Dorsal Skin Microbial Assemblages in Free-ranging Hellbenders, Cryptobranchus Alleganiensis, Associated With Subspecies and Chronic Toe Lesions, but Not With Chytrid Fungal Infection.

Abstract

BackgroundSkin microbiomes are important components of skin health and have been shown to contribute to immunity in amphibians, especially against the chytrid fungus, Batrachochytrium dendrobatidis (Bd). Hellbenders (Cryptobranchus alleganiensis) are large aquatic amphibians (Order Caudata) native to the eastern United States that have experienced population declines of both the Ozark and eastern subspecies, C. a. bishopi and C. a. alleganiensis, respectively. In addition, ulcerative non-healing toe lesions have become increasingly prevalent in C. a. bishopi, in Arkansas (AR) where populations are now reduced to a single river. To evaluate the potential impacts of both chronic toe lesions and Bd on hellbender health, we compared dorsal skin microbial assemblages based on 16S rRNA gene amplicons between a declining Ozark hellbender population in Arkansas (AR) presenting lesions and a reference, recruiting, lesion-free, population of eastern hellbenders in eastern Tennessee (ETN). We further evaluated effects of mass and life stage across both subspecies, as well as toe lesion severity and Bd infection status within AR, to better understand the associations between microbiomes and disease in a wild salamander.ResultsWe found skin of ETN hellbenders to have greater bacterial alpha diversity compared to AR, with this disparity decreasing as Hill number order increased. Conversely, within AR, animals with more severe lesions had decreased alpha diversity than those with mild lesions, which became more pronounced with increasing Hill number. Further, the average microbial assemblage structure differed between ETN and AR. Specifically, AR communities displayed increased beta diversity compared to those from ETN, which appeared to be linked to toe lesion severity. Neither size class (mass) nor Bd infection status had a significant effect on alpha or beta diversity. Taxonomic analysis revealed ETN to have greater OTU abundance of phylum Cyanobacteria 24.3%) compared to AR (5.9%); whereas AR had increased abundance of Proteobacteria (48.5%), Firmicutes (9.1%), and Synergistetes (1.5%), in comparison to ETN (31.5%, 2.6%, 0.2%, respectively).ConclusionsResults demonstrate that eastern hellbenders of ETN have richer and less dispersed dorsal skin bacterial assemblages compared to Ozark hellbenders of AR. Furthermore, we suggest that increased severity of toe lesions may be linked to systemic changes resulting in skin microbial dysbiosis, independent of Bd infection. Although lesions remain to have an unknown etiology, this study is another step towards understanding skin bacterial microbiomes in hellbenders, and their potential associations with chronic disease.