**1. Introduction**

The Gulf of California is one of the most important marine ecosystems in Mexico and one of the most productive and biodiverse in the planet, as well as being one of the least disturbed. There we find 922 islands [1] that stand out for their high diversity in species, its high level of endemic species and a great biological richness, features that have allowed these places to be considered as natural evolutionary laboratories [2]. Mollusks within the marine ecosystem play a big role in the energy flux and community structure, due to the fact that many of them work as ecological regulators [3, 4] and indicators of disruptions that take place in these systems [5]. Besides, they constitute an abundant and ecological important group due to the functions performed by each one of their members within the food web, nutrient recirculation and energy flux [6].

**1.2. Geography and oceanography of the Ohuira lagoon**

Ohuira lagoon, with 125 km2

**1.3. Biodiversity of mollusks**

cial and nutritional values [37].

*1.3.1. Current status*

The Ohuira lagoon connects itself with a 700-m-width cannel at Topolobampo Port. The

the Fuerte River, which extended through the Topolobampo Bay, discharging its waters into this port. It is an area of shallows that during the rain period presents a deep zone of variable location depending on the tides and sediment dragging and presents a branch system that connects it with Navachiste Bay. In total, the system has eight islands: five within the Ohuira lagoon: Patos, Bledos, Bleditos, Tunosa, and Mazocahui (I and II) [29]. The circulation of the maximum currents in the lagoon's mouth is of 1.15 m s−1, and in the channels of 1.10 m s−1 [30].

Mollusks are one of the zoological groups with more biological success, as much for its number of living species as for the habitat diversity they colonize [31]. Within the marine ecosystem, Mollusks play an important role in the energy flux and the community structure, due to the fact that many of them work as ecological regulators [32] and as disturbance indicators inside these systems [5]. In addition, they constitute an abundant and ecologically important group because of the role that each member performs within the food web, nutrient recycling and energy flux [6]. Inside this group, there are primary consumers, both herbivores and detritivores, second-level predators and specialized parasites, as well as opportunistic species, which indicates different answers to habitat modifications and pollution [33]. These organisms possess one of the most widespread distributions in the planet, ranging from the coastline to great sea depths [34]. The highest ability of Mollusks to adapt has given them a huge success along their evolution, and they have colonized terrestrial, damp and freshwater habitats [35], as far as deserts and polar areas, as well as the tropics and great sea depths [36], being widely studied due to their social and economic importance, as well as their commer-

Many diverse studies about Mollusks have been undertaken in the Gulf of California. Nevertheless, the available information regarding the community structure of the group is scarce. Such investigations contribute important information because density variations from specific populations can be known in a specific period of time and locality, as well as the abundance and composition of a community within a natural gradient or when pollution or illness problems exist in the environment [38, 39]. In 2008, in the Guasave municipality, Sinaloa, Mexico, a Mollusk census was performed in the intertidal zone from La Mapachera, Tesobiate, La Huitussera, San Lucas, Guasayeye, Nescoco, El Metate and Las Chivas islands from the lagoon system known as Navachiste, in order to elaborate taxonomic lists and an intertidal species diagnosis. The collected Mollusks were located systematically in four classes (Gastropod, Bivalvia, Polyplacophora and Cephalopoda), 40 families and 81 species. Gastropods represented 59% with 24 families and 46 species, bivalves constituted 43% with 14 families and 34 species, polyplacophora comprised 3% with 2 families and 2 species and the

remaining 1% corresponded to cephalopods with 1 family and 1 species [40].

(12,500 has) of surface, was the river basin of an ancient canal of

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Mollusks are mainly used in benthic studies to relate their presence/absence and/or dominance, with the aim to set their relationship with types of seabed and substrate [7]. Furthermore, they help to establish a baseline for future follow-up and evaluation programs [8]. Life cycles, a high level of stress tolerance [7], an intimate relationship with the sediment and a high response toward disturbances [9] make them ideal organisms to study natural and anthropogenic environmental changes [10, 11].

As a framework, most of the published research about Mollusks in the Mexican Pacific has to do with faunistic studies and taxonomy, whereas others talk about diversity aspects and variation through time [12–17]. Additional studies relate to distribution and abundance [18–24] and ecology [25–27]. Based on these previous studies, there is a lack of current information about the biology and ecology of the community of Mollusks in the intertidal zone from the islands of the Gulf of California; henceforth, it is necessary to do research that can increase and deepen the knowledge about the composition, abundance, and diversity of Mollusks.

#### **1.1. Geography and oceanography in the Gulf of California**

The Baja California Peninsula encloses the Gulf of California and is one of the most remote peninsular areas of the world. The gulf is a big semi-enclosed sea with more than 1100 km in length, 100–200 km in width, and with 258,593 km (99,843 mi<sup>2</sup> ) of surface which comprises more than 9° of latitude which cross the Tropic of Cancer in its southern part, extending to Cabo Corrientes (Jalisco, Mexico). It is the home of more of 900 islands and islets; it gives place to a highly rich and diverse habitat region for the evolutionary forces which in turn shape its flora and fauna. The northeast part of the gulf covers around 60,000 km<sup>2</sup> (24,000 mi2 ) of sea surface, it extends to 3° latitude, and it is a unique water body in many ways. The weather is very dry, with an annual rainfall of less than 100 mm. The array of average monthly air temperatures in the northern gulf is of 18°C. The northern gulf presents some of the biggest tides in the world. The annual tide (amplitude) in San Felipe and Puerto Peñasco comprises around 7 m and in the Delta of the Colorado River, at the highest part of the gulf reaches almost 10 m [28].

#### **1.2. Geography and oceanography of the Ohuira lagoon**

The Ohuira lagoon connects itself with a 700-m-width cannel at Topolobampo Port. The Ohuira lagoon, with 125 km2 (12,500 has) of surface, was the river basin of an ancient canal of the Fuerte River, which extended through the Topolobampo Bay, discharging its waters into this port. It is an area of shallows that during the rain period presents a deep zone of variable location depending on the tides and sediment dragging and presents a branch system that connects it with Navachiste Bay. In total, the system has eight islands: five within the Ohuira lagoon: Patos, Bledos, Bleditos, Tunosa, and Mazocahui (I and II) [29]. The circulation of the maximum currents in the lagoon's mouth is of 1.15 m s−1, and in the channels of 1.10 m s−1 [30].

#### **1.3. Biodiversity of mollusks**

#### *1.3.1. Current status*

**1. Introduction**

120 Biological Resources of Water

and energy flux [6].

Mollusks.

(24,000 mi2

genic environmental changes [10, 11].

part of the gulf reaches almost 10 m [28].

**1.1. Geography and oceanography in the Gulf of California**

in length, 100–200 km in width, and with 258,593 km (99,843 mi<sup>2</sup>

The Gulf of California is one of the most important marine ecosystems in Mexico and one of the most productive and biodiverse in the planet, as well as being one of the least disturbed. There we find 922 islands [1] that stand out for their high diversity in species, its high level of endemic species and a great biological richness, features that have allowed these places to be considered as natural evolutionary laboratories [2]. Mollusks within the marine ecosystem play a big role in the energy flux and community structure, due to the fact that many of them work as ecological regulators [3, 4] and indicators of disruptions that take place in these systems [5]. Besides, they constitute an abundant and ecological important group due to the functions performed by each one of their members within the food web, nutrient recirculation

Mollusks are mainly used in benthic studies to relate their presence/absence and/or dominance, with the aim to set their relationship with types of seabed and substrate [7]. Furthermore, they help to establish a baseline for future follow-up and evaluation programs [8]. Life cycles, a high level of stress tolerance [7], an intimate relationship with the sediment and a high response toward disturbances [9] make them ideal organisms to study natural and anthropo-

As a framework, most of the published research about Mollusks in the Mexican Pacific has to do with faunistic studies and taxonomy, whereas others talk about diversity aspects and variation through time [12–17]. Additional studies relate to distribution and abundance [18–24] and ecology [25–27]. Based on these previous studies, there is a lack of current information about the biology and ecology of the community of Mollusks in the intertidal zone from the islands of the Gulf of California; henceforth, it is necessary to do research that can increase and deepen the knowledge about the composition, abundance, and diversity of

The Baja California Peninsula encloses the Gulf of California and is one of the most remote peninsular areas of the world. The gulf is a big semi-enclosed sea with more than 1100 km

prises more than 9° of latitude which cross the Tropic of Cancer in its southern part, extending to Cabo Corrientes (Jalisco, Mexico). It is the home of more of 900 islands and islets; it gives place to a highly rich and diverse habitat region for the evolutionary forces which in turn shape its flora and fauna. The northeast part of the gulf covers around 60,000 km<sup>2</sup>

ways. The weather is very dry, with an annual rainfall of less than 100 mm. The array of average monthly air temperatures in the northern gulf is of 18°C. The northern gulf presents some of the biggest tides in the world. The annual tide (amplitude) in San Felipe and Puerto Peñasco comprises around 7 m and in the Delta of the Colorado River, at the highest

) of sea surface, it extends to 3° latitude, and it is a unique water body in many

) of surface which com-

Mollusks are one of the zoological groups with more biological success, as much for its number of living species as for the habitat diversity they colonize [31]. Within the marine ecosystem, Mollusks play an important role in the energy flux and the community structure, due to the fact that many of them work as ecological regulators [32] and as disturbance indicators inside these systems [5]. In addition, they constitute an abundant and ecologically important group because of the role that each member performs within the food web, nutrient recycling and energy flux [6]. Inside this group, there are primary consumers, both herbivores and detritivores, second-level predators and specialized parasites, as well as opportunistic species, which indicates different answers to habitat modifications and pollution [33]. These organisms possess one of the most widespread distributions in the planet, ranging from the coastline to great sea depths [34]. The highest ability of Mollusks to adapt has given them a huge success along their evolution, and they have colonized terrestrial, damp and freshwater habitats [35], as far as deserts and polar areas, as well as the tropics and great sea depths [36], being widely studied due to their social and economic importance, as well as their commercial and nutritional values [37].

Many diverse studies about Mollusks have been undertaken in the Gulf of California. Nevertheless, the available information regarding the community structure of the group is scarce. Such investigations contribute important information because density variations from specific populations can be known in a specific period of time and locality, as well as the abundance and composition of a community within a natural gradient or when pollution or illness problems exist in the environment [38, 39]. In 2008, in the Guasave municipality, Sinaloa, Mexico, a Mollusk census was performed in the intertidal zone from La Mapachera, Tesobiate, La Huitussera, San Lucas, Guasayeye, Nescoco, El Metate and Las Chivas islands from the lagoon system known as Navachiste, in order to elaborate taxonomic lists and an intertidal species diagnosis. The collected Mollusks were located systematically in four classes (Gastropod, Bivalvia, Polyplacophora and Cephalopoda), 40 families and 81 species. Gastropods represented 59% with 24 families and 46 species, bivalves constituted 43% with 14 families and 34 species, polyplacophora comprised 3% with 2 families and 2 species and the remaining 1% corresponded to cephalopods with 1 family and 1 species [40].

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

based on each island and whether the organisms were exposed or not to the tide. To represent

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)):

ni

where ni is the number of organisms from the "i" species and N is the total number of organ-

To identify the dominant species from the community we used the community dominance

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 abun-

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

H ´ = *Pi*<sup>∗</sup> *lnPi* (3)

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

*<sup>N</sup>* (1)

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Y1 <sup>−</sup> Y2N \* <sup>100</sup> (2)

*<sup>H</sup>*´ *max* (4)

the biodiversity of gastropods we used the following indexes:

*Pi* = \_\_

ID = \_\_\_\_\_\_

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

which all species are equally abundant (Eq. (4)) [38]:

J ´ = H´ \_\_\_\_\_\_

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

isms from all gastropod species.

index (ID) (Eq. (2)) [27, 54]:

dant and less frequent [27].

(Eq. (3)) [27, 38, 55, 56]:

#### *1.3.2. Biodiversity of gastropods*

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