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

Average daily movement rates were similar among snails within mid and high-shore habitats at both sites (t = 0.61, DF = 559, P = 0.544), thus these data were pooled to analyze the effects of shore position. N. versicolor and T. antonii moved during 94% and 53% of the daily intervals examined, respectively. The mean daily movement rate of the midshore species (3.12 cm x [h.sup.-1]) was significantly higher than the rate of the high-shore species (0.27 cm x [h.sup-1]; t = 10.79, DF = 58, P < 0.0001).

Interestingly, a storm developed 6 days into the observation period (May 19, 2002) beginning with 2 days of rainfall followed by 4 days of increased sea state resulting increased wave heights and subsequent wetting of high shore zones. This provided a unique opportunity to examine the effects of increased wave action on the behavior of midshore species and an increase in substratum moisture on behavior of the high-shore species. Using the same analysis, but introducing a new variable that divided the daily observations into normal and increased wave heights, the effect of shore position on the rate of movement was re-evaluated with respect to increased substratum moisture. The increased wave heights affected the movement rates of mid and high-shore species differently (Fig. 2). Under heavy sea conditions, the rate of movement in the midshore species was similar to that recorded during normal sea state, averaging 2.79 cm x [h.sup.-1] (t = -0.77, DF = 502, P = 0.444). In contrast, the high-shore species exhibited an increase in mobility to 1.63 cm x [h.sup.-1]. Although the locomotory rates of the high-shore species continued to exceed those of the midshore species (t = 2.83, DF = 502, P = 0.005), the mobility of T. antonii increased 6-fold over that observed during normal sea conditions (t = 6.83, DF = 502, P < 0.0001).

[FIGURE 2 OMITTED]

Daily directional movement was analyzed as a 2-factor experiment with respect to shore position and substratum moisture. Daily calculations having <10 observations were removed from the analysis as suggested by Underwood and Chapman (1985). Midshore N. versicolor exhibited directional movement to a higher shore position during the period of increased wave action, otherwise no specific patterns of movement were observed (Fig. 3). Prior to the wetting of the high shore, few movements (favoring no particular direction) were observed in high-shore T. antonii. As wave action increased, some directional movements were evident, hut followed no discernible pattern (Fig. 3).

[FIGURE 3 OMITTED]

Percentages of home area measurements falling into each of 4 categories (< 1, 1-3, 3-5, or >5 m) indicated that 29% of midshore individuals remained within 1 m of their initial mark, 55% ranged from 1-3 m from their home mark, and 16% ventured farther than 3 m. A few N. versicolor (8%) were observed to travel more than 5 m from their home mark with a maximum distance of 17 m recorded. In contrast, 83% of high-shore individuals were found within 1 m of their home mark and the remaining 17% never moved farther than 3 m.