Early life history of the sheepnose (Plethobasus cyphyus) (Mollusca: Bivalvia: Unionoida)

Abstract

Managing a rare species can be improved with knowledge of its natural history. The sheepnose (Plethobasus cyphyus) is a freshwater mussel recently listed by the US as federally endangered. We used standard methods to study P. cyphyus brooding behaviour, host fishes in the laboratory and under natural conditions, and glochidial morphology. We monitored a population of P. cyphyus in the Chippewa River, WI during spring and summer 2007-2009 and 2011 and found brooding animals between mid-May and early August. Gravid individuals ranged between 5 and 27 yr (mean age ± 1 s.d. = 13 ± 4 yr). Plethobasus cyphyus brooded glochidia in outer gills, which varied in colour from red, orange, pink, cream, or white. We observed mature glochidia more commonly in individuals with cream or white gills and these glochidia were released in a clear, adhesive, mucus matrix. In laboratory trials we found several minnow and topminnow species (29 spp.) served as productive suitable native hosts. The mean number of juvenile mussels released per cyprinid per day was significantly higher for trials conducted at 22-25°C compared with those at 18-20°C , and 83% of trials conducted at 18-20°C using suitable host species produced no juveniles. Glochidia had a unique outline and shell morphometrics that distinguished P. cyphyus from seven other Chippewa River mussel species that produce similar sized glochidia. Using morphometrics we determined that mimic shiners (Notropis volucellus) were natural hosts for P. cyphyus, round pigtoe (Pleurobema sintoxia), and Wabash pigtoe (Fusconaia flava). Releasing mucus-bound glochidia has evolved in a variety of mussel species and may be more common than is currently realized. Our data show that P. cyphyus is a cyprinid host specialist, and propagation efforts for this species can be strengthened through improved access to mature glochidia by using females with cream-coloured gills and increased juvenile production through warmer fish holding temperatures.

Population structure and gene flow in the Sheepnose mussel (Plethobasus cyphyus) and their implications for conservation

“…Isolated Sheepnose populations may be large enough to maintain high level of genetic diversity and buffer populations against the erosive effects of genetic drift (Elderkin et al, 2007; Lande & Barrowclough, 1987). Additionally, Sheepnose are estimated to have lifespans of up to 30 years (Hove et al, 2015), such long‐lived species can also buffer populations from the loss of genetic diversity due to drift (Hoffman et al, 2017). If this is true, efforts and resources aimed at conservation strategies such as propagation and translocations would be better directed toward regaining habitat suitability and connectivity (Olson & Vaughn, 2020).…”

“…These estimations are calculated using a Bayesian approach and MCMC sampling to generate values for m over the last few (<5) generations (Wilson & Rannala, 2003). Given a generation time of approximately 5 years for the Sheepnose (Hove et al, 2015), BAYESASS estimated m values over the past ~25 years. Run lengths and parameters were optimized to ensure convergence and delta parameters were adjusted to accommodate 40-60% acceptance.…”

“…There are three species within the genus Plethobasus , all of which are listed as endangered. Of the three species, the Sheepnose currently occupies the broadest distribution (Hove et al, 2015; Turgeon et al, 1998. All three species of Plethobasus have exhibited range‐wide population declines presumably as a result of anthropogenic changes to their habitat and it is thought that these changes are the most prominent factor in the decline of Plethobasus (Stein & Flack, 1997; USFWS Service, 1984; US ACOE, 2011).…”

“…After development is complete, the juvenile mussels then drop off the gills and establish themselves in the substrate (Parmalee & Bogan, 1998). Originally, the only known natural host fish for the Sheepnose was the Sauger ( Sanders candensis ) (Surber, 1913), however, a more recent study has shown that the Sheepnose appears to be a cyprinid host specialist (Hove et al, 2015). Gene flow is thought to be achieved through the dispersal of sperm and the glochidia larval stage (Ferguson et al, 2013; Hove et al, 2015) so conservation efforts must include consideration of host species availability so the life cycle can remain complete.…”

Investigation of Freshwater Mussel Glochidia Presence on Asian Carp and Native Fishes of the Illinois River

“…Based upon morphometric data and recent mussel community information from the Illinois River, we used a bivariate plot and simple qualitative overlap to determine the most-likely identity of each glochidium, as our sample size was small (Kennedy and Haag 2005). Morphometric data were derived from Waller (1987), Hoggarth (1999), Williams et al (2008) and references therein, Watters et al (2009), Hove et al 2012, Hove et al (2015) and M.C. Hove (Macalester College, personal communication).…”