From Genes to Ecosystems: Integrative Approaches in Unionid Ecology I
Presiding: C Vaughn, Oklahoma Biological Survey; D E Spooner, University of Oklahoma
NB41D-01 INVITED 08:30h
Sex in the Sand: Patterns and Strategies
Research has revealed an unexpected complexity in freshwater mussel reproductive strategies involving methods to ensure that their parasitic larvae encounter the proper fish hosts. Most mussels have evolved means of luring hosts to the mussel or to their larvae. By evolving such lures, mussels increase the chances of the proper host becoming infested. Such specialization entails the loss of the ability to parasitize a wide range of hosts. Mussels having a generalized lure can utilize a broader range of hosts but are inefficient at doing so. These patterns represent tradeoffs between efficiency and host specialization. Mussels also show a variety of strategies involving spawning, parasitization, and juvenile growth. Three patterns are recognized. In the first, spawning takes place in the spring, larvae are parasites in the summer, and juveniles have a short growing season for the first year. In the second, spawning occurs in the autumn, larvae are parasites next spring, and juveniles have a longer growing season than that found in pattern one. Pattern three is similar to pattern two except that larvae are parasites in the winter and juveniles occur in the spring to have the longest growing season. These patterns involve tradeoffs between larval and juvenile mortality.
NB41D-02 08:45h
Seasonal Variation of Glochidia in Stream Drift in the Sipsey River, Alabama
We determined seasonal variation of freshwater mussel larvae (glochidia) in stream drift in the Sipsey River, west-central Alabama, from May 2004 to April 2005. We identified glochidia to species using shell morphometrics and discriminant functions generated from a reference library of known glochidia. Total glochidia abundance was high from mid-May to early August, but low from mid-August through fall and winter. In general, the most frequently occurring species in the drift corresponded to the most abundant mussel species in benthic mussel communities in the Sipsey River. An exception to this pattern was noted for Amblema plicata, which were rare in the benthic community but regularly constituted large proportions of glochidia in the drift. The prevalence of glochidia of A. plicata in the drift coupled with its high fecundity suggests that this species releases glochidia using a broadcasting strategy. Our results indicate that factors influencing glochidia abundance in stream drift include adult density, species-specific fecundity, and mode of host infection.
NB41D-03 09:00h
Mussel Glochidia Identification and Infestation Patterns on Fishes in the Sipsey River, Alabama
We examined fishes monthly for naturally occurring infestations of mussel larvae (glochidia) in the Sipsey River, Alabama, and tested the utility of simple morphometrics to identify glochidia from this assemblage. We examined 41 fish species (n = 654 individuals) representing 9 families but found glochidia on only 13 species from 5 families. Across all fish species, 14% of individuals were infested. The most commonly infested fishes were Ammocrypta beani (57% of individuals), A. meridiana (50%), Cyprinella venusta (51%), and Etheostoma stigmaeum (36%). Number of glochidia on individual fishes ranged from 1-231. We measured shell length, hinge length, and height of known glochidia from 21 mussel species occurring in the Sipsey River. Mean dimensions for the largest species were 5-7 times greater than the smallest species. Glochidia size varied little within species except for Pleurobema decisum which was represented by two glochidial morphotypes. Discriminant functions analysis classified 72% of total glochidia (n = 870) to the correct species. For individual species, percentage of correct classification ranged from 42-100%. Glochidia of closely related species were not necessarily more similar than to distantly related species. For example, species of Quadrulini represented some of the smallest and largest glochidia in the study.
NB41D-04 INVITED 09:15h
Metapopulation Models of Unionid Fish Hosts: Landscape Ecology Theory Applied in the Upper Mississippi River
Explanations for patterns observed in local assemblages of unionids are frequently sought with reference to interactions between fish host species. We compared the ability of a neutral model to reproduce the fish-mediated movement of larval unionids and examine fish host data from the Upper Mississippi River in comparison with this neutral model. The models focus on the condition of mussel beds (species richness, species abundance, mussel bed size, and size classes) and four measures of fish host connectivity. We considered the non-host fish community (n=57 species) within our neutral model and examined the variation in spatial patterns of fish hosts (n=31 species) and differences in connectivity of the models among mussel beds. We found greater connectivity in main channel and side channel border areas among mussel beds. Using Ripley's K-function we determined that the distribution of host fish species indicate spatial clustering at multiple scales which reflects the spatial clustering of mussel beds in the Upper Mississippi River. This spatial clustering lends itself to metapopulation analysis with host fish providing the connection between populations of unionids; we present a metapopulation equation for unionids and suggest reasons why unionid populations are predisposed to metapopulation analysis.
NB41D-05 INVITED 09:30h
Validating the Assumption of Annual Shell Ring Deposition in Freshwater Mussels
We evaluated the assumption of annual shell ring deposition by freshwater mussels in three rivers (Little Tallahatchie, Mississippi, 2000 and 2003; Sipsey, Alabama, 2000; St. Frances, Arkansas, 2003) using 14 species (Amblema plicata, Elliptio arca, Fusconaia cerina, F. flava, Lampsilis cardium, L. teres, Leptodea fragilis, Obliquaria reflexa, Potamilus purpuratus, Quadrula asperata, Q. pustulosa, Q. quadrula, Q, rumphiana, Tritogonia verrucosa). In 2000 we filed a notch in the shell margin, returned animals to the water, and retrieved them one year later. In 2003 we measured shell length and affixed numbered tags, returned animals to the water, and retrieved them one year later. After retrieval, we examined external and internal shell rings using thin-sections. For both methods, all species at all sites grew and deposited a single winter rest line between times of initial and final collection. Handling produced a conspicuous disturbance ring in all species. Characteristics of annual rings, disturbance rings, and false annuli differed qualitatively. Growth of some individuals was reduced after handling but negative growth occurred only in a few large individuals. Annual shell ring formation occurs consistently across species, space, and time. Formation of disturbance rings associated with collection indicates that mussels are extremely sensitive to handling.
NB41D-06 09:45h
Use of Relic Shells to Determine Time Since Mortality
As potential ecosystem impairment and/or degradation indicators, relic freshwater mussel valves may contain valuable information pertaining to life history, mortality, and taphonomy. Relic shells persist in aquatic systems long after mussel mortality and may be used to identify time passed since mortality. Such information may provide evidence in determinations of causality, responsibility, and reciprocity with applications that potentially extends beyond relics to modeling future populations or events. Using Unionids and surrogates, factors influencing decomposition rates for visceral mass, periostracum, nacre, hinge and overall valve condition were measured through both laboratory and field experimentations. Analysis of microbial communities associated with decomposing visceral mass provided short-term decomposition markers. X-Ray diffraction of nacre indicated calcium carbonate polymorph variations within and among different age and species. Digital microscopy with polarized light and geographic information system (GIS) mapping of thin-sections allowed percentage comparisons of aragonite and other calcium carbonate polymorphs and possible mineral replacements within shells. Selection of factors influencing decomposition and decomposition rate will enable measurable changes in shell integrity to be determined for the amount of time since mortality. Currently validation and consideration for decomposition rates incorporate both site and species-specific factors of relic shells whose time since mortality can be validated.