Spatial distribution of Echinolitorina peruviana (Lamarck, 1882) for intertidal rocky shore in Antofagasta (23° S, Chile).

Abstract The intertidal rocky shores in continental Chile have high species diversity mainly in northern Chile (18-27° S), and one of the most widespread species is the gastropod Echinolittorina peruviana (Lamarck, 1822). The aim of the present study is do a first characterization of spatial distribution of E. peruviana in along rocky shore in Antofagasta town in northern Chile. Individuals were counted in nine different sites that also were determined their spectral properties using remote sensing techniques (LANDSAT ETM+). The results revealed that sites without marked human intervention have more abundant in comparison to sites located in the town, also in all studied sites was found an aggregated pattern, and in six of these sites were found a negative binomial distribution. The low density related to sites with human intervention is supported when spectral properties for sites were included. These results would agree with other similar results for rocky shore in northern and southern Chile.


Introduction
The rocky intertidal environments in Chilean coast is characterized by the high species diversity, including molluscs, that has a marked geographic distribution pattern (Santelices, 1992;Broitman et al., 2001;Lee et al., 2008).
The rocky shore is markedly exposed to waves among a wide latitudinal gradient in Chile (17-41°S), whereas in analyses one site that has E. peruviana absence has not considered.
To each specie counting data, was obtained in first instance a variance mean ratio, first for determine if the specie has random if the value is 1, uniform if the value is lower than 1, or aggregated distribution, if the value is upper than 1, (Brower et al., 1998;Zar, 1999;Fernandes et al., 2003;De Los Ríos Escalante, 2017;De Los Ríos Escalante and Mansilla, 2017;Ríos and Arancibia, 2018).Once determined the spatial pattern, random, uniform or aggregated, it determined if the species have Poisson, binomial or negative binomial distribution respectively, the analysis was done manually using Excel software and literature descriptions (Zar, 1999;Fernandes et al., 2003;De Los Ríos Escalante, 2017;De Los Ríos Escalante and Mansilla, 2017;Ríos and Arancibia, 2018).For these analyses one site that has E. peruviana absence has not considered.
Finally, for spectral properties data and E. peruviana abundance mean, a principal component analysis was done using the Factoextra R package (Kassambara and Mundt, 2017) with the aim of determine potential grouping patterns in studied sites.

Results
The obtained results revealed that the sites without human intervention have marked high abundances in comparison to sites located in the town, and the results of spatial distribution, revealed the presence of aggregated pattern for eight sites, and one site with uniform distribution (Barrio Histórico) (Table 1).The results revealed that seven sites has negative binomial distribution, and only two sites (Caleta Coloso and El Lenguado 1) have not negative binomial distribution (Figure 2).
The literature about intertidal invertebrates revealed that these species can have a gregarious behaviour, as protection against drying during low tide, or for efficient use of food resources (Rojas et al., 2000), many of these species inhabits in rocky cracks or under rocks, or macroalgae basal disks.(Santelices, 1980;Camus and Andrade, 1999;Cerda and Castilla, 2001).Also, the distribution patterns can be affected due the topography of rocky shores, involving recruitment patterns of small gastropods (Underwood, 2004).Considering these antecedents, there are interspecific competence between intertidal gastropods and monoplacophora due shelter availabilities (Aguilera and Navarrete, 2011;2012), that can generate that some species can have diurnal or nocturnal activity (Aguilera and Navarrete, 2011).
The northern Chile (18-27° S), is a zone with high species diversity due high productivity of these coasts (Santelices, 1992, Camus andAndrade, 1999), that would have complex trophic interactions between involved species (Camus and Andrade, 1999), one of the most widespread species is the gastropod Echinolittorina peruviana that inhabits among rocky shores along Chilean territory (Santelices, 1992;Lee et al., 2008), specifically in upper levels (Castillo and Brown, 2010) and southern Perú (Paredes, 1974;Tejada-Perez et al., 2018).The aim of the present study is do a first descriptive analysis of E. peruviana adults (10-15 mm total length) in different rocky intertidal environments in Antofagasta town, northern Chile, with the aim of determine the presence of defined spatial distribution patterns.

Material and Methods
Field works and study site: the ten sites are located in the coastal town of Antofagasta, northern Chile, six sites are located within town, whereas as external group, was included a four sites in a rocky shore located at 20 km of the town, with low or practically null human intervention (Figure 1, Table 1).The studied sites were visited in summer 2019, for each site, was thrown out random (Ríos and Arancibia, 2018;Ríos and Carreño, 2020), 10 * 10 cm quadrants (n = 40 for each site), considering the relative small size of considered species (Underwood, 2004;Underwood and Chapman, 2005;Ahmad et al., 2011;Ríos and Carreño, 2020).
Spectral properties: Satellite data was obtained from LANDSAT/ETM+ image obtained dated from January 2018 (Table 1) provided by the Land Processes Distributed Active Archive Center (LP DAAC), U.S. Geological Survey (http://LPDAAC.usgs.gov).The bands of visible, near, and mid-infrarred were calibrated radiometrically to spectral irradiance and then to reflectance with atmospheric correction being applied (Table 1).
Data analysis: in a first step, it was compared the E. peruviana abundances for each site, and for two groups of sites, homocedasticity and normality were determined for data, and due the absence of both conditions it was done a non-parametric tests (Zar, 1999) using software R (R Development Core Team, 2009), Wilcoxon for compare sites with presence or absence of human intervention, and a Kruskall-Wallis for comparison among sites using the PGIRMESS R package (Giraoudoux et al., 2018).For these  The results of correlation matrix only show significant correlations between abundances with B2, B3, B4 and B5 reflectances, B1 with B3, B4 with B2, B4 with B3, B5 with B3, B5 with B4, B6 with B4, B6 with B5, B7 with B4, B7 with B5, and B7 with B6 (Table 2).The results of PCA revealed that abundances are the main factor is abundances (Table 3), and for axis 1 the main factors are all reflectance values, whereas for the axis 2 the main factor is the abundance of E. peruviana (Figure 3).Finally, the PCA results revealed the existence of two main groups, the first group corresponded to sites with high reflectance and low abundances (El Lenguado 1, El Lenguado 2, El Lenguado 3), and one site with relative low abundance (El Lenguado 4)(Figure 3).The second group joined sites with low reflectance, corresponded to sites with marked human intervention and low abundances (Trocadero 2, Carboncillo, Barrio Histórico, Las Garumas, Coloso), and one site with high abundance (Trocadero 1)(Figure 3).
The heat map obtained from PCA, revealed two main groups, one that has sites Trocadero 1, Coloso (with human intervention), Lenguado 1, Lenguado 2 and Lenguado 3 (these three without human intervention).The second group has one main sub-group with human intervention (Trocadero 2, Carboncillo, Barrio Histórico), one without human intervention (El Lenguado 4), and finally one with human intervention (Las Garumas)(Figure 4).

Discussion
The results of abundances, would indicate that many of the studied sites with high E. peruviana abundances corresponded to low or null human intervention, that is associated to low reflectance (El Lenguado 1, El Lenguado 2, El Lenguado 3 and El Lenguado 4) whereas sites located in the town at north, have high reflectance and low E. peruviana abundances (Trocadero 1, Trocadero 2, El Carboncillo, Barrio Histórico), finally an intermediate situation would occur in sites with human intervention with high reflectance and low E. peruviana abundance (Coloso) and absence (Las Garumas).The marked differences between sites with different kind of human intervention, in studied sites probably is due to the presence of Pyura praeputialis that is a kind of key species that regulate the species composition in rocky shores in northern Chile (Castilla et al., 2004), in this context, in the present study the human altered sites have not P. praeputialis.Also, the low abundances in sites with marked human intervention agree with results for central Chilean rocky shore (Durán and Castilla, 1989) that is similar to the observations for European rocky shore (Stevčić et al., 2018) and Arabian Sea in India (Pandey et al., 2018;Savurirajan et al., 2018) The results about negative binomial distribution agree with similar observations for inland water benthic invertebrates (Gray, 2005;De Los Ríos Escalante, 2017;De Los Ríos Escalante and Mansilla, 2017;Ríos and Arancibia, 2018).Also, in recent studies, it has described the use of negative binomial distribution for intertidal environments, specifically in middle intertidal zone, in rocky shores without seaweeds, similar to sites in the present study (Philippe et al., 2016;Checon et al.,  2008).Also, the topographical differences in rocky shores, can generate sites with different insolation or wave gradient exposure that would regulate the aggregated pattern of the individuals (Lauzon-Guay and Scheibling, 2009).If it integrated these antecedents, the study site is characterized by irregularities in topography in rocky shores, and wave exposure that would generate a complex scenario that would have consequences in aggregation patterns in intertidal marine invertebrates (Guiller, 1959;Pacheco and Castilla, 2000).
As conclusion it suggests do more ecological studies considering the importance of the marine invertebrates in these ecosystems as important preys for littoral fishes and/or marine birds, that would understand the ecological community structure and process in Chilean rocky shores.2017; Sibaja-Cordero, 2018;Ríos and Arancibia, 2018).In this context Rojas et al. (2000) studied the aggregated pattern of intertidal gastropod Nodolittorina peruviana, nevertheless they did not focus in interpretative equations for explain its absolute abundance, but they remark the role of aggregation behaviour for avoid dehydration during low tide.Similar description was done in the first studies on intertidal decapods (Bahamonde and López, 1969).The literature for other similar ecosystems proposed as survival strategy the joining of groups for avoid dehydration due temperature increase at low tide (Atta et al., 2014;Shanks et al., 2014;Mortensen and Dunphy, 2016).About the absence of negative binomial distribution observed in sites such as Caleta Coloso and El Lenguado 1 would be probably to interspecific behaviour mediated probably by topographic differences (Underwood, 2004), that would provide shelters for optimizate the use of trophic resources (Hidalgo et al., 2008;Aguilera andNavarrete, 2011, 2012).
Other important factor that would explain differences in aggregated pattern probably would be the insolation exposure that would generate dehydration stress, this condition was studied for other gastropods of the Littorinidae family (Erlandsson et al., 1999;Lauzon-Guay and Scheibling, 2009;Miller and Denny, 2011;Rickards and Boulding, 2015), including E. peruviana (Muñoz et al.,

Figure 2 .
Figure 2. Graphs of distributional patterns for E. peruviana included in the present study.

Figure 4 .
Figure 4. Heat map of PCA for spectral properties and E. peruviana abundances in sites included in the present study.

Table 3 .
Eigenvalue for considered sites in the present study.

Table 2 .
Correlation matrix for variables included in the present study, "p" values lower than 0.05 denotes significant associations.
Figure 3. PCA results for spectral properties and E. peruviana abundances in sites included in the present study.