Eastern pondmussel (Ligumia nasuta) COSEWIC assessment and status report: chapter 6

Biology

Ligumia nasuta, like all freshwater mussels, is a sedentary animal that buries itself partially or completely in the substrates of rivers or lakes. Adult freshwater mussels are filter-feeders that obtain nourishment by siphoning particles of organic detritus, algae and bacteria from the water column and, as recently shown, sediments (Nichols et al. 2005). Juveniles of most species of freshwater mussels live completely buried in the substrate where they feed on similar foods obtained directly from the substrate or from interstitial water (Yeager et al. 1994; Gatenby et al. 1997). Aspects of the life history of L. nasuta summarized in the following sections were derived from a review of the available literature as well as the authors’ own knowledge of the species.

Life cycle and reproduction

The life cycle of the Eastern Pondmussel is similar to that of all freshwater mussels and is described as follows (adapted from Clarke 1981, Kat 1984 and Watters 1999): during spawning, males release sperm into the water and females living downstream filter the sperm out of the water with their gills. Ova are fertilized in a specialized region of the female gills, called marsupia, where they are held until they reach a larval stage called the glochidium. The female mussel then releases the glochidia, which must attach to an appropriate vertebrate host, usually a fish. The glochidia become encysted on the host and are nourished by the host’s body fluids until they metamorphose into juveniles. The juveniles then release themselves from the host and fall to the substrate to begin life as free-living mussels. The proportion of glochidia that survive to the juvenile stage is estimated to be as low as 0.000001%. Mussels overcome the extremely high mortality associated with this life cycle by producing large numbers of glochidia – often more than a million. Juvenile mussels are difficult to find because of their small size and because they quickly burrow into the sediment upon release. Juvenile mussels remain buried until they are sexually mature, at which point they move to the surface for the dispersal/intake of gametes (Watters et al. 2001).

Ligumia nasuta is dioecious and there are subtle differences in the external shell features of males and females (see Morphological description). Eastern Pondmussels are bradytictic (long-term brooders); that is, fertilization occurs in late summer and glochidia are released the following spring (Ortmann (1919) cited in Bogan 2002). Glochidia are subovate with an undulate hinge line and measure approximately 250 µm in length and 290 µm in height (Clarke 1981). According to Hoggarth (1993), glochidia with valve heights greater than valve lengths have a wide valve gape and large area of sweep and are most likely gill parasites. The age at maturity is unknown for this species, but the average age of maturity for unionids is 6-12 years (McMahon 1991).

Glochidial hosts for the Eastern Pondmussel are unknown, but the occurrence of this mussel in coastal rivers along the Atlantic seaboard of North America suggests that at least one of the hosts is tolerant of brackish water. Hosts for the other two species of Ligumia, i.e., Ligumia subrostrata (Pondmussel) and Ligumia recta (Black Sandshell) have been identified using evidence from natural and/or laboratory infestations (OSUM 2005). The Pondmussel uses several centrarchid species as hosts and the more widely distributed Black Sandshell uses a variety of centrarchids, percids and cyprinids as well as the American eel (Anguilla rostrata) and the banded killifish (Fundulus diaphanus). Virtually all hosts for these two species are established in the Great Lakes (Cudmore-Vokey and Crossman 2000) and could theoretically be used by the Eastern Pondmussel in Ontario. Stansbery (1961) suggests the yellow perch (Perca flavescens) as a possible host for the Eastern Pondmussel because “the distribution of L. nasuta along the Atlantic coast lies entirely within and all but duplicates that of P. flavescens.” He goes on to say that the distribution of the yellow perch in Ohio in the early 1800s indicates that it used the same post-glacial dispersal route as the Eastern Pondmussel, which provides further circumstantial evidence of a parasite-host relationship between these two species.

Female Eastern Pondmussels use a visual display to attract their hosts. The display behaviour was described by Corey and Strayer (1999) as follows: “While displaying, the female positioned itself upright in the substrate, with valves gaped and mantle exposed. White papillae rippled up and down the mantle margin in an uninterrupted, synchronized rippling, the appearance of which resembled a swimming amphipod. A complete down and back motion along the mantle margin took an average of 0.8 seconds.” This display was observed to strongly attract fish in both natural and laboratory settings (Corey et al. 2006). When a fish struck at the lure, the female mussel would expel a stream of glochidia in close proximity to the fish, thus facilitating the attachment of glochidia to the fish’s gills.

Predation and parasitism

River otter (Lontra canadensis), mink (Mustela vison) and muskrat (Ondatra zibethicus) have been identified as feeding “more or less heavily” on freshwater mussels (Fuller 1974). The impacts of these predators on mussels in Ontario waters have not been investigated and the extent to which they limit the distribution of Ligumia nasuta in Canada is not known (see Limiting factors and threats). Freshwater mussels are often parasitized by helminths (Order Digenea) and mites (Hydrachnidia), which may have detrimental effects on the infected mussels and the overall population (Esch and Fernandez 1993; Di Sabatino et al. 2000). The impacts of parasitism on L. nasuta have not been investigated.

Physiology

Freshwater mussels are sensitive indicators of environmental conditions in rivers and lakes because many species require optimal water and habitat quality for survival. The specific environmental requirements of Ligumia nasuta are unknown.

Dispersal/migration

Freshwater mussels are basically sedentary as adults, with movement limited to a few metres of the lake or river bottom. The only time that significant dispersal can take place is during the parasitic phase. Infected host fishes can transport larval unionids into new habitats and replenish depleted populations with new individuals. Dispersal is particularly important for genetic exchange between populations.

Interspecific interactions

The larvae of Ligumia nasuta are obligate parasites on fishes. Specific fish hosts for this species have not yet been identified.

Adaptability

Freshwater mussels are particularly sensitive to environmental perturbations because of their complex life cycle. Not only are they threatened by disturbances that impact them directly, but also by those that affect their host fish populations. Recent successes in the captive-rearing of several species of freshwater mussels have been reported (e.g., Hanlon and Neves 2000); however, the authors are not aware of any programs involving the Eastern Pondmussel. The release of artificially reared juvenile mussels of other species is underway in the United States, but the long-term outcome of such releases is still being evaluated.

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