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Molluscan communities of the rocky intertidal zone at two sites with different wave action on Isla La Roqueta, Acapulco, Guerrero, Mexico.

ABSTRACT Molluscan communities were studied on two shores, one exposed and the other sheltered from wave action by determining the diversity, richness, density, dominance, and similarity of the groups of species found in the upper part of middle intertidal zone. The sample area per location and date was 20 [m.sup.2]. The samples were taken quarterly during the year. Species richness was found to he 44 species for both sites combined. The exposed site had a greater richness (41 species) and lower density (64.2 organisms/[m.sup.2]), whereas the sheltered site showed a higher density (88.08 organisms/[m.sup.2]) and smaller richness (33 species). Thirty species (68.8%) were determined to be common. The exposed shore had more confined species. The Shannon-Wiener diversity index (H') was higher for the exposed site (3.165 bits/individual). A comparison of means by the Mann-Whitney tested indicated that the annual means of H' were equal between the two sites. Qualitative (Jaccard) and quantitative (Morisita-Horn) similarity scores were high at 0.682 and 0.622, respectively. The mean density of gastropods was the same at both locations, because bivalves showed significant differences with higher densities at the sheltered site. The malacologic community of the two sites was very similar. However, changes in richness and diversity over time were greater at the sheltered shore, while these changes were more gradual at the exposed shore, reflecting a greater stability of the malacologic community at the latter location.

KEY WORDS: molluscan communities, rocky intertidal, wave action, Acapulco

INTRODUCTION

The Pacific rocky coast has a very variable relief, which determines microenvironmental conditions at each location, which in turn control the distribution of species. Molluscs stand out because of their abundance and variety of forms, and have been connected with the development of many cultures.

It is important to measure some of the ecological attributes of the communities, so that comparison can be made with species found at other locations. The information also has practical application for the purpose of management, appropriate use, conservation, and environmental monitoring. On the overall geographic scale, climate is the principal factor that determines the type of communities, whereas on a smaller scale it is more difficult to establish which factors govern the grouping of species. Various physical factors are known to intervene in the shaping of the structure of communities, one of them being exposure to waves. Studies of molluscan populations and communities along the Mexican Pacific have historically focused on species of commercial interest (Baqueiro & Stuardo 1977, Baqueiro 1979, Rfos-Jara et al. 2001), and the majority of these have been conducted on the continental shelf (Reguero & Garcia-Cubas 1989, Landa-Jaime & Arciniega-Flores 1998). There have been few investigations in the particular case of rocky shores, where most of the authors have carried out taxonomic works (Morris 1966, Keen 1971, Holguin-Quinones & Gonzalez-Pedraza 1989, 1994, Sevilla 1995, Reyes-Gomez & Salcedo-Vargas 2002), and others have been concerned with the zonation, richness and species diversity of molluscs (Villalpando-Canchola 1986, Salcedo-Martinez et al. 1988, Castillo-Rodriguez & Amezcua-Linares 1992, Romfin-Contreras et al. 1991, Esqueda et al. 2000, Villarroel et al. 2000) and the different regions exposed to waves (Garcia-Lopez 1994, Del Rio-Zaragoza & Villarroel-Melo 2001).

The main economic activity in the State of Guerrero is tourism and to a lesser degree fishing. However, there is little known about marine faunistic resources, despite that it involves an extensive coastline that encompasses three marine regions prioritized for the conservation of the coastal and ocean biodiversity of Mexico, and Mexico's National Commission on Biodiversity (CONABIO) has indicated that there is a lack of knowledge about this region (Arriaga et al. 1998). Isla La Roqueta is located within this zone.

The aim of this investigation is to determine the diversity, density, dominance, and similarity among the groups of mollusc species in the high intertidal zone of a shore exposed to wave action and one not, under the supposition that locations that differ in exposure also differ in composition of the malacologic community.

METHODS

Area of Study

Isla La Roqueta is located in Acapulco, Guerrero, and the sample sites were as follows: (a) Playa Zoologico is situated 16[degrees]42'11.2"N and 99[degrees]54'8.8"W, and is a shore exposed to wave action. Its massif is composed of intrusive igneous rocks, and has a firm substrate with a large amount of fissures, cracks, and cavities all over the area; (b) Playa Palmitas is situated 16[degrees]49'25.6"N and 99[degrees]54'41.2"W. It is a sheltered shore with a massif composed of intrusive igneous rocks. It has a firm substrate with a large amount of fissures, cracks, and cavities. The climate is hot and sub-humid, with rains in the summer and with 5% to 10% winter rain (Garcia 1981).

Field Methods

The sample was taken from the upper part of middle intertidal zone of the two locations. The sampling dates were December of 2000 and March, June, September and December of 2001. The calculated area for the sample size was 20 [m.sup.2] for each station, determined by the method of species area (Brower et al. 1998). Collection was at 2 vertical levels of the upper part of the middle intertidal zone, called level I with its lower limit adjacent to the barnacle zone, and level II with its upper limit next to the periwinkle and limpet zone. A strip 30 m long and 2 m wide was used, which was parallel to the coastline, and the sampling unit was a quadrant 1 m squared. Sampling was systematic (Scheaffer et al. 1987), randomly selecting the starting point and placing the quadrant in level I. After finishing the first sampling unit, the quadrant was placed in level II, directly above level I. All the molluscs that were found in the quadrant were identified and counted. After completing both levels, a space of 2 m was left before establishing new quadrants until completing 20 [m.sup.2].

Taxonomic identification of the molluscs was according to Morris (1966), Keen (1971), Skoglund (1991, 1992), Reyes-Gomez and Salcedo-Vargas (2002).

Analysis of Data

The density of the molluscs was determined as number of organisms/[m.sup.2]. Species richness constituted the number of species present per site. A species was considered dominant if it showed a density equal to or greater than 1 organism/[m.sup.2] in at least one of the samplings. Cumulative dominance was defined as the sum of individual dominances. The frequency of appearance was determined as the total number of samples in which each species was recorded and expressed as a percentage. A confined species was defined as the only species present in one of the sites. The difference between means of species richness was evaluated with the Mann-Whitney test (Daniel 2002). Similarity between the sites was measured by using the qualitative presence/absence index of Jaccard and the quantitative Morisita-Horn index based on abundance (Magurran 1989). Species diversity was determined with the Shannon-Wiener index II' (Margalef 1974), and the Mann-Whitney test was used to compare the mean values of diversity H' (Daniel, 2002).

RESULTS

Exposed Shore

A total of 41 species were found (Table 1) corresponding to 31 genera and 19 families, of which 75.6% belonged to the class Gastropoda, 17.1% to the class Bivalvia, and 7.3% to the class Polyplacophora. The lowest richness was found in the samplings of March and June with 21 species and the greatest in December of 2001 with 26 species (Fig. 1).

[FIGURE 1 OMITTED]

The mean density during the annual cycle was 64.2 organisms/ [m.sup.2], which fluctuated among the different sampling times from 29.9 to 87.3 organisms/[m.sup.2].

Sixteen dominant species were recorded and in the annual cycle these species accumulated to 98.02% dominance. The quantity of dominant species fluctuated per date from 5 in December of 2000 to 10 in December of 2001 (Table 2). The frequency of appearance of the dominant species was distributed in the following manner: nine had 100%, three 80%, three 60% and one 40%.

Of the 9 species that showed 100% frequency of appearance, the gastropod Plicopurpura patula pansa (Gould 1853) and the bivalve Chama echinata (Broderip 1835) had densities equal to or greater than 1 for all the sampling dates.

The densities of the class Gastropoda for all the dates were higher than those of the classes Bivalvia and Polyplacophora. The highest density of the class Gastropoda was seen in September of 2001, whereas the lowest was recorded in December of 2000. The class Bivalvia showed the highest density in June and the lowest in September (Table 2).

The Shannon-Wiener index for the annual cycle was 3.165 bits/individual, oscillating among the different samples from 2.432 to 3.250 bits/individual (Fig. 2).

[FIGURE 2 OMITTED]

Sheltered Shore

A total of 33 species (Table 1) were found corresponding to 28 genera and 20 families, of which 78.8% belonged to the class Gastropoda, 18.2% to the class Bivalvia and 3.0% to the class Polyplacophora. The least richness was found in the samples of September with 16 species and greatest in December of 2000 and March and June of 2001 with 22 species (Fig. 1).

The mean density during the annual cycle was 88.08 organisms/[m.sup.2], which varied among the different sampling dates from 62.85 to 123.85 organisms/[m.sup.2].

Eleven dominant species were recorded and these species accumulated over the annual cycle to 98.26% dominance. The quantity of dominant species fluctuated per date from seven in December of 2001 to nine in June and September (Table 3). The frequency of appearance of the dominant species was distributed in the following manner: eight had 100%, two 80% and one 40% frequency. Of the 8 that showed 100% frequency of appearance, the gastropods P. Patula pansa, Mancinella triangularis (Bleainville 1832) and Petaloconchus complicatus Dall, 1908 as well as the bivalves Chama echinata, Brachidontes semilaevis (Menke 1849) and Isognomom janus (Carpenter 1857) had densities equal to or greater than 1 at all the sampling dates.

The densities of the class Gastropoda were higher than those of the classes Bivalvia and Polyplacophora in the samples from June and September, whereas the class Bivalvia showed higher densities than those of the classes Gastropoda and Polyplacophora in samples from December of 2000 and March and December of 2001. The highest density for the class Gastropoda was seen in September of 2001, whereas the lowest was recorded in December of 2000. The class Bivalvia showed a higher density in December of 2001 and the lowest in September of the same year (Table 3).

The Shannon-Wiener index for the annual cycle was 2.863 bits/individual, oscillating among the different samples from 1.972 to 2.911 bits/individual (Fig. 2).

Both Sites

Thirty species (68.8%) were common to both sites. Eleven confined species were found at the exposed location, whereas there were only 3 at the sheltered site (Table 1). In the annual cycle, the Jaccard similarity index was 0.682 and the Morisita-Horn quantitative index was 0.622.

The mean number of species found per date was 23 species (SD = 2.12) for the exposed shore and 19.8 species (SD = 3.03) for the sheltered one. The difference between these mean values for the two sites was not statistically significant (Mann-Whitney = 6.0, P = 0.222).

At the exposed site, the mean total density of dominant molluscs of the class Gastropoda for all the samples was 52.43 organisms/[m.sup.2] (SD = 21.24), whereas at the sheltered site it was 44.05 organisms/[m.sup.2] (SD = 33.03). A comparison of the two mean values showed no statistical difference (Mann-Whitney = 9.0, P = 0.465). For the class Bivalvia, there were 9.48 organisms/m2 (SD = 7.13) at the exposed site and 42.5 organisms/[m.sup.2] (SD = 16.23) on the sheltered shore.

The two locations showed high diversity values, and in comparing the mean Shannon-Wiener indices for the five collections, there were no statistically significant differences (Mann-Whitney = 8.0, P = 0.421) (Fig. 2).

DISCUSSION

This study found a greater number species than that reported to date for the upper part of middle intertidal zone of the study area. Villalpando-Canchola (1986) collected 34 species at Isla la Roqueta, in an area referred to as level I, which included the upper part of middle intertidal zone, whereas Garcia-Lopez (1994) found 36 species in the same zone.

The exposed site showed a greater richness, more confined species, a higher number of dominant species, but a lower density. In all the samples, the class Gastropoda showed the greatest density. This site exhibited greater variation with respect to the number of dominant species among the sampling dates, and only two of these species had the characteristic of persisting during the entire cycle as dominant species. These findings indicate that the dominant community showed more mobility among its species.

The sheltered site showed less richness, fewer confined species, a lower number of dominant species, but a higher density. Among the samples, the highest densities per date alternated between the classes Gastropoda and Bivalvia. This location exhibited less variation with respect to number of dominant species among the sampling dates, and six of these species showed the characteristic of remaining the dominant species throughout the entire cycle. These findings indicate that the dominant community had less mobility among its species.

Lewis (1964) found a greater richness and more confined species in an exposed site, and Del Rio-Zaragoza and Villarroel-Melo (2001) also determined a greater number of species in an exposed site, whereas Spight (1978) reported a greater abundance in a sheltered site. However, Garcia-Lopez (1994) reported a greater number of species at Isla la Roqueta for the sheltered sites studied.

The density of molluscs for the combined dominant species that belonged to the class Gastropoda did not differ between the two locations studied, and gastropods showed the highest abundance of species at both locations. Holguin-Quinones and Gonzalez-Pedraza (1989), Castillo-Rodr/guez and Amezcua-Linares (1992), Garcia-Lopez (1994), and Villarroel et al. (2000) also reported that gastropods displayed the greatest species richness among the classes of molluscs.

On the other hand, the density of molluscs for the combined dominant species belonging to the class Bivalvia did show differences between the two sites, such that the sheltered shore showed a greater number of bivalves.

The molluscs of the class Polyplacophora was recorded as dominant species only at the exposed location.

The percentage of cumulative dominance of the species that had densities equal to or greater than 1 organism/[m.sup.2] was similar for the two sites and was over 98% of the whole malacologic community.

A comparison of the mean values for species richness for all the sampling dates did not show significant differences, indicating that the two locations are similar with regard to richness. This same situation was seen when comparing the mean values for the diversity index H', which indicates that the sites studied over the annual cycle can be considered equally diverse. However, the values recorded for both richness and the H' index of the sheltered site for each of the samples, demonstrated greater changes in the malacologic community, whereas in the exposed site these values showed more gradual changes.

In this study, the value for the Shannon-Wiener diversity measure for the exposed site was greater when considering the annual cycle. In a study of the same locations where the present work was conducted, Garcia-Lopez (1994) recorded a lower H' index value at the exposed site (1.606 bits/individual), and higher value for the sheltered site (3.817 bits/individual).

The number of common species between the two locations was very high, which was reflected in the value for the Jaccard index. However, Garcia-Lopez (1994) reported a lower index (0.375) for these same locations. According to the Morisita-Horn index, the two sites were quantitatively very similar.

The malacologic community of the upper part of middle intertidal zone of two locations with different exposure to waves was found to be very similar, contrary to the proposed hypothesis of this investigation. However, there are some differences that are important to point out. At the sheltered shore, seasonal changes in richness and diversity were greater despite the lesser variation in its dominant community. At the exposed shore, these changes were more gradual over time despite a greater variation in the dominant community, reflecting a greater stability of the malacologic community at this site.
TABLE 1.

Molluscan species observed at both locations of collecting
sites in the upper part of middle intertidal zone on Isla la Roqueta,
Acapulco, Guerrero, Mexico; Dec. 2000-Dec. 2001.

Gastropoda

 1. Cantharus sanguinolentus (Duclos. 1833) (C)
 2. Columbella fuscata Sowerby, 1832 (C)
 3. Conus nux Broderip, 1833 (S)
 4. Crucibulum scutellatum (Wood, 1928) (C)
 5. Crepidula excabata (Broderip, 1834) (E)
 6. Diodora inaequalis (Sowerby, 1835) (E)
 7. Fissurella asperella Sowerby, 1835 (E)
 8. Fissurella gemmata Menke, 1847 (C)
 9. Fissurella nigrocincta Carptenter, 1856 (C)
10. Fissurella obscura Sowerby, 1835 (E)
11. Hoffmannola hansi Marcus & Marcus, 1967 (C)
12. Leucozonia cerata (Wood, 1828) (C)
13. Lottia acutapex (Berry, 1960) (E)
14. Lottia mitella (Menke, 1847) (E)
15. Lottia pediculus (Philippi, 1846) (E)
16. Mancinella speciosa (Valenciennes, 1832) (C)
17. Mancinella triangularis (Blainville, 1832) (C)
18. Mitra tristis Broderip, 1836 (C)
19. Mitrella ocellata (Gmelin, 1791) (C)
20. Nerita scabricosta Lamarck, 1822 (C)
21. Nodilittorina aspera, (Philippi 1846) (C)
22. Nodilittorina modesta (Philippi, 1846) (C)
23. Opeatostoma pseudodon (Burrow, 1815) (C)
24. Petaloconchus complicates Dall, 1908. (C)
25. Petaloconchus macrophragma Carpenter, 1857 (C)
26. Planaxis obsoletus Menke, 1851 (S)
27. Plicopurpura columellaris (Lamarck, 1822) (C)
28. Plicopurpura patulo pansa (Gould, 1853) (C)
29. Siphonaria gigas Sowerby, 1825 (E)
30. Siphonaria palmata Carpenter, 1857 (C)
31. Stramonita haemastoma (Linnaeus, 1758) (C)
32. Tectura fascicularis (Menke, 1851) (C)
33. Thais kioskiformis (Duclos, 1832) (C)
34. Trimusculus stellatus (Sowerby, 1835) (S)

Bivalvia

35. Arca mutabilis (Sowerby, 1833) (E)
36. Brachidontes semilaevis (Menke, 1849) (C)
37. Chama equinata Broderip, 1835 (C)
38. Choromitylus palliopunctatus (Carpenter, 1857) (C)
39. Isognomom janus Carpenter, 1857 (C)
40. Striostrea prismatica (Gray, 1825) (C)
41. Saccostrea palmula (Carpenter, 1857) (C)

Polyplacophora

42. Chiton albolineatus Broderip y Sowerby, 1829 (E)
43. Chiton articulatus, Sowerby, 1832 (C)
44. Tonicia forbessi Carpenter 1857 (E)

(C) Common species in both sites.

(E) Confined species in exposed site.

(S) Confined species in sheltered site.

TABLE 2.

Densities (organisms/[m.sup.2]) of the dominant species for
sampling dates in the exposed site to wave action on Isla La
Roqueta, Acapulco, Guerrero Mexico.

Classes/Species Dec-00 Mar-01 Jun-01 Sep-01 Dec-01

Gastropoda
 Columbella fuscata 0.0 0.0 0.0 1.25 2.45
 Crucibulum
 escutellatum 0.45 0.35 0.2 0.15 2.4
 Fissurella gemmata 1.15 0.20 0.05 0.3 3.25
 Fissurella nigrocincta 0.05 0.0 0.05 2.1 2.1
 Hoffmannola hansi 0.35 0.7 1.4 0.0 0.0
 Nodilittorina aspera 0.0 10.95 10.25 30.8 7.0
 Nodilittorina modesta 0.10 1.95 0.0 0.0 0.1
 Nerita scabricosta 0.0 0.0 0.1 1.55 0.35
 Petaloconchus
 complicatus 0.50 24.6 34.15 16.1 20.3
 Petaloconchus
 macrophracma 0.10 1.2 6.1 0.25 0.25
 Plicopurpura panda
 pansa 10.10 9.3 10.05 12.85 9.8
 Mancinella triangularis 5.7 0.3 2.7 9.0 6.7
 Total 18.5 49.55 65.05 74.35 54.7
Bivalvia
 Chama equinata 9.2 3.2 1.75 2.6 8.4
 Brachidontes semilaevis 0.0 0.05 17.75 0.65 0.45
 Isognomom janus 0.15 0.65 1.7 0.35 0.5
 Total 9.35 3.9 21.2 3.6 9.35
Polyplacophora
 Chiton articulatus 1.3 0.95 0.55 0.5 1.85

TABLE 3.

Densities (organisms/[m.sup.2]) of the dominant species for
sampling dates in the sheltered site to wave action on Isla
La Roqueta, Acapulco, Guerrero Mexico.

Classes/Species Dec-00 Mar-01 Jun-01 Sep-01 Dec-01

Gastropoda 1.3 0.0 0.05 0.0 0.0
 Columbella fuscata 0.0 0.55 22.0 79.45 2.35
 Nodilittorina aspera 1.0 2.05 7.05 3.35 0.95
 Nodilittorina modesta 0.0 0.15 1.4 5.05 0.35
 Nerita scabricosta 5.05 7.15 1.15 2.8 6.8
 Petaloconchus
 complicatus 0.75 6.9 0.95 0.45 0.4
 Petaloconchus
 macrophracma 9.0 6.6 7.4 6.7 9.55
 Plicopurpura patula
 pansa 6.1 2.6 5.15 3.15 4.55
 Mancinella triangularis 23.2 26.0 45.15 100.95 24.95
 Total
Bivalvia 11.4 4.45 15.0 6.05 23.25
 Brachidontes semilaevis 24.5 42.05 20.8 14.25 39.55
 Chama equinata 1.25 1.65 2.75 2.0 3.55
 Isognomom janus 37.15 48.15 38.55 22.3 66.35
 Total

Figure 1. Species richness observed by sampling dates for the exposed
and sheltered sites on Isla La Roqueta. December to December
2001.

 Number of species

Sampling dates Exposed Sheltered

December-00 23 22
March-01 21 22
June-01 21 22
September-01 24 16
December-01 26 17

Note: Table made from bar graph.

Figure 2. Shannon-Wiener diversity index values, observed for the
Exposed and sheltered sites on Isla La Roqueta, December 2000 to
December 2001.

 Shannon-Wiener index H' values

Sampling dates Exposed Sheltered

December-00 2.476 2.71
March-01 2.432 2.336
June-01 2.611 2.911
September-01 2.706 1.972
December-01 3.25 2.377

Note: Table made from line graph.


ACKNOWLEDGMENTS

The authors thank CONACyT for the support offered for the development of this research and Dr. Gorgonio Ruiz Campos for his review of the manuscript and valuable comments. Dr. Enrique Villalpando Canchola, provided invaluable help throughout the study. We appreciate the comments and suggestions from Dr. Emilio Michel Morfin and Dr. Victor Landa Jaime. We also thank Dr. Albert Leyva for his help in the preparation of the manuscript.

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Villalpando-Canchola, E. 1986. Diversidad y zonacion de moluscos de facie rocosa Isla Roqueta, Acapulco, Guerrero. Thesis. Facultad de Ciencias, Universidad Nacional Autonoma de Mexico. Mexico D.F.

Villarroel, M. M. A., M. B. Magana, C. O. Gomez, Z. J. Del Riom, P. J. Lucio & S. Sanchez. 2000. Diversidad de moluscos en el litoral rocoso de Michoacan, Mexico. Revista de divulgacion de investigacion cientifica. Mexicoa, Vol. 2, No. 1.

ARCADIO VALDES-GONZALEZ, (1),* PEDRO FLORES-RODRIGUEZ, (1,2) RAFAEL FLORES-GARZA (1,2) AND SERGIO GARCIA-IBANEZ (1)

* E-mail: acuacu_uanl@yahoo.com.mx

(1) Doctorado en Ciencias Biologicas, Especialidad en Ecologia, Facultad de Ciencias Biologicas, Universidad Autonoma de Nuevo Leon, San Nicolas de los Garza, Nuevo Leon, Mexico. C. P. 66450; (2) Universidad Autonoma de Guerrero, Unidad Academica de Ecologia Marina, Acapulco, Guerrero, Mexico. C. P. 39390
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Author:Garcia-Ibanez, Sergio
Publication:Journal of Shellfish Research
Geographic Code:1MEX
Date:Dec 1, 2004
Words:4613
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