Speckled dace (Rhinichthys osculus) COSEWIC assessment and status report: chapter 3

Species Information

Name and classification

Phylum:
Chordata
Class:
Actinopterygii (ray-finned fishes)
Order:
Cypriniformes
Family:
Cyprinidae
Genus:
Rhinichthys
Species:
Rhinichthys osculus (Girard 1856)

Common name

English:
Speckled Dace (Nelson et al. 2004)
French:
Naseux Moucheté (Coad 1995)

Morphological description

The speckled dace (Rhinichthys osculus) (Fig. 1) is a small minnow (usually < 70 mm; females occasionally > 90 mm (McPhail 2003)) with a robust elongate body. It is grey to brownish grey in colour with small dark flecks.

McPhail (2003) provides the following description: a blunt nosed dace with the snout barely overhanging the upper lip; the mouth is oblique and the upper lip free from the snout; the caudal peduncle is moderately narrow; the dorsal and anal fins, and the lobes of the caudal fin, are rounded; the fork of the caudal fin is shallow with a depth about 6% of the total length; it has 8 or 9 dorsal rays and 59-69 scales along the lateral line; the back and sides of juveniles and females lack irregular, dark spots; however, males develop markings similar to those seen in Umatilla dace.

Figure 1. Speckled dace, Rhinichthys osculus (photograph by P. Mylechreest courtesy of Dr. J.D. McPhail).

Figure 1.  Speckled dace, Rhinichthys osculus

Peden and Hughes (1988) differentiated R. osculus in Canada from all others by its absence of barbels and high scale counts around the caudal peduncle. It differs from other Rhinichthys species in Canada as its snout does not extend beyond the premaxillae and its mouth morphology suggests dissimilar foraging behaviours (Peden and Hughes 1988). McPhail and Carveth (1993) also noted that the speckled dace in British Columbia does not have barbels at the corner of the mouth and the origin of the anal fin is almost directly below the hind end of the base of the dorsal fin. Peden (2002) described differences between Canadian and American populations in additional detail.

Haas (2001) describes additional features to help distinguish R. osculus from R. falcatus and R. umatilla based on appearance. Among these species, speckled dace is the least streamlined and most robust in body form and the darkest in colouration; the eyes are smaller; its fins and fin bases are smaller and rounder in shape; its tail is smaller in size and degree of fork, and caudal peduncle thicker; the pelvic fin stays are generally weaker or absent. He also notes that in Canada, speckling can be greatly reduced or not obvious.

Genetic description

Mitochondrial and ribosomal DNA studies based on sequences from the cytochrome-b region (306 base pairs), internal transcribed spacer (250 base pairs), and the ribosomal region (80 base pairs) support the distinction of R. osculus, R. falcatus and R. umatilla (Haas 2001). Canadian populations of speckled dace also show evidence of meristic differences from other populations in the Columbia River drainage; however, limited DNA studies completed by Haas (2001) have not provided additional support for this distinction. To date, no hybrids between speckled dace and Umatilla dace have been documented in the Canadian sector of the river below Cascade Falls, where they occur together (Peden 2002).

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