Characterizing the early life history of an imperiled freshwater mussel (Ptychobranchus jonesi)

Conservation of imperiled species is frequently challenged by insufficient knowledge of life history and the environmental factors that affect various life stages. The larvae (glochidia) of most freshwater mussels in the family Unionidae are obligate ectoparasites of fishes. We describe the early life history of the federally endangered Southern Kidneyshell, Ptychobranchus jonesi, and compare methods for estimating fecundity and conducting host trials on conglutinate-producing mussel species. Both the glochidial inoculation baths and direct feeding of conglutinates to Percina nigrofasciata, Etheostoma edwini, and Etheostoma fusiforme resulted in successful metamorphosis to the juvenile life stage. Ptychobranchus jonesi glochidia did not metamorphose on the 25 other species of fishes tested representing 11 families. Three juveniles were recovered from Gambusia holbrooki resulting in a metamorphosis rate < 1%. We characterize P. jonesi as a host fish specialist that fractionally releases conglutinates from late January to early June. Intact P. jonesi conglutinate resemble a simuliid fly larva attached to an egg, but the majority of conglutinates were released as segments representing separate egg-- or larva--mimics. Viability of glochidia encased within a conglutinate was > 90% for at least 5 days. Directly feeding conglutinates to fishes allowed us to estimate natural infestation rates and calculate average numbers of juveniles produced per conglutinate, unlike the traditional approach of infesting fish hosts using an inoculation bath. Each method for measuring fecundity produced similar estimates but the regression, which estimated fecundity based on the physical dimensions of each conglutinate or conglutinate segment, was most practical. The distribution information, coupled with early life history description and methods developed for determining fecundity and conducting host trials, may assist in the conservation of P. jonesi, specifically during recovery options that include captive propagation, augmentation, and reestablishment.

濒危物种的保护工作常因对其生活史以及影响不同生活阶段的环境因素认知不足而举步维艰。蚌科（Unionidae）多数淡水蚌类的幼虫（钩介幼虫，glochidia）为鱼类专性体外寄生虫。本研究记述了被联邦列为濒危物种的南方肾蚌（Ptychobranchus jonesi）的早期生活史，并对比了针对产粘状体（conglutinates）蚌类的繁殖力估算方法与宿主试验方法。采用钩介幼虫接种浴法，以及直接向带纹颊鲈（Percina nigrofasciata）、爱德华氏镖鲈（Etheostoma edwini）与梭形镖鲈（Etheostoma fusiforme）投喂粘状体，均成功实现了钩介幼虫向幼体阶段的变态发育。南方肾蚌的钩介幼虫无法在其余25种受试鱼类（隶属于11个科）体内完成变态发育。仅从食蚊鱼（Gambusia holbrooki）体内回收3只幼体，变态率不足1%。本研究将南方肾蚌界定为宿主鱼类专性物种，其粘状体释放周期为1月下旬至6月上旬，且呈分批释放模式。完整的南方肾蚌粘状体形似附着于卵粒的蚋幼虫，但多数粘状体以片段形式释放，分别作为卵粒或幼虫的模拟物。包被于粘状体内的钩介幼虫至少可在5天内保持90%以上的存活率。与传统的通过接种浴感染宿主鱼类的方法不同，直接向鱼类投喂粘状体可使我们估算自然感染率，并计算出每个粘状体所能产生的平均幼体数量。两种繁殖力估算方法所得结果相近，但基于单个粘状体或粘状体片段的物理尺寸进行繁殖力估算的回归分析法最为实用。本研究获得的分布信息，结合早期生活史记述以及开发出的繁殖力测定与宿主试验方法，可为南方肾蚌的保护工作提供助力，尤其适用于圈养繁殖、种群扩增与野外放归这类恢复措施中。