BULLETIN OF MARINE SCIENCE, 34(2): 185-196, 1984

HABITAT SELECTION IN TWO INTERTIDAL SNAILS,

Richard V Bovbjerg

ABSTRACT Adult and 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 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

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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 . . ~.. "...

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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. Four replicate sets of 20 snails of each species over 3-day period. Night record at 2000 h and day record mean of 0800, 1200 and 1600 h. Controls under normal photoperiod and experiments under constant darkness. Data are number active at time of recording

Day Night

Control EJ\.periment Control Experiment N. versicolor Sum 1.3 3.5 106 101 Mean 0.11 ± 0.07 0.29 ± 0.16 8.83 ± 1.73 8.42 ± 1.52 % 1% 1% 44% 42% N. tessel/ata Sum 3.1 8.6 86 99 Mean 0.23 ± 0.06 0.72 ± 0.17 7.17 ± 1.42 8.25 ± 1.71 % 1% 4% 36% 41%

There were both similarities and differences between the responses of the two species to certain physical factors. Both species were nocturnal and wandered, apparently grazing, on any wet surface; N. tessel/ata were submerged in tide pools more than N. versicolor. During the day, some N. tessellata were active in tide pools but most were quiet in crevices at the water's edge or in small pockets of water. N. versicolor was seldom active during the day; they were usually wedged in crevices or even on exposed dry rock. Both species seemed to be photonegative and thigmopositive. They found dark crevices. On sloping surface, N. versicolor was higher than N. tessellata. Both species responded by upward movement when placed in the deeper water of tide pools. The most striking difference between the two species was the response to tidal fluctuation. Day and night, they both moved upward with the tide but N. tessellata moved less so during the day. N. versicolor seemed actually driven upward by rising water; a line of snails could be seen just above the water line and isolated rocks were capped with tight clumps. Both species descended with receding waters at night and they moved downward with receding waters in the day but stopped in the nearest crevice or pit.

EXPERIMENTAL STUDIES Vertical Migration in a Tidal Basin. - Both species were observed in a concrete tidal basin (30 x 15 x 3 m). The surface was essentially smooth but was algal encrusted. Marked snails were followed and mapped, day and night, over a 48 hour period. (N = 200 snails; 3,424 positions were recorded.) The differences noted in the field were documented. Both species oscillated with the tide, N. tessellata never far above or below the water line; but N. versicolor migrated less precisely and always higher than its congener. At night, N. versicolor ranged far higher. During the day, with tide out, both species were very quiet, N. versicolor higher than N. tessellata. However both species migrated upward during the day with rising tide. Both species were actively grazing at night, tide in or out; many N. versicolor moved very high on the wall, but descended before dawn. There was extensive lateral movement in 2 days, a maximum distance of 39 m for N. versicolor and 23 m for N. tessellata. Photoperiod. - In these and subsequent experiments, conclusions are based on large numbers of observations, needed in behavior work of this sort. When dif- BOVBJERG: HABITAT SELECTION IN TWO NERITID SNAILS 189

Table 2. Summary data: Differences in response to light and shade; 40 replicate sets of 20 snails of each species

X. \'('rsicolor N. fessel/ala

Dark Light Dark Light Sum 635 165 691 109 Mean 15.88 ± 0.48 4.12 ± 0.48 17.28 ± 0.40 2.72 ± 0.40 % 79% 21% 86% 14% ferences between species are cited, they are significant; P = 0.0 I or less, based on Chi-square tests. Field observations suggested an endogenous daily rhythm by both species, including apparent anticipation of dawn and dusk. To test this, snails were ob- served in continuous darkness for 3 days as well as under normal photoperiod, the control. The laboratory tanks (55 x 35 x 30 em) were awash with running sea water. The criteria for activity were foot extension or tentacular probing. A total of 320 snails was used and records were made of both activity and position, 7,680 total recordings (Table I). The data support the hypothesis of photoperiodism as an endogenous rhythm for both species. For N. versicolor, there was almost no activity during the day in experimental or in control sets. At night 44% were active in the controls and 42% in the experimental sets; this was an insignificant difference (P = 0.73). The data for N. tessellata were similar. During the day I% were active in the controls and 4% in experimental sets; at night it was 36% and 41%; again the difference was insignificant (P = 0.44). The rhythm did not dampen over the 3 days of total darkness. Response to Light and Dark during Daylight. - Field evidence suggested that both species do find crevices and pits in the rock surface; they clump in the corners of tanks in the laboratory. However, such crevices are also darker, suggesting po- tential negative photoresponse. To test this, white enameled pans (25 x 20 x 6 cm) were placed on an outside

Table 3. Summary data: Responses to deep water by vertical movements: 30 replicate sets of 20 snails for each species during the day and 10 sets of each species at night

Out of Water Top III Lower 112 Bottom

N. versicolor Day Sum 418 8 5 169 Mean 13.93 ± 0.84 0.27 ± 0.12 0.17 ± 0.09 5.63 ± 0.82 % 70% 1% 1% 28% Night Sum 143 7 3 47 Mean 14.3 ± 1.63 0.7 ± 0.26 0.3 ± 0.21 4.7 ± 1.51 % 72% 3% 2% 23% N. lessellala Day Sum 26 478 33 63 Mean 0.87 ± 0.27 15.93 ± 0.54 l.l ± 0.25 2.1 ± 0.35 % 4% 80% 5% 11% Night Sum 7 109 37 47 Mean 0.7 ± 0.52 10.9 ± 0.92 3.7 ± 0.72 4.7 ± 0.91 % 3% 55% 19% 23% 190 BULLETIN OF MARINE SCIENCE, VOL. 34, NO.2, 1984

Table 4. Summary data: Response to 5° slope under three conditions of light and dark. (Data based on position up-slope or down-slope relative to release point in center of tank)

Sunlight nnd Sunhght· Dnrknesst Darknesst No. % No. % No. ~o N. versicolor Up 325 8.13 ± 0.29 81 121 6.05 ± 0.44 60 41 1.03 ± 0.18 10 Down 75 1.87 ± 0.29 19 79 3.95 ± 0.44 40 359 8.97 ± 0.18 90 N. tesse/lata Up 217 5.43 ± 0.35 54 73 3.65 ± 0.43 37 48 1.20 ± 0.23 12 Down 183 4.57 ± 0.35 46 127 6.35 ± 0.43 63 352 8.80 ± 0.23 88 • Day experiments in full sunlight at right angles. 40 replicates of 10 for each species. t Dark experiments at night. 20 replicates of 10 for each species. * Day experiments with upper half in full sunlight and lower half of lank covered. in darkness. 40 replicates of 10 for each species.

bench at right angles to the sun. One half of the pan was covered and the other half was in full sunshine. The position of snails was recorded after 1 h. Forty replicate experiments with 20 snails were done with each species, 1,600 obser- vations (Table 2). Both species were predominately in the shaded half of the pan after 1 h, for N. versicolor 15.9 (80%) to 4.1 (20%); for N. tessel/ata it was 17.3 (87%) to 2.7(13%). The difference between means in each species was significant (P = 0.001). The behavior was a kinesis rather than a taxis. Snails did not seem to perceive the dark and then direct their locomotion toward that region. Response to Water Levels.-Both field and tidal basin data suggested differences between the two species in their response to water levels. Two sets of laboratory experiments were done testing the response to submergence in deep water as opposed to response to a dry tank. A total of 1,600 snails were observed over I-h periods. Upward movement differed in the two species. N. versicolor promptly moved up through 25 cm of water and out onto the dry sides of the tank. Response was . the same day and night, 70% in the day and 72% in the night. N. tessel/ata also migrated vertically but formed in a line just below the water surface. In the day only 4% were out of water, while 86% were up on the sides below water, almost all just below the water line. At night the migration was less pronounced. Again only 4% moved up out of water and 73% were on the sides of the tank below water with a less dense band up at the water's edge. So, while both species had a strong tendency to ascend when under 25 cm of water, only N. versicolor left the water, confirming the field observations. The differences between species on mi- gration out of water were significant (P = 0.001) (Table 3). Since N. versicolor was seen on dry rock surfaces between tides, response to dry surface was tested. Eight groups of 20 of each species were observed in dry laboratory tanks at 4-h intervals from noon to noon in a normal photoperiod, N = 320 snails. All snails of both species became active immediately on being wetted after the 24-h period, but there was remarkably little movement vertically or horizontally, N. versicolor slightly and N. tessel/ata not at all. The important response is the clamping on to the surface with consequent reduction of water loss until flooded again by the next tide. Response to Slope. - While it is clear that the two species migrate in different ways BOVBJERG: HABITAT SELECTION IN TWO NERITID SNAILS 191 on a vertical wall, it remained to test response to a gentle slope. A 5° slope was selected for experiment, a usual slope in nature. A rectangular aquarium (44 x 22 x 26 em) was propped to the 5° angle. Water was 3 em at the lower-end and the bottom was awash at the upper end. Light was at right angles to the length of the aquarium. Initial controls were done in the aquarium with no slope; no particular bias was shown except the usual tendency to aggregate in corners. In the experiments, 10 snails were placed in a row across mid-tank facing in alternating directions; their position was recorded an hour later. This was repli- cated 40 times for each species, a total of 800 snails (Table 4). N. versicolor moved up the slope but N. lessel/ala moved equally in both directions. For N. versicolor, 325 (81%) moved up-slope and 75 (19%) moved down-slope, a significant difference (P = 0.000 I). The corresponding data for N. lessel/ala were 217 (54%) and 183 (46%), an insignificant difference (P = 0.09). Almost half of the N. versicolor were at the extreme up-slope end, while N. lessel/ala had equal numbers at the two extreme ends. There remained the question of response to slope at night. An additional set of 200 snails was tested. N. versicolor oriented to slope in darkness as it did during the days but the up-slope movement was less pronounced at night with the increased general activity; 121 (60%) were up-slope and 79 (40%) were down- slope, still a significant difference (P = 0.00 1). Day or night, N. versicolor appears to respond positively to even a gentle slope. For N. lessel/ala a surprising reversal occurred at night; now the snails went down-slope. The data were, 73 (37%) up-slope and 127 (63%) down-slope, a significant difference (P = 0.00 1). The down-slope response at night suggests that they go down into the tide pools, which they indeed do. Since crevices are combinations of both darkness and down-slope cues to snails, the slope experiments were combined with the dark-light experiments. The same tank was used, with the same slope and water level, but with the down-slope end covered with heavy black cloth. Another set of 800 snails was tested. Both species responded in identical fashion; they moved down-slope into dark- ness. For N. versicolor, the data were: 359 (90%) down-slope and dark, the cor- responding figure for N. lessel/ala, 352 (88%). The data are significant for both species (P = 0.001). This reversal in behavior in the presence of dual stimuli could mean that response to darkness takes precedence over the tendency to move up-slope, or that there is a synergistic effect in that the stimuli are sorted and acted upon in a new way. Figure 4 summarizes the slope experiments.

DATA SUMMARY The data presented from both field and laboratory have been diverse and nu- merous, approximately 24,000 observations. It would be useful to summarize these observations before discussing their significance to habitat selection. Zonation exists, but with extensive overlap; N. versicolor inhabits a band higher in the intertidal than N. lessel/ala. In field manipulation it was seen that both species actively select their habitat, even when released in reversed zones. There are several factors which do not seem to be immediately important in this zonation: water chemistry, surf, temperature, predation, competition, food, substratum and recruitment of juveniles. Both species are crevice inhabitants, as seen in the field and in all experiments; this leads to aggregations, sometimes of mixed species. 192 BULLETIN OF MARINE SCIENCE, VOL. 34, NO.2, 1984

In both the field and in experiments, migration can be seen on vertical slopes but N. versicolor ascends higher, often far out of water, and descends less during the day. It is the more motile species. In nature, N. tessellata is seldom deeper than 0.5 meter and N. versicolor seldom deeper than instantaneous water level. Vertical migration is regulated by tide and photoperiod. Both species are capable of extensive lateral movement within zones. Both species are nocturnal. Both species find the darker sites during the day. Both species graze on wet surface at night. In the day N. tessellata seldom moves on dry surface while N. versicolor may. Ascent of a gentle slope was demonstrated for N. versicolor during the day or night, but not for N. tessellata. Both species responded positively toward the combined stimuli of down-slope, darkness, and contact during the day; this led to crevice occupancy.

DISCUSSION What has been described and examined in this paper is active habitat selection. These snails are motile, capable of moving several meters per day. In their con- tinual dispersal they find their locally optimum zone by receiving cues, filtering them and responding with adaptive behavior. These cues are apparently light, tide and slope. The behavioral responses are sufficient to explain the observed zonation. That this zonation is based on behavior rather than on something inherent in the rocky shore is supported by the observations of Coo mans (1969) that three neritids may be found in West Indies mangrove lagoons where they stratify on mangrove prop roots in the same order as seen on their usual rocky shore habitats: N. tessellata at the water's edge, N. versicolor higher and N. peloronta highest. Initially, it is best to consider response to these physical factors on a vertical slope, though such vertical slopes are only on scattered large rocks, in the Pigeon Key intertidal habitat. The following scheme is derived from experiments but it is firmly consistent with field observation. RISINGTIDE: DAY. Both species ascend with water level, N. versicolor above and N. tessellata below. FALLINGTIDE:DAY.N. versicolor clamps on rock or finds crevice; some descend. N. tessellata descends with tide; some clamp on rock or find crevice. Stratification results. RISINGTIDE: NIGHT. Both species ascend with water level, active wandering and grazing on wetted algae. Stratification tends to blur. FALLINGTIDE: NIGHT. Both species continue grazing on wet rock. Some N. versicolor but most N. tessellata descend at dawn; stratification again apparent at dawn. There remains the translation of this effect from the vertical to the gentle horizontal slope found in the field. There is, in a sense, a refraction from the vertical to the oblique and snail stratification in turn is so refracted to zonation (Fig. 5). Conceptually, decimeters of vertical distance translate to meters on a 50° slope. This is the key to horizontal zonation. The problem of dispersion overlap remains. Two reasons for this may be proposed; there is behavioral overlap in the two species, and the substratum is confused and of variable slope. In complex behavior, variability is the rule and that is the case here. BOVBJERG: HABITAT SELECTION IN TWO NERITID SNAILS 193

Figure 5. Schematic refraction of tidal amplitude from a vertical to sloping surface with shift from snail stratification vertically to zonation horizontally.

Also, these species have the same nocturnal habit which directly results in over- lapping nocturnal dispersion; they graze side by side on wet surfaces. A second reason for dispersion overlap is the snail response to the confused substratum. Only occasionally are there smooth, sloping surfaces such as depicted in Figure 5. The substratum is pitted and jagged. Tide pools are bordered by various slopes, often containing large protruding boulders. Cup sized pits abound and both deep and shallow cracks go in all directions. The overall slope of the shelf itself varies from 5° to almost no slope. Mixed responses by snails to con- tradictory cues could result in dispersion overlap. We are left with a picture of zonation different from that of sessile forms whose zonation is so dictated by physiological tolerance, especially by those factors related to length of exposure between tides and by spatial needs in short supply. The zonation of these snails is fluid; these are overlapping populations of snails which are perceiving, responding and moving. Behavioral differences result in a zonation, so striking during the day at low tide when they are both exposed and visible; the zonation at night is far less obvious.

IMPLICA nONS Causation in dispersion, in this case a zonation, is intricate and may be analyzed at 3 levels: (1) The immediate cause is habitat selection at any stage of the life cycle and this level has a strong behavioral component of perception and response. (2) The ecological cause has a strong physiological component over ecological time; the needs and the tolerance ranges of the species define the local optimum, the framework within which habitat selection operates. (3) The ultimate cause is evolution, the selective forces molding the species over its entire range over geological time. The work reported in this paper was directed at level one, the immediate cause of habitat selection resulting in zonation. Selection of local optimum must be a complex response based on multiple stimuli (Bovbjerg, 1975). lander (1975) makes "summation of the positive and negative stimulus values of different in- tensities" an axiom in spatial orientation. The neritids here in the intertidal zone found their local optimum by responding to the algebraic sum of several stimuli . . In synergIsm. This circumtropical genus has been studied outside of the Caribbean; species differ but zonations are ecologically equivalent. In the Great Barrier Reef of 194 BULLETIN OF MARINE SCIENCE, VOL. 34, NO.2, 1984

Australia, a great deal of work has been done (Frank, 1969; Zann, 1973; Under- wood, 1975; 1976a; b; c; 1977; 1978). Both circadian and circatidal patterns were seen as well as zonation related to exposure and algal zonations. In Japan, N. japonicum wandered up to 2 m/day with negative responses to light and gravity (Suzuki, 1935). In the Indian Ocean, N. plicata showed a photonegative response, resulting in orientation away from the sea onto the shore in Kenya (Warburton, 1973). In Somalia, Vannini and Chelazzi (1978) described the dispersion and behavior of N. textilis which migrated daily on a vertical cliff. Members of the genus are reported in four species zonations in South Africa, East Africa and Mozamabique (Stephenson and Stephenson, 1972). Three species were found zoned in the Red Sea (Safriel, 1969) and five species are zoned in Aldabra Atoll (Hughes, 197Ia). The latter demonstrated vertical stratification on cliffs, but also horiziontal zo- nation on slopes; all but one species were crevice dwellers. From the eastern Pacific, Keen (1958) reports N. scabricosta and N. funiculata from the rocky intertidal of the Gulf of.California south to the equator. In an unpublished study of mine on rocky intertidal zonation of this Panamic Province, these two species were often dense. The larger N. scabricosta were in a higher and more narrow band than N.funiculata which was found down to low water in tidal ranges from I to 5 m; zonation ordinarily overlapped. The ecological equivalency to the two Aorida neritids is striking, including horizontal zonation on gentle slopes. The similarity extends to behavior. N. scabricosta (like N. versicolor) fol- lows the ebbing tide down, then recedes before the incoming tide; N. funiculata (like N. tessellata) keeps below the water level (Bertness et aI., 198 I). Little work has been done at the second level of causation, physiological ecology. Since these snails are found in their characteristic zones, it is a truism to state that their needs are met and that they can tolerate the extremes of their environ- ment. Mattox (1949) found direct correlation with zone and survival in air. N. tessellata survived 17 days without immersion, N. versicolor 36 days, and N. peloronta 77 days. Coleman (1976) measured oxygen consumption ofthree zoned neritids of Australia and found no differences. Hughes (197Ib), in the Barbados, found no differences in respiration between N. versicolor and N. tessellata, nor differences in and out of water. However, Lewis (1971) found that N. tessellata, N. versicolor and N. peloronta all reduced respiration rate after exposure to air; N. peloronta, in the highest zone, dropped the least. All three species increased respiration rate with increased temperature. Adaptive differences in tolerance to lower salinities has been demonstrated for these same three sequentially zoned species (Russell, 1941). There do not seem to be seasonal differences in behavior or zonation in these tropical species. Burkett (Univ. of Miami, personal communication) has seen some of the behavior and patterns described in this report during the summer months. We can only ponder at causation at the third level, evolution. Is this habitat selection an evolved adaptation? The term adaptation has been applied to count- less plausible examples, but three conditions should be verified for a more rigorous definition. The first is repeatability, but more importantly, the trait must be hereditary and confer survival value. The zonation of the Caribbean neritids meets the first condition of repeatability; where they have been found, they zone. For neritids elsewhere, similar zonations have been noted. Neritid habitat selection is surely hereditary. No genetic experiments have been done with reared snails, but Partridge (1978) aptly suggests: "If two animals are BOVBJERG: HABITAT SELECTION IN TWO NERITID SNAILS 195 reared in identical environments and differ in their habitat preferences, then the differences must be hereditary." This fits the two species studied in the Keys. The modal but variable adult behavior suggests polygenic traits. The third condition of adaptation, survival value, remains to be tested. It is doctrinaire to propose partition of resources, and the amelioration of competition. But there are conceptual difficulties in selection dynamics when considering the evolution of a mutually advantageous resource partitioning. For a marine snail, however, it would seem that the very high zone of N. versicolor is the departure from the marine habitat; it then becomes more credible to suggest survival value of zonation for this species alone. Simple field experiments are crying to be done; remove all but one species at a time and trace subsequent growth and dispersion. Potential for a diffuse gastropod competition exists in the tide pools where the juvenile neritids forage with large numbers of Battelaria minima and Cerithium variabile. At this time and place it does not appear that food is in short supply. But it may well be that there is survival value to the zonation of N. versicolor in other parts of its range where food could be in short supply. Since these two neritids are widespread in the Caribbean and they disperse as planktonic veligers in currents, there must be gene flow between local populations. Scheltema (1971) cites Mid-Atlantic plankton hauls with veligers from neritids which have a larval existence of 2 months. Whatever the past selection process or selection elsewhere, there is currently a zonation that does achieve a measure of resource partitioning. And what of the nocturnality and crevice seeking behavior of both species? The obvious explanations are desiccation reduction and predator avoidance. Vermeij (1978) cites crabs and puffers as neritid predators; the puffers are able to feed on N. versicolor up to 23.5 mm. However, neither N. versicolor nor N. tessel/ata show as many shell repairs as Pacific neritids. Garrity and Levings (1981) found that N. scabricosta (ecological equivalent of N. versicolor) was 49% of the diet of Purpura pansa. So, while predation was not prominent at Pigeon Key, the crevice dwelling trait among members of the genus around the world suggests some importance as an adaptation of predator avoidance. The maintenance of zonation by these neritids is probably characteristic of the general category of motile animals that find and maintain position in the intertidal region (Newell, 1970). Whatever the survival value may be, there must be an array of stimuli, sensory perceptions, neural integrations and responses of the kind described in this paper leading to habitat selection.

ACKNOWLEDGMENTS

The University of Miami and Pigeon Key staff were most helpful during these studies. Henry Howe kindly read a first draft of this paper.

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DATEACCEPTED: November 16, 1982.

ADDRESS: Department of Zoology, The University of Iowa, Iowa City, Iowa 52242.