Long-term Isopropanol Storage Does Not Alter Fish Morphometrics

Abstract

Since the discovery of spirit-based preservation, scientists have based innumerable studies of systematics, anatomy, development, biogeography, evolutionary biology, and ecomorphology on fluid specimens held by the world's natural history collections. Though alcohol preservation can alter specimen measurements through dehydration and shrinkage, the magnitude of this problem over decadal timescales has never been estimated. If substantial, long-term preservational effects could compromise studies that depend on the comparability of morphometrics drawn from specimens collected at different times unless the artifacts were explicitly controlled. To evaluate the importance of these potential artifacts, we obtained geometric morphometric data from series of four common fish species collected from the Willamette River drainage over the last 70 years and preserved in the Oregon State Ichthyology Collection in 50% isopropanol after formalin fixation. Most regressions of principal component axes against date of collection revealed no directional trends in specimen morphology, and the two significant regressions lose significance when recent collections from the ecologically distinctive river main stem are excluded. We conclude that specimen shrinkage occurs shortly after preservation and does not exacerbate over time and that much of the observed morphological variation in our sample likely stems from habitat differences among collecting localities. Thus, morphological studies can continue to compare specimens collected over decadal timespans to answer questions about the ecological, evolutionary, and biogeographic causes of morphological variation with minimal concern for major preservational artifacts caused by long-term alcohol storage.