Life history of the endangered fine-rayed pigtoe pearly mussel, Fusconaia cuneolus (Lea, 1840), in the Clinch River, Virginia

Abstract

The periods of gravidity and glochidial release, required fish hosts, and age and growth characteristics of Fusconaia cuneolus were studied in the upper Clinch River, Virginia during 1986 and 1987. This summer brooder is gravid from mid-May through late July, releasing most glochidia in midJune. Gills of gravid females assume the color of enclosed conglutinates, and hues change from pink to peach as embryos mature to glochidia. As judged by diel sampling of stream drift, glochidia of the fine-rayed pigtoe are most abundant in the water column in early morning.

A total of 1,619 fish representing 39 species were electroshocked and examined from the Clinch River at Slant for glochidial attachment. Prevalence of infestation of amblemine glochidia was highest on species of the Cyprinidae (27 to 46%), and six species were identified as possible fish hosts for the glochidia of the fine-rayed pigtoe. Amblemine glochidia were absent on non-cyprinid fish species. Glochidia exhibited host specificity in laboratory experiments, metamorphosing on the following seven minnow and one sculpin species: fathead minnow (Pimephales promelas), river chub (Nocomis micropogon), stoneroller (Campostoma anomalum), telescope shiner (Notropis telescopus), Tennessee shiner (Notropis leuciodus), white shiner (Notropis a/beo/us), whitetail shiner (Notropis galacturus), and mottled sculpin (Cottus bairdi). Tested species of sunfishes, catfishes, and darters did not serve as hosts.

Age and growth characteristics were obtained by thin-sectioning shells collected in muskrat middens at Slant and Pendleton Island, Virginia. As predicted by the von Bertalanffy equation, the fine-rayed pigtoe achieves a maximum length of roughly 90 mm and age of 35 yr in the Clinch River. Annual growth in length averaged 5 mm/yr through age 10, decreasing to a rate of roughly 2 mm/yr thereafter. Based upon an age-length key, most individuals (50%) were of intermediate ages (13 to 16 yr). Specimens less than 10 yr old were uncommon, comprising only 22.4% of the deme at Slant. No individuals less than 6 yr old were found at Slant or Pendleton Island. Based on cohort structure at these sites, the fine-rayed pigtoe population appears to be declining in the Clinch River. A monitoring program is suggested.