Status of Pyrgulopsis robusta, 2001.

Authors(s): D. Gustafson

Publication:

Publisher:

Publication Date: 0000-00-00

Type: progress report

Location:

Abstract: Objectives To evaluate the status of the only known population of Pyrgulopsis robusta, which is now under pressure from the invading New Zealand mudsnail Potamopyrgus antipodarum. This study repeats an intial evaluation that was done in 2000. Findings and Status On June 25 2001, I repeated my June 26, 2000 mollusk sample of Polecat Creek below the bridge near the mouth. The same nine mollusca taxa were found as last year. This year we did find one live Pyrgulopsis robusta in the field (none last year). However, only three were found more with lab sorting (54 total last year). There were also several dead shells that were not counted either year. The 2mm sieve works well to separate these species. The total number of the exotic Potamopyrgus antipodarum collected this year was about 80,000 but this actually reduced from last year's collection of nearly 110,000, but this may be more due to a larger average size and my fixed collection volume (about 500 ml). The Potamopyrgus ratio is now about 1:20,000. This represents a 10 fold reduction since last year. The total catch reflects 13.5 fold decline. No small specimens of Pyrgulopsis robusta among the mass of Potamopyrgus shells were found. The common snail Valata humeralis has also dropped to barely detectable levels. The situation does not look good at this site, however, On August 10, 2001 a better population was found upstream near the Yellowstone boundary. These populations were associated with springs and these may serve as refuges for native species. I have seen this situation in several other areas. The native species holds on near the spring source and the mudsnails dominate further downstream. There is no guarantee that this will be the case. The need to further monitor lower Polecat Creek is greatly lessened with knowledge of the upstream populations. The population in the lower creek cannot get much lower and still be detectable with reasonable samples. I now recommend that the upstream spring sites, as potential refuges, be identified and offered speial protection. These areas may be very small and thus very venerable. The identity of the upstream snails should also be checked to insure they are the same species, but this does not seem most likely.

Keywords: Grand Teton National Park, Jackson Lake, animal, mollusk, population, snail, Gastropoda, mudsnail, New Zealand mudsnail, Potamopyrgus antipodarum, springsnail, Jackson Lake springsnail, Pyrgulopsis robusta, Valata humeralis, common snail