Assessing salinity toxicity of Popenaias popeii (Texas Hornshell) from the Rio Grande, Texas

Authors: Michael Hart, Charles Randklev

Year: 2019

Abstract

Popenaias popeii (Texas Hornshell), is endemic to the Rio Grande drainage, in Texas and New Mexico, and to Mexico in select coastal streams. Currently, only four populations persist in the United States and its status within Mexico remains unknown. In 2018, USFWS listed Texas Hornshell as Endangered under the ESA. A number of factors have been implicated in the decline of this species including degraded water quality due to changes in land use, river impoundment, and ground water pumping, but none of these stressors have been explicitly tested. In particular, salinization of the Rio Grande and its tributaries has long been a concern due to the underlying geology, the effects of which have been exacerbated by agricultural practices, natural gas extraction, and river impoundments. In general, unionid mussels are considered sensitive to even low levels of salinity so increased salinization could be a major contributing factor to the decline of Texas Hornshell in the Rio Grande. The objective of this study was to determine the effects of various concentrations of salinity on survival of adult mussels of Texas Hornshell. We performed acute and chronic toxicity tests at various salinity concentrations for up to 10 days. We found that mussels exposed to concentrations above 4 ppt showed significant mortality, while concentrations below this showed no mortality. Our results demonstrate that Texas Hornshell is tolerant of salinization compared to other unionid mussel species. However, large segments of the Pecos and parts of the Rio Grande are near or exceed 4 ppt, which indicates that these reaches are becoming unsuitable and populations within them at risk.