Variations in density, shell-size and growth with shore height and wave exposure of the rocky intertidal snail, Calyptraea spirata (Forbes, 1852), in the tropical Mexican Pacific.

ABSTRACT This study evaluates the density, shell-size, and growth of the intertidal snail, Calyptraea spirata (Forbes 1852), in the beach of Teopa of Bahia Chamela, Jalisco, located in the tropical Mexican Pacific. The snail is one of the most important resources from the rocky shores of the region. Gastropod density and the size of the shell (maximum diameter and height) were estimated in the upper and lower intertidal zones and in wave-exposed and wave-protected areas of the beach on December 1999. Results showed that density is lower in the upper intertidal zone and increases in the lower intertidal zone (ANOVA, P < 0.05). The species was not registered in the supralittoral zone, while in the infralittoral zone it was observed to a depth of 3 m. There was no difference in the size of the shell between snails from the upper and lower intertidal zones (ANOVA, P = 0.30), although bigger shells (maximum diameter = 5.0 cm) were more frequently observed in the lower zone. Shell diameter was bigger in the exposed than in the sheltered areas of the beach (ANOVA, P < 0.05). The growth rate of snails smaller than 3 cm was higher (0.268 cm/month) than in bigger snails (0.077 cm/month) (ANOVA, P = 0.037). The ANCOVA test indicated that the least squares regressions of maximum diameter versus height of the shell correspond to differentiated morphologic responses to the wave intensity effect but no difference in these associations was detected in response to shore height.

KEY WORDS: density, shell-size, Calyptraea spirata, growth, Mexican Pacific

INTRODUCTION

It has been shown that wave intensity plays a major role in determining the abundance and vertical distribution of organisms living in the intertidal zone (Lewis 1964). In the Pacific shores, wave action intensity is particularly important because the extensive continuous surface area of this ocean makes longer and more intense waves relative to other oceans of the world (Ricketts et al. 1997). The rocky intertidal environment of the tropical Mexican Pacific includes a great variety of mollusks, gastropods being the most abundant and diverse. Many high energy rocky shores of this region are characterized by a dynamic topographic structure with a steep environmental gradient over short distances across the intertidal zone. Thus, the tidal regimen determines temperature, desiccation, and submersion time gradients, those that influence the organisms that inhabit intertidal shores (Levinton 1984). All these factors together with the biologic factors such as density or predation define the abundance and the zonation patterns of gastropod populations.

Differences in the size of the shell and growth of gastropods have been related to several physical conditions of the habitat including wave intensity (Brown & Quinn 1988, Denny 1994, De Wolf et al. 1999, Giraldo-Lopez & Gomez-Schouben 1999) and shore height (Vermeij 1972, Hobday 1995, Giraldo et al. 2002, Tanaka et al. 2002). Furthermore, density is considered responsible for intraspecific differences in shell size (Giraldo et al. 2002) and individual growth rate variability of gastropods (Haven 1973, Black 1977, Underwood 1978, Williamson and Kendall 1981).

The growth of mollusks have been traditionally estimated through the use of direct methods based on measurements of the size of the shell of individuals at different intervals of time (Gomez-Marquez 1994). The mark-recapture method is reliable to estimate mollusk growth (Hughes & Roberts 1980, Phillips 1981, Fletcher 1984, Chow 1987, Katoh 1989, Bowling 1994, Takada 1995, Iijima 2001). In the Mexican Pacific, differences in the growth rate of the wavy turban snail, Astrea undosa, have been related to wave intensity (Cupul-Magana & Torres-Moye 1996, Gluyas-Millan et al. 1999). Other studies describe variations in the size of the shell among populations living in areas with different wave exposition. In populations of the rocky intertidal snail Thais sp. from Baja California, the shell is shorter and heavier in exposed beaches and longer and slender in protected beaches (Kitching 1976). Similarly, the shell of the limpet Siphonaria gigas is wider when living in more wave exposed rocky beaches (Giraldo-Lopez & Gomez-Schouben 1999), and the size of the chiton Katharina tunicata from northern California may be smaller in areas with stronger wave action (Stebbins 1988).

In the coast of Jalisco, Mexico, there are few studies on growth and shell size of snails. Probably, the purple snail Plicopurpura pansa is the only species studied so far (Rios-Jara et al. 1994, Michel-Morfin et al. 2002). One of the most important potential resources from the rocky shores of Jalisco is the so-called "gorrito" (small cap) snail Calyptraea spirata (Forbes 1852). The snail is easily distinguished for its conical limpet-like shell with large, coarsely ribbed, whitish or grayish outside, dark brown within without operculum. Its distribution ranges from Mazatlan to the Gulf of Tehuantepec, Mexico (Keen 1971) and the Gulf of California (Morris 1966). In the coast of Jalisco, the species has been registered in the beaches of El Tamarindo, La Calechosa, Los Angeles (Gonzalez-Villarreal 1977); Yelapa (Fonseca Madrigal 1998); Bahia Tenacatita (Hernandez 1998), and in Bahia Cuastecomate (Esqueda et al. 2000). The muscular foot of C. spirata is most esteemed by fishermen who used it for food. The species has also a cultural traditional value in the region. It was used as a burial offering in pre Hispanic funerals recovered in the archaeological site of Salagua, Colima, dated between 900-1530 years B.C. (Fieldman 1968). Recently, local populations have been diminished and in some beaches the species has disappeared, probably because of the intense fishing and deficient management activities. Although the taxonomic composition and abundance of rocky intertidal mollusk communities from the coast of Jalisco is well known, only a few studies focus on certain species, particularly on the population ecology of Plicopurpura patula pansa, a commercially important rocky intertidal snail (see Rios-Jara et al. 1994, 2001, Perez Pena & Rios-Jara 1998, Esqueda et al. 2000 for a list of this research).

There are no previous studies on the population ecology of C. spirata. Therefore, the major objective of this study is to evaluate the abundance, vertical distribution, size, and growth of this species in Bahia Chamela, Jalisco, in the tropical Mexican Pacific. The study examines a population located in the beach of Teopa, in the southern part of the bay, which is well represented in terms of abundance, distribution, and size of the organisms. The size of the shell and the rate of growth were estimated in the upper and lower intertidal levels and at two different wave exposition conditions. This study provides basic knowledge concerning the factors that affect the growth and development of the species, which may be used to suggest future recommendations needed for the sustainable harvesting of the snail.

MATERIALS AND METHODS

Study Site

Chamela Bay is located in the central portion of Jalisco, Mexico. The bay spreads 23.3 by 3 km between Punta Rivas and Punta Farallon (19[degrees]34'50"N to 19[degrees]23'20"N, 105[degrees]117'30"W to 105[degrees]01'40"W) and has a surface area of approximately 6,690 hectares. The region has warm-wet climate with the rainy season occurring mostly daring the summer. Annual temperature ranges from 32.3[degrees]C in September to 20.6[degrees]C in January (mean = 25.2[degrees]C). Cumulative monthly precipitation ranges between 587.5 and 967.3 mm. September records the highest precipitation values with 301.7 mm and February the lowest with 1.6 mm (Garcia 1973). There is a mixed tidal cycle with two unequal high and two unequal low tides each day.

The shoreline of the bay is composed of sandy and rocky beaches. The rocky beaches usually have solid blocks sometimes mixed with fixed or loose boulders and pebbles. The rocky beach of Teopa (19[degrees]23'33"N, 105[degrees]01'24"W) is located in the southern portion of the bay. The topography of the beach is very irregular with steep and smooth surfaces and heterogeneous rock platforms with water retaining cracks, crevices, and tidal pools. This irregular profile (slope = 60[degrees] to 90[degrees]) has many habitats with different degrees of exposure to wave action from sheltered to very exposed conditions through the intertidal zone. This beach was chosen because the population of Calyptraea spirata is well represented in terms of abundance, distribution, and size of the organisms. Preliminary observations were necessary to characterize the zonation of conspicuous species in the intertidal where C. spirata lives. Intertidal levels were determined using sessile forms, particularly macroalgae, crustaceans, and mollusks.

Methods

The beach of Teopa has a narrow intertidal zone (3-5 m). Two different levels were determined (upper and lower intertidal) for the study of gastropod density, shell size, and growth. At the same time, 2 areas of the beach with different degrees of exposure to wave action were chosen: (1) a more sheltered part of the beach with relatively less wave intensity, and (2) a very exposed surf sweeping area with strong wave action. Sample size was estimated previously according to the cumulative means technique (Elliott 1977). For this technique, the cumulative mean density of snails was plotted as a function of the cumulative number of replicate samples or the cumulative area sampled. As the number of samples increased, the fluctuation of the mean decreased. The number of replicates was considered sufficiently large when such fluctuations were so slight that the cumulative mean became insensitive to variations in the data.

Density and size structure of the population found within the upper and lower intertidal zones and two wave exposure conditions (exposed and protected) were estimated during low tides in December 1999. The plot method was used for density estimations (Brower & Zar 1977). Line transects of 30 m and square plots (quadrats) of 0.25 [m.sup.2] were used. The procedure involved placing a series of 30 quadrats along each transect line, 15 on each side. The line was marked and numbered every 50 cm (60 numbers on each side) and a table of random numbers was used to choose the position of the quadrats along the transect. The number and size of snails found within each quadrat were estimated. The optimal sample size (25-30 quadrats) suggested by the cumulative means technique was used. A two-way ANOVA (P < 0.05) was used to determine whether the mean density (ind./[m.sup.2]) differed between intertidal zones and among the two wave intensity conditions. A posthoc test (Dunnet) was used to contrast mean densities estimated in the upper and lower intertidal zones and the protected and exposed areas.

The snails were measured to nearest 0.1 mm with a hand vernier caliper (maximum diameter and height of the shell). Maximum diameter values were used to make histograms of the size frequency distributions. Bimodality was tested by applying a 5% level of significance hypothesis test (Reschenhofer 2001). Differences in the size of the shell related to tidal level and wave exposure conditions were tested using a two-way ANOVA (P < 0.05). The normality and the homocedasticity of data (Bartlett's test) was examined, and the logarithmic transformation (log X + 1) was performed when necessary (Zar 1999). If these conditions were not fulfilled, the non parametric mean comparison test of Kruskal-Wallis was used. The relationship between maximum diameter and height of the shell of individuals from different tidal levels and wave exposure conditions was described with least-squared regressions (Statistica Program SYNTAX 0.5 for Windows). The resulting equations were contrasted using an ANCOVA analysis (SPSS and SigmaStat Programs).

The growth rate was estimated during monthly samplings from October 1999 to May 2000 at the same areas and intertidal levels of the beach of Teopa where gastropod density and shell-size were studied. During the first sampling event, the shells of 300 snails from the upper and lower intertidal levels were measured (maximum diameter of the shell). Each individual was marked using plasticized epoxy glue (Plastiloca) and labels with printed progressive numbers. The labels were then coated with transparent instant nail varnish to preserve the numbers. All snails were released to their original position. The following months, marked individuals were measured and the numbered labels renewed when necessary. Increments in the size of the shell were documented and expressed in cm/week. New shells were labeled when the number of marked individuals decreased, so the total number was always similar to the original. Sometimes, the somehow lost or not found snails were found again in later samplings. Individual growth rates (G) of the recaptured snails were estimated using the equation (Iijima 2001):

G = [DELTA]X/X (30/T)

where T is the time (month) from the last release to recapture, X the maximum diameter of the shell at the last release. [DELTA]X the increment in maximum diameter during the period T.

For comparative purposes, snails were divided in 2 groups: Group 1, included snails smaller than 3 cm of maximum diameter and Group 2, those snails of more than 3 cm of maximum diameter. A one-way ANOVA (P < 0.05) was performed to compare the growth rate of both groups of snails.

RESULTS

Mean density of Calyptraea spirata, increases from the upper to the lower level of the intertidal zone (Fig. 1). The density was lower in the upper intertidal zone compared with the lower intertidal zone of both the protected and the exposed areas (ANOVA, Dunnet test, P < 0.05). The species was not registered in the supralittoral zone in any area of the beach, whereas in the infralittoral zone it was observed (but not sampled for density estimations) to a depth of 3 m. Snails in the infralittoral were also attached to rocky substrata, defining a wide vertical distribution from the upper intertidal to the shallow infralittoral zones. Within the same shore height (upper or lower intertidal), no significant variation in gastropod density was detected between the sheltered and the exposed parts of the beach (ANOVA, P > 0.05).

[FIGURE 1 OMITTED]

Histograms of the size frequency distributions of individuals show at least one mode in upper intertidal zone of both the protected and the exposed areas of the beach (Fig. 2). In the protected area, the mode extends along all size ranges of the histogram, from 0.4-5.0 cm; in the exposed area, the mode includes the ranges of 1.0-4.5 cm. According to the hypothesis test (P < 0.05), histograms of individuals from the lower intertidal are bimodal. In the protected area, the modes occur in the size ranges of 1.0-1.5 and 3.1-3.5 cm of shell diameter; in the exposed area, the modes occur in the size ranges of 1.6-2.0 and 3.1-3.5 cm. During the study, the diameter of the shell ranged from 0.40 cm the smallest individual to 5.0 cm for the largest individual measured. The mean diameter was found to be 2.86 [+ or -] 0.75 mm ([+ or -] one standard deviation). The most common repeated value of diameter was 3.1 cm (by 3.5%), 3.0 cm (by 2.8%), and 2.8 cm (by 2.5%).

[FIGURE 2 OMITTED]

The homoscedasticity condition was not fulfilled by shell diameter even with logarithmic transformation (Bartlett test, P < 0.01), so the non parametric test of Kruskal-Wallis was used to detect differences in shell size. Although the biggest individuals were always found in the lower intertidal zone, no significant difference in shell diameter between snails from the upper and lower intertidal zones was detected (ANOVA, P > 0.30) (Table 1). However, the shell-diameter of C. spirata was bigger at the most wave-exposed areas compared with the wave-protected areas of the beach (ANOVA, P < 0.05).

Individual growth rates did not varied greatly between individuals from different shore heights and wave exposure conditions (Table 2). The growth rates were significantly different in only 2 situations: (1) when comparing all snails of group 1 versus group 2, the growth rate of individuals smaller than 3 cm (Group 1) was significantly higher than the growth rate of larger individuals (Group 2) (ANOVA, P = 0.037); and (2) in the case of snails of group 2 inhabiting the upper intertidal, results indicated that there is a significant difference in the growth rate between exposed and protected conditions (ANOVA, P = 0.018).

The least squares regressions of maximum diameter versus height of the shell were examined by shore height and by wave exposure conditions (Fig. 3). All associations were significant (P < 0.05). The ANCOVA test indicated that these associations correspond to differentiated morphologic responses to the wave intensity effect (P < 0.05), whereas regressions calculated in the two different intertidal zones were not significantly different (P > 0.05).

[FIGURE 3 OMITTED]

DISCUSSION

The intertidal area of Teopa beach shows important environmental differences that explain the vertical gastropod variation found in this study. Several biotic factors should have an important role in the explanation of the observed density, shell size, and growth rate variations; including the human impact on gastropod populations; and the natural predation and competition.

The harvesting pressure by humans was considered responsible for the small size of Monodonta labio, an edible rocky intertidal snail in Amakusa, Japan (Iijima 2001). Furthermore, the growth rate in Amakusa was slower than in other localities possibly because the selective collection of large M. labio by human. In Chamela Bay, fishermen use different mollusk species (limpets, snails, mussels, chitons and octopuses) from the rocky shores. Calyptraea spirata is preferred over all other intertidal and shallow subtidal gastropods. The last report of the capture of the snail from the goverment's Fishery Office in Puerto Vallarta, Jalisco, indicate a very small volume of 106 Kg in 1995 (Esquivel & Plascencia 1999). In recent years, the capture of the snail is no longer reported by the Mexican government, probably because of a decrease of the previously exploited populations and the absence of adequate mechanisms to obtain accurate information on the harvesting of the snail. Nowadays, the species is found abundantly only in a few beaches of Jalisco that may be considered as relics, because access is difficult for fishermen. Although access to Teopa beach is restricted mainly because the entrance walkways go across private properties, the beach is regularly visited by local people. The snail is mostly harvested from the upper intertidal zone where it is easier to reach and there are less hazardous conditions. The strong waves and heavy surf of the lower intertidal prevent fishermen from collecting the snails even in the sheltered areas of the beach. On the other hand, natural predation also affects gastropod density and shell size in tropical intertidal habitats (Palmer 1979, Garrity & Levings 1981, Menge & Lubchenco 1981). Shell-breaking predators of gastropods include fishes and crustaceans (Vermeij 1978, Bertness & Cunningham 1981). In the rocky shores of Jalisco, several mammals and sea birds have been observed as predators of snails (Rios-Jara et al. 1994). During the high tide periods, predation by fishes and crustaceans may be more intense while during the low tide periods, predation by mammals and birds is more important. Consequently, the predation intensity should be influenced by intertidal shore height and it is probably related to the vertical variation in density and shell-size found for C. spirata in Teopa beach.

The interspecific and intraspecific competition for food has a considerable effect on gastropod density (Underwood 1979, Creese & Underwood 1982, Underwood & Denley 1984) and growth rates (Paine 1969) in rocky intertidal habitats. Food resources have been also considered as responsible for differences in growth rates in intertidal gastropods (Takada 1995). The members of the Family Calyptraeidae are filter-feeding gastropods that consume microplankton and suspended organic matter from water (Ruppert & Barnes 1996). Field observations indicate that C. spirata may have this feeding strategy, because snails were seen rising the shell from the substratum only during the upcoming and lowering tides. Little or no movements of marked individuals were detected during the study, indicating that snails do not need to move much to filter food items from the surrounding waters. Therefore, gastropod density difference through the intertidal zones may be related to the tidal regimen, which determines an emergence and submersion time gradient, and influence the food availability for gastropods.

Previous studies have suggested that the density of intertidal gastropods is affected by abiotic factors such as wave action, temperature, and desiccation (Atkinson & Newbury 1984, Raffaelli & Hawkins 1996, Gluyas-Millan et al. 1999, Tanaka et al. 2002). Differences in shell-size and growth rate have been explained in response to the effects of wave exposure and shore height (Crothers 1992, Giraldo et al. 2002). In the case of C. spirata, the growth rate of individuals smaller than 3 cm was significantly higher than the growth rate of larger individuals. Similarly, Williamson and Kendall (1981) found that the growth rate of the rocky intertidal snail Monodonta lineata from the coast of England decreases with the size of the individuals. The same situation was later found for M. labio in Japan (Iijima 2001). Differences in the growth rate between smaller and bigger individuals may be related to the reproductive effort of adults. Thus, sexual maturity may imply more energy used for reproduction instead of shell growth.

The tropical intertidal zone experiences continuous severe physical conditions during daytime exposures to sun and air. Substratum temperatures in tropical rocky shores may be extreme in sunny weather, and lethal stress levels can be reached on open surfaces on a clear day (Giraldo et al. 2002). Although, C. spirata live on the exposed areas of the rocks where the wave action and the sunlight are important restrictive factors (Santes-Alvarez & Hernandez 1983, Williams & Morritt 1995). Adaptations to the exposed areas of C. spirata include the wide cap-shaped hydrodynamic shells that diminish the effect of direct wave action and tidal currents; a wide shell aperture, and an extensive foot that gives a stronger attachment to the rocky substratum. The rigor of desiccation and extreme temperatures in the rocky intertidal increases according to a gradient from low to high shore heights. Therefore, those gastropod species living through the intertidal zone may have density variations related to this gradient. Calyptraea spirata has a wide vertical distribution, but the number of snails increases from the upper to the lower level of the intertidal zone. Other studies have shown variable vertical distribution patterns in the rocky beaches of Jalisco; many gastropod species also increase their numbers from the supralittoral to the lower tidal zones whereas the opposite occurs in other species (Esqueda et al. 2000, Rios-Jara et al. 2001). Marked differences in vertical distribution of species or their absence in some intertidal levels closely relate to the transient nature of the littoral environment, which changes frown nearly terrestrial to completely marine conditions. Environmental cues that govern distribution of these species were not evaluated, but we predict that they respond to features of the habitat, which are very variable across the shore.

Several other factors (e.g., mating, feeding, and interspecific relationships) may be important regulating factors. Vertical distribution of many gastropod species closely relate to the presence of other invertebrates in the rocky beaches of California (Sleder 1981) and Baja California (Rios-Jara 1983). Other species are similarly associated with the mats of macroalgae and the aggregations of mussels (Brachidontes sp. and Chloromytilus sp.) in the coast of Jalisco (Esqueda et al. 2000). Calyptraea spirata form aggregations of individuals regularly distributed one next to the other. The snails have a rich epibiota growing on their shells. This epibiota consist mostly of algae together with acorn (balanomorpha) and goose (lepadomorpha) barnacles, polychaetes, amphipods, bivalves, limpets, and briozoans that moisten and shade the snails and incidentally give them protection against the sun and the predators. The aggregations of C. spirata are therefore protected from high temperatures and desiccation, the two most restrictive physical factors throughout the long periods of exposure during the low tides in the rocky intertidal.

There is strong evidence that wave intensity is an important agent of shell-size variation in intertidal gastropods. Recently, Gomez et al. (2001) reported the variation in shell-thickness of the limpet Notoacmea biradiata due to wave intensity, being the shell thicker in exposed rocky shores. The shell size of C. spirata was bigger at the most wave-exposed areas compared with the wave-protected areas of the beach. The same condition has been reported for the turbinid snail Astrea undosa in 2 localities of Bahia Tortugas, Baja California Sur, Mexico (Gluyas-Millan et al. 1999) and for the trochid snail Monodonta labio in the Pacific Coast of Central Japan (Iijima 2001). Calyptraea spirata, is a tenacious snail very well adapted to high wave energy habitats, particularly in the rocky intertidal and shallow subtidal zones of exposed coasts. Specimens are found in the most surf-beaten rocks (Keen 1971) where strong waves and currents are definite factors. The wide shell aperture and extensive foot gives to the organisms a better attachment to the rocky substratum. The irregular margin of the shell adapts well to the irregular microtopography of the rocky substratum, so the water trapped inside during the low tide periods prevents from desiccation and high temperatures, giving oxygen supply for low metabolism. During the periods of low tide, when C. spirata is exposed to desiccation and strong variations in temperature, the snails remain strongly attached to the rocky substratum apparently retaining seawater in the cavity of the mantle. Water retention is an important mechanism to avoid an excessive increase in body temperature and allows minimal gas exchange during these periods of low activity and metabolism. Therefore, many intertidal gastropods species change the size of the shell in response to desiccation and temperature (Lowell 1984, Branch 1985). Vermeij (1973) reported a negative relationship between shell size and intertidal height. However, in this study, the shell size of C. spirata was not affected by its intertidal position; it is probable that desiccation and temperature do not sufficiently affect the gastropods so their shells do not express a morphologic response to these factors.

In conclusion, the results of this study suggest that the density of C. spirata was significantly different between intertidal zones, increasing from the upper to the lower areas of the beach of Teopa, Jalisco. However, there is no gastropod density difference between the sheltered and the exposed parts of the beach. On the other hand, the shell size variation was associated with wave intensity, with bigger shells (greater shell diameter) being found in the most exposed areas of the beach compared with the protected areas. Although the bigger shells were found in the lower interidal zone, shore height did not show a significant effect on the size of the shell for C. spirata. Individual growth rate of snails smaller than 3 cm of diameter was higher than the rate of larger individuals. The larger snails inhabiting the upper intertidal had significant differences in growth rate when comparing exposed to protected conditions. In contrast, the smaller snails, showed no difference in the growth rate related to wave exposure. Finally, shore height did not show a significant effect on growth rate, because individuals from the upper and lower intertidal zones have similar growth rates. Of many possible factors, shore height, wave intensity, and human impact might be at least partly responsible for these differences. This study, however, was conducted during one season only. Thus, more research is necessary to verity whether these patterns remain common over several seasons. Furthermore, the significance of other biotic factors such as gastropod density, natural predation, and competition should also be taken into consideration in future studies, together with physical factors, principally the desiccation and temperature gradients through the intertidal zone during the low tide periods.

TABLE 1.
Mean size of the shell (maximum diameter and height, cm [+ or -] one
standard deviation) of Calyptraea spirata at 2 intertidal zones of
the beach of Teopa, Jalisco with different degrees of exposure to
wave intensity (exposed and protected) (December, 1999).

Intertidal Zone                       n         Exposed          n

     Upper        Maximum diameter   216   2.79 [+ or -] 0.74   212
                  Height             216   1.85 [+ or -] 0.57   212
     Lower        Maximum diameter   218   2.85 [+ or -] 0.70   220
                  Height             218   1.85 [+ or -] 0.51   220
     Mean         Maximum diameter   434   2.82 [+ or -] 0.72   432
                  Height             434   1.85 [+ or -] 0.54   432

Intertidal Zone                          Protected

     Upper        Maximum diameter   2.83 [+ or -] 0.67
                  Height             2.02 [+ or -] 0.47
     Lower        Maximum diameter   2.97 [+ or -] 0.87
                  Height             1.92 [+ or -] 0.68
     Mean         Maximum diameter   2.90 [+ or -] 0.78
                  Height             1.96 [+ or -] 0.59

Intertidal Zone                             Mean

     Upper        Maximum diameter   2.81 [+ or -] 0.70
                  Height             1.93 [+ or -] 0.53
     Lower        Maximum diameter   2.91 [+ or -] 0.79
                  Height             1.91 [+ or -] 0.60
     Mean         Maximum diameter   2.86 [+ or -] 0.75
                  Height             1.91 [+ or -] 0.57

TABLE 2.
Growth rate (cm/week) of Calyptraea spirata from different
tidal levels and wave exposure conditions.

Tidal Level and                     Group 1
Wave Exposure                     Shells <3 cm

                          Growth Rate (cm/month)

                       Exposed            Protected

Upper intertidal        0.180               0.312
                       (n = 42)           (n = 69)
Lower intertidal        0.360               0.220
                       (n = 30)           (n = 27)
Total individuals of              0.268
  each group

                                 Group 2
                               Shells >3 cm

                          Growth Rate (cm/month)

                       Exposed            Protected
Upper intertidal        0.040               0.028
                       (n = 48)           (n = 26)
Lower intertidal        0.080               0.160
                       (n = 27)           (n = 39)
Total individuals of              0.077
  each group

                        n         ANOVA Test             P Value   n

Upper intertidal       111   Exposed vs. Protected        0.155    74
Lower intertidal        57   Exposed vs. Protected        0.747    66
Exposed                 72   Upper zone vs. Lower zone    0.543    75
Protected               96   Upper zone vs. Lower zone    0.723    65
Total individuals of   308                                Group 1 vs.
  each group                                                Group 2

                               ANOVA Test          P Value

Upper intertidal       Exposed vs. Protected       0.018 *
Lower intertidal       Exposed vs. Protected       0.812
Exposed                Upper zone vs. Lower zone   0.942
Protected              Upper zone vs. Lower zone   0.124
Total individuals of                               0.037 *
  each group

* Significant difference in individual growth rate, ANOVA, P < 0.05.

Figure 1. Mean density (ind/[m/.sup.2] [+ or -] one standard
deviation) of Calyptraea spirata in the beach of Teopa, Jalisco
(December, 1999). Estimated densities of two areas of the beach
with different degrees of exposure to wave intensity (exposed
and protected) at two intertidal zones are shown.

INTERTIDAL ZONE AND EXPOSURE   DENSITY (ind/[m.sup.2])

Exposed Upper Intertidal            [+ or -] 3.35
Protected Upper Intertidal          [+ or -] 3.49
Exposed Lower Intertidal            [+ or -] 7.61
Protected Lower Intertidal          [+ or -] 5.03

Note: Table made from bar graph.

LITERATURE CITED

Atkinson, W. D. & S. F. Newbury. 1984. The adaptations of the rough winkle, Littorina rudis, to desiccation and dislodgement by wind and waves. J. Anim. Ecol. 53:93-106.

Bertness, M. D. & C. Cunningham. 1981. Crab shell crushing predation and gastropod architecture. J. Exp. Mar. Biol. Ecol. 50:213-230.

Black, R. 1977. Population regulation in the intertidal limpet Patelloida altidostata (Angas, 1968). Oecologia 30:9-22.

Bowling, C. 1994. Habitat and size of the Florida crown conch (Melongena corona Gmelin): Why big snails hang out at bars? J. Exp. Mar. Biol. Ecol. 175:181-195.

Branch, G. M. 1985. Limpets: evolution and adaptation. The Mollusca 10:187-220.

Brower, J. E. & J. H. Zar. 1977. Field and laboratory methods for general ecology. WMC Brown Co. Publishers. Dubuque, IO. 194 p.

Brown, M. K. & J. F. Quinn. 1988. The effect of wave action on growth in three species of intertidal gastropods. Oecology. 75:420-25.

Creese, R. G. & A. J. Underwood. 1982. Analysis of inter- and intra-specific competition amongst intertidal limpets with different methods of feeding. Oecologia. 53:337-346.

Chow, V. 1987. Patterns of growth and energy allocation in northern California populations of Littorina (Gastropoda:Prosobrancia) J. Exp. Mar. Biol. Ecol. 110:69-89.

Crothers, J. H. 1992. Shell size and shape variation in Littorina littorea (L.) from west Somerset. In: J. Grahame, P. J. Mill & D. G. Reid, editors. Proceedings of the 3rd International Symposium on Littorinid Biology. London: The Malacological Society of London. pp. 91-97.

Cupul-Magana, F. G. & G. Torres-Moye. 1996. Age and growth of Astraea undosa Wood (Mollusca: Gastropoda) in Baja California, Mexico. Bull. Mar. Sci 59(3):490-497.

De Wolf, H., C. Brito, S. Van Dongen & T. Backeljau. 1999. Did heavy storm affect the aperture area of wave-exposed and sheltered Littorina striata? J. Mar. Biol. Ass. UK. 79:1129-1130.

Denny, M. W. 1994, Roles of hydrodynamics in the study of life on wave-swept shores. P. 169-205. In: P. C. Wainwright & S. M. Reilly, editors. Ecological morphology: integrative organismal biology. Chicago: Univ. of Chicago Press. 368 pp.

Elliott, J. M. 1977. Some methods for the statistical analyses of samples of benthic invertebrates. Freshwater Biol. Ass., Scientific Publication 25. 160 pp.

Esqueda, M. C., E. Rios-Jara, J. E. Michel-Morfin & V. Landa-Jaime. 2000. The vertical distribution and abundance of gastropods and bivalves from rocky beaches of Cuastecomate Bay, Jalisco, Mexico. Rev. Biol. Trop. 48(4):765-775.

Esquivel Hernandez, M. A. & E. C. Plascencia Reyes. 1999. Analisis de la problematica de la pesca costera de los estados de Jalisco y Colima, Mexico. Tesis de Licenciatura en Biologia. Univ. de Guadalajara. Mexico. 221 pp.

Fieldman, L. 1968. Some West Mexican archaeological molluscs. In: C. W. Meighan and L. J. Foote, editors. Excavations at Tizapan El Alto, Jalisco. Univ. of California, Los Angeles. Col. Latin American studies. pp. 165-173.

Fletcher, W. J. 1984. Intraspecific variation in the population dynamics and growth of the limpet, Cellana trasomerica. Oecologia 63:110-121.

Fonseca Madrigal, J. F. 1998. Algunos aspectos de dinamica poblacional del caracol Plicopurpura patula pansa (Gould, 1853) en una playa rocosa de Yelapa, Bahia de Banderas, Jalisco. Tesis de Licenciatura en Biologia. Univ. de Guadalajara. Mexico. 43 pp.

Garcia, E. 1973. Modificaciones al sistema de clasificacion climatica de Koppen. Instituto de Geografia, UNAM. Mexico, D. F. 246 pp.

Garrity, S. D. & S. C. Levings. 1981. A predatory-prey interaction between two physically and biologically constrained tropical rocky shore gastropods: direct, indirect and community effects. Ecol. Monogr. 51:267-286.

Giraldo-Lopez, A. & C. Gomez-Schouben. 1999. Variacion en la concha de Siphonaria gigas (Sowerby, 1825) come respuesta al efecto de la intensidad de los olas. Ciencias Marinas 25(2):213-224.

Giraldo, A., C. Gomez & E. Rodriguez. 2002. Shell size variation of Notoacmea biradiata (Archeogastropoda: Acmaeidae) as a response to gastropod density and height in shore in the Colombian Pacific coast. Ciencias Marinas 28(3):237-246.

Gluyas-Millan, M. G., C. Quinonez-Velazquez, J. A. Masso-Rojasy & F. N. Melo-Barrera. 1999. Diferencias en la relacion talla-edad del caracol panocha Astrea undosa (Wood, 1828) entre dos localidades de Bahia Tortugas, Baja California Sur, Mexico. Ciencias Marinas 25(1):91-106.

Gomez-Marquez, J. L. 1994. Metodos para determinar la edad en los organismos acuaticos. Universidad Nacional Autonoma de Mexico. Mexico, D. F. 89 pp.

Gomez, C., A. Giraldo & E. Rodriguez. 2001. Efecto de la intensidad del oleaje sobre la morfometria de Notoacmea biradiata (Archaeogastropoda: Acmaeidae) en el Pacifico colombiano. IX Congreso Latinoamericano sobre Ciencias del Mar. San Andres Isla, Colombia. 83 pp.

Gonzalez Villarreal, L. M. 1977. Estudio taxonomico de los gasteropodos marines de la bahia de Tenacatita. Tesis de Licenciatura en Biologia Univ. Autonoma de Guadalajara. Mexico. 171 pp.

Haven, S. B. 1973. Competition for food between the intertidal gastropods Acmaea scabra and Acmaea digitalis. Ecology 54:143-151.

Hernandez Herrera, R. M. 1998. Analisis de comunidades de macroalgas en ambientes intermareales del sureste de Bahia Tenacatita, Jalisco. Tesis de Licenciatura en Biologia. Univ de Guadalajara. Mexico. 74 pp.

Hobday, A. 1995. Body-size variation exhibited by an intertidal limpet: influence of wave exposure. Tidal height and migratory behavior. J. Exp. Mar. Biol. Ecol. 189:29-45.

Hughes, R. N. & D. J. Roberts. 1980. Growth and reproductive rates of Littorina neritoides (L.) in north Wales. J. Mar. Biol. Ass. UK. 60: 591-599.

Iijima, A. 2001. Growth of the intertidal snail, Monodonta labio (Gastropoda, Prosobranchia) on the Pacific Coast of Central Japan. Bull. Mar. Sci. 68(1):27-36.

Katoh, M. 1989. Life history of the golden ring cowry Cypraea annulus (Mollusca:Gastropoda) on Okinawa Island. Japan. Mar. Biol. 101:227-233.

Keen, M. A. 1971. Sea shells of tropical Western America. Stanford University, Stanford. California. 1064 pp.

Kitching, J. A. 1976. The distribution and changes in shell form of Thais sp. (Gastropoda) near Bamfield. B. C. J. Exp. Mar. Biol. 23:109-126.

Levinton, J. S. 1984. Marine Ecology. Prentice Hall, Inc. New Jersey. 526 pp.

Lewis, J. R. 1964. The ecology of rocky shores. London: The English University Press. 323 pp.

Lowell, R. B. 1984. Desiccation of intertidal limpets: effects of shells size, fit to substratum and shape. J. Exp. Mar. Biol. Ecol. 77:197-207.

Menge, B. A. & J. Lubchenco. 1981. Community organization in temperate and tropical rocky intertidal habitats: prey refuges in relation to consume pressure gradients. Ecol. Monogr. 51:429-50.

Michel-Morfin, J. E., E. L. Chavez & Gonzalez. 2002. Estructura de la poblacion, esfuerzo y rendimiento de tinte del caracol Plicopurpura pansa (Gould, 1853) en el Pacifico Mexicano. Ciencias Marinas. 28(4): 357-368.

Morris, P. A. 1966. A field guide to shells of the Pacific coast and Hawaii. Boston, Massachusetts: Houghton Mifflin. 297 pp.

Paine, R. T. 1969. The Pisaster-Tegula interaction: prey patches. Predator food preferences and intertidal community structure. Ecology 50:950-962.

Palmer, A. R. 1979. Fish predation and the evolution of gastropod shells sculpture: an experimental and geographic evidence. Evolution 33: 697-713.

Perez Pena, M. & E. Rios Jara. 1998. Gastropod mollusks from the continental shelf off Jalisco and Colima, Mexico: Species collected with a trawling net. Revista Ciencias Marinas. 24(4):425-442.

Phillips, D. W. 1981. Life history features of the marine intertidal limpet Notoacmea scutum (Gastropoda) in central California. Mar. Biol. 64: 95-103.

Raffaelli, D. & S. Hawkins. 1996. Intertidal ecology. London: Chapman & Hall. 356 pp.

Reschenhofer, E. 2001. The bimodality principle. J. Statistics Education. 9(1):1-16.

Ricketts, E. F., J. Calvin & J. W. Hedgpeth. 1997. Between Pacific tides. Stanford, California: Stanford Univ. Press. 652 pp.

Rios-Jara, E. 1983. Estructura poblacional y actividad de Acanthina lugubris (Prosobranchia:Thaididae) en una playa rocosa de Bahia Todos Santos, Baja California, Mexico. Tesis de Maestria en Ciencias en Ecologia Marina. Centre Investigacion Cientifica y Educacion Superior de Ensenada, Baja California (CICESE). Mexico. 140 pp.

Rios-Jara, E., H. G. Leon-Alvarez, L. J. E. Lizarraga-Chavez & Michel-Morfin. 1994. Produccion y tiempo de recuperacion del tinte de Plicopurpura patula pansa (Neogastropoda: Muricidae) en Jalisco, Mexico. Rev. Biol. Trop. 42:537-545.

Rios-Jara, E., M. Perez-Pena, R. Beas-Luna, E. Lopez-Uriarte & E. Juarez-Carrillo. 2001. Gastropods and bivalves of commercial interest from the continental shelf of Jalisco and Colima, Mexico. Revista de Biologia Tropical 49(3):785-789.

Ruppert, E. E. & R. D. Barnes. 1996. Zoologia de los Invertebrados. 6th Edition. Mexico: Mac Graw-Hill Interamericana, 1114 pp.

Santes-Alvarez, R. & S. V. Hernandez-Cardona. 1983. Tamano poblacional y organizacion espacial de un gasteropodo predador: Acanthina punctulata (Sowerby). Ciencias Marinas 9:125-136.

Sleder, J. 1981. Acanthina punctulata (Neogastropoda: Muricacea): Its distribution, activity, diet and predatory behavior. Veliger 24:172-180.

Stebbins, T. D. 1988. Variable population structure and tenacity in the intertidal chiton Katharina tunicata (Mollusca: Polyplacophora) in Northern California. The Veliger 30(4):351-357.

Takada, Y. 1995. Variation in growth rate with tidal level in the gastropod Monodonta labio on a boulder shore. Mar. Ecol. Prog. Ser. 117:103-110.

Tanaka, M. O., T. E. M. Duque-Estrada & C. A. Magalhanes. 2002. Dynamics of the acmaeid limpet Collisella subrugosa and vertical distribution of size and abundance along a wave exposure gradient. J. Mol. Stud. 68:55-64.

Underwood, A. J. 1978. An experimental evaluation of competition between three species of intertidal prosobranch gastropods. Oecologia 33:185-202.

Underwood, A. J. 1979. The ecology of intertidal gastropods. Adv. Mar. Biol. 16:111-210.

Underwood, A. J. & E. J. Denley. 1984. Paradigms, explanations and generalizations in models for the structure of intertidal communities on rocky shores, pp. 151-180. In: D. Strong, R. Thistle. A. B. & D. Simberloff, editors. Ecological communities: conceptual issues and the evidence. New Jersey: Princeton Univ. Press. 613 pp.

Vermeij, G. J. 1972. Intraspecific shore-level size gradients in intertidal mollucs. Ecology 53:693-700.

Vermeij, G. J. 1973. Morphological patterns in high-intertidal gastropods: adaptive strategies and their limitations. Mar Biol. 20:319-346.

Vermeij, G. J. 1978. Biogeography and adaptation. Patterns of marine life. Massachusetts: Harvard Univ. Press. 332 pp.

Williams, G. A. & D. Morritt. 1995. Habitat partitioning and thermal tolerance in a tropical limpet, Cellana grata. Mar. Ecol. Prog. Ser. 124:89-103.

Williamson, P. & M. A. Kendall. 1981. Population age structure and growth of the trochid Monodonta lineata determined from shell rings. J. Mar. Biol. Ass. UK. 61:1011-1026.

Zar, J. H. 1999. Biestatistical analysis. 4th ed. New Jersey: Prentice Hall. 663 pp.

EDUARDO RIOS-JARA, CINTHYA CAROLINA HERNANDEZ CEDILLO, EDUARDO JUAREZ CARRILLO AND ILDEFONSO ENCISO PADILLA

Laboratorio de Ecosistemas Marinos y Acuicultura. Departamento de Ecologia. Centro Universitario de Ciencias Biologicas y Agropecuarias, Universidad de Guadalajara. Apartado Postal 52-114, Zapopan, Jalisco 45035 Mexico.

Corresponding author. E-mail: [e-mail]