Life History and Habitat of the Endangered Dwarf Wedgemussel Alasmidonta heterodon (Bivalvia:Unionidae)

Abstract

The reproductive cycle, population demographics, and habitat use of the dwarf wedgemussel (Alasmidonta heterodon) were studied in populations from the Tar River, North Carolina, and Aquia Creek, Virginia, during 1991 and 1992. The mussel is a long-term brooder, spawning in late summer and becoming gravid in fall. Laboratory infestations with glochidia on 15 species of fish confirmed three hosts for A. heterodon: tessellated darter (Etheostoma olmstedi), Johnny darter (E. nigrum), and mottled sculpin (Cottus bairdi). Six species of sunfishes, four minnows, one sucker, and one catfish tested in the laboratory did not serve as hosts. Age and growth characteristics were calculated using a thin sectioning technique on valves collected from the Tar River, Aquia Creek, and Neversink River, New York. As computed by the von Bertalanffy equation, the dwarf wedgemussel reaches a maximum length of 38.70 mm, 45.26 mm, and 45.84 mm in the Tar River, Aquia Creek, and Neversink River, respectively. There were significant differences in growth rates of A. heterodon from the Tar River and Aquia Creek and between populations in the Neversink River and Aquia Creek. There was no significant difference in growth rate of populations from the Tar and Neversink rivers. Age-class distribution of live mussels collected at the three sites was estimated using age-length keys. The Tar River population included year classes from 4 to >6 years. Excessive shell erosion prevented the aging of older individuals and thus, a key to include larger live individuals. The age class distribution of the Aquia Creek population ranged from 2 to 9 years, while the Neversink River population was represented by age 1 to 8 year classes. The von Bertalanffy growth equations for the Tar River, Aquia Creek, and Neversink River populations were as follows: L_t = 38.70 mm (1-e^{-0.299(t+0.251)}; L_t = 45.26 mm (1-e^{-0.260(t+0.244)}); and L_t = 46.84 mm (1-e^{-0.207(t+0.335)}), respectively. Habitat-suitability experiments were conducted in the laboratory to determine preferred substratum size and water velocity. Dwarf wedgemussels always preferred the finer substratum offered, but little preference was shown for lotic versus lentic flow regimes.