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 in·ter·tid·al
Of or being the region between the high tide mark and the low tide mark.
in 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 gastropod, member of the class Gastropoda, the largest and most successful class of mollusks (phylum Mollusca), containing over 35,000 living species and 15,000 fossil forms. 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 anova
see analysis of variance.
ANOVA Analysis of variance, see there , P < 0.05). The species was not registered in the supralittoral zone The supralittoral zone, also known as the spray zone, is the area above the spring high tide line that is regularly splashed, but not submerged by ocean water. <1> See also
1. Thurman H.V. , 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 ANCOVA Analysis of Covariance 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
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 An environmental gradient is a gradual and continuous change in communities and environmental condition.
The gradients can be related to environmental factors such as altitude, temperature and moisture supply.
See also: Biome, thermocline, cline (population genetics). over short distances across the intertidal zone. Thus, the tidal regimen determines temperature, desiccation des·ic·ca·tion
The process of being desiccated.
desic·ca , and submersion submersion
the act of placing, or the condition of being under, the surface of a liquid. 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 predation
Form of food getting in which one animal, the predator, eats an animal of another species, the prey, immediately after killing it or, in some cases, while it is still alive. Most predators are generalists; they eat a variety of prey species. define the abundance and the zonation zo·na·tion
1. Arrangement or formation in zones; zonate structure.
2. Ecology The distribution of organisms in biogeographic zones. 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 Hobday is a surname, and may refer to:
This page or section lists people with the surname Hobday. 1995, Giraldo et al. 2002, Tanaka et al. 2002). Furthermore, density is considered responsible for intraspecific in·tra·spe·cif·ic also in·tra·spe·cies
Arising or occurring within a species: intraspecific competition. 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 mollusk: see Mollusca.
Any of some 75,000 species of soft-bodied invertebrate animals (phylum Mollusca), many of which are wholly or partly enclosed in a calcium carbonate shell secreted by the mantle, a soft 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 Baja California, state, Mexico
Baja California (Span.: bä`hä kälēfōr`nyä), state (1990 pop. 1,660,855), 27,628 sq mi (71,576 sq km), NW Mexico, on the Baja California peninsula. Mexicali is the capital. , the shell is shorter and heavier in exposed beaches and longer and slender in protected beaches (Kitching 1976). Similarly, the shell of the limpet limpet, marine gastropod mollusk with a simple, flattened, conical shell, found in cooler waters of the Atlantic and the Pacific oceans. Certain species creep over rocks, feeding on algae during high tides, but when the tide recedes they return instinctively to the Siphonaria gigas is wider when living in more wave exposed rocky beaches (Giraldo-Lopez & Gomez-Schouben 1999), and the size of the chiton chiton (kī`tən), common name for rock-clinging marine mollusks of the class Polyplacophora. Chitons are abundant on rocky coasts throughout most of the world, from the intertidal zone to a depth of about 1,200 ft (400 m). Katharina tunicata from northern California Northern California, sometimes referred to as NorCal, is the northern portion of the U.S. state of California. The region contains the San Francisco Bay Area, the state capital, Sacramento; as well as the substantial natural beauty of the redwood forests, the northern 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 conical /con·i·cal/ (kon´i-k'l) cone-shaped.
con·i·cal or con·ic
Of, relating to, or shaped like a cone. limpet-like shell with large, coarsely ribbed, whitish or grayish outside, dark brown within without operculum operculum /oper·cu·lum/ (o-per´ku-lum) pl. oper´cula [L.]
1. a lid or covering.
2. the folds of pallium from the frontal, parietal, and temporal lobes of the cerebrum overlying the insula. . Its distribution ranges from Mazatlan to the Gulf of Tehuantepec Noun 1. Gulf of Tehuantepec - an arm of the Pacific in southern Mexico
Mexico, United Mexican States - a republic in southern North America; became independent from Spain in 1810
Pacific, Pacific Ocean - the largest ocean in the world , Mexico (Keen 1971) and the Gulf of California Noun 1. Gulf of California - a gulf to the west of the mainland of Mexico
Sea of Cortes
Mexico, United Mexican States - a republic in southern North America; became independent from Spain in 1810 (Morris 1966). In the coast of Jalisco, the species has been registered in the beaches of El Tamarindo, La Calechosa, Los Angeles Los Angeles (lôs ăn`jələs, lŏs, ăn`jəlēz'), city (1990 pop. 3,485,398), seat of Los Angeles co., S Calif.; inc. 1850. (Gonzalez-Villarreal 1977); Yelapa (Fonseca Madrigal madrigal, name for two different forms of Italian music, one related to the poetic madrigal in the 14th cent., the other the most common form of secular vocal music in the 16th cent. 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 tax·o·nom·ic also tax·o·nom·i·cal
Of or relating to taxonomy: a taxonomic designation.
tax 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 Population ecology
The study of spatial and temporal patterns in the abundance and distribution of organisms and of the mechanisms that produce those patterns. 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
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 sessile /ses·sile/ (ses´il) attached by a broad base, as opposed to being pedunculated or stalked.
Permanently attached or fixed; not free-moving. forms, particularly macroalgae, crustaceans, and mollusks.
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 according to
1. As stated or indicated by; on the authority of: according to historians.
2. In keeping with: according to instructions.
3. 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 In mathematics, the phrase sufficiently large is used in contexts such as:
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 tran·sect
tr.v. tran·sect·ed, tran·sect·ing, tran·sects
To divide by cutting transversely.
[trans- + -sect. 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 quad·rat
1. Printing A piece of type metal lower than the raised typeface, used for filling spaces and blank lines. Also called quad2.
2. 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 vernier (vûr`nēr), auxiliary scale, either straight or an arc of a circle, designed to slide along a fixed scale. Its unit divisions, usually smaller than those on the fixed scale, permit a far more precise reading. caliper caliper
Instrument that consists of two adjustable legs or jaws for measuring the dimensions of material parts. Spring calipers have an adjusting screw and nut; firm-joint calipers use friction at the joint to hold the legs unmoving. (maximum diameter and height of the shell). Maximum diameter values were used to make histograms of the size frequency distributions. Bimodality Bimodality is the simultaneous use of two distinct pitch collections. It is more general than bitonality since the "scales" involved need not be traditional scales; if diatonic collections are involved, their pitch centers need not be the familiar major and minor-scale tonics. 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 Bartlett's test (Snedecor and Cochran, 1983) is used to test if k samples have equal variances. Equal variances across samples is called homoscedasticity or homogeneity of variances. ) was examined, and the logarithmic logarithmic
pertaining to logarithm.
when the logs of two variables plotted against each other create a straight line. 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 A statistical package from SPSS, Inc., Chicago (www.spss.com) that runs on PCs, most mainframes and minis and is used extensively in marketing research. It provides over 50 statistical processes, including regression analysis, correlation and analysis of variance. 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 Noun 1. epoxy glue - a thermosetting resin; used chiefly in strong adhesives and coatings and laminates
epoxy, epoxy resin
adhesive, adhesive agent, adhesive material - a substance that unites or bonds surfaces together (Plastiloca) and labels with printed progressive numbers. The labels were then coated with transparent instant nail varnish varnish, homogeneous solution of gum or of natural or synthetic resins in oil (oil varnish) or in a volatile solvent (spirit varnish), which dries on exposure to air, forming a thin, hard, usually glossy film. 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 Growth Rates
The compounded annualized rate of growth of a company's revenues, earnings, dividends, or other figures.
Remember, historically high growth rates don't always mean a high rate of growth looking into the future. (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.
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
Protected areas , the mode extends along all size ranges of the histogram histogram
or bar graph
Graph using vertical or horizontal bars whose lengths indicate quantities. Along with the pie chart, the histogram is the most common format for representing statistical data. , 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 bi·mod·al
1. Having or exhibiting two contrasting modes or forms: "American supermarket shopping shows bimodal behavior . 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 In statistics, the average amount a number varies from the average number in a series of numbers.
(statistics) standard deviation - (SD) A measure of the range of values in a set of numbers. ). 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]
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 Puerto Vallarta (pwār`tō väyär`tä), city (1990 pop. 93,503), Jalisco state, W Mexico. Located on the expansive Bahía de Banderas [Bay of Flags], Puerto Vallarta has been used since the 16th cent. , Jalisco, indicate a very small volume of 106 Kg in 1995 (Esquivel & Plascencia 1999). In recent years, the Years, The
the seven decades of Eleanor Pargiter’s life. [Br. Lit.: Benét, 1109]
See : Time 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 in·ter·spe·cif·ic
Arising or occurring between species.
interspecific also interspecies
Arising or occurring between species.
Adj. 1. and intraspecific competition Intraspecific competition is a particular form of competition in which members of the same species vie for the same resource in an ecosystem (e.g. food, light, nutrients, space). This can be contrasted with Interspecific competition, in which different species compete. 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 sub·stra·tum
n. pl. sub·stra·ta or sub·stra·tums
a. An underlying layer.
b. A layer of earth beneath the surface soil; subsoil.
2. A foundation or groundwork.
3. 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 a·bi·ot·ic
Nonliving: The abiotic factors of the environment include light, temperature, and atmospheric gases.
a 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 hy·dro·dy·nam·ic also hy·dro·dy·nam·i·cal
1. Of or relating to hydrodynamics.
2. Of, relating to, or operated by the force of liquid in motion. 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 rigor /rig·or/ (rig´er) [L.] chill; rigidity.
rigor mor´tis the stiffening of a dead body accompanying depletion of adenosine triphosphate in the muscle fibers. 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 littoral /lit·to·ral/ (lit´ah-r'l) pertaining to the shore of a large body of water.
pertaining to the shore. 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 In population ecology, a regulating factor is something that keeps a population at equilibrium (neither increasing nor decreasing in size over time).
An example of a regulating factor would be food supply. . 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 algae (ăl`jē) [plural of Lat. alga=seaweed], a large and diverse group of primarily aquatic plantlike organisms. These organisms were previously classified as a primitive subkingdom of the plant kingdom, the thallophytes (plants that 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 Baja California Sur (sr), state (1990 pop. 317,764), 27,571 sq mi (71,428 sq km), NW Mexico, on the Baja California peninsula. La Paz is the capital. , 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 seawater
Water that makes up the oceans and seas. Seawater is a complex mixture of 96.5% water, 2.5% salts, and small amounts of other substances. Much of the world's magnesium is recovered from seawater, as are large quantities of bromine. 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.
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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.
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