Currently, studying the environment is important because of the phenomena that take place on the earth every day. That is why it is a priority to carry out studies that relate environmental changes to the biology of organisms. This allows us to know the interactions with the environment, and in this way solve, reduce or prevent ecological and economic problems, if they are organisms with a commercial value. The objective of this investigation is to determine ecological parameters of the gastropod community from the intertidal zone on five islands from the Gulf of California, México, to model the diversity, distribution and abundance of malacological fauna. We considered to evaluate the Shannon-Wiener diversity (H′), Pielou’s of evenness (J) and the Margalef species richness indexes, in order to evaluate through an analysis from biotic and abiotic factors, the species status that was collected from the exposed and non-exposed zone tidal. The generated data were contemplated from a year-based biodiversity project (2016–2017) on the following islands: Patos, Bledos, Bleditos, Tunosa, and Mazocahui which belong to the Ohuira lagoon in Ahome, Sinaloa, southeast of the Gulf of California, México. Likewise a status about the importance of gastropods is mentioned for the study area.
- Gulf of California
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  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 . 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 . 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 .
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 . Furthermore, they help to establish a baseline for future follow-up and evaluation programs . Life cycles, a high level of stress tolerance , an intimate relationship with the sediment and a high response toward disturbances  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, 13, 14, 15, 16, 17]. Additional studies relate to distribution and abundance [18, 19, 20, 21, 22, 23, 24] and ecology [25, 26, 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 mi2) 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 km2 (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 .
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) . 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 .
1.3. Biodiversity of mollusks
1.3.1. Current status
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 . 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  and as disturbance indicators inside these systems . 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 . 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 . These organisms possess one of the most widespread distributions in the planet, ranging from the coastline to great sea depths . 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 , as far as deserts and polar areas, as well as the tropics and great sea depths , being widely studied due to their social and economic importance, as well as their commercial and nutritional values .
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 .
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
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, 43, 44, 45].
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 m2 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, 47, 48, 49, 50, 51, 52, 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:
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)):
where ni is the number of organisms from the “i” species and N is the total number of organisms from all gastropod species.
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 .
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]:
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)) :
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)):
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 :
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)):
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 .
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:
In the areas where there was tidal exposure a total of 19 gastropod species were found:
2.1.2. Biodiversity in Patos Island
A total of 11 gastropod species were registered in Patos Island, where
2.1.3. Biodiversity in Bledos Island
Bledos Island was the one that showed a higher mollusk diversity with 13 species in which
18.104.22.168. Biodiversity in Bleditos Island
Bleditos Island had a gastropod diversity of 12 recorded species.
22.214.171.124. Biodiversity in Mazocahui Island
On Mazocahui Island a total of 9 species were registered from which
2.1.4. Biodiversity in Tunosa Island
Tunosa Island had a diversity of 10 species, from which
2.2. Ecological indexes
2.2.1. Dominant species (ID) and Shannon-Wiener’s diversity (H´)
2.2.2. Pielou’s equity
126.96.36.199. Comparison between dominant species in tide exposed areas (J´1) and non-exposed areas (J´2)
188.8.131.52. Diversity index or Margalef’s richness (Dmg)
On Patos Island we found Dmg = 1.80, on Bledos Island Dmg = 1.68, Bleditos Island Dmg = 1.53, Tunosa Island Dmg = 1.95, and on Mazocahui Island, Dmg = 1.81, which suggests that on Patos and Mazocahui Islands, the diversity or richness is similar, being higher on Tunosa Island and lower on Bleditos.
3. Importance of gastropod biodiversity in the study area
Due to the lack of biological information from the Gulf of California islands, specifically from the ones found in Ohuira lagoon and their ecological and economic importance, it is necessary to perform research to increase the knowledge about them and to contribute on the elaboration of management methods and alternatives for the sustainable use the marine resources found on the islands. While it is true that in the past years the number of investigations have increased, the marine studies that have been performed on the islands of the Ohuira lagoon do not provide enough information about the species that inhabit the area, such as their biology, ecology, reproduction, physiology and taxonomy. This is why this current research pretends to set a baseline for future studies of the gastropod mollusk community from these islands throughout time, in order to evaluate possible environmental changes whether they are natural or anthropogenic.
3.1. Importance in ecology, biology and taxonomy
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
3.1.1. Gastropods black murex Hexaplex (Muricanthus) nigritus (Philippi, 1845)
The distinctive features of the gastropod
3.1.2. Pink-mouthed murex gastropod Phyllonotus Erythrostoma (Swainson, 1831)
It belongs to the Muricidae family, the synonyms used are
3.1.3. Pacific melongena gastropods Melongena Patula (Broderip and Sowerby, 1829)
With only a single species in the study area, it belongs to the Melongenidae family. It presents a large and heavy, piriformed shell, the most recent rounds gradually enveloping the oldest, forming an irregular and deeply grooved suture and a very small spiral coil. Young organisms (less than 60 mm in length) are therefore more fusiform. The sculpture consists of a single-spaced row of short spines (although this feature might be absent) on the rounded shoulder, as well as numerous fine spiral grooves, mainly in the lower part of the shell. It has a rather smooth and thick periostraco, a very large opening, with a short and wide channel, an internal smooth and satin lip and a simple outer lip. The corneum operculum has a claw-like shape, with a terminal nucleus. The color is dark brown with cream spiral bands, just below the widest part of the last lap. The opening’s color is yellowish to pinkish. The families of gastropods with similar appearance present in the area are: Fasciolariidae, with more fusiform shells, longer and narrow siphon canals, with few folds sometimes present in the columella. The maximum reported size was 260 mm; nevertheless, the most common size was up to 160 mm. Its habitat is in sandbars and mud from the high levels of the intertidal zone. It is a carnivorous species that especially feeds on other gastropod Mollusks .
3.2. Elaboration of crafts (tourism)
Several gastropods are used to create crafts (Port of Mazatlán, Sinaloa) such as jewelry boxes, picture frames, key holders, reliquaries, candles, lithographs, among others, by using shells of the gastropods belonging to the genus
3.3.1. Current status
Some organisms of the gastropod are a very important fishery resource worldwide and have a significant economic impact through the generation of resources at the level of artisanal fishermen, local trade and the export of fishery products of international value. The gastropod fishery destined for human consumption in the study area is based on the black murex
184.108.40.206. Gastropods black murex Hexaplex (Muricanthus) nigritus (Philippi, 1845)
The fishery of the gastropods black murex
220.127.116.11. Pink-mouthed murex gastropod
Phyllonotus erythrostoma(Swainson, 1831)
The pink-mouthed murex gastropod fisheries
18.104.22.168. Pacific melongena gastropod
Melongena patula(Broderip and Sowerby, 1829) and incidentals gastropods
Captures in the study area of the Pacific melongena gastropod
3.4.1. Current status
The reproductive cycle of the snail
A difference was found among the ecological indexes (H′, J′, Dmg) from the intertidal Mollusks community between the present study from the Ohuira lagoon, Ahome, Sinaloa (2016–2017 period) and previous studies undertaken at the Guasave municipality in Sinaloa (Navachiste lagoon)  and in the Topolobampo and Ohuira lagoons (mangrove zone) . The abundance distribution of organisms and species on the collecting sites was heterogeneous. In the gastropod mollusk community in Ohuira lagoon, Ahome, there was a certain type of association with the type of substrate, which is composed of rocky and sandy zones (beach), zones with small rocks and in less proportion mangroves. The sampling methods showed that the gastropod
The authors thank Dra. Mercedes Eugenia Guerrero Ruiz for translating the manuscript into English.