Morphological Identification Overestimates the Number of Pallid Sturgeon in the Lower Mississippi River due to Extensive Introgressive Hybridization

Abstract

Pallid Sturgeon Scaphirhynchus albus and Shovelnose Sturgeon S. platorynchus are congeners occupying overlapping habitats within the Missouri and Mississippi River basins. The Pallid Sturgeon is listed as federally endangered under the Endangered Species Act in the United States, whereas the Shovelnose Sturgeon is not. Observational data across the range of both species include specimens that appear to be phenotypically and genotypically intermediate between Pallid Sturgeon and Shovelnose Sturgeon, though these occurrences are most frequent in the Mississippi River. Based on both morphological and genetic data, the variability and overlap within Scaphirhynchus spp. phenotypes has resulted in debate about the accuracy of existing identification methods of Pallid Sturgeon within the Mississippi River portion of its range (i.e., distinguishing Pallid, Shovelnose, and hybrid sturgeons). As a result, increasing questions have surfaced relative to species identification, implications of purported hybridization, the accuracy of local demographic estimations, and how those effect management actions. This project is the first comprehensive analysis to compare genotypic, phenotypic, and field identification data in order to evaluate the accuracy of existing field identification of the two congener Scaphirhynchus spp. and their purported hybrids in the lower Mississippi River. Our results indicate current field identification methods are not reliable for identifying Pallid Sturgeon in the lower Mississippi River, likely due to introgressive hybridization between Pallid and Shovelnose sturgeons and the resulting continuum of morphotypes and genotypes in hybridized individuals. We conclude that reliance on current morphological‐based field identification methods to quantify Pallid Sturgeon may lead to inaccurate assessments that could negatively impact the conservation and management of this endangered species, including the use of hybrid broodstock for conservation stocking and erroneous estimates of stock and recruitment.

Improved genetic identification of acipenseriform embryos with application to the endangered pallid sturgeon Scaphirhynchus albus

“…In contrast, evidence of shovelnose sturgeon spawning was relatively uncommon at this location in the present study. In other portions of the pallid sturgeon's range where habitat is greatly altered by channelization and bank stabilization, the species has been documented to hybridize with the related shovelnose sturgeon with increasing frequency downstream in the Missouri River (Jordan et al ., ). The near absence of hybridization in this portion of the species range, combined with the low occurrence of shovelnose sturgeon spawning at this site, may provide insight into the mechanism responsible for hybridization rangewide.…”

“…Because pallid sturgeon and shovelnose sturgeon do not exhibit fixed mtDNA differences (Campton et al ., ), nuclear DNA markers ( e.g ., microsatellites and single nucleotide polymorphisms; SNP) are needed to distinguish the two species. Microsatellites reliably discriminate between pallid sturgeon and shovelnose sturgeon in the Missouri River, but the presence of introgressive hybridization between the two species in the Mississippi River makes it difficult to distinguish among pure species and advanced hybrids (Jordan et al ., ).…”

“…This methodology will falsely identify approximately 3% of pallid sturgeon as shovelnose sturgeon (Figure ), but because pallid sturgeon are so rare and because SNPs are far more efficient than microsatellites, it allows us to screen far more embryos and larvae, and detect more pallid sturgeon than would be possible using microsatellites alone (Eichelberger et al ., ). While microsatellites are not reliable for distinguishing among the many hybrid classes present in the lower Mississippi River where introgressive hybridization is common, they are very effective in the upper Missouri River where hybridization is rare (Jordan et al ., ). Using this approach, wild‐spawned pallid sturgeon larvae have been matched to previously genotyped wild parents to validate successful spawning at locations identified using telemetry of reproductive adults (DeLonay et al ., ; Eichelberger et al ., ).…”

Pallid sturgeon seasonal habitat selection in a large free‐flowing river, the lower Mississippi River

“…Pallid sturgeon and sympatric shovelnose sturgeon, Scaphirhynchus platorynchus (Rafinesque, 1,820), share morphological similarities throughout both species' ranges (Jordan et al, ). Recent studies document extensive hybridization between pallid sturgeon and shovelnose sturgeon in the LMR (Jordan et al, ). Some genetically pure pallid sturgeon continue to occupy the LMR, but only genetic analysis can accurately distinguish pure pallid sturgeon from introgressed specimens (Jordan et al, ).…”

“…Recent studies document extensive hybridization between pallid sturgeon and shovelnose sturgeon in the LMR (Jordan et al, ). Some genetically pure pallid sturgeon continue to occupy the LMR, but only genetic analysis can accurately distinguish pure pallid sturgeon from introgressed specimens (Jordan et al, ). The significance of introgressed pallid sturgeon in the LMR and to the range‐wide conservation of the species has not been addressed.…”