Integrative taxonomy resolves taxonomic uncertainty for freshwater mussels being considered for protection under the U.S. Endangered Species Act

Nathan A Johnson,Charles R Randklev,James D Williams,John M Pfeiffer,James D Austin,Chase H Smith

doi:10.1038/s41598-018-33806-z

Abstract

Objectively delimiting species boundaries remains an important challenge in systematics and becomes urgent when unresolved taxonomy complicates conservation and recovery efforts. We examined species boundaries in the imperiled freshwater mussel genus Cyclonaias (Bivalvia: Unionidae) using morphometrics, molecular phylogenetics, and multispecies coalescent models to help guide pending conservation assessments and legislative decisions. Congruence across multiple lines of evidence indicated that current taxonomy overestimates diversity in the C. pustulosa species complex. The only genetically and morphologically diagnosable species in the C. pustulosa species complex were C. pustulosa and C. succissa and we consider C. aurea, C. houstonensis, C. mortoni, and C. refulgens to be synonyms of C. pustulosa. In contrast, all three species in the C. nodulata complex (C. necki, C. nodulata, and C. petrina) were genetically, geographically, and morphologically diagnosable. Our findings have important conservation and management implications, as three nominal species (C. aurea, C. houstonensis, and C. petrina) are being considered for protection under the Endangered Species Act.

Highlights

A robust taxonomy has profound implications for inferring common biological characteristics, understanding shared organismal responses, and is required to effectively set conservation priorities[1,2]
Each taxon was represented by c oxidase subunit 1 (CO1), NADH dehydrogenase subunit 1 (ND1), and internal transcribed spacer 1 (ITS1) (951 nt with avg. ≈ 49.13% gaps) and the concatenated three-locus alignment consisted of 2397 nt
Five partitions and nucleotide substitution models were selected by Partitionfinder for implementation in both IQ-TREE and BEAST: CO1 and ND1 1st position- TrNef+I+G, CO1 and ND1 2nd position- HKY+I+G, CO1 3rd position- HKY+G, ND1 3rd position- TrN+G, and ITS1- K80+I+G

Summary

Introduction

A robust taxonomy has profound implications for inferring common biological characteristics, understanding shared organismal responses, and is required to effectively set conservation priorities[1,2]. Recent authors have called for more integrative approaches that draw inference from multiple independent lines of evidence[14,15,16,17,18,19,20,21] This has developed in part from the recognition that morphological characters or geographic distributions alone are not necessarily diagnostic at the generic and species levels[8,9,22,23,24]. The distributions of nominal species in the C. pustulosa complex in western Gulf of Mexico drainages have been revised with all species considered allopatric and previous reports of sympatry attributed to misidentifications[38]. Of particular importance is the taxonomic validity of three species being considered for protection under the Endangered Species Act (ESA)[42]: C. aurea, C. houstonensis, and C. petrina

Methods

Results

Conclusion