BULLETIN OF MARINE SCIENCE, 34(2): 185-196, 1984 HABITAT SELECTION IN TWO INTERTIDAL SNAILS, GENUS NERITA Richard V Bovbjerg ABSTRACT Adult Nerita versicolor and Nerita tessellata occupy higher and lower but overlapping zones of the rocky intertidal coast of the Florida Keys. Field and laboratory experiments suggest that the zonation is an active habitat selection based on responses to largely physical factors of light, watcr depth and slope. Both species are nocturnal, photonegative and are crevice dwellers. Both species migrate with the tide but N. versicolor retreats higher above the rising tide. Evidence for competitive exclusion is lacking but the zonation, though blurred at night when both species wander, does achieve some measure of resource partitioning. This report attempts to establish causality in the zonation of congeneric snails of a gently sloping, rocky shore in the Florida Keys. The question is more knotty when it is appreciated that the environment is not zoned; it is rather uniform though the rock is furrowed and pitted. Nevertheless, these snails are motile and they select different habitats. The work was done at the University of Miami Field Station at Pigeon Key, past the midway point of the Florida Archipelago (24°42'N and 81009'W). The two species studied were Nerita versicolor, Gmelin, in an inner zone, and Nerita tessel/ata, Gmelin, in an outer zone. Both species were present in large numbers and shared the shore with two other neritids present at much lower densities: Nerila .fulgurans and the striking "bleeding tooth," Nerita peloronta. Pigeon Key is a very small, low island, an exposed patch of recent fossil coral. It is fringed with an intertidal shelf of coral rock with less than a 5° slope. The tidal range of about 0.5 m exposes this shelf out to about 10m where the ragged face of the shelf drops less than a meter to the subtidal. The ebbing tide leaves an array of pools over the entire shelf, from cup sized to larger tide pools. The water in this zone was refreshed twice daily and flowed freely through the zones of both snails with no temperature or salinity differences. Neither snail inhabited the extreme inner tide pools which become very stagnant and highly variable in temperature and salinity. Due to the presence of reefs and turtle grass shoals off- shore, the surf is gentle except in storms. This strip of intertidal shelf and its water is not conducive to enforcing zonation on its inhabitants. Nevertheless, the two neritid snails do occupy different zones. DENSITY, DISPERSION AND DISPERSAL Snail zonation is immediately apparent; only N. versicolor is found up to the high water line and only N. tessel/ata is found out to the edge of the shelf. Yet, more careful examination reveals a considerable overlap. Kolpinski (1964)1 de- scribed the zonation ofthese species to the north on Key Largo and Virginia Key; Stephenson and Stephenson (1950, 1972) found this pattern throughout the Keys, with some variability in density. Elsewhere in the Caribbean the same pattern has been seen for all four neritid species (Mattox, 1949; Lewis, 1960, 1971; Hughes, 1971b; Atsatt, 1972; Ekdale, 1974). 1 Kolpinski. M. C'. 1964. The life history. growth and ecology of four intertidal gastropods (genus A'er;/a) of southeast Florida. Ph.D. Dissertation. Univ. of Miami. 131 pp. Unpublished. 185 186 BULLETIN OF MARINE SCIENCE, VOL. 34, NO.2, 1984 •• L 10 I. Figure I. Number of snails/m2 in a I-m transect of the intertidal zone with a 5° slope. Black = N. versicolor (N = 68), white = N. lessellala (N = 71). Figure 2. Dispersion of two sets of marked snails after 4 days from release point T for N. lessel/ala (white dots) and V for N. versicolor (black dots). Dashed line = high water line and dotted line = outer edge awash at high water. N = 100. On Pigeon Key, densities were determined in two transects from the high water mark out to the shelf edge, one with a 50 slope and another with almost no slope. The work was done during the day with the tide out. N = 343 snails (Fig. 1). In the transect of greater slope, N. versicolor had a maximum density of 331m2 between 1 and 2 m from the shore, while that of N. tessellata was 241m2 between 4 and 5 m out. This dispersion was more characteristic than that of the very nearly level transect when there was more overlap between the two species. Here the rock was awash at high water out to 6 m. However, N. versicolor stopped there while N. tessellata ranged out to 9 m. These data agree with other studies. Kolpinski (1964)' cites densities of 401m2 for N. tessellata and 921m2 for N. versicolor at Key Largo with their modal densities 4 m apart. Lewis (1960) found 200 N. tessellata under one rock and McLean (1967) counted 220/m2 in the Barbados. Stephenson and Stephenson (1950) note that densities vary from Key to Key. Field experiments were done to test motility of the two snails and their ability to select their habitats. Fifty snails, tipped with paint, were released and their position was recorded after 4 days, N. versicolor was released 3.5 m from the shore and N. tessel/ata 5.5 m from the shore; these were the modal points for these species at this site. The experiment was repeated at the same site. Figure 2 maps the recovery site of each snail. Each species dispersed in all directions up to several meters. However, after 4 days, the two species had achieved the characteristic bimodal dispersion. After a month, some of these marked snails were still near the site but others were found all up and down one side of the island. A more severe test was devised; again snails were marked, released and recap- tured, but this time released in the reverse position-N. versicolor farther out (6 m) than N. tessel/ala (3.5 m). This time 100 of each species were marked and released at low tide and recovered the following day at the same time. Again the experiment was duplicated with another set of snails, N = 400. Recovery was good, 91 and 99% (Fig. 3). In only 1 day the two marked groups had threaded their way through each other and reversed their dispersion; all but 4 N. versicolor had moved inward and N. lessel/ala had moved outward more than inward. Again there was lateral move- BOVBJERG: HABITAT SELECT[ON IN TWO NERIT[D SNAILS 187 0,,,", l . ~.. "... ,•. j --- o It 40 n Figure 3. Dispersion of marked snails after I day; release at point R, the reverse of the usual zonation. No. released = 400; No. recovered = 375,. Figure 4. Response to gentle slope; position after I h in either the upper or lower half of tank on a 5" slope~ release at center. Three experimental conditions: full daylight, full darkness and upper half in full daylight with lower half covered. Means of 100 replicate sets, 10 of each species. N = 2000. ment and the dispersion pattern after I day strongly reflected the immediate topography of the rock surface which was riddled with pocks and fissures. N. versicolor found higher elevations, most of which were closer to the shore. NATURAL HISTORY Neritids have a basic similarity of structure but these two species are readily distinguished by the tooth arrangement of the shell opening. Seen from above, N. versicolor is a banded amber color while N. tessellata is a banded grey to a checkered black and white. To the human eye, both blend well with the back- ground. N. versicolor is slightly larger than N. tessellata. The mean adult size (shell measurement at greatest dimension) of N. versicolor was 20.8 mm (N = 81); N. tessellata was 18.8 mm (N = 120). The size difference is significant (P = 0.001). Kolpinski (1964)[ notes the same size disparity at other sites in the Keys. The smallest neritids studied were 10 mm; no juveniles of either species were found with the adults. They were finally discovered in the debris at the bottom of the larger permanent tide pools. They did not migrate up onto the rocks. Nothing in the biology of these species seemed critical to the zonation observed. An apparent abundance of food and space suggests that competitive exclusion is unlikely as a strong force on this island at this time, a conclusion shared with Kolpinsky (1964).1 Connell (1975) suggests that intertidal species do not reach densities sufficient to compete for resources, especially in species where the greatest attrition is in the larval stages. Only one predator was seen taking these snails, Octopus vulgaris. Wading birds and gulls did not appear to feed on these snails in the intertidal area. The fish and crabs in the tidal channels were too small to feed on adult neritids. Larger crabs, stomatopods and thaid snails did not get up onto the shelf. Both species appeared to graze in the same fashion, often side by side. Scrapings of the surface film in tide pools were examined and found to be largely of very fine marl particles and detritus. Also present were: sheets of algal cells, masses of fine and coarse algal filaments, flagellates, diatoms and nematodes. The contents offecal pellets were strikingly like those of the rock scrapings. Food does not seem to be limiting population density, nor does it seem to be different in the two species; nor does it seem to be a factor contributing to zonation. 188 BULLETIN OF MARINE SCIENCE, VOL. 34, NO.2, 1984 Table I. Summary data of photoperiod experiments.