Holopedium amazonicum Stingelin, 1904

Rowe, Chad L., Adamowicz, Sarah J. & Hebert, Paul D. N., 2007, Three new cryptic species of the freshwater zooplankton genus Holopedium (Crustacea: Branchiopoda: Ctenopoda), revealed by genetic methods, Zootaxa 1656, pp. 1-49 : 30-32

Holopedium amazonicum Stingelin, 1904

Historical literature descriptions. Holopedium amazonicum is restricted to the Amazon River basin. Individuals from North America previously identified as this species are either H. atlanticum or H. acidophilum .

Stingelin (1904 a): 54–64, Table 1, Figs. 1–2

Stingelin (1904 b): 577–578, Table 20, Figs. 1–2

Thomasson (1955): No figures (paper incorrectly identifies specimens as H. gibberum )

Korovchinsky (1992): 77–78, Figs. 378–381

Paggi (1995): 912, 930–931. Figs. 14–15

Korovchinsky (2004): 345–348, Fig. 141

Etymology. amazonicum refers to the distribution of this species in waterbodies throughout the Amazon River basin.

Type locality. Rio Aramá Grande on Marajó Island at the mouth the Amazon River in Brazil ( Stingelin 1904 a; approx. 1.02° S, 49.24° W).

Type specimens. Stingelin’s (1904 a) type material is housed in the Naturmuseum Olten (Kirchgasse 10, CH –4600 Olten, Switzerland; Dr. D. Vallen, curator). The material consists of two mounts, with some adult females and many juveniles from “Amazonas, 1900”. One of the females is designated as the lectotype, and the others as paralectotypes.

Voucher specimens. Twenty ovigerous females from Lago Coari, Amazonas, Brazil (collected by P. Mera, INPA, Manaus, Amazonas, on May 24, 1996; 4.08° S, 63.14° W) were deposited in the CMN under accession number CMNC 2007-0740 and an additional 30 ovigerous females from this collection were deposited at INPA, Manaus, Amazonas.

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

Morphological description. FEMALE. Representative photomicrographs are shown in Fig. 8 . In this study, due to specimens’ preservation in formalin, jelly coat sculpturing was not discernable.

Females are small, with adult carapace lengths ranging from 0.39–1.27 mm (mean 0.74 mm), while carapace heights range from 0.32–1.17 mm (mean 0.71 mm). The H/L ratios range from 0.66–1.64 (mean 0.99). The ventral carapace margin has many, tightly-spaced spinules posteriorly, but is smooth anteriorly.

Anal spine number ranges from 5–12 (mean 8.37). Holopedium amazonicum lacks a basal spine on each postabdominal claw. Each claw ordinarily has a row of denticles running laterally from the base of the claw to its midpoint, although individuals were observed lacking claw denticulation.

MALE. Two males were discovered by CLR in a collection from Lago Coari, Amazonas, Brazil (4.08º S, 63.14º W), which constitutes their first report from South America. The jelly coat was present, but due to its preservation in formalin, sculpturing was not discernable. The specimens were small, with a mean length and height of 0.36 mm and 0.21 mm, respectively. The ventral carapace margin has many, tightly-spaced spinules posteriorly, but is smooth anteriorly.

Differential diagnosis. Holopedium amazonicum is morphologically indistinguishable from H. atlanticum , but these species have allopatric distributions ( Fig. 4 c,e). H. amazonicum is distinguished from H. acidophilum by its smaller size and smaller number of anal spines. It differs from members of the H. gibberum complex by its absence of a basal spine on either postabdominal claw. COI mtDNA sequence divergence between H. amazonicum and H. atlanticum averages 12.3%, while the divergence between H. amazonicum and H. acidophilum averages 8.7%. Levels of genetic variation within this species remain poorly known, but, based on patterns in the other Holopedium species, individuals showing less than 4% divergence from a representative COI mtDNA sequence (GenBank AF 245351 ) are likely to belong to H. amazonicum .

Distribution. H. amazonicum appears restricted to the Amazon River basin (see also Rowe 2000), from which it was described ( Stingelin 1904 a) ( Fig. 4 e). It has been reported from several ria lakes throughout the Amazon River basin, typically associated with highly humic-stained “blackwater”. Records indicate that H. amazonicum is present throughout the year, with highest densities during the semi-annual rising and lowering of the water level (E.R. Hardy, INPA, Manaus, Amazônia, unpubl. data). It has occasionally been collected in low densities from whitewater ria lakes, but it has likely been washed into these habitats from blackwater habitats during periodic flooding.

Breeding system. Members of this species were not included in allozyme analyses, and so the breeding system cannot be diagnosed in this fashion. However, males of H. amazonicum were detected by CLR in a collection made by Pedro Mera (INPA, Manaus, Amazônia) from Lago Coari, Brazil (4.077° S, 63.140° W; May 24, 1996), suggesting that cyclic parthenogenesis may be the mode of reproduction.

Figures

FIGURE 1. Maps of Holopedium populations sampled for genetic analyses. Exact geographic coordinates and habitat names are provided in Appendix A. Populations sampled from outside North America are shown on the inset global map. Type localities for the two initially described species of Holopedium are indicated with stars on the inset map and were not included in this study, although (relatively) nearby collections were available. North American freshwater biogeographic provinces (after Burr & Mayden 1992) are shown, and the names of those from which samples were collected are listed in the legend.

FIGURE 2. UPGMA phenogram showing genetic distances, based upon allozyme data, among 121 Holopedium populations. Only those collections known to contain a single species, due to being either invariant or in H-W equilibrium, are included here. Nei’s (1972) genetic distance is indicated on the scale bar. The geographic distributions of the two major groups are consistent with those of the two initially described species of Holopedium, and these groups are therefore named after these species, H. gibberum and H. amazonicum. New species names are assigned to two clusters on the basis of allozyme, mtDNA, distributional, and morphological information (see text for further information and justification). Asterisks designate those populations for which mtDNA results are available; allozyme and mtDNA clusters are concordant. Double asterisks and haplotype numbers are given for those populations having sequence data presented in Fig. 4.

FIGURE 4. Putative geographic distribution of Holopedium species. Dots indicate populations where species assignments were confirmed by genetic analyses. Dark shaded areas represent the hypothesized range of each species based on results from this study and from distribution data from 1,827 localities inhabited by Holopedium, obtained from literature reports or by sampling (see Rowe 2000). Since several species are morphologically cryptic while the species complexes can be readily distinguished, the areas between or adjacent to genetic localities are tentatively marked as that same species, provided that there are Holopedium records there belonging to the same complex. Pending further evidence, all South American localities are here shaded as H. amazonicum and most Eurasian localities as H. gibberum s. s., but further cryptic species may be detected in the future. Definitive species assignments for populations in Greenland and India, which are currently described as separate species, require genetic evidence (see text), but morphological traits indicate that they do belong to the H. gibberum complex. Their distributions are shown along with H. gibberum s. s. (inset map in part b). a) H. glacialis n. sp., b) H. gibberum s. s., c) H. atlanticum n. sp., d) H. acidophilum n. sp., and e) H. amazonicum s. s.

FIGURE 8. Representative photomicrographs of Holopedium amazonicum. (a) Lateral view of ovigerous female with jelly coat removed. (b) Lateral view of brooding female with jelly coat removed. (c) Lateral view of female head. (d) Partial lateral view of female. (e) Lateral view of female head. (f) Lateral view of female postabdomen. Lago Coari, Amazonas, May 24, 1996. (g) Lateral view of brood pouch margin and eggs. (h) Lateral view of ventral carapace margin. (a – e, g – h) from Lago Caju, Amazonas, September 24, 1998.