The Commission for Environmental Cooperation of North America determined the existence of 24 marine eco- regions. Seven ecoregions are located in Mexico. The Mexican Pacific Transitional (MPT) forms part of these seven regions and is composed of the states of Jalisco, Colima, Michoacán, Guerrero and Oaxaca and the south- ern tip of Baja California Sur. It is characterized as a region of high productivity, as a result of warmer ocean temperatures; tropical marine fauna is very different to that found in areas influenced by the Humboldt and Cali- fornia currents.

The coasts of MPT are comprised of cliffs and have rocky and sandy bottoms. These sites are suitable for de- velopment of biotic communities. Mollusks are part of these communities and this group has pointed out that knowledge is scarce and we have a great ignorance of the status these organisms in the intertidal zone of the MPT [1] .

For the State of Oaxaca the National Commission for the Knowledge and Use of Biodiversity (Comision Na- cional Para el Uso y Conocimiento de la Biodiversidad; CONABIO) stated that there are five marine priority areas for the conservation of coastal and ocean biodiversity; about these regions CONABIO has highlighted the lack of knowledge about the diversity of marine species [2] , hence it is necessary to conduct research to gain in- formation on the species diversity and in addition to analyze the ecological aspects of populations and communi- ties.

There are reports of mollusks for different MPT sites that deal with biodiversity [3] - [9] and ecological para- meters, among which species richness, density, diversity index, dominance and sizes were considered [10] - [24] . Other reports provide information on the biology and ecology of mollusks and commercial use [25] [26] , mol- lusks coral area [27] [28] and continental shelf [29] [30] . Mollusk zonation reports were also found [12] [31] [32] .

The lack of knowledge about mollusks in many places of the MPT, was the principal motivation for carrying out this research and focusing on the GASTROPODA, BIVALVIA and POLYPLACOPHORA Classes and ob- jectives were: 1) to determine species richness; 2) to analyze the composition of the community based on the most represented families; 3) to estimate the density; 4) to determine the distribution of the species in the study area; 5) to analyze the size composition of populations; and 6) to estimate the diversity index.

5862 specimens were examined. 68 mollusks species were identified: 49 species of GASTROPODA Class, eight BIVALVIA and 11 POLYPLACO2

N.: Number of test organism per site; 1: Carrizalillo; 2: Coral; 3: Punta Cometa; De.: Density; D.: Distribution; W.: Wide; R.: Regular; I.: Irregular; Min.: Minimum; Max.: Maximum; Ave.: Average; and Sd.: Standard deviation.

N.: Number of test organism per site; 1: Carrizalillo; 2: Coral; 3: Punta Cometa; De.: Density; D.: Distribution; W.: Wide; R.: Regular; I.: Irregular; Min.: Minimum; Max.: Maximum; Ave.: Average; and Sd.: standard deviation.

On tours conducted outside of the quadrants, they found four species of OPISTHOBRANCHIA Subclass: Berthelina chloris (Dall, 1918), Elysia sp1, Glossodoris sedna (Marcus y Marcus, 1967) and Mexichromis tura (Marcus y Marcus, 1967).

The GASTROPODA Class, showed the highest relative abundance (67.66%), followed by BIVALVIA (27.31%) and POLYPLACOPHORA (5.03%).

26 Families were identified, of which 17 are of the GASTROPODA Class, four are from the BIVALVIA Class and five are of the POLYPLACOPHORA Class.

The Families of the GASTROPODA Class better represented in species richness were FISSURELLIDAE (nine species), MURICIDAE (eight species), LOTTIIDAE (six species) (Figure 1). The Family best represented in relative abundance was LOTTIIDAE with 38.25%, followed by SIPHONARIDAE with 17.57%. In the BIVALVIA Class, the best represented families in species richness were MYTILIDAE (three species) and CHAMIDAE (two species).

The best represented in relative abundance was MYTILIDAE with 87.75%. In the POLYPLACOPHORA Class, the best represented families in species richness were ISCHNOCHITONIDAE and CHITONIDAE with 3 species each. The best represented Family in relative abundance was CHITONIDAE with 57.28%.

The estimated density was 117.84 organisms/m², which is divided into 79.32 gastropods/m², 32.02 bivalves/m² and 5.9 polyplacophorans/m².

The species of the GASTROPODA Class that showed the highest density were Lottia discors (Philippi, 1849) (13.48 organisms/m²), Siphonaria palmata Carpenter, 1857 (10.64 organisms/m²), Lottia acutapex (Berry, 1960) (8.46 organisms/m²), and Nodilittorina modesta (Philippi, 1846) (6.44 organisms/m²). In BIVALVIA Class, spe- cies that showed the highest density were Chormoytilus pallopunctatus (Carpenter, 1857) (14.5 organisms/m²), Brachidontes adamsianus (Dunker, 1857) (13.56 organisms/m²), Isognomon janus Carpenter, 1857 (1.84 organ- isms/m²), and species POLYPLACOPHORA Class showing the highest density were Chiton articulatus Sower- by, 1832 (2.38 organisms/m²) and Ischnochiton muscarius Reeve, 1847 (2.24 organisms/m²) (Figure 1).

The species with the widest distribution (Figure 1) were: L. mesoleuca, L. mitella, L. discors, F. gemmata, H. hansi, S. palmata, B. adamsianus and C. articulatus and represent 11.76%. With regular distribution was rec- orded at 29.41% and irregular distribution was found in 58.82% of the species.

In GASTROPODA Class Crucibulum monticulus had the greatest size with 59.20 mm (minimum = 21.84 mm, mean = 35.48 mm and standard deviation = 10.08). In GASTROPODA Class Crucibulum monticulus had the greatest size, and Nodilittorina modesta the smallest size, with 1.13 mm (maximum = 13.72 mm, mean = 4.45 mm and standard deviation = 1.74). In BIVALVIA Class Choromytilus palliopunctatus showed the greatest size, 55.99 mm (minimum = 4.90 mm, mean = 17.19 mm and standard deviation = 7.70 mm) and B. adamsianus the smallest size and was 2.16 mm (maximum = 13.81 mm, mean = 5.87 mm and standard deviation = 1.26 mm), (Figure 1).

In POLYPLACOPHORA Class, Chiton articulatus showed the greatest size and was 57.69 mm (minimum = 5.71 mm, mean = 27.56 mm and standard deviation = 13.09 mm) and showed the smallest size Lepidochitona sp1 and was 3.19 mm (maximum = 3.19 mm, mean = 3.19 mm).

The value of diversity index (H’) calculated for the study area was H’ = 4.29 bits/ind and value Pielou index (J’) was J’ = 0.72. By study site: Carrizalillo H’ = 3.70 bits/ind. and J’= 0.0.68; Coral H’ = 3.57 bits/ind. and J’= 0.68; Punta Cometa H’ = 2.68 bits/ind. and J’ = 0.62.

In research conducted in one sampling site in the rocky intertidal zone of Oaxaca [7] [11] [20] , they found fewer species which is reported in this research.

The greater number of species recorded in this study is attributed to sampling that was conducted at three sites that vary in type of substrate, stability and wave exposure, so the composition of the microhabitat is considera- bly varied and thus, a greater number of species is supported.

Reference [9] investigated the marine mollusks in the state of Oaxaca. Sampling was carried out by at sites with rocky and sandy substrates, located in the intertidal and subtidal zone, including a greater number of habi- tats, they found more species than is reported in this research.

In composition classes, all research was conducted at the respective sites in the rocky intertidal zone in the State of Oaxaca agree that the GASTROPODA Class has the highest species richness, a result that was also re- ported in the present investigation.

The BIVALVIA Class has been reported with higher species richness after GASTROPODA Class [7] [9] [11] . This research does not match this result, because after the GASTROPODA Class, found the highest species richness POLYPLACOPHORA Class, this result is equal to that reported for Acapulco [17] [22] [24] , who sam- pled only in the rocky intertidal zone of Acapulco.

In the GASTROPODA Class, the families best represented in species richness were FISSURELLIDAE and MURICIDAE, [17] also reported these families as being among the greatest in species richness. Other research reports to the MURICIDAE Family as the best represented in species richness [7] [11] [16] [24] .

In the BIVALVIA Class, MYTILIDAE and CHAMIDAE Families were better represented in species richness, as was reported for Acapulco by Galeana-Rebolledo et al. (2012). The CHAMIDAE Family was also reported by [17] as being the best represented in species richness.

In POLYPLACOPHORA Class families who turned out to be better represented in species richness were ISCHNOCHITONIDAE and CHITONIDAE, as was reported for Acapulco by [22] . The ISCHNOCHITONIDAE Family was also reported by [17] as the best represented in species richness.

The Families with higher abundance in the GASTROPODA Class were LOTTIDAE and SIPHONARIDAE, in BIVALVIA Class was the MYTILIDAE Family and POLYPLACOPHORA Class had the highest abundance CHITONIDAE Family. For the GASTROPODA Class, this research does not agree with what has been reported in research conducted on other sites in the Mexican Pacific Transitional, as occurs with [24] , who found a COLUMBELLIDAE and TROQUIDAE as most abundant. In the BIVALVIA Class, MYTILIDAE Family also was reported by [11] and [20] as the most abundant, which differs from that reported for Acapulco by [22] [24] , who found the CHAMIDAE Family to be the most abundant. In the POLYPLACOPHORA Class CHITONIDAE Family was also reported to be the best represented in abundance by [18] [22] [24] .

The density values estimated for the study area are high. Other studies have also reported high densities in the region such as [15] [20] [17] [24] . In this research, the species with the highest density values for GASTROPODA, BIVALVIA and POLYPLACOPHORA Classes do not coincide with those reported in other research carried out in the Mexican Pacific Transitional.

Reference [21] reported for the state of Guerrero that 48% of mollusks species in the rocky intertidal zone of Guerrero had a wide distribution and 22.6% of the species showed irregular distribution. In the sites analyzed in this investigation, the number of species with wide distribution is considerably lower than that reported in Guer- rero and the percentage of species with irregular distribution is significantly higher.

The GASTROPODA Class has the largest number of species with irregular distribution.

Regarding the size of the species, is coincides with reports from [17] [22] - [24] , in which the largest Class size in GASTROPODA recorded in the genera Crucibullum, Lottia and Leucozonia; In BIVALVIA Class size is presented in most of the genera Choromytilus and Chama. The largest size in POLYPLACOPHORA Class, oc- curred in Chiton articulatus.

The results of this investigation indicate that mollusk species associated with the rocky intertidal zone, the maximum size rarely exceeds seventy millimeters in length. The same was reported for other sites that are lo- cated in the Mexican Pacific Transitional.

In the Mexican Transitional Pacific for mollusks associated with the rocky intertidal zone, have had high val- ues reported for the diversity index of Shannon-Weaver [20] [22] [24] . For mollusks associated with the rocky intertidal zone in the Transitional Mexican Pacific, there are reports of high values for the Shannon-Weaver in- dex.

The value of diversity index found by this research is high and corresponds to that expected in the rocky sites where this research was conducted.

With regard to uniformity for Transitional Mexican Pacific reports values were found around J’ > 0.60 [10] [17] [22] [24] . The values of J’ reported in this study are similar to what has been reported in the above men- tioned reports.

Species richness found in the study sites corresponds to that expected for rocky intertidal zones located in the Mexican Pacific Transitional. The GASTROPODA Class provides a greater number of species followed by the POLYPLACOPHORA Class.

Families considered as representative in species richness on the rocky intertidal zone of the study sites were FISSURELLIDAE and MURICIDAE (GASTROPODA), CHAMIDAE and MYTILIDAE (BIVALVIA) and CHITONIDAE (POLYPLACOPHORA).

Families considered as representative in abundance on rocky intertidal zones of the study sites were TRO- CHIDAE and COLUMBELLIDAE (GASTROPODA), MYTILIDAE (BIVALVIA) and CHITONIDAE (POLY- PLACOPHORA).

The density found is high and corresponds to that expected in sites that are located in the Mexican Pacific transitional.

Species considered representative by high density values recorded in the study area were, C. pallopuntatus, B. semilaevis and L. discors.

The mollusks community in the rocky intertidal study sites is characterized by small size, because they rarely exceed seventy millimeters. Among the species with larger size are Cruibullum monticulus; Ch. palliopunctatus and C. articulatus.

The values of H’ and J’ indicate that the mollusk community in the study area has a high diversity and high uniformity, corresponding to mature and stable communities in a tropical region.