formerly recognized as being in a separate genus, Placopharynx, due to its large molariform pharyngeal teeth and associated trophic adaptations. It has since been synonymized with Moxostoma, partially due to the larger pharyngeal arch and teeth observed in copper redhorse (Moxostoma hubbsi) (Jenkins and Burkhead 1993; Stagliano 2001). Although similar to members of the Catostomus genus, species of Moxostoma are generally larger, more laterally compressed and possess a three-chambered swim bladder (Scott and Crossman 1973; Parker 1988).

Figure 1: The River Redhorse (Moxostoma carinatum)

The river redhorse is referred to by a variety of common names including redhorse sucker, river mullet, greater redhorse, redfin redhorse, pavement-toothed redhorse, and big-sawed sucker (Scott and Crossman 1973; Becker 1983; Stagliano 2001). Until recently, the official French name for the river redhorse was “suceur ballot”. In February 1998, this name was replaced with “chevalier de rivière” (Branchaud et al. 1998).

Morphological description

Parker (1988) provided a description of the river redhorse in the previous updated status report. Jenkins (1970), Scott and Crossman (1973) and Jenkins and Burkhead (1993) have also described this species in detail. The dorsum of the river redhorse is brown or olive-green, with sides brassy, yellowish-green or coppery and undersides white. Crescent-shaped dark spots are present on each dorso-lateral scale. Scale counts for the river redhorse are usually 12 around the caudal peduncle and between 42 and 47 scales along the lateral line. The caudal and dorsal fins are crimson in colour, while the lower fins are orange to reddish. The lips of the river redhorse are deeply plicate with transverse ridges and papillae (bumps) absent. The lower lip is broader than the upper lip with a virtually straight posterior margin (sometimes scallop-shaped). The dorsal fin is straight or slightly concave in shape. The caudal fin is forked, with the upper lobe usually longer and more pointed than the lower lobe.

The river redhorse is similar in external appearance to all redhorse species but particularly to the shorthead redhorse (M. macrolepidotum) and greater redhorse (M. valenciennesi). Each of these species exhibits red-tinted fins, large scales, subterminal mouth, complete lateral lines (38-48 scales), a single dorsal fin (11–17 rays), and lack buccal teeth.

The primary feature distinguishing the river redhorse from other Moxostoma is the heavy pharyngeal arch bearing molariform teeth (Jenkins and Burkhead 1993). The only sympatric species with a heavier pharyngeal arch and teeth is the copper redhorse while the other Canadian species (M. anisurum, M. macrolepidotum, M. valenciennesi, M. erythrurum and M. duquesnei) have a pharyngeal arch with comblike teeth (Eastman 1977, Jenkins 1970, Jenkins and Burkhead 1993, Mongeau et al. 1986).

Adult river redhorse are sometimes confused with the greater redhorse but can be easily distinguished by caudal peduncle scale count (greater redhorse: 15-16 scales). However, most of the misidentification occurs between the river redhorse and the shorthead redhorse. Both species have the same caudal peduncle scale counts (12). Shorthead redhorse has a thinner head, transverse ridges across the plicae (folds) of the lower lip and a falcate dorsal fin (Mongeau 1984).

Nuptial males are adorned with tubercles on the snout and anal and caudal fins (Scott and Crossman 1973; Trautman 1981; Becker 1983; Jenkins and Burkhead 1993; Reid 2003). The scales of nuptial males are thick and rough during the spawning period (Reid 2003). As well, breeding males usually exhibit a midlateral, dark stripe extending from the snout to above the anal region (Hackney et al. 1967; Jenkins 1970; Becker 1983). A thickening of the epidermis on the lower area of the caudal peduncle has been observed in breeding females as well as tubercle formation on the anal fin (Scott and Crossman 1973; Becker 1983). Both sexes in breeding condition exhibit intensification of body and fin colouration (Hackney et al. 1967; Jenkins 1970).

Genetic description

Canadian populations are found within the Great Lakes-Western St.Lawrenceecozone of the freshwater ecozone classification adopted by COSEWIC. Population structure is unknown; however, one of the authors (SR) is currently conducting an examination of the genetic structure of river redhorse in the Grand River and Trent River.