Recovery Strategy for the Dwarf Wedgemussel
1.1 Species Assessment Information from COSEWIC
Date of Assessment: May 2000 Common Name: Dwarf wedgemussel Scientific Name: Alasmidonta heterodon COSEWIC Status: Extirpated Reason for designation: This freshwater mussel was previously known in Canada from only one river drainage. It has disappeared subsequent to the building of a causeway across the river in 1967/68, and has not been found despite intensive systematic searches of its former habitat Canadian Occurrence: New Brunswick COSEWIC Status History: Extirpated by 1968. Designated Extirpated in April 1999. Status re-examined and confirmed in May 2000. Last assessment based on an existing status report.
The dwarf wedgemussel is a small freshwater mussel that is roughly trapezoidal in shape. Clarke (1981 cited in Hanson and Locke 1999: 3-4) provided the following detailed description of the species:
Shell up to about 45 mm long, 25 mm high, 16 mm wide, and with shell wall about 1 mm thick in the mid anterior region; more or less ovate or trapezoidal, roundly pointed posterio-basally, thin but not unduly fragile, with rounded posterior ridge, and of medium inflation. Females more inflated posteriorly than males. Sculpturing is absent except for lines of growth and beak sculpture. Periostracum is brown or yellowish-brown, and with greenish rays in young or pale-colored specimens. Nacre bluish or silvery white, and iridescent posteriorly. Beak sculpture composed of about 4 curved ridges, which are angular on the posterior slope. Hinge teeth small but distinct; pseudocardinal teeth compressed, 1 or 2 in the right valve and 2 in the left; lateral teeth gently curved and reversed, that is, in most specimens, 2 in the right valve and 1 in the left.
Age and growth of the dwarf wedgemussel are little known. Neither size nor age at maturity has been determined. Aging is difficult because shell erosion in larger animals degrades annuli. Ten-year-old animals have been recorded and ages of 12 to 18 years are theoretically possible. In the Petitcodiac, no individual dwarf wedge mussels were found in 1984, 16 years after construction of the Moncton causeway. This suggests rapid mortality and disappearance of individuals living at the time of causeway construction (thus a relatively short life span), and rapid failure of recruitment.
1.3 Populations and Distribution
The dwarf wedgemussel was historically found at about 70 locations in 15 major drainages from North Carolina to New Brunswick, and has always been considered naturally uncommon or rare throughout this range. The historical distribution (Figure 1) is more or less continuous in the USA from North Carolina to the Connecticut River in Vermont, but there is a substantial range disjunction between the northern occurrence in the USA (Vermont) and the single historic occurrence in the Petitcodiac River (New Brunswick) in Canada. In the USA the species is currently found only in 20 of the 70 historic locations and most populations are considered small or declining (Nedeau 2005). The species was listed as federally endangered under the US Endangered Species Act in 1990.
In Canada the dwarf wedgemussel was restricted to several locations in the Petitcodiac River of New Brunswick, where it was described as “common” based on a 1960 survey. Subsequent surveys in 1984 and 1997/98 did not locate specimens in the Petitcodiac. The species is not known from any other systems draining into the Bay of Fundy, nor from the state of Maine, despite suitable habitat conditions (Nedeau 2005). Because of the range disjunction with US populations, it has been suggested (Nedeau 2005) that the Petitcodiac population originated in a different glacial refugium from US populations and may thus have been genetically isolated from US populations for as long as 50,000 years. Several other mussel species show similar disjunct distributions including the yellow lampmussel (Lampsilis cariosa), tidewater mucket (Ligumia ochracea), brook floater (Alasmidonta varicosa) and eastern lampmussel (Lampsilis radiata).
1.4 Needs of the Dwarf wedgemussel
1.4.1 Habitat and biological needs
Habitat requirements of the dwarf wedgemussel have been well documented as a result of US recovery efforts. The species lives in running waters, where currents are moderate to slow, in streams from less than 5 m wide to shallow rivers more than 100 m wide.
Figure 1: Historic range of the dwarf wedgemussel in North America (Nedeau 2005, based on data from the United States Fish and Wildlife dwarf wedgemussel federal recovery plan, 1993).
It always occurs in patches of sand or fine gravel, often in areas of stones or cobble, and shows very low tolerance for silt or low oxygen conditions. It is often found near riverbanks under overhanging trees.
Little information on the biology of dwarf wedgemussel is available, and what is known of the species is consistent with the general biology of freshwater mussels. Eggs are fertilized in mid-summer or autumn and the glochidia (larvae) mature within the specialized regions of the females’ gills known as marsupia. Glochidia are released into the water in spring, attach to the fins and gills of fish with tiny hooks, and then encyst on the host fish. After several weeks the cyst ruptures and the juvenile mussel drops to the bottom. Successful settlement depends on the presence of suitable substrate of sand or fine gravel. Juvenile mussels bury in the sediments and feed by filtering algae and fine organic debris from the water. Following settlement mussels are essentially sedentary, with maximum movements measured in meters.
Fish hosts for wild dwarf wedgemussel are not known with certainty, although several species are capable of carrying larvae based on laboratory studies. The species is more specific in its fish host requirements than many other freshwater mussel species, and the tessellated darter (Etheostoma olmstedi) is believed to be the primary host in US populations (Nedeau 2005). Of species known to be capable of hosting dwarf wedgemussel, only Atlantic salmon parr (Salmo salar) were historically present in the Petitcodiac River. The most likely host for dwarf wedgemussel in the Petitcodiac system, however is thought to be American shad (Alosa sapidissima), which was present at sites where the dwarf wedgemussel was historically found and was immediately eliminated from the Petitcodiac system after construction of the causeway (Hanson and Locke 1999). Atlantic salmon are unlikely to have been the principal host, since a remnant population supplemented by stocking of parr persisted in the Petitcodiac system until the 1990s, long after the dwarf wedgemussel had disappeared.
1.4.2 Ecological role
Given its relatively low historic abundance throughout its range, the dwarf wedgemussel may not have played a large role in aquatic ecosystems. The Petitcodiac system was one of two North American locations where the species was considered historically common, but given the lack of knowledge on the species and its biology in this system, it is not possible to describe its ecological role here.
Freshwater mussels, as benthic filter feeders, play a role in transforming planktonic production to benthic biomass and ultimately to aquatic, avian and terrestrial predators. They are considered good indicators of water quality in aquatic systems because of their sensitivity to siltation, low oxygen conditions, and changes in fish communities.
1.4.3 Limiting factors
As with other freshwater mussels, the dwarf wedgemussel has two critical early life history stages: the parasitic stage requiring a specific fish host, at which dispersal can occur; and, the settlement stage where specific microhabitat conditions are required for survival. Fish host abundance and the availability of juvenile and adult habitat (sand or fine gravel) are therefore potential limiting factors.
The dwarf wedgemussel is sensitive to habitat degradation including reduced water flow, siltation, and low oxygen. Muskrats are the only known predators of adult freshwater mussels but are not considered to have been a significant factor in the extirpation of the Canadian population.
1.5.1 Description of threats
No detailed assessment of threats is possible since the species is extirpated in Canada and historical information is sparse. Available information suggests that the extirpation of dwarf wedgemussel was primarily caused by the elimination of the fish host due to a lack of fish passage at the Moncton-Riverview causeway constructed in 1968. Fish communities changed substantially following the construction of the causeway; populations of several species disappeared (American shad, Alosa sapidissima; Atlantic salmon, Salmo salar; Atlantic tomcod, Microgadus tomcod; and striped bass, Morone saxatilis) and others were greatly reduced (alewife, Alosa pseudoharengus; blueback herring, Alosa aestivalis; rainbow smelt, Osmerus mordax; and sea-run brook trout, Salvelinus fontinalis). Available information suggests that suitable habitat for dwarf wedgemussel remains in the Petitcodiac system despite degradation in some areas. Several other freshwater mussel species, including two from the same genus as the dwarf wedgemussel, (Alasmidonta undulata and A. varicosa) have been recently collected in the system.
With the exception of Part 1.1 (Species Information from COSEWIC), this Background section is based on Hanson and Locke (1999) unless other sources are cited.
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