**2. Biodiversity of gastropods in Ohuira lagoon**

Considering the period between October 2016 and 2017, organisms were collected by some of the authors from this present chapter in order to evaluate the biodiversity of Mollusks within the project named "Community structure of the Mollusks found in the islands of the north of Sinaloa, México" (Register number DSA/103.5/16/10277). Sampling stations were established in Patos (25°20′450″ N, 109°00′531″ W), Bledos (25°18¨350″ N, 109°00′458″ W), Bleditos (25°14′566″ N, 109°00′664″ W), Tunosa (25°15′785″ N, 109°00′924″ W) and Mazocahui (25°34′154″ N, 109°00′855″ W) islands in the Ohuira lagoon. To collect gastropod Mollusks we took as reference six quadrants of 1 m<sup>2</sup> dimension in the zone exposed to the waves and three quadrants in the area not exposed to the tides. The organisms were collected from the sand, mud, silt-clay and rocky soil, which were representative from the study area. Thereupon, in soft substrates the harvest was made manually, and those organisms that were found adhered to rocky substrates were removed with a scraper, chisel or hammer. In addition, those organisms that were found at a greater depth were collected by snorkeling. The collected Mollusks were stored in plastic bags with their corresponding label, according to the type of sample method. The organisms were conserved in ice to be transferred to the biology lab at the Universidad de Occidente Unidad Los Mochis, Ahome, Sinaloa, Mexico. Taxonomic keys were used to identify the gastropod Mollusks [46–53].

#### **2.1. Alpha diversity measurement**

The analysis of the community structure of gastropod Mollusks was based on ecological indexes that quantified the information given by the lagoon system, which were applied based on each island and whether the organisms were exposed or not to the tide. To represent the biodiversity of gastropods we used the following indexes:

In previous studies held in 2014 in the intertidal rocky zone (beach and mangrove area) from the Ohuira and Topolobampo Bays (Ahome), Sinaloa, the collected organisms represented a highly important trophic phase. The biodiversity and distribution of the community of epibenthic invertebrates was composed by a specific richness (S) of 168 species, divided in 10 taxonomic groups: 3 porífera, 2 cnidarians, 2 platyhelminths, 35 annelids, 2 sipunculids, 74 mollusks, 46 crustaceans, 1 pycnogonida, 1 ectoprocta, and 2 echinoderms, where Mollusks were the most predominant group with 74 species. The dominant Mollusks species were *Neritina* sp., and *Cerithium stercusmuscarum*. The epibenthic distribution was influenced by

The interest in studying biodiversity is linked to the lack of knowledge that exists over its magnitude, the processes that determine it and the constant loss due to human actions or climate change effects; thus, it is important to know and understand the processes that determine the abundance and distribution of biodiversity under different spatial and temporal scales in the

Considering the period between October 2016 and 2017, organisms were collected by some of the authors from this present chapter in order to evaluate the biodiversity of Mollusks within the project named "Community structure of the Mollusks found in the islands of the north of Sinaloa, México" (Register number DSA/103.5/16/10277). Sampling stations were established in Patos (25°20′450″ N, 109°00′531″ W), Bledos (25°18¨350″ N, 109°00′458″ W), Bleditos (25°14′566″ N, 109°00′664″ W), Tunosa (25°15′785″ N, 109°00′924″ W) and Mazocahui (25°34′154″ N, 109°00′855″ W) islands in the Ohuira lagoon. To collect gastropod Mollusks we

quadrants in the area not exposed to the tides. The organisms were collected from the sand, mud, silt-clay and rocky soil, which were representative from the study area. Thereupon, in soft substrates the harvest was made manually, and those organisms that were found adhered to rocky substrates were removed with a scraper, chisel or hammer. In addition, those organisms that were found at a greater depth were collected by snorkeling. The collected Mollusks were stored in plastic bags with their corresponding label, according to the type of sample method. The organisms were conserved in ice to be transferred to the biology lab at the Universidad de Occidente Unidad Los Mochis, Ahome, Sinaloa, Mexico. Taxonomic

The analysis of the community structure of gastropod Mollusks was based on ecological indexes that quantified the information given by the lagoon system, which were applied

dimension in the zone exposed to the waves and three

salinity and organic matter, enhancing the differences in the Ohuira lagoon [41].

gastropod species, as well as their transformation due to the environment [42–45].

**2. Biodiversity of gastropods in Ohuira lagoon**

keys were used to identify the gastropod Mollusks [46–53].

*1.3.2. Biodiversity of gastropods*

122 Biological Resources of Water

took as reference six quadrants of 1 m<sup>2</sup>

**2.1. Alpha diversity measurement**

The species richness (S) was estimated by counting the number of species because it is the easiest way to measure biodiversity, since it is based on the number of species that are present without considering their importance. The abundance (A) was estimated by counting the number of organisms that were registered in each sampling station. The relative abundance (Pi) represented the existing relation between the organisms of a single species and the total number of organisms from all the species encountered, by using the following equation (Eq. (1)):

$$\text{Pi} = \frac{\text{m}}{\text{N}} \tag{1}$$

where ni is the number of organisms from the "i" species and N is the total number of organisms from all gastropod species.

To identify the dominant species from the community we used the community dominance index (ID) (Eq. (2)) [27, 54]:

$$\text{ID} = \frac{\text{Y1} - \text{Y2}}{\text{N}} \cdot 100\tag{2}$$

where Y1 is the abundance of the most common species, Y2 is the abundance of the species that occupied the second place in abundance, and N is sum of the abundance of all species.

In accordance with the estimators, Pi, ID, H′, as well as with the dominance level, the main species for each island was determined. To represent the sui generis characteristic from the community, we analyzed jointly the abundance (A) and frequency (F) to establish four categories of species which are cataloged as (AF)—highly abundant and very frequent, (aF)—less abundant and highly frequent, (Af)—highly abundant and less frequent, and (af)—less abundant and less frequent [27].

The Shannon-Wiener diversity ecology index measured the average uncertainty degree to predict to which species a randomly chosen individual could belong to within a collection (Eq. (3)) [27, 38, 55, 56]:

$$\mathbf{H}^{\prime} = \Sigma \mathbf{P} i^\* \ln \mathbf{P} i \tag{3}$$

where Pi is proportional abundance of the "i" species.

Pielou's equity measured the proportion of the observed diversity from the maximum expected diversity. Its value ranges from 0 to 1, where 1 corresponds to those situations in which all species are equally abundant (Eq. (4)) [38]:

$$\mathbf{J}\_{\text{a}} = \frac{\mathbf{H}\_{\text{a}}}{\mathbf{H}\_{\text{a}}} \tag{4}$$

where H´: Shannon-Wiener's diversity and H′max: maximum diversity.

Species diversity under conditions of maximum equity, in other words, the species diversity from a sample if all the species (S) had the same abundance equity (Eq. (5)):

$$H^\prime \max \mathbf{w} = \ln \mathbf{S} \tag{5}$$

In the areas where there was tidal exposure a total of 19 gastropod species were found: *Cerithium stercusmuscarum* (n = 704, Pi = 0.130), *Neritina* sp. (n = 271, Pi = 0.050), *Nerita scabricosta* (n = 319, Pi = 0.059), *Nerita funiculata* (n = 505, Pi = 0.0930), *Nassarius luteostoma* (n = 11, Pi = 0.0021), *Crucibulum spinosum* (n = 14, Pi = 0.0026), *Crepidula onix* (n = 10, Pi = 0.00184), *Crepidula rostrata* (n = 8, Pi = 0.0015), *Littorina aspera* (n = 1, Pi = 0.000184), *Littorina modesta* (n = 50, Pi = 0.0092), *Crepidula lessoni* (n = 69, Pi = 0.0127), *Diodora digueti* (n = 4, Pi = 0.00074), *Murex (Recurvirostris) lividus* (n = 11, Pi = 0.0021), *Turritella gnostoma* (n = 1, Pi = 0.000184), *Thais biceralis* (n = 1, Pi = 0.000184), *Hexaplex (Muricanthus) nigritus* (n = 557, Pi = 0.1030), *Hexaplex eristrosthomus* (n = 76, Pi = 0.0140), *Phyllonotus brassica* (n = 15, Pi = 0.0028), and *Melongena patula* (n = 10,

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A total of 11 gastropod species were registered in Patos Island, where *Cerithium stercusmuscarum* showed the highest abundance with 214 organisms in the zone that was not exposed to the tide, whereas a total of 395 organisms were found in the area exposed to the tide. The least abundant species were *Crepidula onyx*, *Crucibulum scutellatum*, *Nassarius gallegosi*, *Nassarius luteostoma*, *Littorina modesta*, and *Murex (recurvirostris) lividus*, which were present with one single organism collected, both in the area exposed to the tide and the one not exposed to it (**Figure 1**).

Bledos Island was the one that showed a higher mollusk diversity with 13 species in which *Cerithium stercusmuscarum* was the most abundant with 41 organisms, whereas *Nerita funiculata* was the least abundant, being represented by only one collected individual (**Figure 1**).

Bleditos Island had a gastropod diversity of 12 recorded species. *Cerithium stercusmuscarum* and *Nerita scabricosta* were the most abundant species in the intertidal area, being represented by 130 organisms. *Crucibulum spinosum* had 180 organisms in the area not exposed to the tide

On Mazocahui Island a total of 9 species were registered from which *Nerita funiculata*, and *Neritina* sp. had the highest abundance with 109 and 124 organisms, respectively in the intertidal area. On the other hand, *Nassarius gallegosi* (17 organisms) and *Cerithium stercusmuscarum* (26 organisms) had the highest abundance in the area not exposed to the waves (**Figure 1**).

Tunosa Island had a diversity of 10 species, from which *Nerita funiculata*, *Nerita scabricosta*, and *Hexaplex (Muricanthus) nigritus* were the most abundant in both zones, the one exposed

Pi = 0.00184).

(**Figure 1**).

*2.1.2. Biodiversity in Patos Island*

*2.1.3. Biodiversity in Bledos Island*

*2.1.3.1. Biodiversity in Bleditos Island*

*2.1.3.2. Biodiversity in Mazocahui Island*

*2.1.4. Biodiversity in Tunosa Island*

The diversity index or Margalef's richness (Dmg), transformed the number of species per sample into a proportion in which species are added by the expansion of the sample. This index assumes that there is a functional relation between the number of species and the total number of organisms [38]:

$$\mathbf{S} = \mathbf{k}\sqrt{N} \tag{6}$$

where k is the constant.

If the constant is not maintained, then the index varies with the sample size in an unknown manner. By using s-1 instead of S, we get Dmg = 0, when there is only one single species (Eq. (7)):

$$Dmg = \frac{S - 1}{\ln N} \tag{7}$$

where S: number of species; and N: total number of organisms.

In order to calculate these indexes the abundance data were transformed into a natural algorithm [27].

#### *2.1.1. Richness (S), abundance (A), and relative abundance (Pi)*

At the Ohuira lagoon we collected a total of 5431 gastropods, being Patos Island the one with the highest abundance of 2135 organisms (39.35%), followed by Bleditos Island with 1471 (27.12%), Tunosa Island with 768 (14.14%), Bledos Island with 649 (11.95%), and Mazocahui Island with 408 organisms, representing 7.44%.

In general, within all the islands that were studied in Ohuira lagoon a total of 22 species of gastropods were collected. In those areas where there was nonexposure to tides, the species that were found were: *Cerithium stercusmuscarum* (n = 333, Pi = 0.0613), *Neritina* sp. (n = 208, Pi = 0.0383), *Nerita scabricosta* (n = 301, Pi = 0.0554), *Nerita funiculata* (n = 207, Pi = 0.0381), *Nassarius luteostoma* (n = 22, Pi = 0.0041), *Nassarius gallegosi* (n = 20, Pi = 0.0037), *Crucibulum spinosum* (n = 217, Pi = 0.040), *Eupleura* sp. (n = 5, Pi = 0.000921), *Crepidula onix* (n = 1, Pi = 0.000184), *Crepidula rostrata* (n = 6, Pi = 0.001105), *Fisurella* sp. (n = 1, Pi = 0.000184), *Littorina aspera* (n = 5, Pi = 0.000921), *Littorina modesta* (n = 14, Pi = 0.0026), *Crepidula lessoni* (n = 10, Pi = 0.00184), *Tegula corteziana* (n = 5, Pi = 0.000921), *Diodora* sp. (n = 3, Pi = 0.00055), *Scurria mesoleuca* (n = 3, Pi = 0.00055), *Diodora digueti* (n = 3, Pi = 0.00055), *Crucibulum scutellarum* (n = 7, Pi = 0.00130), *Murex (Recurvirostris) lividus* (n = 3, Pi = 0.00055), *Terebra* sp. (n = 10, Pi = 0.00184), and *Hexaplex (Muricanthus) nigritus* (n = 10, Pi = 0.00184).

In the areas where there was tidal exposure a total of 19 gastropod species were found: *Cerithium stercusmuscarum* (n = 704, Pi = 0.130), *Neritina* sp. (n = 271, Pi = 0.050), *Nerita scabricosta* (n = 319, Pi = 0.059), *Nerita funiculata* (n = 505, Pi = 0.0930), *Nassarius luteostoma* (n = 11, Pi = 0.0021), *Crucibulum spinosum* (n = 14, Pi = 0.0026), *Crepidula onix* (n = 10, Pi = 0.00184), *Crepidula rostrata* (n = 8, Pi = 0.0015), *Littorina aspera* (n = 1, Pi = 0.000184), *Littorina modesta* (n = 50, Pi = 0.0092), *Crepidula lessoni* (n = 69, Pi = 0.0127), *Diodora digueti* (n = 4, Pi = 0.00074), *Murex (Recurvirostris) lividus* (n = 11, Pi = 0.0021), *Turritella gnostoma* (n = 1, Pi = 0.000184), *Thais biceralis* (n = 1, Pi = 0.000184), *Hexaplex (Muricanthus) nigritus* (n = 557, Pi = 0.1030), *Hexaplex eristrosthomus* (n = 76, Pi = 0.0140), *Phyllonotus brassica* (n = 15, Pi = 0.0028), and *Melongena patula* (n = 10, Pi = 0.00184).

#### *2.1.2. Biodiversity in Patos Island*

Species diversity under conditions of maximum equity, in other words, the species diversity

*H*´ *max*= *lnS* (5)

The diversity index or Margalef's richness (Dmg), transformed the number of species per sample into a proportion in which species are added by the expansion of the sample. This index assumes that there is a functional relation between the number of species and the total num-

S = k√*N* (6)

If the constant is not maintained, then the index varies with the sample size in an unknown manner. By using s-1 instead of S, we get Dmg = 0, when there is only one single species (Eq. (7)):

In order to calculate these indexes the abundance data were transformed into a natural algo-

At the Ohuira lagoon we collected a total of 5431 gastropods, being Patos Island the one with the highest abundance of 2135 organisms (39.35%), followed by Bleditos Island with 1471 (27.12%), Tunosa Island with 768 (14.14%), Bledos Island with 649 (11.95%), and Mazocahui

In general, within all the islands that were studied in Ohuira lagoon a total of 22 species of gastropods were collected. In those areas where there was nonexposure to tides, the species that were found were: *Cerithium stercusmuscarum* (n = 333, Pi = 0.0613), *Neritina* sp. (n = 208, Pi = 0.0383), *Nerita scabricosta* (n = 301, Pi = 0.0554), *Nerita funiculata* (n = 207, Pi = 0.0381), *Nassarius luteostoma* (n = 22, Pi = 0.0041), *Nassarius gallegosi* (n = 20, Pi = 0.0037), *Crucibulum spinosum* (n = 217, Pi = 0.040), *Eupleura* sp. (n = 5, Pi = 0.000921), *Crepidula onix* (n = 1, Pi = 0.000184), *Crepidula rostrata* (n = 6, Pi = 0.001105), *Fisurella* sp. (n = 1, Pi = 0.000184), *Littorina aspera* (n = 5, Pi = 0.000921), *Littorina modesta* (n = 14, Pi = 0.0026), *Crepidula lessoni* (n = 10, Pi = 0.00184), *Tegula corteziana* (n = 5, Pi = 0.000921), *Diodora* sp. (n = 3, Pi = 0.00055), *Scurria mesoleuca* (n = 3, Pi = 0.00055), *Diodora digueti* (n = 3, Pi = 0.00055), *Crucibulum scutellarum* (n = 7, Pi = 0.00130), *Murex (Recurvirostris) lividus* (n = 3, Pi = 0.00055), *Terebra* sp. (n = 10, Pi = 0.00184), and *Hexaplex (Muricanthus)* 

*S* − 1

*lnN* (7)

from a sample if all the species (S) had the same abundance equity (Eq. (5)):

ber of organisms [38]:

124 Biological Resources of Water

where k is the constant.

rithm [27].

*Dmg* = \_\_\_\_

where S: number of species; and N: total number of organisms.

*2.1.1. Richness (S), abundance (A), and relative abundance (Pi)*

Island with 408 organisms, representing 7.44%.

*nigritus* (n = 10, Pi = 0.00184).

A total of 11 gastropod species were registered in Patos Island, where *Cerithium stercusmuscarum* showed the highest abundance with 214 organisms in the zone that was not exposed to the tide, whereas a total of 395 organisms were found in the area exposed to the tide. The least abundant species were *Crepidula onyx*, *Crucibulum scutellatum*, *Nassarius gallegosi*, *Nassarius luteostoma*, *Littorina modesta*, and *Murex (recurvirostris) lividus*, which were present with one single organism collected, both in the area exposed to the tide and the one not exposed to it (**Figure 1**).

#### *2.1.3. Biodiversity in Bledos Island*

Bledos Island was the one that showed a higher mollusk diversity with 13 species in which *Cerithium stercusmuscarum* was the most abundant with 41 organisms, whereas *Nerita funiculata* was the least abundant, being represented by only one collected individual (**Figure 1**).

#### *2.1.3.1. Biodiversity in Bleditos Island*

Bleditos Island had a gastropod diversity of 12 recorded species. *Cerithium stercusmuscarum* and *Nerita scabricosta* were the most abundant species in the intertidal area, being represented by 130 organisms. *Crucibulum spinosum* had 180 organisms in the area not exposed to the tide (**Figure 1**).

#### *2.1.3.2. Biodiversity in Mazocahui Island*

On Mazocahui Island a total of 9 species were registered from which *Nerita funiculata*, and *Neritina* sp. had the highest abundance with 109 and 124 organisms, respectively in the intertidal area. On the other hand, *Nassarius gallegosi* (17 organisms) and *Cerithium stercusmuscarum* (26 organisms) had the highest abundance in the area not exposed to the waves (**Figure 1**).

#### *2.1.4. Biodiversity in Tunosa Island*

Tunosa Island had a diversity of 10 species, from which *Nerita funiculata*, *Nerita scabricosta*, and *Hexaplex (Muricanthus) nigritus* were the most abundant in both zones, the one exposed

**Figure 1.** Diversity and abundance of gastropod mollusks on islands of Ohuira lagoon.

to the tide and the one not exposed to it, while *Turritellla gnostoma*, *Thais biceralis*, *Hexaplex (Muricantus) nigritus*, *Crepidula onix*, *Crucibulum spinosum* and *Neritina* sp. were the least abundant species in the area exposed to the tide (**Figure 1**).

**2.2. Ecological indexes**

Ohuira lagoon, Ahome, Sinaloa, Mexico.

*2.2.1. Dominant species (ID) and Shannon-Wiener's diversity (H´)*

*Cerithium stercusmuscarum* (ID = 32.20%, AF, H´ = 0.3161) was found on five islands with 1037 collected organisms, being Isla Patos the island with the highest abundance and Mazocahui the one with the lowest abundance (35 organisms). *Nerita funiculata* and *Nerita scabricosta* were

**Figure 2.** Dominance of *Cerathium stercusmuscarum, Nerita funiculata*, *Nerita scabricosta*, and *Neritina* sp. on the islands of

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**Figure 2.** Dominance of *Cerathium stercusmuscarum, Nerita funiculata*, *Nerita scabricosta*, and *Neritina* sp. on the islands of Ohuira lagoon, Ahome, Sinaloa, Mexico.

#### **2.2. Ecological indexes**

to the tide and the one not exposed to it, while *Turritellla gnostoma*, *Thais biceralis*, *Hexaplex (Muricantus) nigritus*, *Crepidula onix*, *Crucibulum spinosum* and *Neritina* sp. were the least abun-

dant species in the area exposed to the tide (**Figure 1**).

126 Biological Resources of Water

**Figure 1.** Diversity and abundance of gastropod mollusks on islands of Ohuira lagoon.

#### *2.2.1. Dominant species (ID) and Shannon-Wiener's diversity (H´)*

*Cerithium stercusmuscarum* (ID = 32.20%, AF, H´ = 0.3161) was found on five islands with 1037 collected organisms, being Isla Patos the island with the highest abundance and Mazocahui the one with the lowest abundance (35 organisms). *Nerita funiculata* and *Nerita scabricosta* were the most abundant species on Tunosa Island with 712 (ID = 24.53%, Af, H´ = 0.2662) and 620 (ID = 21.67%, Af, H´ = 0.2478) collected organisms, respectively, in the intertidal zone. *Neritina* sp. was the dominant species and presented the highest abundance on Patos Island with 479 collected organisms (ID = 13.07%, aF, H´ = 0.2142), the sum of the dominance of these species was of 91.47%, where 8.53% corresponds to the rest of the remaining gastropod species. The biometrics of the dominant species on the islands of the Ohuira lagoon, Ahome, Sinaloa, México were as follows: *Cerithium stercusmuscarum* with 26.65 ± 0.146 mm in length*, Nerita funiculata* with 16.03 ± 0.118 mm in length*, Nerita scabricosta* with 39.03 ± 1.46 mm in length*, Neritina* sp. with 8.9 ± 0.205 mm in length (**Figure 2**).

**3.1. Importance in ecology, biology and taxonomy**

each case [41, 52].

coastal shallow waters [52].

In Ohuira lagoon, Ahome, Sinaloa, the ecological importance in particular of the species of gastropod Mollusks is related to trophic levels, since there are organisms that are of carnivorous feeding habits such as the black murex *Hexaplex (Muricanthus) nigritus*, the prince murex *Hexaplex (Muricanthus) prínceps*, the cabbage murex *Phyllonotus brassica*, the pinkmouthed murex *Phyllonotus erythrostoma*, Regal murex *Phyllonotus regius*, the Pacific melongena *Melongena patula*, the giant Eastern Pacific conch *Strombus galeatus*, and the Pacific cask shell *Malea ringens,* which can feed on other smaller gastropods such as *Cerithium stercusmuscarum*, the onyx slipper shell *Crepidula onyx*, *Terebra armillata, Hormospira maculosa*, and *Fusinus (Barbarofusus) colpoicus,* to mention some species. It is important to emphasize that both gastropods, the black murex *Hexaplex (Muricanthus) nigritus* and the ambiguous murex *Hexaplex (Muricanthus) ambiguus*, are sometimes considered as northern subspecies of the species radish murex *Hexaplex (Muricanthus) radix* [52]. Considering the gastropod species with the greatest economic-commercial importance in the study area, the biology is described for

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The distinctive features of the gastropod *Hexaplex (Muricanthus) nigritus*, are that it belongs to the family Muricidae, it has a relation of synonymy with *Murex nigritus* and *Muricanthus nigritus*; it has a very large, robust, bulbous shell with a moderately prominent conical spiral and a wide body turn. It presents six to nine strong spinal mandibular varices in the back of the body, crossed by spiral ribs intermixed with smaller ribs. It has relative scarce thorny acute varices, and those located on the shoulder and on the basis of the longer shell. It has a wide oval opening with a small rear channel and a wide siphoned channel, fairly well developed and slightly curved. It presents a strongly crenulated outer lip and an internal lip with a columellar adherent callus and a spiral crest on its back, and the nucleus of the operculum presents an anterior position. The color of the outer surface is white opaque, with a blackish-brownish dye on the ribs, spirals and thorns, and the opening is porcelain white. The maximum size reported was 150 mm, although the most common size is 120 mm. The reported habitats of the species were reefs or sandy bottoms in the intertidal zone and sub-

*3.1.1. Gastropods black murex Hexaplex (Muricanthus) nigritus (Philippi, 1845)*

*3.1.2. Pink-mouthed murex gastropod Phyllonotus Erythrostoma (Swainson, 1831)*

It belongs to the Muricidae family, the synonyms used are *Murex erythrostoma*, *Chicoreus erythrostoma,* and *Hexaplex erythrostoma*. The distinctive features are a large, robust, globose-oval shell, with a short conical spiral and a wide body turn; four or five thick axial varices around the body, alternating with tubercular axial ridges; six to seven spiral crests that form nodules in the intervarical ridges and become open and sharp spines on the varices, being stronger in the shoulder; oval opening with a small posterior canal and a wide siphonel canal, relatively short and curved; an external erect and crenulated lip; an internal lip with a thin, expanded columellar callus and the nucleus of the operculum with an anterior position. The outer surface is opaque white and it has a bright pink opening. The maximum size reported was 150 mm,

#### *2.2.2. Pielou's equity*

#### *2.2.2.1. Comparison between dominant species in tide exposed areas (J´1 ) and non-exposed areas (J´2 )*

The gastropods *Cerithium stercusmuscarum* (J´<sup>1</sup> = 0.5413, J´<sup>2</sup> = 0.8390) was recorded on five islands with 1037 collected organisms, being Patos Island the one with the highest abundance and Mazocahui Island the one with the lowest (35 organisms), finding a higher tendency of the proportion of the observed diversity in those nonexposed areas. *Nerita funiculata* and *Nerita scabricosta* were the most abundant species on Tunosa Island with 712 (J´<sup>1</sup> = 0.4677, J´<sup>2</sup> = 0.8298) and 620 (J´<sup>1</sup> = 0.6469, J´<sup>2</sup> = 0.6739) respectively, recording the highest equity index in the areas exposed to the tides. *Neritina* sp. dominated with the highest abundance on Patos Island with 479 collected organisms (J´<sup>1</sup> = 0.5831, J´<sup>2</sup> = 0.6993) showing the same tendency of high equity in the nonexposed areas.
