Reestablishing a host-affiliate relationship: migratory fish reintroduction increases native mussel recruitment.

Co-extirpation among host-affiliate species is thought to be a leading cause of biodiversity loss worldwide. Freshwater mussels (Unionida) are at risk globally and face many threats to survival, including limited access to viable host fish required to complete their life history. We examine the relationship between the common eastern elliptio mussel (Elliptio complanata) and its migratory host fish the American eel (Anguilla rostrata), whose distribution in the Chesapeake Bay watershed is limited, in part, by dams. We examined population demographics of E. complanata across locations in the Chesapeake Bay watershed, primarily in the Susquehanna River in the absence of American eels, and conducted experimental restocking of eels to assess potential impacts on mussel recruitment. Compared to surveys completed ~20 yr prior, E. complanata could be experiencing declines at several historically abundant sites. These sites also had extremely limited evidence of recruitment. Restoration of host fish improved recruitment, but results were not equivalent between stocking sites, indicating that host reintroduction alone may not be fully effective in reestablishing mussel populations. One site where eels were introduced (Pine Creek, Tioga County, Pennsylvania, USA) experienced an increase from 0 juveniles found during quantitative surveys prior to eel stocking to 151 (21% of individuals collected during quantitative surveys) E. complanata juveniles found four years after stocking. A second site (Buffalo Creek, Union County, Pennsylvania) experienced a more moderate increase from 2 to 7 juveniles found during 2010 and 2014 quantitative surveys, respectively. Continued examination of other potential interacting factors affecting recruitment, including water quality or habitat conditions, is necessary to target favorable sites for successful restoration.

The effects of salinity exposure on multiple life stages of a common freshwater mussel, Elliptio complanata.

There is growing concern over the effects of increased salinization on freshwater organisms, which are largely unknown for unionid mussels. Adult and larval Elliptio complanata were exposed to low-level salt concentrations to determine the effects on mussel survival, physiology, and reproduction. Adults were exposed to salt concentrations of 0 parts per thousand (ppt), 2 ppt, 4 ppt, and 6 ppt NaCl and monitored over 7 d for mortality. Treatment groups exposed to 6 ppt and 4 ppt experienced 50% mortality at day 3 and day 4, respectively, with complete mortality by day 7. No mortality was observed in the other treatments. Adults were also exposed to sublethal salinity levels of 1 ppt and 2 ppt NaCl for 4 wk to determine physiological consequences of prolonged salinity exposure. Mussels exposed to 1 ppt and 2 ppt experienced reduced metabolic rates within the first 24 h of exposure that recovered to control levels in the 1-ppt treatment within 7 d. Metabolic recovery did not occur in the 2-ppt treatment by the end of 28 d. Glochidia exposed to 3-ppt NaCl during attachment to their host fish suffered a reduction in attachment success and metamorphosis, resulting in a 10-fold reduction in the number of juveniles produced per host fish. The present study demonstrates that low levels of salt can have a dramatic effect on the reproduction, physiology, and survival of freshwater mussels.