A survey of restriction site polymorphism in the mitociibndrial DNA (mtDNA) of the American eel Anguilla rostrata showed no genetic divergence among samples from a 4000-km stretch of North America coastline. Lack of geographic differentiation in mtDNA over such a large area contrasts sharply with results for terrestrial and freshwater vertebrates and is most likely attributable to the extraordinary life history of these catadromous fishes, which involves perhaps a single spawning population in the western tropical mid-Atlantic Ocean and subsequent widespread dispersal of larvae by ocean currents. However, samples of the European eel (nominally Anguilla anguilla) are highly distinct from A. rostrata in mtDNA genotype (distinguishable by 11 of 14 restriction endonucleases), contradicting some previous suggestions that the two forms belong to the same panmictic population. Results of this study emphasize the importance of life history in shaping population genetic structure.

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Opaque bands on otoliths were validated as annuli for bluegills Lepomis macrochirus throughout their lives in Par Pond, a nuclear reactor cooling reservoir in South Carolina. Whole mounts of otoliths were suitable for age determination of young bluegills (less than 5 years old), but both whole mounts and otolith sections were necessary for age determination of older bluegills. Marginal increment analyses indicated that annulus formation began as early as February and was complete by June in bluegills of ages 1–9. Annulus formation began earlier in younger (age 1–2) bluegills than in older individuals. Similarity of observed and back‐calculated sizes at age supported identification of opaque bands as annuli. Input of warm water from reactor operation occasionally produced opaque bands (false annuli) on otoliths; however, the faintness and incompleteness of such bands clearly distinguished them from true annuli. Identification of the first annulus of some otoliths was problematic due to the occurrence of two types – opaque and translucent – of otolith center (incremental area around the primordium extending out to the first contrasting band). Use of daily otolith increments established that opaque otolith centers were not annuli, Bluegills with opaque otolith centers hatched during the period of annulus formation and were about 1 month older on average than bluegills with translucent otolith centers, which hatched after the period of annulus formation. Comparison of growth rates in Par Pond and elsewhere, though complicated by other researchersˈ use of scale information, suggests that errors in age determination were unlikely, This study extends previous research into validating annuli of bluegill otoliths to older fish and confirms the utility of otolith information for age assessment of another centrarchid species.

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