Simpsonaias ambigua, the salamander mussel or mudpuppy mussel, is a species of bivalve in the family Unionidae. It is unique among freshwater mussels in using mudpuppies as its glochidial host. It is the only freshwater mussel known to have a non-fish host.[2]
Salamander mussels are a unique freshwater mussel species notable for their obligate parasitic relationship with the mudpuppy salamander (Necturus maculosus), making it the only known unionid mussel to use an amphibian host rather than a fish.[3] Its larvae (glochidia) attach to the gills of mudpuppies, where they receive necessary nutrients and dispersal, eventually metamorphosing before dropping off into suitable benthic substrate. The species tends to occur in low-density populations, often associated with mudpuppy parameters such as population size and microhabitats that support them (e.g., refugia such as flat rocks and crevices).[4] Adult S. ambigua typically inhabit clean, well-oxygenated, fine-sand or silt substrates in medium- to large-sized rivers with stable flow.[4]
Like many freshwater mussels, S. ambigua contributes to ecosystem processes such as nutrient cycling and substrate stabilization. By filtering suspended particles, individuals improve water clarity and support aquatic food webs.[5] Its life history, however, makes it particularly vulnerable to disturbances that affect both its host and habitat.
Distribution
The historical range of the salamander mussel includes the Mississippi, Ohio, and Great Lakes drainages, spanning 25 U.S. states and parts of southern Ontario, Canada.[4] Adult S. ambigua typically inhabit clean, well-oxygenated, fine-sand or silt substrates in medium- to large-sized rivers with stable flow.[4] It has been documented in rivers such as the Wabash, Maumee, Ohio, and Tennessee, though it is now considered rare or extirpated from much of its former range.[6]
Recent records show persistence in several areas once thought to have lost populations. For example, S. ambigua was rediscovered in Illinois using environmental DNA (eDNA) analysis,[7] and continues to occupy isolated sites in southern Ontario, where populations exhibit small size structure and low density but evidence of continued reproduction.[5] A 2020 record from the Harpeth River in Tennessee extended the known distribution within the lower Cumberland system.[6]
Because the species’ occurrence is closely tied to that of the mudpuppy salamander, its range mirrors the distribution of this host. Declines in mudpuppy populations due to habitat degradation and pollution are likely to contribute to the mussel’s reduced distribution.[8]
Conservation
The Salamander Mussel is considered rare throughout its range, listed as a Special Concern in most U.S. states and Threatened in Canada.[3][5] Recovery is limited by the species’ dependence on both pristine benthic habitats and healthy populations of its mudpuppy salamander host.[4] Modern molecular and habitat-based tools are increasingly used to aid in the detection and management of the species. Environmental DNA (eDNA) surveys have confirmed its persistence in regions where it was previously thought extirpated, including Illinois and Ontario.[7][9] Research continues to refine methods for identifying suitable habitats and assessing the ecological conditions that support viable populations.[6][4]
Threats
Major threats to the Salamander Mussel include habitat degradation, pollution, and declines in its amphibian host. Channelization, sedimentation, and dam construction have reduced suitable substrates and altered natural flow regimes, limiting available habitat.[5] Runoff from agricultural and industrial activities further degrades water quality, negatively affecting both the mussel and the mudpuppy host.[4] Declines in mudpuppy populations, linked to pollution and habitat loss, directly limit mussel reproduction and dispersal.[8][4] Surviving mussel populations are often small and isolated, leaving them vulnerable to stochastic events and local extirpation.[9]
Like many unionid mussels, the Salamander Mussel was historically affected by overharvesting during the pearl button industry boom of the late 19th and early 20th centuries. Mussels were collected extensively in river systems of the Mississippi Basin, particularly around Muscatine, Iowa, then known as the “Pearl Button Capital of the World.” This exploitation contributed to widespread declines in native mussel populations before modern conservation regulations were established.[10]
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