Mapleleaf mussel (Quadrula quadrula) COSEWIC assessment and status report: chapter 3

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Species Information

Name and classification

The current authority for the classification and scientific nomenclature of freshwater mussels in North America is Turgeon et al. (1998). The classification currently accepted is:

A complete list of synonyms and the nomenclatural history for Quadrula quadrula is provided by Parmalee and Bogan (1998).

Morphological description

The Mapleleaf (Quadrula quadrula) is a medium to large freshwater mussel species that can be recognized by 2 bands of nodules radiating in a V-shape from the umbo to the ventral margin (Figure 1). One band is located centrally and the other is located on the posterior ridge. The rows are separated by a shallow groove. The accompanying species description has been adapted from Clarke (1981), Cummings and Mayer (1992), Parmalee and Bogan (1998), Graf and Ó’Foighil (2000), Cicerello and Schuster (2003), Haag and Staton (2003) and Hay et al. (2003). Morphology of Canadian specimens consistent with the range of variation reported from American specimens. Conchological morphology of sexes is alike. Shell is thick, moderately inflated, quadrate in outline. Anterior end rounded, posterior end squared or truncated. Umbos are small and slightly raised above the hinge line. Umbo sculptured; double loops or zigzags radiate to ventral margin as 2 rows of raised nodules. Nodules on the umbo are small and crowded, generally eroded on older specimens. Nodules distal to umbo may be elongated or rounded. Nodules radiating from umbo are separated by a wide, shallow sulcus or furrow. One row is centrally located, the other posterior. Occasional variations in nodule patterns range from small nodules scattered over shell surface to absent (Neel 1941). Periostracum variable, ranging from yellowish green to light brown in young individuals, older individuals ranging from greenish brown through dark brown. Annual growth lines on the shell surface are obvious and well defined, especially in younger individuals; can be crowded and difficult to distinguish at the shell margin of older individuals.

Pseudocardinal teeth are well developed, vertically elongated and serrated; two in the left valve, one in the right (Figure 2). Pseudocardinal tooth in the right valve heavy, thick and triangular. Pseudocardinal teeth in left valve rough, solid and sometimes joined dorsally. Two lateral teeth in the left valve; erect, fairly long and straight, may be serrated. Single lateral tooth in the right valve: high, long and straight, may be serrated. Interdentum is wide. Beak (umbo) cavity is deep but open. Nacre is white and iridescent posteriorly.

Parmalee and Bogan (1998) report mature Quadrula quadrula reaching 120 mm in length. In Canada, Clarke (1981) reports Q. quadrula reaching 125 mm in length, 100 mm in height and 50 mm in width. In Manitoba, individuals have been recorded up to 121 mm in length, 88 mm in height and 52 mm in width (Carney 2003a).

In Ontario, Quadrula quadrula is most similar to Quadrula pustulosa. The two can be distinguished as follows. Quadrula quadrula has nodules restricted to 2 bands whereas they are scattered and more uniformly distributed on Q. pustulosa. Quadrula quadrula is quadrate in outline whereas Q. pustulosa is rounded in outline. There are no other mussels in Canada with which Q.quadrula can be confused.

The following description of soft-tissue morphology is derived from Lydeard et al. (1996) and Graf and Ó’Foighil (2000). Quadrula quadrula uses all four gill demibranchs as marsupia. Interlamellae of gill demibranchs connected by complete septa. Marsupial water tubes undivided, not tripartite. No swelling of marsupial interlamellar septa into water tubes. Margin of gravid marsupium sharp. Ventral margin of marsupium does not extend beyond non-marsupial portion of gill. Mantle ventral to incurrent siphon simple, not modified with papillae or lure-like elaborations.

Genetic description

Genetic approaches to freshwater mussel systematics is a growing area of research and many of the reported conclusions depend on the species included, the geographic scale from which specimens are collected, the type of genetic data examined and how those data are analyzed. Genetic approaches include allozyme electrophoresis (Davis et al. 1981; Berg et al. 1998), analysis of restriction fragment length polymorphisms (RFLP) (White et al. 1996), and analyses of gene sequence data from the nuclear and mitochondrial genomes (Lydeard et al. 1996; Graf and Ó’Foighil 2000; Serb  et al. 2003). Gene sequences investigated have been derived from NADH dehydrogenase (NDI) gene, cytochrome c oxidase subunit I (COI) gene, 16S rRNA, and 28S rDNA and have been used to address questions pertaining to character evolution, species identification, phylogeography and systematic relationships across a range of taxonomic scales.

Genetic information specific to Quadrula quadrula are scattered and have been used to address questions relating to higher level Unionidae phylogenetic relationships, or questions including the evolution of specific traits such as reproductive characters (e.g. Davis and Fuller 1981; Lydeard et al. 1996; Graf and Ó’Foighil 2000). For example, Serb et al. (2003) used the mitochondrial ND1 gene to assess the molecular systematics of the genus Quadrula. Their results indicated the genus was monophyletic only if Tritogonia verrucosa, ‘Fusconaia’ succissa and ‘Quincuncina’ infucata were included in the genus. They also revealed that Q.quadrula is not monophyletic but may form a species group that includes Q.rumphiana, Q. apiculata and Q.nobilis (Serb et al. 2003). However, they were able to demonstrate repeated geographic structuring especially in the quadrula species group and predicted that phylogeographic studies would result in populations being differentiated by drainage. A study by Krebs et al. (2003) using a fragment of the mitochondrial 16S rRNA gene supported the idea that Q.quadrula may not be monophyletic. They (Krebs et al. 2003) found that an individual Q.quadrula from the Lake Erie drainage was more similar to Q.apiculata than to a Q.quadrula from a southern location. Krebs et al. (2003) also reported that Q.quadrula had a higher degree of molecular variation than the other unionid species they investigated.

These gene sequence data are generally not presented in a way that allows assessments of population genetic structure, population variation and gene flow between and among populations. There are, however, some allozyme data available specific to these questions for Quadrula quadrula.

Davis (1984) and Johnson et al. (1998a, b, c) using allozyme electrophoresis described genetic variation in populations of Quadrula quadrula from Wisconsin and Arkansas, respectively. They described similar alleles per locus (1.6) and mean polymorphism (0.35-0.36) for these populations (Table 1).  Mean heterozygosity was almost double in the Wisconsin populations relative to the Arkansas populations. These data are in contrast to those reported by Berg et al. (1998) who investigated allozyme variation in Q.quadrula populations from Ohio and the lower Mississippi and found much higher values for mean polymorphism, mean heterozygosity and alleles per locus than reported by Davis (1984) and Johnson et al. (1998a, b, c) (Table 1). Berg et al. (1998) also reported much lower values for F_st and Nei’s genetic distance than reported by Johnson et al. (1998a, b).  These genetic distance data indicate there is very little genetic variation among the Q. quadrula populations examined by Berg et al. (1998) despite being separated by up to 2,000 km of river. Berg et al. (1998) suggested this may be a consequence of high gene flow resulting from using a highly vagile host for the glochidia.

Recent data have become available analyzing Cytochrome Oxidase I gene region of the mitochondrial genomeof Quadrula quadrula populations from regions in the United States and from Manitoba and Ontario (Levine, email comm.) (Figure 3). Preliminary analysis of these data reveals that the Manitoba and Ontario populations share the majority of their haplotypes with some populations in the United States, suggesting the Canadian populations are subsets derived from the American populations. Both the Manitoba and Ontario populations appear to have low genetic diversity. However, both populations respectively have haplotypes that are unique and not shared with any other region. This indicates there is unique genetic information in each of the Manitoba and Ontario populations.

It is clear that data derived from allozymes and data derived from gene sequences are not always in agreement. Allozymes suggest little genetic variability in Quadrula quadrula whereas gene sequence data indicate high variability, possibly more than one species and phylogeographic structuring.  In many cases the number of individuals sampled is very small (e.g. n = 1; Krebs et al. 2003) and the geographic coverage is limited. The majority of available data are from the Mississippi drainage (but see Krebs et al. 2003) with few data from the Great Lakes/St. Lawrence drainage or from the Red River/Hudson Bay drainage. Clearly there is a pressing need for a more comprehensive continental coverage of the genetic structure of these taxa that includes larger sample sizes.

Designatable units

COSEWIC recognizes units below the species level based on taxonomically recognized subspecies or varieties; genetic distinctiveness; separation of units by range disjunction; and presence in biogeographically different ecoregions. The populations of Quadrula quadrula in Ontario and Manitoba merit distinction as separate designatable units according to the criteria outlined by COSEWIC. First, they can be recognized as being genetically distinct. Despite being low in genetic diversity and the majority of individuals sharing a haplotype common throughout North American populations, each population has haplotypes unique to each population. Manitoba populations have 3 distinct haplotypes and Ontario populations have 2 distinct haplotypes. Second, they are units separated by a major range disjunction. The Manitoba populations are part of the Hudson Bay watershed and the Ontario populations are part of the Great Lakes-St. Lawrence watershed. As aquatic organisms they are limited to dispersal only within these continental scale watersheds. Effectively this means they are disjunct from each other and from other North American Q.quadrula populations in the Ohio-Mississippi watershed in the United States and dispersal among these watersheds is not possible. Third, they occupy biogeographically distinct different eco-geographic regions. The Manitoba populations occupy the Saskatchewan-Nelson National Ecological Freshwater Area whereas the Ontario populations occupy the Great Lakes-Western St. Lawrence Ecological Area. As such, these populations meet 3 of the 4 criteria outlined by COSEWIC for recognition as distinct designatable units.