Unionoid Bivalves Research Articles

Influence of body size and gender on valve movement responses of a freshwater bivalve to uranium

Body size, age, and gender (male or female) are important variables influencing many aspects of the ecology, physiology, and survival of animals. However, no data are available on the influence of these variables on bivalves exposed to sublethal metal concentrations. This study tested whether the valve movement responses (measured in terms of the duration or frequency of valve opening) of the Australian tropical freshwater unionoid bivalve, Velesunio angasi, exposed to sublethal uranium (U) concentrations in a standard synthetic water (pH, 6.0; hardness and alkalinity, 4 mg/L as CaCO(3)) were influenced by body size (shell length, shell breadth, or dry tissue weight), age, and/or gender. The valve movement responses of V. angasi to U were independent of gender; the sensitivity of males to U was not significantly (p > 0.05) different from that of females. In contrast, the valve movement responses of V. angasi to U were size- and age-dependent; smaller and younger individuals (median shell length = 36.8 mm; median age = 0.7 years) were 22% more sensitive (p </= 0.05) than larger and older individuals (median shell length = 61.3 mm; median age = 14 years). By correcting for differences in body size, age, and/or gender among individuals, using multiple linear regression analyses, the mean variability in valve movement responses was reduced by 81%. Consequently, errors (95% confidence limits) associated with valve movement response (EC(50)) values were substantially reduced (75%). Hence, the ability to demonstrate statistically significant (p </= 0.05) differences in valve movement responses between U exposure concentrations was enhanced. This, in turn, may permit better estimates of the risk of the effects of U exposures to bivalve populations.

Molluscan palaeoecology and biostratinomy in a Lower Cretaceous meanderplain succession (Wessex Formation, Isle of Wight, southern England)

Five categories of shelly sediment are documented from the upper part of the alluvial Wessex Formation (Barremian) on the Isle of Wight, southern England. They represent biostratinomic ‘windows’ within an essentially corrosive and intermittently depositional, warm to very hot meanderplain environment. The fauna is dominated by unionoid bivalves with rarer viviparid and physid gastropods. These molluscs are considered to have adopted similar life modes to their present-day freshwater counterparts. Thick, regularly spaced growth lines in some bivalves probably reflect alternating wet and dry phases and solution-pitted shells indicate acidic surface waters on the floodplains. Autochthonous concentrations characterize well-oxygenated pond or lake deposits and consist of Margaritifera preserved in their inferred life position. Such water bodies were subjected to wet-period sediment influx and disturbance and to dry-period desiccation. Poorly sorted muddy conglomerates representing allochthonous mudflow sediments contain closed, variably oriented unionoids, indicating their catastrophic exhumation and subsequent rapid burial during periodic floods. Channel sediments are poorly fossiliferous. Sand bodies containing viviparid gastropods are possibly crevasse splay deposits.

Freshwater Bivalve Extinctions (Mollusca: Unionoida): A Search for Causes

The freshwater bivalves (Mollusca: Order Unionoida) are classified in six families and about 165 genera worldwide. Worldwide rate of extinction of freshwater bivalves is poorly understood at this time. The North American freshwater fauna north of Mexico is represented by 297 taxa in two families. There are 19 taxa presumed extinct, 44 species listed or proposed as federally endangered, and there are another 69 species that may be endangered. A number of these endangered species are functionally extinct (individuals of a species surviving but not reproducing). Extinction of North American unionoid bivalves can be traced to impoundment and inundation of riffle habitat in major rivers such as the Ohio, Tennessee and Cumberland and Mobile Bay Basin. Damming resulted in the local loss of the bivalves' host fish. This loss of the obligate host fish, coupled with increased siltation, and various types of industrial and domestic pollution have resulted in the rapid decline in the unionoid bivalve fauna in North America. Freshwater communities in Europe have experienced numerous problems, some local unionoid populations have been extirpated, but no unionoid species are extinct. Three taxa from Israel are now reported as extinct. Other nations such as China that have problems with soil erosion and industrial pollution or have numerous dams on some of the rivers (e.g., South America: Rio Parana) are probably experiencing problems of local extirpation if not the extinction of their endemic freshwater bivalve fauna.

Impoundment of rivers: Sediment regime and its effect on benthos

1. A major effect of impoundment of rivers is the coarsening of the sediment of the bed (‘armouring’). The effect occurs over many tens of kilometres at least and develops over a period of years to tens of years. 2. It is postulated that coarsening of the sediment may prevent burrowing benthos from finding suitable habitats and may thus severely diminish the fauna of impounded rivers. 3. The upper Delaware River has a depleted fauna of gomphid dragonflies and unionoid bivalves which probably results from the armouring of the bed resulting from impoundment.