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The larvae of freshwater mussels in the order Unionoida are obligate parasites on fishes. Because adult mussels are infaunal and largely sessile, it is generally assumed that the majority of gene flow among mussel populations relies on the dispersal of larvae by their hosts. The objective of this study was to compare the genetic diversity and the degree of congruence between the population structures of two related freshwater mussels Leptodea leptodon and Leptodea fragilis and their fish host, Aplodinotus grunniens. Host specificity in parasites has been shown to result in greater congruence between the population structures of the two interacting species, and assessing the congruence of genetic structure of the endangered L. leptodon with its sister species L. fragilis and their sole host is an important step in understanding the impact of host dispersal on population structure. Analysis of microsatellite data indicated that despite its imperiled status, L. leptodon displayed greater genetic diversity than the more common L. fragilis. However, the population structures of all three species were incongruent even in the presence of substantial gene flow. Other factors such as habitat specificity may play a role in generating the differences in population structure observed. This study indicates that barriers to gene flow or lack of available host fish are not the cause of decline of the federally endangered L. leptodon, and suggests that alternative explanations should be considered.

Acute (24-h) toxicity tests were used in this study to compare lethality responses in early life stages (glochidia) of six freshwater mussel species, Leptodea fragilis, U. imbecillis, Lampsilis cardium, Lampsilis siliquoidea, Megalonaias nervosa, and Ligumia subrostrata, and two standard test organisms, Ceriodaphnia dubia and Daphnia magna. Concentrations of carbaryl, copper, 4-nonylphenol, pentachlorophenol, permethrin, and 2,4-D were used in acute exposures to represent different chemical classes and modes of action. The relative sensitivities of species were evaluated by ranking their LC50 values for each chemical. We used these ranks to determine the extent to which U. imbecillis (one of the most commonly used unionids in toxicity tests) was representative of the tolerances of other mussels. We also calculated geometric mean LC50s for the families Unionidae and Daphnidae. Rankings of these data were used to assess the extent to which Daphnidae can be used as surrogates for freshwater mussels relative to chemical sensitivity. While no single chemical elicited consistently high or low toxicity estimates, carbaryl and 2,4-D were generally the least toxic to all species tested. No species was always the most sensitive, and Daphnidae were generally protective of Unionidae. Utterbackia imbecillis, while often proposed as a standard unionid mussel test species, did not always qualify as a sufficient surrogate (i.e., a substitute organism that often elicits similar sensitivity responses to the same contaminant exposure) for other species of mussels, since it was usually one of the more tolerant species in our rankings. U. imbecillis should be used as a surrogate species only with this caution on its relative insensitivity.

We conducted a series of toxicological treatments with 3trifluoromethyl-4-nitrophenol (TFM) and a TFM:1% 2',5dichloro-4'-nitrosalicylanilide (niclosamide) mixture, two compounds used to control larval sea lamprey ( Petromyzon marinus) in Great Lakes tributaries, to evaluate the acute toxicity of the lampricides to a number of nontarget species of concern. Treatments were conducted with yellow stage American eel ( Anguilla rostrata ), adult and larval haliplid water beetles ( Haliplus spp.), a surrogate for the endangered Hungerford's crawling water beetle ( Brychius hungerfordi ), and adults of three unionid species-giant floater ( Pyganadon grandis ), fragile papershell ( Leptodea fragilis ), and pink heelsplitter ( Potamilus alatus ). Treatments were conducted using a serial dilution system consisting of nine test concentrations and an untreated control with 20% dilution between concentrations. Narcosis was evident among giant floaters exposed to the TFM and the TFM:1% niclosamide mixture and among pink heelsplitters exposed to the TFM:1% niclosamide mixture only but mostly at concentrations greater than 2-fold that required to kill 100% of larval sea lamprey (minimum lethal concentration (MLC)). Tests with the haliplid beetle suggest the risks to the Hungerford's crawling water beetle associated with TFM applications are minimal. Concentrations over 2-fold the sea lamprey MLC did not kill adult or larval water beetles. Preliminary behavioral observations suggest water beetles may avoid treatment by crawling out of the water. Adult water beetles exposed to TFM at 3-fold the sea lamprey MLC were observed above the water line more often than controls. The lampricide TFM was not acutely toxic to American eel. Mortalities were rare among American eel exposed to TFM concentrations up to 7-fold the observed sea lamprey MLC. Similarly, for the TFM:1% niclosamide mixture, mortalities were rare among American eel exposed to nearly 5-fold the observed sea lamprey MLC. Overall, acute TFM toxicity was not evident among any of the species examined in this study at concentrations targeted to control larval sea lamprey. Results for the adult unionids should be viewed with caution due to the lack of replication in the treatments.

Unionid (Mollusca: Unionidae) densities have declined dramatically throughout the Laurentian Great Lakes after the introduction of dreissenid mussels (Mollusca: Dreissenidae). Recent surveys in some Great Lake coastal wetlands have found abundant unionid populations, but the factors that reduce zebra mussels on unionids in these habitats are not well understood. In 2001-2002, we tested effects of predation and unionid burrowing on corbiculids, sphaeriids and dreissenids in a Great Lake coastal wetland in western Lake Erie. In one experiment, we reduced access by molluscivores using exclosures with two mesh sizes (1.3 cm × 1.3 cm; 5 cm × 10 cm) and sampled bivalves after 15 months. Small mesh exclosures had higher numbers of dreissenids, Corbicula fluminea and sphaeriids (54.9, 3.8, 22.6 individuals/m^sup 2^, respectively) than large mesh exclosures (0.0, 1.13, 0.13 individuals/m^sup 2^, respectively) or open controls (0.3, 1.0, 0.1 individuals/m^sup 2^, respectively). Numbers of dreissenids on C. fluminea were higher in small mesh exclosures (3.8 dreissenids/Corbicula) than in large mesh exclosures (0.1 dreissenids/Corbicula) or cageless controls (0dreissenids/Corbicula). In a second experiment, we held two species of live unionids (Leptodea fragilis, Quadrula quadrula) and immobile Pyganodon grandis shells in exclosures (2.5 cm × 2.5 cm mesh) with either 5 cm, 10 cm, or 20 cm deep sediments and sampled bivalves after 2 months. There were fewer dreissenids on L. fragilis than P. grandis shells, but there was no difference in the number of dreissenids on Q. quadrula and P. grandis shells. Numbers of attached dreissenids were higher inside (189-494 dreissenids/unionid) than outside (8-11 dreissenids/unionid) exclosures, and densities of sphaeriid and C. fluminea clams were also higher inside (21.8, 4.7 individuals/m^sup 2^, respectively) than outside (0.4, 0.9 individuals/m^sup 2^, respectively) exclosures. Numbers of attached dreissenids were higher on unionids that could burrow below the sediments (20 cm depth) than unionids in shallow sediments (5 cm depth) for unexplained reasons. Our data suggest that molluscivores can play a pivotal role in limiting numbers of bivalves including dreissenids in coastal wetlands.[PUBLICATION ABSTRACT]

Freshwater unionid clams in North America have been virtually eliminated from waters that are colonized by zebra mussels. Near total mortality has been reported in western Lake Erie 1 , 2 , 3 , 4 , but we have now discovered a large population of native clams in a Lake Erie wetland that shows little sign of infestation. Field observations and laboratory experiments show that warm summer water temperatures and soft, silt-clay sediments trigger burrowing by clams. This discourages infestation and physically removes any attached zebra mussels.

The Little Arkansas River extends 102 miles through south-central Kansas to its confluence with the Arkansas River at Wichita. Historical studies along the most urban reach of this river at Wichita reported 12 unionid species. Unionid mussel surveys were conducted at 250 sites within the Little Arkansas River basin in 1978–79. Along the reach at Wichita, seven species were found alive: Lasmigona complanata, Leptodea fragilis, Potamilus ohiensis, Pyganodon grandis, Quadrula pustulosa, Quadrula quadrula, and Uniomerus tetralasmus. In 2008–09, surveys were conducted at the Wichita sites between 11th Street North and 109th Street North. Live representatives of five of these species were detected, missing P. ohiensis and U. tetralasmus. An exotic species, Corbicula fluminea, first was reported in the 2008–09 study. Mussel abundance in the current study was significantly lower than in the 1978–79 survey. In addition, there was a significantly different species distribution in this reach between the two surveys. Mussel length class frequency was not significantly different between surveys for any of the mussel species. A combination of environmental changes during the urban development of this river reach may have caused the decline in mussel populations.

In 1996, thousands of live Leptodea fragilis were collected from a marsh located in the western basin of Lake Erie that was infested with zebra mussels (Dreissena polymorpha). Despite the presence of zebra mussels at this site for a number of years, this L. fragilis population showed no signs of competition-induced changes in population dynamics. Biofouling was limited: fewer than 1% of the L. fragilis showed evidence of recent or past zebra mussel colonization. Successful recruitment occurred yearly, with multiple year classes collected that ranged in age from 1 to 12 years. However, age and shell length were not well correlated. Seventy-one percent of the individuals collected were 51-80 mm long, but ranged in age from 2 to 4.5 years. Three different patterns of growth or shell deposition were found. Some individuals grew rapidly, reaching 105 mm in 3.5 years, while others grew only 4.5 mm over the same time period. A few grew poorly during some years but very rapidly in others. Individuals with a shell length of 41 mm or more were sexually mature and females were more common than males. The strong recruitment and steady growth of this population showed no change between the years before and after the zebra mussel invasion, indicating that this marsh is functioning as a natural refugium from potential problems caused by zebra mussels.

Relocation of unionid mussels into refuges (e.g., hatchery ponds) has been suggested as a management tool to protect these animals from the threat of zebra mussel (Dreissena polymorpha) invasion. To evaluate the efficacy of relocation, we experimentally relocated 768 mussels, representing 5 species (Leptodea fragilis, Obliquaria reflexa, Fusconaia flava, Amblema plicata,andQuadrula quadrula) into an earthen pond at a National Fish Hatchery or back into the river. In both locations, mussels were placed into 1 of 4 treatments (mesh bags, corrals, and buried or suspended substrate-filled trays). Mussels were examined annually for survival, growth (shell length and wet mass), and physiological condition (glycogen concentration in foot and mantle and tissue condition index) for 36 mo in the pond or 40 mo in the river. We observed significant differences in mortality rates between locations (mortality was 4 times greater in the pond than in the river), among treatments (lowest mortality in the suspended trays), and among species (lower mortality in the amblemines than lampsilines). Overall survival in both locations averaged 80% the 1st year; survival in the pond decreased dramatically after that. Although length and weight varied between locations and over time, these changes were small, suggesting that their utility as short-term measures of well being in long-lived unionids is questionable. Mussels relocated to the pond were in poor physiological condition relative to those in the river, but the magnitude of these differences was small compared to the inherent variability in physiological condition of reference mussels. These data suggest that relocation of unionids into artificial ponds is a high-risk conservation strategy; alternatives such as introduction of infected host fish, identification of mussel beds at greatest risk from zebra mussels, and a critical, large-scale assessment of the factors contributing to their decline should be explored.

Native unionid mussel populations have recently declined throughout North America as a result of zebra mussel (Dreissena polymorpha) fouling. Periodic cleaning of fouled unionids and cleaning followed by translocation have been suggested as methods for reducing mortality.Leptodea fragilisandPotamilus alatuswere used to determine survival, recovery of energetic stores, and accumulation of newly settled zebra mussels after cleaning and replacement in situ. Both species had high survival, andL. fragilisincreased energetic stores after cleaning.Elliptio complanataandLampsilis radiatawere used to compare conservation strategies for unionids fouled by zebra mussels. Survival and glycogen content were used to evaluate stress induced by cleaning and replacement in situ, cleaning and translocation, and cleaning, quarantine, and translocation, relative to the stress in fouled unionids and control (never fouled) unionids. New zebra mussel settlement was assessed to estimate the frequency of cleanings needed. CleanedE. complanataandL. radiatamaintained significantly higher glycogen levels and had higher survival than fouled unionids in all treatments; however, 30% ofL. radiatadied while in quarantine but noE. complanatadied. Translocated unionids were difficult to relocate in the riverine refugium. The inability to find translocated unionids, coupled with high survival and energetic stores in cleaned and replaced unionids, indicate that cleaning and replacement is an effective conservation strategy. Cleaning and replacement may be used as the 1st step to conserve small populations of fouled unionids living in environments where food is not limiting and where collection and cleaning are logistically feasible.