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Nathan A Johnson, Ph.D.

Nathan Johnson is a Research Biologist at the USGS Wetland and Aquatic Research Center. 

EDUCATION

Ph.D., Fisheries and Aquatic Sciences, University of Florida

M.S., Fisheries and Wildlife Sciences, Virginia Tech University

B.S., Fisheries Science, Virginia Tech University

RESEARCH

Nathan Johnson's research focuses on investigating species boundaries, distributions, and conservation priorities of freshwater mussels in the Southeast.  Raised in eastern Virginia, Nate received both his bachelor’s and master’s degrees from the Fisheries and Wildlife Department at Virginia Tech and earned his Ph.D. at the University of Florida.  His undergraduate research focused on conservation genetics and propagation of endangered freshwater mussels and fishes.  For his master’s, Nate looked at the genetics of disease resistance in rainbow trout at the USDA-National Center for Cool and Coldwater Aquaculture in Leetown, WV. By developing new molecular markers to trace pedigrees and populations structure, he identified associations between major histocompatibility genes and disease resistance. His dissertation research resolved long-standing uncertainties about the systematics of freshwater mussels that will aid in the development of more effective conservation management strategies.  This includes documenting mussel diversity in rivers of the North American Coastal Plain using DNA barcoding and evaluating the processes responsible for their current geographic distribution (phylogeography).  He is active in a variety of professional societies including the American Fisheries Society and the Freshwater Mollusk Conservation Society and currently partnered with the Florida Museum, the Smithsonian Institute, Florida Fish and Wildlife Conservation Commission, and the US Fish and Wildlife Service. Outside of professional interests, he enjoys traveling, mountain biking, fishing, and surfing.

Science and Products

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Inflated Heelsplitter (Potamilus inflatus)

Development and Implementation of Environmental DNA (eDNA) Tools to Aid Listing and Recovery Efforts for Imperiled and Common Freshwater Mussels

Researchers will develop and optimize an eDNA assay to delineate the current distribution of P. inflatus. The assay will then be used to provide up-to-date distributional information and detection rates for P. inflatus in the Pearl River basin.
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Species Management Research Program, Wetland and Aquatic Research Center
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Development and Implementation of Environmental DNA (eDNA) Tools to Aid Listing and Recovery Efforts for Imperiled and Common Freshwater Mussels

Researchers will develop and optimize an eDNA assay to delineate the current distribution of P. inflatus. The assay will then be used to provide up-to-date distributional information and detection rates for P. inflatus in the Pearl River basin.
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Figure 2. Photo illustrating difficulties distinguishing between federally endangered and common freshwater mussel species

Using Molecular Tools to Recalibrate Freshwater Mussel Taxonomy with a Focus on Imperiled Species

Freshwater mussels of the family Unionidae, also known as naiads, pearly mussels, freshwater clams, or unionids, are a diverse group of bivalve mollusks that are distributed on every continent except Antarctica. Approximately 300 species are known from the United States, with most of this diversity residing in rivers of the Southeast where many endemic taxa have evolved.
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Ecosystems Mission Area, Species Management Research Program, Wildlife Program (unpublished), Wetland and Aquatic Research Center
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Using Molecular Tools to Recalibrate Freshwater Mussel Taxonomy with a Focus on Imperiled Species

Freshwater mussels of the family Unionidae, also known as naiads, pearly mussels, freshwater clams, or unionids, are a diverse group of bivalve mollusks that are distributed on every continent except Antarctica. Approximately 300 species are known from the United States, with most of this diversity residing in rivers of the Southeast where many endemic taxa have evolved.
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Map of habitat for Imperiled Freshwater Mussel Species in the S

Life History Characterization and Host Fish Identification for Federally Listed and Imperiled Freshwater Mussel Species in the Suwannee River Basin in Georgia and Florida

Freshwater mussels are considered the most imperiled group of animals in the United States. These animals provide valuable ecological services by filtering water, sequestering nutrients, and providing forage for migratory birds, small mammals, and turtles. They also have a unique and complex life cycle that makes them especially vulnerable to human disturbances. It includes a parasitic larval...
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Ecosystems Mission Area, Fisheries Program (unpublished), Species Management Research Program, Wetland and Aquatic Research Center
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Life History Characterization and Host Fish Identification for Federally Listed and Imperiled Freshwater Mussel Species in the Suwannee River Basin in Georgia and Florida

Freshwater mussels are considered the most imperiled group of animals in the United States. These animals provide valuable ecological services by filtering water, sequestering nutrients, and providing forage for migratory birds, small mammals, and turtles. They also have a unique and complex life cycle that makes them especially vulnerable to human disturbances. It includes a parasitic larval...
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Evaluation of Stream Reaches for Mussel Reintroduction in the Upper Coosa Watershed, NW Georgia

Evaluation of Stream Reaches for Mussel Reintroduction in the Upper Coosa Watershed, NW Georgia

The Conasauga River in northwest Georgia and southeast Tennessee harbors the majority of mussel diversity still found in the Georgia portion of the Upper Coosa Basin. While the Conasauga historically supported at least 44 mussel species, only about 20 species remain.
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Ecosystems Mission Area, Species Management Research Program, Wetland and Aquatic Research Center
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Evaluation of Stream Reaches for Mussel Reintroduction in the Upper Coosa Watershed, NW Georgia

The Conasauga River in northwest Georgia and southeast Tennessee harbors the majority of mussel diversity still found in the Georgia portion of the Upper Coosa Basin. While the Conasauga historically supported at least 44 mussel species, only about 20 species remain.
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Red Lionfish DNA data collected from Florida, USA and around the invasive distribution from 2007 to 2016

Red lionfish (Pterois volitans) have become a successful invasive predator across the Northwestern Atlantic, Caribbean, and Gulf of Mexico (GoM). Previous investigations have identified the southeast coast of Florida as the original site of introduction, but no region-wide genetic study has directly addressed the question of introduction location(s). This dataset includes previously unpublished re
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Wetland and Aquatic Research Center

Novel genetic resources for Clubshell freshwater mussels (Pleurobema clava, P. oviforme) for enhanced conservation

Here we provide public access to two mitochondrial DNA sequence alignments, multi-locus microsatellite DNA genotypes, morphometric measurements and details on all specimens utilized in Morrison et al., 2021 (In press).
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Eastern Ecological Science Center

Molecular data to investigate phylogeographic patterns, species boundaries, and demographic history of a North American freshwater mussel species complex (Bivalvia: Unionidae)

Here we provide public access to three DNA sequence alignments (COI, ND1, ITS1) and details on all specimens utilized in Keogh et al. (in review).
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Wetland and Aquatic Research Center

Novel genetic resources to facilitate future molecular studies in freshwater mussels (Bivalvia: Unionidae)

Here we provide public access to six DNA sequence alignments and details on all specimens utilized in Smith and Johnson (2020).
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Wetland and Aquatic Research Center

Molecular and morphological data to resolve species boundaries in the critically imperiled freshwater mussel species, Fusconaia mitchelli

We generated multilocus DNA sequence data and traditional morphometric measurements to evaluate species boundaries in Fusconaia mitchelli. We sequenced three loci: the protein-coding mitochondrial DNA genes cytochrome oxidase subunit 1 and NADH dehydrogenase 1, and the nuclear ribosomal internal transcribed spacer 1. We also took three measurements, maximum length, maximum width, and maximum heigh
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Wetland and Aquatic Research Center

Anchored hybrid enrichment in Potamilus mussels

Freshwater mussels are a species-rich group with biodiversity patterns shaped by life history strategies that include an obligate parasitic larval stage. Anchored hybrid enrichment (AHE) and ancestral character reconstruction (ACR) revealed convergence of life history strategies involving larval morphologies that reduce size to increase fecundity and require growth during encapsulation to increase
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Wetland and Aquatic Research Center

Molecular, morphometric, and distribution information for the freshwater mussel genus Potamilus and newly described species, Potamilus streckersoni sp. nov.

Inaccurate systematics confound our ability to determine evolutionary processes that have led to the diversification of many taxa. The North American freshwater mussel tribe Lampsilini, Ihering, 1901, is one of the more well-studied groups in Unionidae; however, many supraspecific relationships between lampsiline genera remain unresolved. Two genera that have been largely overlooked are Leptodea a
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Wetland and Aquatic Research Center

Cytonuclear discordance in the Florida Everglades invasive Burmese python (Python bivitatus) population indicates possible hybridization with the Indian python (Python molurus)

abstract
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Wetland and Aquatic Research Center

Salinity tolerance among three freshwater mussels (Bivalvia: Unionidae) from Gulf Coastal Plain drainages

An important physiological constraint influencing distributions of coastal freshwater organisms is their tolerance for saline conditions. We experimentally evaluated salinity tolerance for three freshwater mussel species (Utterbackia imbecillis, Elliptio jayensis, and Glebula rotundata). Mussels were transferred abruptly from well water to one of five treatments (0 [control], 6, 12, 18 or 24 parts
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Wetland and Aquatic Research Center

Molecular and morphological data on two species complexes in the freshwater mussel genus Cyclonaias

We examined species boundaries within an imperiled group of freshwater mussels in the genus Cyclonaias (Bivalvia: Unionidae) using morphometrics, molecular phylogenetics, and multispecies coalescent models to help guide pending conservation assessments and legislative decisions. Congruence across all lines of evidence indicated that current taxonomy overestimates diversity in the C. pustulosa spec
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Wetland and Aquatic Research Center

Genetics rayed creekshell

This dataset supports the inclusion of Anodontoides radiatus in the genus Strophitus, introducing the binomial Strophitus radiatus. These data also provide strong support for paraphyly in Strophitus and advocate the resurrection of the genus Pseudodontoides to represent the binomials Pseudodontoides connasaugaensis and Pseudodontoides subvexus.
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Wetland and Aquatic Research Center

Cumulative spring discharge and survey effort influence threatened Suwannee moccasinshell, Medionidus walkeri, occupancy and detection

Freshwater mussels (Unionidae) are among the most imperiled groups of organisms in the world. Unionids are plagued with a lack of basic information regarding species distributions, life history characteristics, and ecological and biological requirements. We assessed the influence of hydrologic factors on the occurrence of the Suwannee Moccasinshell, Medionidus walkeri, a federally threatened fresh
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Wetland and Aquatic Research Center
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Genetic and morphological characterization of the freshwater mussel clubshell species complex (Pleurobema clava and Pleurobema oviforme) to inform conservation planning

The shell morphologies of the freshwater mussel species Pleurobema clava (federally endangered) and Pleurobema oviforme (species of concern) are similar, causing considerable taxonomic confusion between the two species over the last 100 years. While P. clava was historically widespread throughout the Ohio River basin and tributaries to the lower Laurentian Great Lakes, P. oviforme was confined to
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Ecosystems Mission Area, Wetland and Aquatic Research Center

Biogeography of freshwater mussels (Bivalvia: Unionida) in Texas and implications on conservation biology

AimBiogeography seeks to identify and explain the spatial distributions of species and has become an important tool used by conservationists to protect and manage aquatic organisms. Texas, located in the southwestern United States, is home to 52 species of freshwater mussels, 9 of which are endemic to Texas and 7 that are endemic to Texas and neighboring states or countries. There have been two ma
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Ecosystems Mission Area, Wetland and Aquatic Research Center

Using a multi-model ensemble approach to determine biodiversity hotspots with limited occurrence data in understudied areas: An example using freshwater mussels in México

Species distribution models (SDMs) are an increasingly important tool for conservation particularly for difficult-to-study locations and with understudied fauna. Our aims were to (1) use SDMs and ensemble SDMs to predict the distribution of freshwater mussels in the Pánuco River Basin in Central México; (2) determine habitat factors shaping freshwater mussel occurrence; and (3) use predicted occup
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Ecosystems Mission Area, Wetland and Aquatic Research Center

Genetic analysis of red lionfish Pterois volitans from Florida, USA, leads to alternative North Atlantic introduction scenarios

The red lionfish Pterois volitans is a successful invasive predator across the western North Atlantic, Caribbean, and Gulf of Mexico. The southeast coast of Florida (USA) has been identified as the original introduction location, but genetic analyses including Florida lionfish have yet to investigate introduction scenarios. Here, we assessed the potential lionfish invasion pathways using 1795 sequ
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Ecosystems Mission Area, Wetland and Aquatic Research Center

Establishing conservation units to promote recovery of two threatened freshwater mussel species (Bivalvia: Unionida: Potamilus)

Population genomics has significantly increased our ability to make inferences about microevolutionary processes and demographic histories, which have the potential to improve protection and recovery of imperiled species. Freshwater mussels (Bivalvia: Unionida) represent one of the most imperiled groups of organisms globally. Despite systemic decline of mussel abundance and diversity, studies eval
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Ecosystems Mission Area, Wetland and Aquatic Research Center

Rediscovery and genetic confirmation of the Threeridge Mussel, Amblema plicata (Say, 1817) (Bivalvia, Unionidae), in the Choctawhatchee River, Florida, USA

Recent freshwater mussel research has resulted in rediscovery of several species presumed extinct. We report the rediscovery of Amblema plicata (Say, 1817) in 2019 from the Choctawhatchee River, Florida, USA. Amblema plicata has not been reported in the Choctawhatchee river basin since 1958, more than 61 years ago. This species was collected during the long-term monitoring of freshwater mussels in
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Ecosystems Mission Area, Wetland and Aquatic Research Center

Gulf Coast vicariance shapes phylogeographic history of a North American freshwater mussel species complex

AimFreshwater mussels share habitat and are parasites of freshwater fishes during the larval life stage. Therefore, models of fish biogeography may also explain the historical biogeography of freshwater mussels. We tested this assumption using predictions of three biogeographic models constructed for northern Gulf of Mexico drainages on a freshwater mussel species complex. Specifically, we tested
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Ecosystems Mission Area, Wetland and Aquatic Research Center

Resolving species boundaries in the critically imperiled freshwater mussel species, Fusconaia mitchelli (Bivalvia: Unionidae)

Species are a fundamental unit of biology, and defining accurate species boundaries is integral to effective conservation and management of imperiled taxa. Freshwater mussels (Bivalvia: Unionidae) are among the most imperiled groups of organisms in North America, yet species boundaries remain uncertain for many taxa. The False Spike, Fusconaia mitchelli (Simpson in Dall, 1895), is a freshwater mus
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Ecosystems Mission Area, Wetland and Aquatic Research Center

Novel molecular resources to facilitate future genetics research on freshwater mussels (Bivalvia: Unionidae)

Molecular data have been an integral tool in the resolution of the evolutionary relationships and systematics of freshwater mussels, despite the limited number of nuclear markers available for Sanger sequencing. To facilitate future studies, we evaluated the phylogenetic informativeness of loci from the recently published anchored hybrid enrichment (AHE) probe set Unioverse and developed novel San
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Ecosystems Mission Area, Wetland and Aquatic Research Center

The utility of zooarchaeological data to guide listing efforts for an imperiled mussel species (Bivalvia: Unionidae: Pleurobema riddellii)

The status of species in freshwater systems shift over time due to natural and anthropogenic causes. Determining the magnitude and cause of these shifts requires a long‐term perspective. This process is complicated when there are also questions about the taxonomic validity of a species. Addressing these issues is important because both can undermine conservation and management efforts if incorrect
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Ecosystems Mission Area, Wetland and Aquatic Research Center

A new species of freshwater mussel in the genus Popenaias Frierson, 1927, from the Gulf coastal rivers of central Mexico (Bivalvia: Unionida: Unionidae) with comments on the genus

The Gulf coastal drainages of central Mexico are a faunal transition zone between North and South America and harbor a unique assemblage of freshwater mussels (Bivalvia: Unionida). However, little information is available regarding the taxonomy, distribution, and evolutionary history of the Mexican mussel fauna due to limited sampling over the last 100 years. To address these knowledge gaps, we ev
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Ecosystems Mission Area, Wetland and Aquatic Research Center

A comparative phylogeographic approach to facilitate recovery of an imperiled freshwater mussel (Bivalvia: Unionida: Potamilus inflatus)

North American freshwaters are among the world’s most threatened ecosystems, and freshwater mussels are among the most imperiled inhabiting these systems. A critical aspect of conservation biology is delineating patterns of genetic diversity, which can be difficult when a taxon has been extirpated from a significant portion of its historical range. In such cases, evaluating conservation and recove
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Ecosystems Mission Area, Wetland and Aquatic Research Center

Non-USGS Publications**

Grobler, P. J., Jones, J.W., Johnson, N.A., Neves, R.J., and E.M. Hallerman. 2011. Homogeneity at Nuclear Microsatellite Loci Masks Mitochondrial Haplotype Diversity in the Endangered Fanshell Pearlymussel (Cyprogenia stegaria). Journal of Heredity 102(2): 196-206. [Link]

**Disclaimer: The views expressed in Non-USGS publications are those of the author and do not represent the views of the USGS, Department of the Interior, or the U.S. Government.

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