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SAVING AN ENDANGERED MUSSEL: HABITAT OF THE HIGGINS EYE PEARLYMUSSEL

SAVING AN ENDANGERED MUSSEL: HABITAT OF THE HIGGINS EYE PEARLYMUSSEL

Cassie Warren, Department of Biology, Macalester College
Daniel J. Hornbach, Department of Biology, Macalester College
Mark C. Hove, Department of Biology, Macalester College

The Higgins Eye Pearlymussel (Lampsilis higginsi), a federally endangered freshwater bivalve, can be found in the St. Croix, Mississippi, and (rarely) Wisconsin Rivers. Facing a shrinking habitat range and a growing threat of invasion by zebra mussels (Dreissena polymorpha), the survival of the Higgins Eye species requires immediate but carefully calculated human intervention. To address the increasing threat of Higgins Eye extinction, some have considered the merits of relocating individuals to areas less vulnerable to human-related disturbances and zebra mussel invasion. The purpose of this study was to determine the characteristics of typical habitat for Higgins Eye so that appropriate habitats can be chosen for relocation.

Since 1990, Dan Hornbach and his team have monitored mussel populations and the natural environment surrounding them in several regions of the St. Croix River. Data on mussel density, substrate composition, bottom flow, and depth were collected at 9 sites along the St. Croix River between 1990 and 2007. In eighteen years of monitoring, a mere 35 Higgins’ Eye mussels have been found, along with 15,988 mussels of other species. Using the data set compiled in those eighteen years, we have attempted to create a model predicting the presence or absence of L. higginsi using the variables available.

Given our statistical tools and the limited availability of locations with Higgins Eye present, it proved impossible to create an accurate model predicting the presence of Higgins Eye. However, since there was a significant relationship between total mussel density and the presence of Higgins Eye, we created a model predicting mussel density. We infer that habitat characteristics correlated with high mussel density will also then be associated with the presence of Higgins Eye.

This study revealed that greater water depth, lower bottom flow, and coarser substrate are all positive contributors to greater mussel density, supporting the theory that greater substrate stability provides higher quality mussel habitat. Therefore, if relocation of Higgins Eye becomes the last option to save them from habitat destruction and zebra mussel invasion, areas with high levels of mussel density (areas with greater depth, lower bottom flow, and coarser substrate) would be ideal choices for relocation habitat.