Locomotory behavior and habitat selection in littoral gastropods on Caribbean limestone shores

Journal of Shellfisheries Research, Jan, 2005 by Thomas W. Bates, David W. Hicks

Seawater spray resulting from crashing waves was collected using rain gauge-type devices constructed from PVC pipe and secured into cement bases that were arbitrarily placed above and below the mean high water line. At each site, the devices were deployed for a period of 1 h on three different days to capture the day-to-day variation in wave strength over the course of the 12-day experiment (3 devices x 3 different days x 2 zones = 18 total measurements at each site). The volume (mL) of spray collected above and below the mean high water line was used as an additional indicator to define varying degrees of moisture conditions occurring among mid and high-shore zones. One-way analyses of variance (ANOVA) were used to compare mid and high-shore zones with respect to rugosity, zone width, rock surface temperature, and desiccation potential (sponge-survey and seawater spray collection).

Movement and Microhabitat Selection

Field observations were made from May 12-25, 2002. Three subsites from mid and high-shore positions were sampled at each study site (3 midshore x 3 high-shore x 2 study sites = 18 total subsites). Within each subsite, five adult individuals of each species were arbitrarily selected and marked with a uniquely colored enamel paint to ensure individual distinction (methodology after Britton & McMahon 1992, Gochfeld & Minton 2001). Marked specimens of N. versicolor and T. antonii had similar size distributions as individuals in the natural population (16.6 [ or -] 1.8 mm & 13.9 [ or -] 1.6 mm shell length, respectively). Initial location of each snail was marked by drilling a hole approximately 5 cm south of each individual and installing a colored nail corresponding to the color of the painted snail (home mark). Three initial observations were recorded for each marked specimen: (1) distance to the water (m); (2) compass heading to the water (degrees); and (3) a description of the microhabitat in which it was found. All distances were estimated using a fiberglass tape measure. Microhabitat was classified based upon surface topography and substratum moisture (after Minton & Gochfeld 2001). Surface topography was classified as exposed surface (flat rock), crevice (depression in the rock surface large enough for two or more individuals to occupy), or pit (depression in rock large enough for a single individual to occupy). Microhabitats were further classified as wet (wet but no standing water), at the waterline, underwater, or dry.

Each subsequent day, as many individual snails as possible were recovered. A second nail, of the same color, was used to mark the new location of each individual (day mark). Distance and direction traveled from the previous day's mark (colored nail) as well as the microhabitat occupied were recorded dally for each individual located. Distance and direction from the home mark was also recorded to determine a "home area" measurement. For those individuals that moved, the day mark was repositioned at the snail's new location.