Black nerite – an aquatic snail reintroduction in Hungary

Summary

The black nerite (Theodoxus prevostianus), an Endangered freshwater snail confined to hypothermal springs of Central Europe, has vanished from most of its 15-20 historical populations over the past 50 years, leaving just four sites, including one at Kács, Hungary, that remains vulnerable because it sits on private land outside any national park. Researchers first proposed reintroduction in 1999 and confirmed the value of an insurance population after genetic analysis showed the Kács population represents a distinct mitochondrial lineage. After testing water chemistry at three historically occupied sites, Vízfő Spring in Sály was chosen as the closest chemical match to Kács; Hungarian authorities permitted collection of only 200 snails per year for three years, so researchers released 200 hand-collected specimens in 2010 and 400 more in 2012, hand-carried in wet tissue paper and gradually acclimated to the recipient stream's temperature before release at two points near the spring's outflow, sheltered initially under clay pots to prevent drifting. Twice-yearly monitoring found no living snails or shells for almost two years, but by October 2015 numerous adults were detected, and by October 2016 the population had spread to a 400-500 metre stream section and upstream of the release point, with quadrat sampling estimating 5,000-20,000 adult and sub-adult snails, many carrying freshly hatched egg capsules. The long undetected latency period underscored the need for sustained monitoring in gastropod reintroductions. The project rates itself successful, crediting well-matched habitat chemistry, prioritizing snails with unhatched eggs given the low collection quota, and timing translocations to late autumn to match the species' egg-laying cycle.