The endemic spring snail Pyrgulopsis montezumensis in a high CO2 environment: Importance of extreme chemical habitats as refugia

1. We examined the distribution of the endemic spring snail, Pyrgulopsis mo ntezumensis, along a gradient of dissolved free CO2 and on selected substra ta in the collapsed travertine springmound of Montezuma Well, Arizona, U.S. A. 2. Dissolved CO2 concentration ranged from 480 (SE +/- 56) mg L-1 near the bottom spring vents in the limnetic zone (12 m deep) of the well to < 20 mg L-1 in an irrigation canal about 1400 m downstream from the well outlet. S pring snails occurred in habitats with 110-315 mg L-1 dissolved CO2. 3. Laboratory experiments showed that P. montezumensis tolerated a dissolve d CO2 concentration up to about 400 mg L-1 for 11 days, but displayed 40% ( SE +/- 4) mortality after 3 days at 728 mg L-1 and 100% mortality after 24 h at 1200 mg L-1. We also measured 100% mortality after 10 days at ambient concentrations (< 10 mg L-1) of dissolved CO2. 4. Although the concentration of dissolved CO2 was within the tolerance ran ge for spring snails in the irrigation canal, inadequate substrata and incr eased predation probably restricted their distribution in the lower reaches of the canal. Both field measurements and laboratory selection experiments showed a significant (P < 0.001) preference for limestone substrata over s ubmerged vegetation and fine sediments. 5. We propose that springs with high dissolved CO2 may provide refugia from invertebrate and/or vertebrate predators not physiologically adjusted to t he extreme chemical environment. These extreme chemical habitats probably r educe competition for resources and predation and allow resistant populatio ns like P. montezumensis to attain high density.