Eastern pondmussel (Ligumia nasuta) COSEWIC assessment and status report: chapter 3

Species information

Name and classification

Scientific name:

Ligumia nasuta (Say, 1817)

English common name:

Eastern Pondmussel

French common name:

Ligumie pointue

The recognized authority for the classification of aquatic molluscs in the United States and Canada is Turgeon et al. (1998). The current accepted classification of this species is as follows:

Phylum:

Mollusca

Class:

Bivalvia

Subclass:

Paleoheterodonta

Order:

Unionoida

Superfamily:

Unionoidea

Family:

Unionidae

Subfamily:

Lampsilinae

Genus:

Ligumia

Species:

Ligumia nasuta

Morphological description

The following description of Ligumia nasuta was adapted from Clarke (1981), Strayer and Jirka (1997), Nedeau et al. (2000) and Bogan (2002). The Eastern Pondmussel (Figure 1) is a medium-sized to large mussel with a long-elliptical, laterally compressed shell that is thin but strong. The posterior ridge is well developed, distinct, and angled near the beak, but becoming rounded posteriorly. The anterior end is rounded; the ventral margin is broadly curved; and the posterior end is rounded and drawn out into a blunt point near the midline of the shell. Females can be distinguished from males by a swelling along the posterior ventral margin. The beaks are low, barely raised above the hinge line and located in the anterior quarter of the shell. Beak sculpture consists of 5-8 fine double-looped bars. Lateral and pseudocardinal teeth are well-developed and sharp, but delicate. The surface of the shell (periostracum) is rough with concentric wrinkles and clearly visible lines of growth. The colour of the periostracum varies from yellowish- or greenish-black in juveniles to dark brown or black in adults. Narrow green rays, concentrated posteriorly, are often visible in juveniles and light-coloured adults. The nacre is usually silvery-white or bluish-white in specimens from the Great Lakes basin, and pinkish or purple in specimens from the Atlantic drainage.

Ligumia nasuta reaches a maximum length of approximately 100 mm in Canada. The average length of an adult shell is about 70 mm based on over 200 live specimens measured by the authors and their associates between 1997 and 2004. The Eastern Pondmussel can be distinguished from all other species of freshwater mussel in Canada by its elongate shell with distinctive, bluntly-pointed posterior end, rough periostracum and delicate hinge teeth.

Figure 1. (A) Line drawing of the external features of the shell and internal structure of the left valve of Ligumia nasuta. Reproduced with permission from Burch (1975). 
(B) Photograph of live specimens collected from East Lake in Prince Edward County, Ontario in 1996. (Photo credit: S. Staton, NWRI)

Figure 1.  (A) Line drawing of the external features of the shell and internal structure of the left valve of Ligumia nasuta. Reproduced with permission from Burch (1975).  (B) Photograph of live specimens collected from East Lake in Prince Edward County, Ontario in 1996. (Photo credit: S. Staton, NWRI)

Genetic description

There are no data available on the genetic structure of the Canadian population of Ligumia nasuta. The Eastern Pondmussel has been lost from over 90% of its former range in Canada (see Canadian range); thus, it is likely that genetic diversity has been greatly reduced. The remaining two populations of L. nasuta in Canada are separated by more than 800 km of zebra mussel-infested waters of the lower Great Lakes and their connecting channels. There would be no gene flow between these populations.

Ligumia nasuta was included in a recent study investigating the evolution of active host-attraction strategies in freshwater mussels. Zanatta and Murphy (2006) used mitochondrial DNA sequence data (COI, 16S and ND1) to create a molecular phylogeny for several species of unionids (freshwater mussels belonging to the Family Unionidae). Their analyses revealed that L. nasuta was most closely related to members of Potamilus and Leptodea and not closely related to Ligumia recta. As L. recta is the type species of the Ligumia, Zanatta and Murphy (2006) concluded that L. nasuta should be re-designated into an existing or newly described genus.

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