Effects of current velocity on habitat suitability of exotic giant ramshorn snails (Marisa cornuarietis) in Comal Springs, Texas
The effect of velocity on habitat suitability of Marisa cornuarietis (Pilidae) was studied over a four month (28 Feb 98 - 12 June 98) period at Landa Lake, New Braunfels, Comal Co., TX, and in the laboratory. Well withdrawals and drought conditions cause decreases in springflows that lead to subsequent decreases in current velocities within the lake. Springflows during this study ranged from 9.4 to 6.7 m3 /s. Except for two instances of M. cornuarietis observed in Saggitaria platyphylla, no snails were observed in macrophyte beds other than Vallisneria americana. A negative relationship between velocity and snail abundance was found in field observations (r = -0.335). The greatest numbers of snails were found in velocities less than 0.03 meters per second (mps). The partitioning velocity in the lake was predicted to be 0.17 mps by way of a simple linear regression. No snails were found in velocities greater than 0.16 mps. In laboratory experiments, t-tests for independent samples revealed that of four size classes of snails, the three smallest were significantly impacted with respect to mobility by a velocity of 0.18 mps. The largest size class was also effected, though to a lesser degree. These findings show that there is a relationship between current velocity and habitat suitability for M. cornuarietis in Landa Lake.
Marisa cornuarietis, Streamflow velocity, Habitats
Ourso, R. T. (1998). <i> Effects of current velocity on habitat suitability of exotic giant ramshorn snails (Marisa cornuarietis) in Comal Springs, Texas</i> (Unpublished thesis). Southwest Texas State University, San Marcos, Texas.