Molluscan Communities of the Rocky Intertidal Zone at Two Sites with Different Wave Action on Isla La Roqueta, Acapulco, Guerrero, Mexico

MOLLUSCAN COMMUNITIES OF THE ROCKY INTERTIDAL ZONE AT TWO SITES WITH DIFFERENT WAVE ACTION ON ISLA LA ROQUETA, ACAPULCO, GUERRERO, MEXICO

ARCADIO VALDÉS-GONZÁLEZ,* PEDRO FLORES-RODRÍGUEZ,^'^ RAFAEL FLORES-GARZA* ^ AND SERGIO GARCÍA-IBÁÑEZ' ^Doctorado en Ciencias Biológicas, Especialidad en Ecología, Facultad de Ciencias Biológicas, Universidad Autónoma de Nuevo León, San Nicolás de los Garza, Nuevo León, México. C. P. 66450; ^Universidad Autónoma de Guerrero, Unidad Académica de Ecología Marina, Acapulco, Guerrero, México. C. P. 39390 Journal of Shellfish Research, Vol. 23, No. 3, 875-880, 2004.

MOLLUSCAN COMMUNITIES OF THE ROCKY INTERTIDAL ZONE AT TWO SITES WITH DIFFERENT WAVE ACTION ON ISLA LA ROQUETA, ACAPULCO, GUERRERO, MEXICO

ARCADIO VALDÉS-GONZÁLEZ,*'* PEDRO FLORES-RODRÍGUEZ,* ^ RAFAEL FLORES-GARZA*^ AND SERGIO GARCÍA-IBÁÑEZ' ^Doctorado en Ciencias Biológicas, Especialidad en Ecología, Facultad de Ciencias Biológicas, Universidad Autónoma de Nuevo León, San Nicolás de los Garza, Nuevo León, México. C. P. 66450; ^Universidad Autónoma de Guerrero, Unidad Académica de Ecología Marina, Acapulco, Guerrero, México. C. P. 39390

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 middie intertidal zone. The sample área per location and date was 20 m^. The samples were taken quarterly during the year. Species richness was found to be 44 species for both sites combined. The exposed site had a greater richness (41 species) and lower density (64.2 organisms/m~), whereas the sheltered site showed a higher density (88.08 organisms/m") 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 bit,s/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-Hom) similarity scores were high at 0.682 and 0.622, respectively. The inean 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 to waves (García-Lopez 1994, Del Río-Zaragoza & Villarroel- Melo 2001). The Pacific rocky coast has a very variable relief, which de• The main economic activity in the State of Guerrero is tourism termines microenvironmental conditions at each location, which in and to a lesser degree fishing. However, there is little known about turn control the distribution of species. Molluscs stand out because marine faunistic resources, despite that it involves an extensive of their abundance and variety of forms, and have been connected coastline that encompasses three marine regions prioritized for the with the development of many cultures. conservation of the coastal and ocean biodiversity of México, and It is important to measure some of the ecological attributes of México's National Commission on Biodiversity (CONABIO) has the communities, so that comparison can be made with species indicated that there is a lack of knowledge about this región (Ar- found at other locations. The Information also has practical appli- riaga et al. 1998). Isla La Roqueta is located within this zone. cation for the purpose of management, appropriate use, conserva- The aim of this investigation is to determine the diversity, tion, and environmental monitoring. On the overall geographic density, dominance, and similarity among the groups of moUusc scale, climate is the principal factor that determines the type of species in the high intertidal zone of a shore exposed to wave communities, whereas on a smaller scale it is more difficult to action and one not, under the supposition that locations that differ establish which factors govern the grouping of species. Various in exposure also differ in composition of the malacologic commu• physical factors are known to intervene in the shaping of the struc- nity. ture of communities, one of them being exposure to waves. Studies METHODS of molluscan populations and communities along the Mexican Pa• cific have historicatly focused on species of commercial interest Area of Study (Baqueiro & Stuardo 1977, Baqueiro 1979, Ríos-Jara et al. 2001), and the majority of these have been conducted on the continental Isla La Roqueta is located in Acapulco, Guerrero, and the shelf (Reguero & García-Cubas 1989, Landa-Jaime & Arciniega- sample sites were as follows: (a) Playa Zoológico is situated Flores 1998). There have been few investigations in the particular 16°42'11.2"N and 99°54'8.8"W, and is a shore exposed to wave case of rocky shores, where most of the authors have carried out action. Its massif is composed of intrusive igneous rocks, and has taxonomic works (Morris 1966, Keen 1971, Holguín-Quiñones & a firm substrate with a large amount of fissures, cracks, and cavi- González-Pedraza 1989, 1994, Sevilla 1995, Reyes-Gómez & Sal• ties all over the área; (b) Playa Palmitas is situated 16°49'25.6"N cedo-Vargas 2002), and others have been concerned with the zo- and 99°54'41.2"W. It is a sheltered shore with a massif composed nation, richness and species diversity of molluscs (Villalpando- of intrusive igneous rocks. It has a firm substrate with a large Canchola 1986, Salcedo-Martinez et al. 1988, Castillo-Rodríguez amount of fissures, cracks, and cavities. The climate is hot and & Amezcua-Linares 1992, Román-Contreras et al. 1991, Esqueda sub-humid, with rains in the summer and with 5% to 10% winter et al. 2000, Villarroel et al. 2000) and the different regions exposed rain (García 1981). Field Methods The sample was taken from the upper part of middie intertidal E-mail: acuacu_uanl @yahoo.com.mx zone of the two locations. The sampling dates were December of

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2000 and March, June, September and December of 2001. The mined as the total number of samples in which each species was calculated área for the sample size was 20 m^ for each station, recorded and expressed as a percentage. A confined species was determined by the method of species área (Brower et al. 1998). defined as the only species present in one of the sites. The differ• CoUection was at 2 vertical levéis of the upper part of the middie ence between means of species richness was evaluated with the intertidal zone, called level I with its lower limit adjacent to the Mann-Whitney test (Daniel 2002). Similarity between the sites barnacle zone, and level 11 with its upper limit next to the peri- was measured by using the qualitative presence/absence index of winkle and limpet zone. A strip 30 m long and 2 m wide was used, Jaccard and the quantitative Morisita-Hom index based on abun• which was parallel to the coastline, and the sampling unit was a dance (Magurran 1989). Species diversity was determined with the quadrant 1 m squared. Sampling was systematic (Scheaffer et al. Shannon-Wiener index H' (Margalef 1974), and the Mann- 1987), randomly selecting the starting point and placing the quad• Whitney test was used to compare the mean valúes of diversity H' rant in level 1. After finishing the first sampling unit, the quadrant (Daniel, 2002). was placed in level II, directly above level 1. All the molluscs that were found in the quadrant were identified and counted. After RESULTS completing both levéis, a space of 2 m was left before establishing new quadrants until completing 20 m^. Exposed Shore Taxonomic identification of the molluscs was according to A total of 41 species were found (Table 1) corresponding to 31 Morris (1966), Keen (1971), Skoglund (1991, 1992), Reyes- genera and 19 families, of which 75.6% belonged to the class Gómez and Sal cedo-Vargas (2002). Gastropoda, 17.1% to the class Bivalvia, and 7.3% to the class Analysis of Data Polyplacophora. The lowest richness was found in the samplings of March and June with 21 species and the greatest in December The density of the molluscs was determined as number of or• of 2001 with 26 species (Fig. 1). ganisms/m^. Species richness constituted the number of species The mean density during the annual cycle was 64.2 organisms/ present per site. A species was considered dominant if it showed a m^, which fluctuated among the different sampling times from density equal to or greater than 1 organism/m^ in at least one of the 29.9 to 87.3 organisms/m^. samplings. Cumulative dominance was defined as the sum of in• Sixteen dominant species were recorded and in the annual cycle dividual dominances. The frequency of appearance was deter• these species accumulated to 98.02% dominance. The quantity of

TABLE 1. Molluscan species observed at both locations of collecting sites in the upper part of middie intertidal zone on Isla la Roqueta, Acapulco, Guerrero, México; Dec. 2000-Dec. 2001.

Gastropoda

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

35. Arca mutabilis (Sowerby, 1833)'* 39. Isognomom janus Carpenter, 1857"= 36. Brachidontes semilaevis (Menke, 1849)*= 40. Striostrea prismática (Gray, 1825)'= 37. Chama equinata Broderip, 1835*= 41. Saccostrea pálmala (Carpenter, 1857)*= 38. Choromitylus palliopunctatus (Carpenter, 1857)*^ Polyplacophora

42. C/iííon a/fcoím^aíMí Broderip y Sowerby, 1829^ 44. Tonicia forbessi Carpenter \S51^ 43. Chiton articulatus, Sowerby, 1832"=

'^Common species in both sites. ^Confined species in exposed site. '"^Confined species in sheltered site. MOLLUSCAN COMMUNITIES ON LA ROQUETA, ACAPULCO, MÉXICO 877

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).

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 organ• December-OO March-01 June-01 September-01 December-01 isms/m^, which varied among the different sampling dates from Sampling dates 62.85 to 123.85 organisms/m^. [•Exposed a Sheltered I Eleven dominant species were recorded and these species ac• Figure 1. Species richness observed by sampling dates for the exposed cumulated over the annual cycle to 98.26% dominance. The quan• and sheltered sites on Isla La Roqueta. December 2000 to December tity of dominant species fluctuated per date from seven in Decem• 2001. ber of 2001 to nine in June and September (Table 3). The fre• quency of appearance of the dominant species was distributed in dominant species fluctuated per date from 5 in December of 2000 the following manner: eight had 100%, two 80% and one 40% to 10 in December of 2001 (Table 2). The frequency of appearance frequency. Of the 8 that showed 100% frequency of appearance, of the dominant species was distributed in the foUowing manner; the gastropods P. Patula pansa, Mancinella triangularis (Bleain- nine had 100%, three 80%, three 60% and one 40%. ville 1832) and Petaloconchus complicatus Dalí, 1908 as well as Of the 9 species that showed 100% frequency of appearance, the bivalves Chama echinata, Brachidontes semilaevis (Menke the gastropod Plicopurpura patula pansa (Gould 1853) and the 1849) and Isognomom janus (Carpenter 1857) had densities equal bivalve Chama echinata (Broderip 1835) had densities equal to or to or greater than 1 at all the sampling dates. greater than 1 for all the sampling dates. The densities of the class Gastropoda were higher than those of The densities of the class Gastropoda for all the dates were the classes Bivalvia and Polyplacophora in the samples from June higher than those of the classes Bivalvia and Polyplacophora. The and September, whereas the class Bivalvia showed higher densities highest density of the class Gastropoda was seen in September of than those of the classes Gastropoda and Polyplacophora in 2001, whereas the lowest was recorded in December of 2000. The samples from December of 2000 and March and December of

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

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 0.2 0.05 0.3 3.25 Fissurella nigrocincta 0.05 0.0 0.05 2.1 2.1 Hojfmannola hansi 0.35 0.7 1.4 0.0 0.0 Nodilittorina áspera 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 patula 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 1.85 878 VALDÉS-GONZÁLEZ ET AL.

the two sites was not statistically significant (Mann-Whitney = 6.0, P = 0.222). At the exposed site, the mean total density of dominant mol• luscs of the class Gastropoda for all the samples was 52.43 organ• isms/m^ (SD = 21.24), whereas at the sheltered site it was 44.05 r organisms/m^ (SD — 33.03). A comparison of the two mean val• i úes showed no statistical difference (Mann-Whitney = 9.0, P = 0.465). For the class Bivalvia, there were 9.48 organisms/m^ (SD = 7.13) at the exposed site and 42.5 organisms/m^ (SD = 16.23) on the sheltered shore. The two locations showed high diversity valúes, and in comparing the mean Shannon-Wiener índices for the five coUections, -Exposed there were no statistically significant differences (Mann-Whitney = 8.0, P = 0.421) (Fig. 2).

DISCUSSION December-OO June-01 Seplember-01 Sampling dates This study found a greater number species than that reported to Figure 2. Sliannon-Wiener diversity index valúes, observed for the date for the upper part of middie intertidal zone of the study área. exposed and sheltered sites on Isla La Roqueta, December 2000 to Villalpando-Canchola (1986) collected 34 species at Isla la Ro• December 2001. queta, in an área referred to as level L which included the upper part of middie intertidal zone, whereas García-Lopez (1994) found 200 L The highest density for the class Gastropoda was seen in 36 species in the same zone. September of 2001, whereas the lowest was recorded in December The exposed site showed a greater richness, more confined of 2000. The class Bivalvia showed a higher density in December species, a higher number of dominant species, but a lower density. of 2001 and the lowest in September of the same year (Table 3). In all the samples, the class Gastropoda showed the greatest den• The Shannon-Wiener index for the annual cycle was 2.863 sity. This site exhibited greater variation with respect to the num• bits/individual, oscillating among the different samples from 1.972 ber of dominant species among the sampling dates, and only two to 2.911 bits/individual (Fig. 2). of these species had the characteristic of persisting during the entire cycle as dominant species. These findings indícate that the Both Sites dominant community showed more mobility among its species. The sheltered site showed less richness, fewer confined species, Thirty species (68.8%) were common to both sites. Eleven a lower number of dominant species, but a higher density. Among confined species were found at the exposed location, whereas there the samples, the highest densities per date alternated between the were only 3 at the sheltered site (Table 1). In the annual cycle, the classes Gastropoda and Bivalvia. This location exhibited less Jaccard similarity index was 0.682 and the Morisita-Horn quanti• variation with respect to number of dominant species among the tative index was 0.622. sampling dates, and six of these species showed the characteristic The mean number of species found per date was 23 species (SD of remaining the dominant species throughout the entire cycle. = 2.12) for the exposed shore and 19.8 species (SD = 3.03) for These findings indícate that the dominant community had less the sheltered one. The difference between these mean valúes for mobility among its species.

TABLE 3. Densities (organisms/m^) of the dominant species for sampling dates in the sheltered site to wave action on Isla La Roqueta, Acapulco, Guerrero México.

Classes/Species Dec-00 Mar-01 Jun-01 Sept-01 Dec-01 Gastropoda Columbella fuscata 1.3 0.0 0.05 0.0 0.0 Nodilittorina áspera 0.0 0.55 22.0 79.45 2.35 Nodilittorina modesta 1.0 2.05 7.05 3.35 0.95 Nerita scabricosta 0.0 0.15 1.4 5.05 0.35 Petaloconchus complicatus 5.05 7.15 1.15 2.8 6-8 Petaloconchus macrophracma 0.75 6.9 0.95 0.45 0.4 Plicopurpura patula pansa 9.0 6.6 7.4 6.7 9.55 Mancinella triangularis 6.1 2.6 5.15 3.15 4.55 Total 23.2 26.0 45.15 100.95 24.95 Bivalvia Brachidontes semilaevis 11.4 4.45 15.0 6.05 23.25 Chama equinata 24.5 42.05 20.8 14.25 39.55 Isognomom janus 1.25 1.65 2.75 2.0 3.55 Total 37.15 48.15 38.55 22.3 66.35 MOLLUSCAN COMMUNITIES ON LA ROQUETA, ACAPULCO, MÉXICO 879

Lewis (1964) found a greater richness and more confined spe• logic community, whereas in the exposed site these valúes showed cies in an exposed site, and Del Río-Zaragoza and Villarroel-Melo more gradual changes. (2001) also determined a greater number of species in an exposed In this study, the valué for the Shannon-Wiener diversity mea• site, whereas Spight (1978) reported a greater abundance in a sure for the exposed site was greater when considering the annual sheltered site. However, García-Lopez (1994) reported a greater cycle. In a study of the same locations where the present work was number of species at Isla la Roqueta for the sheltered sites studied. conducted, García-Lopez (1994) recorded a lower H' index valué The density of molluscs for the combined dominant species that at the exposed site (1.606 bits/individual), and higher valué for the belonged to the class Gastropoda did not differ between the two sheltered site (3.817 bits/individual). locations studied, and gastropods showed the highest abundance of The number of common species between the two locations was species at both locations. Holguín-Quiñones and González- very high, which was reflected in the valué for the Jaccard index. Pedraza (1989), Castillo-Rodríguez and Amezcua-Linares (1992), However, García-Lopez (1994) reported a lower index (0.375) for García-Lopez (1994), and Villarroel et al. (2000) also reported that these same locations. According to the Morisita-Hom index, the gastropods displayed the greatest species richness among the two sites were quantitatively very similar. classes of molluscs. The malacologic community of the upper part of middie inter• On the other hand, the density of molluscs for the combined tidal zone of two locations with different exposure to waves was dominant species belonging to the class Bivalvia did show differ• found to be very similar, contrary to the proposed hypothesis of ences between the two sites, such that the sheltered shore showed this investigation. However, there are some differences that are a greater number of bivalves. important to point out. At the sheltered shore, seasonal changes in The molluscs of the class Polyplacophora was recorded as richness and diversity were greater despite the lesser variation in dominant species only at the exposed location. its dominant community. At the exposed shore, these changes were The percentage of cumulative dominance of the species that more gradual over time despite a greater variation in the dominant had densities equal to or greater than 1 organism/m^ was similar community, reflecting a greater stability of the malacologic com• for the two sites and was over 98% of the whole malacologic munity at this site. community. ACKNOWLEDGMENTS A comparison of the mean valúes for species richness for all the sampling dates did not show significant differences, indicating that The authors thank CONACyT for the support offered for the the two locations are similar with regard to richness. This same development of this research and Dr. Gorgonio Ruiz Campos for situation was seen when comparing the mean valúes for the diver• bis review of the manuscript and valuable comments. Dr. Enrique sity index H', which indicates that the sites studied over the annual Villalpando Canchóla, provided invaluable help throughout the cycle can be considered equally diverse. However, the valúes re• study. We appreciate the comments and suggestions from Dr. corded for both richness and the H' index of the sheltered site for Emilio Michel Morfín and Dr. Víctor Landa Jaime. We also thank each of the samples, demonstrated greater changes in the malaco• Dr. Albert Leyva for his help in the preparation of the manuscript.

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