DNA sorts look-alike southern hickorynut mussels into five new species with narrow ranges

Freshwater mussels are hard to count because they are hard to tell apart. According to the study's announcement, surveys across the southern US have been held back by similar-looking shells, historical misidentifications and incomplete sampling ^[5]. The paper's title calls the five "cryptic species" of Obovaria from the Mississippi Embayment and Gulf coastal rivers ^[12]. When taxonomists call a species cryptic, they mean it is too similar to its relatives to be sorted reliably by eye.

That problem decided how the study was built. A survey that sorts mussels by shell repeats whatever errors earlier identifications made. Testing DNA, as the Shedd Aquarium, Arkansas State University and USGS team did ^[2], can find a separate lineage inside one familiar shape. It rules out the mistake this group is most prone to: two species filed under one name because their shells match ^[5].

The conservation case depends on where the species live. The authors describe all five as rare and found only in specific geographic areas ^[3]. Splitting look-alikes changes the denominator in an extinction-risk assessment, because each species is now judged on its own populations. According to the announcement, recognizing distinct species is what lets managers assess extinction risk and decide which habitats to protect first ^[7]. "Conservation efforts can only protect what we recognize, and without this work, we risk certain species going extinct before they're even known to science," said Kentaro Inoue, a research biologist at Shedd Aquarium and the paper's lead author ^[8].

The thing this doesn't tell you is how rare. The announcement does not report how many mussels were sequenced, which genetic markers separated the lineages, how far apart those lineages sit, or how many populations of each species remain. Those figures decide whether a species needs emergency recovery work or routine monitoring. Until they are public, the region-by-region case is the authors' argument from rarity and range, and I think it is a sound one.

"Our discovery and descriptions of these new freshwater mussel species demonstrate how much we still have to learn about the biodiversity in our rivers," said Nathan Johnson, a co-author and research biologist at the U.S. Geological Survey Wetland and Aquatic Research Center ^[10].