Holopedium glacialis Rowe, Adamowicz & Hebert, 2007, n. sp.

Holopedium glacialis n. sp.

Synonymy. All previous descriptions of H. gibberum from temperate North America are properly assigned to H. glacialis.

Forbes (1882): 641–642, Plate IX, Figs. 12–15

Herrick (1884): 22–23, Plate N, Fig. 11

Birge (1918): 693, Figs. 1060, 1061a

Brooks (1959): 603, Figs. 27.12a,b

Pennak (1953): 364–365, Figs. 227a,b

Pennak (1978): 365–366, Figs. 254a,b

Pennak (1989): 386–387, Figs. 12a,b

Dodson & Frey (1991): 746–747, Fig. 20.7

Etymology. glacialis refers to the near restriction of this species to regions of North America that were glaciated during the Pleistocene.

Type locality. Wren Lake, Ontario (45.183º N, 78.866º W). This lake is situated near Carnarvon, Ontario, approximately 3 km past the Leslie M. Frost Natural Resources Centre on Hwy 35N.

Type specimens. Holotype: an ovigerous female in ethanol deposited in the CMN under accession number CMNC 2007-0745 (collection date Sept. 29th 2007).

Paratypes: Twenty ovigerous females, also from Wren Lake, Ontario, preserved in ethanol, deposited in the CMN under accession number CMNC 2007-0746(collection date Sept. 29th 2007).

Material examined. Other habitats with H. glacialis are listed in Appendix A.

Morphological description. FEMALE. Representative photomicrographs are shown in Figure 7. The jelly coat is of the Z type, in which the anterior jelly curl is relatively straight and ends in a short curve posteriorly toward the carapace, and the lateral lobes are divided (see Montvilo et al. 1987).

Adult female carapace lengths range from 0.68–1.30 mm (mean 0.98 mm), while carapace heights range from 0.75–1.82 mm (mean 1.26 mm). The H/L ratios range from 0.61–1.95 (mean 1.30). The ventral carapace margins usually have many, tightly-spaced spinules posteriorly, but are often smooth anteriorly.

Anal spine number is less variable than in H. gibberum, ranging from 11–20 (mean 15.1). H. glacialis typically possesses at least one basal spine on each postabdominal claw. Each claw has a row of denticles running laterally from the base of the claw to its midpoint.

MALE. Males were encountered in autumn collections. The jelly coat is present and resembles that of the female of this species. Mature males are often half the size of adult females, with individuals ranging from 0.40–0.63 mm in length (CLR, pers. obs.). The ventral carapace margin is spinulated posteriorly, but is smooth anteriorly.

The postabdomen is long and terminates posteriorly with two postabdominal claws. A single row of anal spines runs ventro-laterally on each side of the postabdomen (range 13–17). There is one basal spine on each postabdominal claw, and each claw invariably has a row of denticles running laterally from the base of the claw to its midpoint.

Differential diagnosis. Holopedium glacialis can be distinguished from all members of the H. amazonicum species complex by its possession of at least one basal spine on each postabdominal claw. It is morphologically indistinguishable from H. gibberum, although they have largely allopatric distributions (Fig. 4 a,b). Holopedium glacialis can be biochemically distinguished from North American H. gibberum at the Gpi locus. Holopedium glacialis was never found to possess Gpi 114, while H. gibberum was monomorphic for this allele. COI mtDNA sequence divergence between H. glacialis and H. gibberum averages 13.1%. Based on current evidence, individuals showing less than 4.3% divergence from a representative COI mtDNA sequence (GenBank AF 245355) belong to H. glacialis.

Distribution. Holopedium glacialis presumably occurs over most of the formerly glaciated regions of North America (Fig. 4 a), from the Coastal and Rocky Mountain ranges in the west through to the Atlantic coast in the east, excepting the central Great Plains, where habitats with suitable water chemistry are absent (see Rowe 2000 for a discussion of the ecological requirements of Holopedium). It occurs as far south as the Sierra Nevada mountains of California and the Rocky Mountains of Colorado, but in the east there are no confirmed populations south of New York State. It occurs sympatrically with H. atlanticum in New Brunswick and Maine, with no evidence of hybridization. H. glacialis is also present in the Canadian arctic, but not north of 68.5° N latitude. The northernmost habitats were a few lakes on the Melville Peninsula, including three in which it was sympatric with H. gibberum, again with no evidence of hybridization.

Breeding system. Many populations were invariant, but 43 of 59 polymorphic populations were in H-W equilibrium, suggesting that at least these populations reproduce by cyclical parthenogenesis. Males were discovered in several populations in the spring and in larger numbers in the autumn. The few single-species populations in H-W disequilibrium were due to occasional heterozygote excesses or deficits, patterns that may result from extended bouts of asexual reproduction.