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

Wetting Experiment

A wetting experiment was conducted to estimate the probability of snails moving under different environmental conditions (dry-substratum, freshwater moistened substratum, and seawater moistened substratum) (modified from Gochfeld & Minton 2001). Forty-five individuals of each species were collected from their respective mid- and high-shore zones at Site 1. Five snails were randomly assigned to each of the three treatment groups (15 total) and placed on an outdoor testing surface (concrete platform). Ten minutes prior to testing, each group of snails was placed onto one of three 50 x 50 cm treatment squares that were drawn on the testing surface. Five minutes prior to testing, the snails were moved to the middle of their respective squares and left untouched for the duration of the test. One of three treatments was randomly applied to each group. Treatments tested were dry-substratum (control), 250 mL of fresh water, and 250 mL of saltwater. Freshwater and saltwater treatments were designed to represent 1 mm of rainfall and saltwater inundation and/or spray, respectively and were applied evenly to the entire treatment box using a perforated-capped water bottle. Specimens were observed for periods of 10-min during which the total number of individuals exhibiting movement was recorded. The experiment was replicated three times for each species (15 snails x 3 replicates = 45 individuals of each species). A weighted-least-squares analysis was used to model the probability of moving under the various treatments via the CATMOD procedure in SAS (Cary, North Carolina).

RESULTS

Site and Shore Level Characterization

The rocky shore habitats of mid and high-shore species were similar with respect to substrate morphology (measured by rugosity) and area (zone width; Table 1). The substratum surface in both zones was rough, weathered rock with many small pits and sharp edges. In contrast, mid- and high-shore zones differed significantly with respect to desiccation and heat stress. Rock temperature increased significantly with increasing shore height as did desiccation potential, measured as the amount of seawater lost by evaporation (or gained from sea spray) using the sponge-survey method (Table 1). On average, sponges in midshore zones absorbed an additional 0.27 g of seawater, whereas sponges in the high-shore zone lost 6.97 g of seawater (Table 1). No wave splash and spray were collected in high-shore zones during the 3-day survey period, whereas a total of 337 mL were collected in low-shore zones. The lack of wave splash and spray in high-shore areas precluded comparing zones by ANOVA, yet demonstrated the dry conditions prevailing in high-shore habitats.

Daily Movement and Direction

All 60 snails marked at Sites 1 and 2 (15 mid and high-shore specimens at each site) were subsequently recovered at least once over the course of the 12-day observation period. A total of 593 (86%) of the 690 possible observations (30 individuals x 11 days at Site 1 30 individuals x 12 days at Site 2) were recorded over both sites. Similar numbers of mid and high-shore individuals were subsequently recovered during surveys (e.g., 84% of N. versicolor and 89% of T. antonii).