File:American malacological bulletin (1988) (17533859324).jpg

Title: American malacological bulletin
Identifier: americanmal6719881990amer (find matches)
Year: 1983 (1980s)
Authors: American Malacological Union
Subjects: Mollusks; Mollusks
Publisher: (Hattiesburg, Miss. ?) : (American Malacological Union)
Contributing Library: Smithsonian Libraries
Digitizing Sponsor: Biodiversity Heritage Library

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NEVES AND MOYER: AGING OF FRESHWATER MUSSELS 185
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0 5 10 15 20 25 THIN-SECTION AGE (yr) Fig. 4. A comparison of age estimates for two species aged by the thin-sectioning and external growth ring methods. Data points below the 45° line represent underestimates of specimen ages by the growth ring method. exhibit shell bands, but the causes for their formation are pro- bably different from those for temperate species (McMichael, 1952). This apparent regularity in banding could lead some investigators to assume that annulus formation is a universal phenomenon and that age validation might not be necessary. However, we caution that annual periodicity of growth line deposition is a hypothesis that should be confirmed for each species and locality before it is accepted. The slow growth of most tagged specimens (96% grew less than 2 mm per year) was the major handicap in age validation. Growth increments along the shell margin of these specimens were insufficient to allow clear separation of growth during the year after tagging from growth in the penultimate year. Ages of most of the tagged specimens, determined later by thin-sections, were 8 to 20 years. These older, larger specimens proved to be unsuitable, in retrospect, for this com- ponent of the study. Our age validation efforts were most suc- cessful with mussels of the relatively faster growing, younger age-classes. Therefore, a range of size classes of sufficient number should be used in age validation to overcome the dif- ficulties posed by the slow growth of adults of riverine species. Other problems associated with slow growth included accuracy of caliper measurements and growth layer detach- ment. Unnotched mussels that grew less than 1 mm per year had to be excluded because rough shell margins contributed to measurement error with calipers, and annulus deposition could not be confidently ascertained. The narrow growth band along the shell margin often became brittle after the specimens were killed and occasionally broke during measurement or thin-sectioning. Despite these problems with age validation, successes and failures provided experience that improved precision in age determinations of shells. For shells that grew sufficiently for measurement during the 1 year period, the formation of a single growth band per year was confirmed. The identification of both internal and external growth bands for a specimen facilitated the recognition of true versus false annuli and contributed to our confidence in age determinations. As judged by counts of annuli on mussel shells and growth measured for up to 4 years at study sites, adults of riverine species in Virginia grow slowly and reach maximum ages greater than those reported for lentic species (Grier, 1938; Stansbery, 1961). Longevities of the species aged by thin-sections ranged from 22 to 56 years. These ages exceed those reported for some species in the Mississippi River (Coon et al., 1977), are less than the extreme age (> 100 yr) reported for Margaritifera margaritifera L. in Europe (Hendelberg, 1960), but are apparently similar to ages of other slow-grcwing species (Isley, 1914; Stansbery, 1971). Isley (1914) and Coker et al. (1921) reported that light-shelled species grow rapidly, and subsequent studies on Anodonta spp. and other thin- shelled species have confirmed their observations (Stansbery, 1961; Negus, 1966; Haukioja and Hakala, 1978). In com- parison, they noted that growth in length of heavy-shelled species is most rapid in early life but slows considerably, such that growth lines become tightly spaced and difficult to dif- ferentiate. Coker et al. (1921) computed mean growth rates of roughly 6 mm/yr for medium-sized individuals of thick- shelled species (Quadrula spp.), and Isley (1914) observed shell growth to be roughly 1 mm/yr for older (larger), riverine individuals. Riverine populations of at least some mussel species therefore contain many older, slow-growing cohorts. Based on the slow growth, closely spaced annuli, and con- siderable longevity of mussels, it is imperative that specimens be accurately aged if exploitation potential or population statistics are to be assessed from age-class structure and abundance (Moyer, 1984). Although the formation of growth bands is the key pro- cess that allows age determination, it is not well understood. Band patterns on freshwater mussel shells occur in two varieties, wide, dark bands at fairly regular intervals, and lighter bands that are irregularly spaced (Tevesz and Carter, 1980). The mechanism through which these bands are incor-

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