Natural Resources, 2012, 3, 248-260
http://dx.doi.org/10.4236/nr.2012.34032 Published Online December 2012 (http://www.SciRP.org/journal/nr)
1
Mollusk Species Richness on the Rocky Shores of the State
of Guerrero, Mexico, as Affected by Rains and Their
Geographical Distribution
Flores-Rodríguez Pedro1, Flores-Garza Rafael1, García-Ibáñez Sergio1, Valdés-González Arcadio2,
Violante-González Juan1, Santiago Cortés Enedina1, Galeana-Rebolledo Lizeth1,
Torreblanca-Ramírez Carmina1
1Unidad Académica de Ecología Marina, Universidad Autónoma de Guerrero, Acapulco, México; 2Universidad Autónoma de Nuevo
León, Laboratorio de Acuacultura, Facultad de Ciencias Biológicas, Ciudad Universitaria, San Nicolás de los Garza, México.
Received August 18th, 2012; revised September 26th, 2012; accepted October 11th, 2012
ABSTRACT
Natural processes in ecosystems shows significant variations in species richness with regular cycles that can be altered
by natural or anthropogenic activities; the present research was in order to understand on some of these variations. The
objectives were: 1) determine the species richness of rocky intertidal mollusks; 2) track their geographical distribution
at State level; and 3) determine the changes in species richness as a result of rainfall on an annual cycle. For these pur-
poses it was sampled in March, June, September and December 2005, the sampled area was 20 m2 for each collecting
site; the sampling unit was a PVC rectangle frame one by two meter per side. The mollusks found within the sampling
units were collected, identified and counted. The species distribution was analyzed by sites and regions, considered spe-
cies richness per season, and the distributional patterns by rainy season historical records. A total of 62 mollusks species
were found, its richness associated with substrate stability and wave intensity on each site, Gastropods had the highest
species richness. When analyzing the regional distribution and sites, it was found a pattern of species richness roughly
constant in the proportion of species with wide distribution (generalists) versus those of restricted spatial representation
(specialists). Gastropods had the highest number of species with restricted spatial representation. Seasonal rainfall dif-
ferences did not quantitatively altered on the overall the species of mollusks in the rocky intertidal sampled zone how-
ever, in the analysis per Class, Gastropoda indicated changes in species richness influenced by rainfall, which were not
observed in Bivalvia and Polyplacophora, suggesting that those changes might depend on adaptive processes or the lack
of adaptation of gastropod species to the characteristics of the rocky intertidal habitat.
Keywords: Rocky Shore; Species Richness; Geographic Distribution; Rain Effects; Mollusks
1. Introduction
The State of Guerrero has a humid tropical climate with
different types of rocky substrates that occur along the
coasts, causing great species richness, which can be af-
fected by changes in the amount of rainfall during the
year. Species richness shows significant variations that
conform to natural processes in ecosystems, and its cy-
cles can be altered by natural or anthropogenic activities
such as pollution, overexploitation of habitat, and intro-
duction of exotic species, which among other things are
considered as the main forces leading to changes in spe-
cies richness and community structure [1]. In nature the
dispersion can be influenced by many factors, including
intrinsic characteristics of some species (e.g. mobility,
specificity for different resources or abilities to perceive
the quality, size or structure of the parcels they inhabit)
and/or characteristics of the habitat itself [2]. Identifying
changes in diversity, either in the number of species, abun-
dance, distribution, or dominance, can alert us of detri-
mental processes [3]. There are several reports in the lit-
erature on rocky coast mollusks for different parts of the
Americas, which analyzes the relationship between spe-
cies richness, wave intensity and type of substrate [4-8].
Regarding the Mexican Pacific coasts there have also
been studies on this subject [9-12]. From the reports in
the literature on species richness associated with rocky
substrate in the State of Guerrero, most of them were
made on Acapulco [13-21]; literature reports are scarce
for other places along its coasts [22-24]. The aforemen-
tioned reports analyzed the species richness locally, their
vertical distribution in the intertidal zone, and ecological

Mollusk Species Richness on the Rocky Shores of the State of Guerrero, Mexico,
as Affected by Rains and Their Geographical Distribution
249
features, but do not cover the geographical distribution of
species, nor changes in species richness caused by rains,
nor do they include temporary component analysis. In the
State of Guerrero the need has been recognized to expand
the level of understanding and knowledge about marine
mollusk fauna and because of it, the scope of this re-
search were: 1) determine the species richness of rocky
intertidal mollusks; 2) track their geographical distribu-
tion at State level; and 3) determine the changes in spe-
cies richness as a result of rainfall on an annual cycle.
2. Materials and Methods
2.1. Study Area
The coastline of the State of Guerrero is located in the
Mexican Tropical Pacific (Figure 1); The coastline of
the State of Guerrero is located in the Mexican Tropical
Pacific (Figure 1); the climate is warm, sub-humid year
round with considerable summer rains (Aw type) and has
low winter rains, rating 5% to 10% of the total yearly rain
fall. The coast has an approximate length of 470 km, and
according to regional characteristics it is located in the
morphotectonic unit number VIII in front of which is a
very narrow continental shelf. The coastline is parallel to
the Mesoamerican Mexican pit. It is classified as conti-
nental collision coast, dominated by primary coasts formed
by diastrophic movements, with faults and fault scarps.
However there are secondary coasts, to a lesser extent
formed by erosion of waves, coastal headlands and wave-
cut terraces; and higher secondary coasts of marine depo-
sition, and barrier beaches and hooks [25]. The surface
water temperature on an annual cycle fluctuates between
26˚C and 28˚C, the semidiurnal tides are mixed and show
a variation of ±1 m [26]. The average sea level has a
seasonal variation with a decline between December and
May, and a rise from June to November [27]. Significant
rains occur during the summer, hurricane season offi-
cially runs from May 15, when the start of the rainy sea-
son, to November 30th when the end of the rainy season
[28]. For this work, nine sites with rocky coast consid-
ered as representative of different types of substrates and
distributed in the three coastal regions of Guerrero were
selected; collecting sites were: in the Costa Chica region
were three locations: Punta Maldonado, Las Peñitas, and
Playa Ventura; in the Acapulco region there are two lo-
cations: Zoológico at Isla La Roqueta, and Palmitas at
Isla La Roqueta, and for the Costa Grande region there
were four locations: Piedra de Tlacoyunque, Barra de
Potosi Expuesto, Barra de Potosi Protegido, and Playa
Troncones (Table 1).
Figure 1. Map of the coast of Guerrero, Mexico.

Mollusk Species Richness on the Rocky Shores of the State of Guerrero, Mexico,
as Affected by Rains and Their Geographical Distribution
250
Table 1. Description of sampling sites and substrate types.
Sites Coodrenates MPIO Sustrate description W.I.
Region costa Chica
Punta
Maldonado
16˚19'35,49''N
98˚34'12,65''W Cuajinicuilapa Sedimentary Rocks sandstone type. Fragile substrate, has many cavities of varying diameter and depth, widely distributed High
Las Peñitas 16˚33'14,4''N
98˚46'22,0''W Marquelia Metamorphic rocks gneiss type. Wave resistant, stable fixed substrate, rough surface made of protrusions, cracks and crevices. Media
Playa Ventura 16˚32'8,3''N
99˚54'44,6''W Copala
Intrusive igneous rocks graneodorite type, wave resistant compact
substrate, cracks, fissures and cavities heterogeneously distributed,
rough surface.
High
Region acapulco
Zoológico 16˚49'11,2''N
99˚54'88,8''W Acapulco Igneous rocks of varying size, fixed substrate, semi rough with many cracks, crevices and holes. High
Palmitas 16˚49'25,9''N
99˚54'40,5''W Acapulco Igneous fixed substrate, with variable size and many cracks, crevices and holes. Low
Region costa grande
Piedra de
Tlacoyunque
16˚19'35,49''N
98˚34'12,65''W
Tecpan de
Galeana
Large blocks of intrusive igneous rocks, rough surface with folds,
fissures and cracks in a heterogeneous distribution. High
Barra de Potosí
Expuesto
17˚31'43,51''N
101˚27'8,22''W Petatlán Large metamorphic fixed rocks, hardly displaced by waves, with some cracks and crevices, between a few smooth loose boulders. High
Barra de Potosí
Protegido
17˚32'17,4'N
101˚27'19,1''W Petatlán Metamorphic rocks, smooth boulders around 1 m large with small holes, some cracks and fissures, some displacement by wave’s impact.Media
Troncones 17˚47'35''N
101˚44'46,6''W La Unión Sedimentary and volcano-sedimentary compact mounds, with abundan
t
cracks and crevices. Media
Wave intensity = W.I. Municipalyte = MPIO.
2.2. Fieldwork
Sites were geo-referenced and their description based on
[29] geological maps of the National Institute of Statis-
tics, Geography and Informatics INEGI (Acapulco E14-11,
1:50,000) complemented with field observations. Wave
intensity was classified according to the following crite-
ria: 1) high, when the wave freely hits the substrate; 2)
middle, when the wave impact on the substrate is ham-
pered by smoothing or barriers; and 3) low, when the
wave does not hit the substrate directly since the site is
protected by different types of barriers. Each site was
sampled on four occasions. The first sampling was con-
ducted in March (drought season <5 mm of rain), the
second in June (start of the rainy season <200 mm), the
third in September (end of the rainy season >200 mm)
and the last in December (time of scarce rains <10 mm in
total). According to historical records [30] established for
the period 1941 to 2005 the average annual rainfall was:
January 2.9 mm, in June 198.4 mm, September 254.9
mm, and December 6.2 mm. Sampling was done at low
tide on new moon days; the area sampled per site was of
20 m2, calculated by the method of species area [31]. The
sampling unit consisted of a rectangle 2 m long × 1 m
wide and delimited by a PVC frame. The starting point in
the intertidal zone was randomly selected where the
frame was set, with the sampling unit accommodated
perpendicular to the coast and parallel to it by setting a
rope 30 m in length. All mollusks within the sampling
unit were collected, identified and counted. Once the first
sampling unit was completed, the PVC frame was reset
two meters further down the rope. This operation was re-
peated until the completion of 20 m2 for each site.
2.3. Laboratory Work and Analysis
The species that were not recognized in situ, were trans-
ferred to the laboratory for their processing to identify
them by the use of proper literature [32], the nomencla-
ture was updated [33-35]. The specimens were deposited
in the “Colección de Moluscos de la Unidad Académica
de Ecología Marina de la Universidad Autónoma de
Guerrero.” Species richness was measured in number of
total species. The geographical distribution by site was
analyzed based on the frequency of the species, and clas-
sified as: 1) wide distribution, when the species was pre-
sent in at least seven sites; 2) regular distribution, when
present on five or six sites; 3) limited distribution, when
the species was present on three or four places; and 4)
irregular distribution, when it was present between one
and two sites out of the nine sites visited. The distribu-
tion by region and season of the species was analyzed on
the basis of frequency of appearance. To analyze sea-
sonal differences between the mean values of species

Mollusk Species Richness on the Rocky Shores of the State of Guerrero, Mexico,
as Affected by Rains and Their Geographical Distribution
251
richness, a single-factor ANOVA test was used, previ-
ously checking the homogeneity of variances by the
Levene test [36]. To analyze the similarity of values on
species richness per season, the hierarchical analysis
(clusters) with the method of Euclidean squared distance
test was used.
3. Results
3.1. Species Richness
Altogether 67,430 specimens were analyzed, and found a
richness of 62 species in the rocky intertidal zone of the
State of Guerrero, distributed in 50 Gastropoda species,
nine Bivalvia and three for Polyplacophora. The highest
overall species richness was at Barra de Potosi Protegido
with 42 species and the lowest was at Piedra Tlacoyun-
que with 29. The highest species richness for the Class
Gastropoda also was found in Barra de Potosi Protegido
with 35 species, for Bivalvia was in Las Peñitas with
eight species; and Polyplacophora presented equal domi-
nance at three sites: Punta Maldonado, Barra de Potosi
Protegida and Barra de Potosi Expuesta with three spe-
cies on each site (Figure 2).
3.2 Geographical Distribution
Analyzing the distribution of species by coastal region, it
was found that 46.8% of species were present in all three
regions, 30.6% in two and 22.6% in one (Table 2). For
the Class Gastropoda 46% of species were found in all
Figure 2. Mollusk species richness of nine sites in the rocky coast of the State of Guerrero.
Table 2. Species richness, frequency, site of collection, distributional type and seasonal dist ribution of the Class Gastropoda,
Bivalvia and Polyplacophora in the intertidal rocky shores in the State of Guerrero, Mexico.
Families/Species SFD C.CHIC. ACA. C. GRANDE DS
Gastropoda 1 2 3 4 5 6 7 8 9 1 2 3 4
LOTTIIDAE Gray, 1840)
Lottia acutapex (Berry, 1960) R ■ ■ ■ ■ ■ ■ ■ ■ ■
Lottia mitella (Menke, 1847) W ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■
Lottia pediculus (Philippi, 1846) I ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■
Lottia mesoleuca (Menke, 1851) W ■ ■
Tectura fascicularis (Menke, 1851) W ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■
Tectura filosa (Carpenter, 1865) W ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■
FISSURELLIDAE
Diodora inaequalis (Sowerby, 1835) L ■ ■ ■ ■ ■ ■
Fissurella (C.) asperella (Sowerby, 1835) W ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■
Fissurella (C.) gemmata (Menke, 1847) W ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■
Fissurella (C.) nigrocincta (Carpenter, 1856) W ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■
Fissurella (C.) obscura (Sowerby, 1835) W ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■

Mollusk Species Richness on the Rocky Shores of the State of Guerrero, Mexico,
as Affected by Rains and Their Geographical Distribution
252
Continued
TURBINIDAE
Astraea (U.) buschii (Philippi, 1844) I ■
■
Eulithidium phasianella (Philippi, 1849) I ■ ■ ■ ■
TROCHIDAE
Tegula (A.) globulus (Carpenter, 1857) L ■ ■ ■ ■ ■
Tegula (A.) maculostriata (C. B. Adams, 1845) L ■ ■ ■ ■ ■
NERITIDAE
Nerita (R.) scabricosta (Lamark, 1822) W ■ ■ ■ ■ ■ ■ ■
Nerita (T.) funiculate (Menke, 1851) L ■ ■ ■ ■ ■ ■ ■
LITTORINIDAE
Nodilittorina (N.) aspera (Philippi, 1846) W ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■
Nodilittorina modesta (Philippi, 1846) W ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■
VERMETIDAE
Petaloconchus (P.) complicates (Dall, 1908) W ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■
Petaloconchus macroprhacma (Carpenter, 1857) W ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■
CERITHIDAE
Cerithium (T.) menkei (Carpenter, 1857) I ■ ■ ■ ■ ■
Liocerithium judithae (Keen, 1971) I ■ ■
PLANAXIDAE
Planaxis obsoletus (Menke, 1851) L ■ ■ ■ ■ ■ ■ ■ ■
HIPPONICIDAE
Hipponix A. panamensis (C. B. Adams,1852) I ■
■
Pilosabia pilosa (Deshayes, 1832) L ■ ■ ■ ■ ■
CALYPTRAIDAE
Crucibulum (C.) monticulus (Berry, 1969) I ■ ■
Crepidula excavata (Broderip, 1834) I ■
■
Crucibulum (C.) scutellatum (Wood, 1828) ■ ■ ■ ■ ■ ■ ■ ■ ■ ■
MURICIDAE
Mancinella speciosa (Valenciennes, 1832) R ■ ■ ■ ■ ■ ■ ■ ■ ■
Mancinella triangularis (Blainville, 1832) W ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■
Stramonita biserialis (Blainville, 1832) R ■ ■ ■ ■ ■ ■ ■ ■
Acanthais brevidentata (Wood, 1828) L ■ ■ ■ ■ ■ ■ ■
Thais (T.) kiosquiformis (Duclos, 1832) W ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■
Vasula (V.) melones (Dulcos, 1832) I ■
■
Plicopurpura columellaris (Lamark, 1822) L ■ ■ ■ ■ ■ ■ ■
Plicopurpura pansa (Gould, 1853) W ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■
BUCCINIDAE
Cantharus (P.) sanguinolentus (Duclos, 1833) L ■ ■ ■ ■ ■ ■ ■ ■
FASCIOLARIIDAE
Leucozonia cerata (Wood, 1828) L ■ ■ ■ ■ ■
■ ■
Opeatostoma pseudodon (Burrow, 1815) L ■ ■ ■ ■ ■ ■ ■ ■

Mollusk Species Richness on the Rocky Shores of the State of Guerrero, Mexico,
as Affected by Rains and Their Geographical Distribution
253
Continued
COLUMBELLIDAE
Columbella fuscata (Sowerby, 1832) W ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■
Costoanachis nigrofusca (Carpenter, 1857) I ■
■
Mitrella ocellata (Gmelin, 1791) A ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■
MITRIDAE
Mitra (S.) tristis (Broderip, 1836) L ■ ■ ■ ■ ■ ■
CONIDAE
Conus (S.) nux (Broderip, 1833) I ■ ■ ■ ■ ■
ONCHIDIIDAE
Hoffmannola hansi (Marcus and Marcus, 1967) W ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■
SIPHONARIDEA
Siphonaria (H.) gigas (Sowerby, 1825) R ■ ■ ■ ■ ■ ■ ■ ■
Siphonaria (H.) palmata (Carpenter, 1857) W ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■
Siphonaria maura (Sowerby, 1835) I ■ ■ ■
■
TRIMUSCULIDAE
Trimusculus stellatus (Sowerby, 1835) L ■ ■ ■ ■ ■ ■ ■
BIVALVIA
ARCIDAE
Arca (A.) mutabilis (Sowerby, 1833) I ■ ■ ■ ■ ■
MYTILIDAE
Brachidontes adamsianus (Dunker, 1857) W ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■
Choromytilus palliopunctatus (Carpenter, 1857) W ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■
ISOGNOMONIDAE
Isognomon (M.) janus (Carpenter, 1857) W ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■
OSTREIDAE
Undulostrea megodon (Hanley, 1846) I ■ ■ ■ ■ ■ ■
Crassostrea palmula (Carpenter, 1857) R ■ ■ ■ ■ ■ ■ ■ ■ ■
Crassostrea prismática (Gray, 1825) R ■ ■ ■ ■ ■ ■ ■ ■ ■
CARDITIDAE
Carditamera affinis (Sowerby, 1833) L ■ ■ ■ ■
■
CHAMIDAE
Chama (C.) echinata (Broderip, 1835) W ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■
POLYPLACOPHORA
Ischnochitonidae ( Dall, 1889)
Ischnochiton (I.) muscarius (Reeve, 1847) I ■ ■ ■ ■ ■ ■
CHITONIDAE ■ ■ ■ ■
Chiton (C.) al boline atus (Broderip and Sowerby, 1829) L ■ ■ ■ ■ ■ ■ ■ ■
Chiton (C.) articulatus (Sowerby 1832) W ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■
Regions: C.CHIC. = Costa Chica; ACA. =Acapulco; C. GRANDE = Costa Grande; DS = Distribution per season; Sites: 1 = Punta Maldonado; 2 = Las Peñitas;
3 = Playa Ventura; 4 = Roqueta Zoológico; 5 = Roqueta Palmitas; 6 = Piedra de Tlacoyunque; 7 = Barra Potosí Expuesta; 8 = Barra Potosí Protegida; 9 =
Troncones; Species present = ■. Species Frequency of Distribution: W = Wide; R = Regular; L = Limited; I = Irregular; Seasons: 1 = Start of the rainy season; 2 =
End of the rainy season; 3 = Time of scarce rains; 4 = Drought season.

Mollusk Species Richness on the Rocky Shores of the State of Guerrero, Mexico,
as Affected by Rains and Their Geographical Distribution
254
three regions, 28% in two and 26% in one. Bivalvia pre-
sented 44.44% in three regions, the same percentage in
two and 11.11% at one. Polyplacophora had 66.66% of
species found in three regions and 33.3% in one. When
examining the distribution of species by collection site, it
was found that for Gastropoda 38.7% of the species had
wide distribution, 14.5% regular distribution, 22.6% lim-
ited and 24.2% irregular.
Bivalvia had 44.4% with wide distribution, 22.22%
with regular distribution, 11.1% limited and 22.22% ir-
regular. On Polyplacophora 33.33% of the species pre-
sented wide distribution, 33.33% limited and 33.33% ir-
regular.
3.3. Changes in Species Richness as a Result of
Rains
Species richness per season was 50 species at the start of
the rainy season, 49 by the end of the rainy season, 50 in
time of scarce rains and 52 species in drought season.
Also by comparing richness values for each Class sepa-
rately, there were minimal variations (Table 3).
Considering the species distribution per season, it was
found that 62.9% of all species were present during all
four seasons (Figures 3(a) and (b)), 14.5% were present
in three, 8.1% in two and 14.5% in one, (Table 2). (a)
Examining the distribution of species per season for
each Class, it was found that Gastropoda had 58% of spe-
cies in all seasons, Bivalvia 77.8% and Polyplacophora
100% (Figure 4). Examining the distribution of species
per season for each Class, it was found that Gastropoda
had 58% of species in all seasons, Bivalvia 77.8% and Po-
lyplacophora 100% (Figure 4).
We found an average of 23.36 species per season (min
14, max 36, SD 4.35). The average species richness at
the beginning of rain season was 23.88 (min 19, max 30,
SD 3.37), end of the rainy season in 21.55 (min 14, max.
25, SD 4.19), in time of scarce rains 23.11 (min 16, max
32, SD 4.8) for drought season 24.88 (min 20, max 36,
SD 4.9) (Table 4). The average species richness during
different seasons did not show statistically significant
Table 3. Values of species richness by season and class of
mollusk on the rocky coast of the State of Guerrero, Mexi-
co.
Season Gas (n) BIV (n) Poly (n)T.
Start of the rainy season. 39 8 3 50
End of the rainy season. 39 7 3 49
Time of scarce rains. 39 8 3 50
Drought season. 41 8 3 52 (b)
Figure 3. Species present in all four seasons with wide dis-
tribution.
GAS = Gastropoda; BIV = Bivalvia; POLY = Polyplacophora; T. = Total; N =
species richness found by class.

Mollusk Species Richness on the Rocky Shores of the State of Guerrero, Mexico,
as Affected by Rains and Their Geographical Distribution
255
Figure 4. Species percentage of Gastropoda, Bivalvia and Polyplacophora, according to their seasonal distribution in the
rocky coast of the State of Guerrero, Mexico.
Table 4. Species richness value per site and per rainy season in the rocky coast of the Staste of Guerrero, Mexico.
Site Start of the rainy season
(n)
End of the rainy season
(n) Time of scarce rains (n) Drought season (n)
Punta Maldonado 24 25 24 20
Las Peñitas 19 25 23 26
Playa Ventura 27 23 22 25
Roqueta Zoológico 22 25 26 21
Roqueta Palmitas 22 16 17 22
Piedra de Tlacoyunque 21 14 16 21
Barra Potosí Expuesto 26 23 22 26
Barra Potosí Protegido 30 24 32 36
Troncones 24 19 26 27
AVERAGE 23.88 21.55 23.11 24.88
Standar deviation 3.37 4.18 4.83 4.91
(n) = species richness found by season.
differences either (ANOVA, P = 0.436). For Gastropoda
(ANOVA, P = 0.647), Bivalvia (ANOVA, P = 0.862) and
Polyplacophora (ANOVA, P = 0.714) mean values of spe-
cies richness for different seasons did not show statistic-
cally significant differences either.
Furthermore, hierarchical cluster test applied to the val-
ues of species richness for the four seasons produced the
formation of two groups: The first consisted of the values
of species richness in drought seasons and start of the
rainy season; these values had higher relative similarity.
The second group was formed by the values of species
richness of the end of the rainy season and the time of
scarce rains (Figure 5).
The hierarchy Class species richness analysis for Gas-
tropoda established a group of greater similarity with
drought season and the time of scarce rains one. A sec-
ond group was formed with the values of species richness
at the start of the rainy season plus the first group formed
(Figure 6). It is also noted that the value of species rich-
ness of the end of rainy season did not show relative
similarity with the two groups formed. For Bivalvia, the
hierarchical analysis determined the formation of two
groups with approximately the same similarity. The first
group was established by the values of species richness
with the end of the rainy season plus the drought season
period. A second group was formed by the relative simi-
larity of the values of species richness in the start of the
rainy season and the time of scarce rains (Figure 7).
On Polyplacophora the species richness values in the
different seasons analyzed, were very homogeneous and

Mollusk Species Richness on the Rocky Shores of the State of Guerrero, Mexico,
as Affected by Rains and Their Geographical Distribution
256
Figure 5. Richness values dendr ogram of Class Gastropoda, Bivalvia and Polyplacophora by rainy season on the rocky coast
of the State of Guerrero, Mexico , using a halflink between groups.
Figure 6. Richness values dendrogram of Class Gastropoda by rainy season on the rocky coast of the State of Guerrero,
Mexico, using a halflink betwe en groups.
Figure 7. Richness values de ndrogram of Class Bivalvia by rainy season on the rocky coast of the State of Guerrero, Mexico,
using a half link between groups.

Mollusk Species Richness on the Rocky Shores of the State of Guerrero, Mexico,
as Affected by Rains and Their Geographical Distribution
257
were not possible to see the formation of groups using the
cluster test.
4. Discussion and Conclusion
4.1. Species Richness
Species richness found was higher than previously re-
ported in works that had deployed the same field meth-
odology [16,23,24], Also species richness was higher
than previously reported [22], even when the collecting
methodology was different. For the Acapulco region [18,
19], previous species richness results were lower than
here reported. The high species richness found, corre-
sponds to that expected in a tropical area characterized
by low variations in sea water temperature, it also hap-
pens that the substrate is complex, and it has rocks that
vary greatly in dimensions, from gravel to large blocks,
creating diverse habitats where many organisms can be
protected and become established. Besides the sampled
sites were representative of the different types of sub-
strates present in the intertidal rocky coast of the State of
Guerrero. The species richness proportion for each Class
was similar to that previously reported for the State of
Guerrero, except on [18,19] where Polyplacophora had
higher species richness than Bivalvia. This difference
could be caused by the type of environment, which fa-
vors chitons and is less conducive for Bivalves by its
instability or harshness. It also coincided with beaches [7]
composed of large physically stable boulders, more so
than those beaches composed of small rocks or loose
rock fragments, so it points to a direct correlation in this
type of beaches between habitat stability and species
richness. For Costa Rica [5], a richness of 42 species was
reported for habitat consisting of pebble substrate com-
pared to 25 species on the site with firm cliff substrate.
Highest species richness showed up on Barra de Potosi
Protegido, a place where waves had medium intensity,
and the substrate was somewhat unstable when the waves
hit, formed by smooth boulders and scarce presence of
cracks and fissures, this arrangement generates many
interspaces between stones and rocks where mollusks can
be located. There are two sites as second in species rich-
ness that differ from the above mentioned, one of them
Barra de Potosi Expuesto, with high intensity waves,
presents a substrate composed by a combination of rocky
cliffs with cracks and fissures, large boulders stable to
wave impact; the other site, Troncones, has medium wave
intensity and the substrate composed by volcanic sedi-
mentary rocks, with compact mounds and a great amount
of fissures and cracks, all of which provides potential
refuge to many organisms. On the other hand, the lowest
species richness was presented at Piedra de Tlacoyunque,
a zone with high intensity waves and stable substrate,
formed of large blocks with folds, fissures and cracks ir-
regularly distributed.
4.2. Geographical Distribution
Almost half of the species found in the rocky intertidal
zone of the State of Guerrero were present in all three
regions; these species [6] were considered as generalists,
while the rest were confined to one or two regions, indi-
cating that almost half of the species present at the com-
munity, is made up of specialist species, those with a
higher level of adaptation. By applying distributional ana-
lysis by site, the same results were obtained indicating
that most species showed broad distribution, associated
with those which had regular distribution constituted more
than 50%. These were considered to be broadly distrib-
uted and generalist, present in all the types of habitat
analyzed. Almost the same percentage of species was
found associated with fewer sites, indicating that these
species can be considered as specialists. A comparative
study of intertidal tropical and temperate species [6] had
indicated that tropical Gastropoda were found in fewer
habitats and were more specialized, consistent with the
findings in this work. On the analysis of geographical
distribution by region and site, at the State of Guerrero
those species that have high spatial representation and
are generalists, contribute with the greatest representation.
On the other hand, species with spatial representation
restricted to a region or a few sites, being only slightly
less than the proportion of generalists, the difference was
not significant. The balance between the number of spe-
cies of large spatial representation and those with limited
representation is benefited from the variety of substrates
and the differences in wave intensity found.
4.3. Changes in Species Richness as a Result of
the Rains
The analysis of mollusk species richness in the different
rain seasons shows roughly constant values; this implies
that the difference in the amount of rain per season does
not change the number of species present in the rocky
intertidal ecosystem, and supports the idea that species
richness is quantitatively constant throughout the year.
The same analysis done for each Class, had exhibited the
same behavior. Regarding species distribution per rainy
season, Gastropoda had the lower amount of species in
all the four seasons, which means that major changes in
species richness as result of the rains were observed in
this Class. The proportion of Bivalvia species present in
the four seasons was higher, so this Class supports fewer
changes under the effect of the rains. No changes in spe-
cies richness were observed for Polyplacophora, since
the 100% of its species were present in all four seasons.

Mollusk Species Richness on the Rocky Shores of the State of Guerrero, Mexico,
as Affected by Rains and Their Geographical Distribution
258
The fact that the species of Polyplacophora were present
in all seasons could be attributed to the greater adaptabil-
ity of them to this kind of environment, to which they are
well adapted by having the body structured as articulated
plates, belt, large muscular foot, flattened body and oval
shape, also are not displaced by the increase in algal
cover grown by rains, since it was common to find them
seeking refuge among algae into the cavities and crevices.
The Bivalvia appeared as the second group present in all
seasons. This is understandable because most of its spe-
cies remain fixed to the substrate and can hardly be dis-
placed by algae growth. The biggest change in species
richness as a result of rainfall was observed in Gastro-
poda. The characteristics of this Class such as smaller
muscular foot compared to the chitons, the varied shape
of its shell and the variety of feeding habits, caused them
to have a lesser degree of adaptability to the conditions
during rain at the intertidal. This coincides with prelimi-
nary findings [19] concerning better adaptation to these
sorts of conditions by Class and species of Polyplaco-
phora and Bivalvia, and the one with lower adaption,
Gastropoda. Therefore based on species composition,
which report Class Polyplacophora at the highest number
of dominant species by percentage, making clear that
members of this Class are more efficient to withstand the
rocky intertidal harsh conditions. Bivalvia show the sec-
ond percentage in dominant species, they are fixed with
byssus or cemented to the substrate, where they were the
major dominant species; and finally the Gastropoda,
among whose dominant species are those characterized
by having shielded or ear shaped shells. By comparing
the species richness analyzed by season, the averaged
values for the three Class altogether, there were no statis-
tically significant differences observed. The same hap-
pened when analyzed for each Class. No quantitative
change in species richness was influenced by rainfall on
an annual cycle, and as has been stated [3], the species
stability indicates that the environment is far from suf-
fering processes of degradation or deterioration caused
by rain. Applying cluster analysis for species richness for
each season to the three Classes altogether, the formation
of two groups was found, the first group made up by the
species richness of two sequential seasons, the drought
season and the start of the rainy season, associated to the
gradual increase in precipitation. The second group was
formed with species richness of the start of the rainy
season and time of scarce rains, which happened to be
sequential at the same time, the formation of this group
was associated with the decrease in precipitation. The
formation of groups reflects the amount of rainfall during
different seasons. It was noted that the start of the rainy
season and the end of the rainy season, were not pooled,
assuming that the intensity of rainfall causes significant
alterations of habitat and thus the species richness changes,
in [7] a direct correlation between the habitat stability
and species richness. By analyzing species richness per
season for each of the Classes by cluster analysis, it was
found that, for Class Gastropoda groups were formed cor-
responding to the amount of precipitation, the greater
similarity group made up by the two seasons of lower
amount of rain (drought season and time of scarce rains),
and followed by a second group that had lesser similarity
and presented an increment in the amount of precipita-
tion, which includes the previous group plus the group
formed at the start of rainy season (time of scarce rains-
drought season—start of the rainy season). Species rich-
ness of rainy season did not join or form any group. This
could be related to the significant habitat changes due to
higher amount of rainfall, the increment of wave inten-
sity on the substrate, the considerable increment of algae
cover overlying cracks and cavities, which in turn induce
the species of Gastropoda to move or seek to be accom-
modated in other type of habitat. In agreement [8] to
rocky shore communities that are affected by waves and
this in turn determines the growth of algae, and subse-
quently animals, causing large differences in the distribu-
tion of coastal organisms from one place to another be-
cause of geographical, geological and climatic conditions.
Moreover the adaptation of Gastropoda to the rocky in-
tertidal zone shows that, because it has fewer amount of
families that has shells with shielded shape or ear type
forms, which can stand quite well the impact of the
waves; While a large number of families present shells
with spiral or coiled form that could be at disadvantage
in the middle of the intertidal. When comparing two
beaches, [4] a protected one and another exposed, its biota
was probably distributed according to their abilities to
withstand wave and air conditions exposure. Bivalvia Class
formed two groups, one composed of species present dur-
ing drought season and end of the rainy season; and an-
other group formed by the one at the start of the rainy
season and the one at time of scarce season. These groups
do not responded to gradual changes in the amount of
rain. It is particularly important to mention that, the mem-
bers of the Class Bivalvia are essentially sessile, so pro-
vably they can hardly be displaced by algae or dislodged
by the waves, so it could not be concluded that there is a
change in the species richness of bivalves influenced by
the amount of precipitation.
5. Final Conclusions
The mollusk community associated with the intertidal
rocky substrate at the State of Guerrero is very diverse,
and by studying species richness change due to rain does
not produced quantitative effect, and presented similar
proportions of species with large spatial representation

Mollusk Species Richness on the Rocky Shores of the State of Guerrero, Mexico,
as Affected by Rains and Their Geographical Distribution
259
(generalist) and those with restricted representation (spe-
cialists). However, the analysis for each Class showed
significant differences, mainly for Gastropoda, which had
the highest proportion of species with restricted spatial
representation, forming groups in species richness that
respond to changes in the amount of rainfall for each sea-
son studied. This sort of behavioral trait for Gastropoda
with species sensible to the amount of rain could lead to
rate some of them as possible “bioindicators” for changes
in the environment, and is worth to keep for closer look
on them for further analysis.
6. Acknowledgements
Thanks are due to the Faculty Improvement Program (Pro-
grama de Mejoramiento del Profesorado PROMEP) by
the Secretary of Public Education (Secretaria de Educa-
ción Publica SEP)/“Subsecretaría de Educación Supe-
rior” SES, Mexico, that funded this project as part of the
support for the incorporation of former fellow students,
authors of this work.
Thanks are due to Arcadio Valdes Gonzales for over-
seeing, reviewing and translation services, and to Kit Sto-
well for correcting and improving the final manuscript.
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