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Assessment of mortality in toxicological trials of dreissenid mussel veligers is often time consuming and can be erroneous. Recommended methods for scoring mortality or survival of veligers include observing each individual to detect external or internal movements or lack thereof. Post-mortem infestation of tissue with decomposers frequently mimics movements of internal organs and leads to inaccuracy in assessment. Conversely, studies determining the toxic effect of potassium have found that veligers may appear dead after exposure, with shells agape, vellum exposed and no movement. However, after a recovery period in potassium free water, the test subjects may recover and move. Differential stains have been used to aid in scoring mortality or survival and we evaluated fast green stain compared to direct observation without stain. Fast green stain is used as a food coloring dye, and is a differential stain that penetrates dead tissues only, allowing for easy and accurate determination of live and dead tissues. The stain is used for assessment of saltwater mussel spat viability and more recently with freshwater dreissenid larval and juvenile life stages. Fast green allows for a differentiation between paralyzed and dead veligers following exposure to potassium without the need for a recovery period. In this study, we exposed dreissenid veligers to elevated levels of potassium chloride and evaluated the proportion of live and dead, and empty shells using two methods: one with the fast green stain and one without. We found the staining method resulted in more rapid and precise results without need for a long recovery step. We recommend this technique as beneficial to both novice and experienced microscopists. The fast green stain method is easy to use in the field or laboratory setting.

Use of a differential simple stain to confirm mortality of dreissenid mussel veligers in field and laboratory experiments