Neritina snails hierarchical distribution

Snails were collected, counted, and measured within 0.5 x 0.5 m quadrats placed on the streambed as described in data set "Neritina snails upstream migrations":  Snails on and under rocks were counted within 0.5 x 0.5 m quadrats (n=10) placed on the streambed on a weekly basis.  Sampling quadrats were placed ˜1 m apart across the main channel (MC), and in five pairs along the side channel (SC). At SC, one quadrat of each pair was placed on the thalweg and another on the shallow part.  In addition, a random sample of individuals was collected from each quadrat to measure shell size. Shell size was measured as aperture width (straight line running along the columnella from the base of the operculum to the shell shoulder on body whorl). Total length (TL, perpendicular to AW) is related to aperture width (AW) but cannot be measured in individuals with broken outer lips (TL=1.24AW+0.04, r2=0.98).

Four habitats (point-bar and straight channel pools, a riffle, and a high flow pond) in Río Mameyes near Route 3 Bridge were weekly sampled between August and December 2000. Water depth and substrate type were measured within each quadrat. The three-most abundant substrates were classified as boulders, cobbles, pebbles, gravel or sand/fines. Plots covered >1/3 by Elodea were classified as “mats”. Nearbed water velocity was measured along cross-sections within each habitat as described in the data set "Neritina snails upstream migrations".  In addition, the presence and inland extent of distribution of N. virginea were surveyed in 32 coastal rivers around the island during summer 2001 and 2003. Water chemistry and river discharge data were obtained from the US Geological Survey (http://nwis.waterdata.usgs.gov/usa/nwis/).