On July 25, 2023, the U.S. Fish and Wildlife Service (Service) published a proposed rule to list two mussel species, the Salina mucket (Potamilus metnecktayi) and Mexican fawnsfoot (Truncilla cognata), as endangered under the Endangered Species Act (ESA). The Service also proposed to designate critical habitat for these species under the ESA. The proposed critical habitat for the Salina mucket would amount to approximately 200 river miles in the Texan counties of Brewester, Terrell, and Val Verde, and the proposed critical habitat for the Mexican fawnsfoot would amount to approximately 185 river miles in the Texan counties of Maverick, Webb, and Zapata.

Both mussel species are filter feeders (meaning they siphon nutrients suspended in the water column) and have historically occurred in the Texas portion of the Rio Grande drainage. The Service estimates that the Mexican fawnsfoot currently occupies about 48% of its historic range and that the Salina mucket occupies 16% of its historic range. The Salina mucket was previously thought to be extirpated entirely from Texas, until 2003, when the species was rediscovered upstream of the Amistad Reservoir. Members of both species prefer medium to large river systems and anchor themselves in the sandy material that forms in rock crevices along river banks to avoid being swept away by the current, especially during high flow or large flood events.

The Service identified increased fine sediment, water quality impairment, loss of flowing water, barriers to fish movement (these mussel species rely on host fish to reproduce), and climate change as the main threats to the species. The Service points out that some of these threats are interconnected—for example, the decrease in flowing or high-flowing water leads to increased fine sediment and concentrated pollution, because the high-flow events are needed to discharge the sediment and other contaminants from the riverbed.

The proposed rule serves as the Service’s 12-month finding on the petition to list these species. The proposed rule states that the comment period will be open for sixty days, until September 25, 2023. The Federal Register notice and supporting documents are available at regulations.gov, under Docket Number FWS-R2-ES-2023-0026.