Movement and growth of Unionidae (Mollusca: Bivalvia) in the Little Brazos River, Texas

Typescript (photocopy).This study investigated movement and growth of the Unionidae (Mollusca: Bivalvia) of the Little Brazos River, Texas. Turnover rates of individuals (all species pooled) were similar at widely separated pool and riffle sites, and tended to vary jointly in time, suggesting that turnover is linked to large scale disturbance processes (such as flooding). Although I observed maximum turnover after flood events (approximately 1.0% day"^), at least 30% of all individuals persisted in the study sites to the end of the sampling year. In contrast with turnover, movement patterns were site-dependent. Direction and magnitude of movement was random in the pool site. At the riffle site, clams moved preferentially downstream and moved significantly (p = 0.05) greater distances downstream than in other directions. I compared a new mark/recapture sampling technique (tagging clams with small metal tags and "recapturing" with a metal detector) with hand sampling and repetitive quadrat sampling. Hand sampling missed approximately half of the individuals present. Repetitive quadrat sampling by excavating and sieving substrate was laborious, but efficient in detecting all individuals within small areas. The metal detector technique offers promise of efficient, long-distance movement detection, but problems with tag loss and detection efficiency remain to be solved. I measured growth rates for 3 common species (Amblema perplicata, Quadrula quadrula, and Q. pustulosa) over all times of the year. Growth did not correlate significantly with sample date, days between samples, initial temperature, average temperature, or season. Growth did correlate significantly (p = 0.05) with initial length. I observed negative growth of individuals of all 3 species