Habitat heterogeneity in the assemblages and shell use by the most abundant hermit crabs ( Anomura : Diogenidae and Paguridae ) : does the occupied shell species di er according to gender and species ?

Th e goal of this study was to identify patt erns of shell occupation by diff erent species of hermit crabs from the southern Brazilian coast. In total, 644 individuals were collected, represented by six hermit species. Isocheles sawayai Forest & Saint Laurent, 1968 showed the highest abundance, with 575 individuals, followed by Loxopagurus loxochelis (Moreira, 1901) (n = 56). Th e other species were Petrochirus diogenes (Linnaeus, 1758), Dardanus insignis (Saussure, 1858), Pagurus exilis (Benedict, 1892) and Pagurus leptonyx Forest & Saint Laurent, 1968. Loxopagurus loxochelis was found associated with shells of 12 gastropod species, with 75% of males occupying shells of Olivancilaria urceus (Roding, 1798) and 78% of females inhabiting shells of Semicassis granulata (Born, 1778). Shells of Semicassis granulata were the lightest of all gastropod shells, demonstrating diff erential resource utilization. Additionally, I. sawayai occupied shells of 10 species, highlighting Stramonita haemastoma (Linnaeus, 1767) with the highest occupation percentage in all demographic classes, confi rming a patt ern of occupation with a strong relationship to the availability of the resource. Th e comparison of our results with those of other studies corroborated the infl uence of region and gastropod diversity on gastropod shell occupation. keY words Environmental resource, fi tness, sexual selection.


IntroductIon
The habitual use of shells by hermit crabs has led this group of crustaceans to develop remarkable behavior in the biological environment and has also contributed to their evolutionary success (Mantelatto and Sousa, 2000).
The diversity of hermit crabs and their abundance in a particular locality is strongly related to the geographic distribution, availability and suitability of gastropod shells (Reese, 1962;Pereira et al., 2009).Thus, the availability of shells constitutes an essential and limiting factor in the population structure of hermit crabs (Meireles et al., 2003).Agonistic conflicts between individuals of the same species or different species over resources (shells) are common in this group (Mantelatto and Garcia, 2000), but the selection and shell occupation do not occur randomly and follow some precepts, such as competition, because the availability of shells is generally low in environment (Bertness, 1980), shell size (Vance, 1972;Bertness, 1981a), internal volume (Conover, 1978), shell weight (Mantelatto and Dominciano, 2002), external architecture and adornments, which increase protection against predation and minimize energy costs transport to the hermit crabs (Vance, 1972;Rotjan et al., 2004).
Therefore, knowledge concerning the availability and variety of shells in the environment provides important information about communities of hermit crabs, because in addition to providing protection, they are closely related to the somatic growth rate, gonadal development and longevity of hermit crabs (Lancaster, 1988).
Thus, the objective of the study was to characterize the patterns of use of gastropod shells by the more abundant hermit crab species in a coastal region of the southern coast of Brazil.For this, we analyzed the percentage of shells of gastropod species that are most commonly used.To detect biometric parameters that provide indications about the occupation of such shells we established a correlation between the shell dimensions (weight and aperture width) and the weight and size of the respective occupants.

MaterIal and Methods
Hermit crabs were collected monthly from July 2010 to June 2011 at five sampling sites parallel to the shoreline and at distinct depths (5, 8, 11, 14 and 17 m).The trawls were conducted during 30-min, in each sampling site, using a shrimp boat outfitted with double-rig nets, in adjacent areas in Babitonga Bay (Fig. 1) (northern coast of the state of Santa Catarina).

Laboratory procedures
After collection, all the specimens were sorted, frozen and transported to the laboratory, where they were removed from their shells, counted, and identified according to Melo (1999).The sexes were recognized by verifying the position of the gonopores (female and male gonopores on the coxae of the third and fifth pairs of pereopods, respectively) and the reproductive condition was observed by the presence of ovigerous females (fertilized eggs attached to pleopodal setae) (Mantelatto and Garcia, 1999).These analyses were performed using a stereomicroscope.
The Cephalothoracic Shield Length (CSL) of the animals was measured with a caliper (0.01 mm), and the Hermit Wet Weight (HWW) was obtained using precision scales (0.01 g).The shells were identified (Rios, 1994;Gofas, 2014), and the largest Shell Aperture Width (SAW) (between inner lip and outer lip) (0.01 mm) and Shell Wet Weight (SWW) were measured with precision scales (0.01 g).

Data analyses
Statistical analyses were performed using the species that were found in more than 10% of samples, i.e., only Isocheles sawayai Forest &Saint Laurent, 1968 andLoxopagurus loxochelis (Moreira, 1901) that were the most abundant species, mainly in depths of 5 m.The Kruskal-Wallis and Dunn a posteriori tests were applied to compare the dimensions of Shell Aperture Width (SAW) and Shell Weight (SW), and to compare the dimensions in the occupied shells by hermit crabs of different sexes.Linear Regressions were performed to evaluate the relationships between parameters of the occupied shells relative to the size of the hermit crabs (Zar, 1999).The values of the variables were log (1+x)-transformed to satisfy the assumptions of the statistical tests (Castilho et al., 2008), which were evaluated at the 5% level of significance.

results
A total of 644 individuals of six hermit crab species was collected, with four belonging to Diogenidae

Linear regression: Isocheles sawayai
Linear regressions with animal variables (CSL, HWW) and shell variables (SAW, SWW) were performed with shells of the four most abundant gastropod species, which all resulted in significant relationships (p < 0.05).The higher coefficients of determination (R2) between CSL vs. SWW and HWW vs. SAW were obtained for shells of S. haemastoma (Fig. 3).The regressions performed between HWW vs. SWW showed high coefficients of determination (Fig. 4).
Loxopagurus loxochelis was represented by 24 (43%) females, 31 males (55%) and one ovigerous female (2%).These animals occupied shells of 12 gastropod species.Segregation in the pattern of shell occupation according to the sex was observed; O. urceus was occupied by 75% of males, whereas S. granulata was

dIscussIon
Interspecific competition should be a non-significant factor in the pattern of shell occupation for the two most abundant and coexistent species of hermit crabs in the study area.The apparent absence of competition for resources between both species might be a consequence of a difference in their spatial distribution: Isocheles sawayai occurs in coastal regions, preferably at sites with depth of 5 m or less (Fantucci et al., 2009), whereas Loxopagurus loxochelis is found between 12 and 30 m of depth (Ayres-Peres and Mantelatto, 2008;Fransozo et al., 2008).This spatial occupation difference facilitates coexistence and minimizes niche overlap (Fransozo et al., 1998) in relation to the occupation of resources.
Although the four other species of hermit crabs were represented by few individuals, they demonstrated a spatial overlap with L. loxochelis: Pagurus exilis inhabits depths from 15 to 35 m (Meireles et al., 2006), Pagurus leptonyx from 10 to 20 m (Lemaitre et al., 1982), and Dardanus insignis and Petrochirus diogenes are found in areas deeper than 20 m (Fransozo et al., 2008).Despite some overlap in certain deep areas, Meireles et al. (2012) proposed that the competition for resources among species is more pronounced in the juvenile stage when individuals have similar body sizes, and thus, compete for empty shells with the same dimensions.According to these authors, competition for resources tends to decrease in the adult phase, because L. loxochelis, D. insignis and P. diogenes reach larger sizes than P. exilis and P. leptonyx.
Males and females of all species occupied shells of different dimensions (weight and aperture width).This is because the sexes have different sizes, with males being larger than females, which has already been described in several studies on hermit crabs (Branco et al., 2002).In general, two hypotheses have been proposed for the size difference (see references below): males invest more energy in growth to reach larger sizes than females, allowing them to use larger and heavier shells, whereas females direct energy into the production of eggs and incubation, resulting in less somatic growth and therefore, they occupy lighter shells that require less energy to carry (Bertness, 1981a).Alternatively, the difference in body size might be the result of an evolutionary selection of reproduction prowess, where hermit crabs compete for reproductive partners and the largest individuals tend to win intraspecific combats, which might represent a selective factor over generations (Bertness, 1981a;Abrams, 1988, Mantelatto andSousa, 2000;Garcia and Mantelatto, 2001;Branco et al., 2002;Martinelli et al., 2002;Mantelatto et al., 2005;Biagi et al., 2006;Ayres-Peres et al., 2008).
Another factor that benefits the population is differential growth.Manjón-Cabeza and García-Raso (1998) confirmed that the growth period of males of Dardanus pugilator (Roux, 1829) begins about five months before that of females, which privileges their development and avoids competition for shells between the sexes at the juvenile stage.
Therefore, the difference in body size according to sex reduces the intraspecific competition for shelter among demographic groups and other resources possibly become beneficial and decisive factors for the establishment, growth and maintenance of the population in the environment (Yoshino et al., 2001).This appears to be the case for the species studied here.

Isocheles sawayai
The richness of empty shells (10 species of gastropods) occupied by I. sawayai in the study area (26ºS) was greater than that described by Pinheiro et al. (1993), in Ubatuba, São Paulo (23ºS) (six species), but was lower than that identified at the same latitude by Fantucci et al. (2008) (17 species) in a neighboring area (Caraguatatuba), and was similar to that found by Masunari et al. (2008) (eight species) on the Paraná coast (25ºS), thereby demonstrating the absence of a latitudinal pattern in determining shell occupation.The competence in using resources is probably directly linked to the regional abiotic and biotic conditions and the plasticity of species of hermit crab to occupy gastropod shells of different species (Galindo et al., 2008).This plasticity might be related to the body size of the hermit crab, with small animals such as I. sawayai having an enormous variety of species of gastropod shells available, because smaller shells are commonly found for many species in the natural environment (Bertini and Fransozo, 2000) and are rarely damaged, enabling their widespread use (FLM, pers.obs.).
The shells of Stramonita haemastoma were occupied at a high percentage by all demographic groups (males, females and ovigerous females), similar to the pattern found by Masunari et al. (2008) (males 57.14% and females 62.33%) and suggesting that no competition between the sexes exists for the higher availability of this resource in the environment (Abrams, 1988).In contrast, Mantelatto and Dominciano (2002) described a different shell occupation pattern by Paguristes tortugae Schmitt, 1933, with low coefficient of determination: Pisania auritula (Link, 1807) (R2 = 0.46), Cerithium atratum (Born, 1778) (R2 = 0.51), and Morula nodulosa (Adams, 1845) (R2 = 0.45).This is due to the lack of adequate shells available in the environment, which generates intense intraspecific competition in species that occupy rocky shore areas.
The shell of S. haemastoma provided the best fit for I. sawayai, as shown by the higher coefficients of determination for the animal variables CSL and HWW with the shell variables (CSL vs. SWW; HWW vs. SAW) and the excellent association between the variables HWW vs. SWW.High biometric coefficients were also identified by other authors for I. sawayai and S. haemastoma, for example, Pinheiro et al. (1993) (CSL vs. SAW, R2 = 0.72), and Fantucci et al. (2008) (CSL vs. SWW, R2 = 0.76).
An appropriate balance between the dimensions of the animal and the shell is extremely important for the occupation choice.However, the shell dimension might be of secondary importance after the availability of this resource in the environment (Mantelatto and Meireles, 2004).This explains the high occupation of S. haemastoma recorded in this study, because it combines a high availability in the environment and provides the most suitable dimensions for occupation by hermit crabs.
Olivancillaria urceus was the second most abundant shell used (31.3%), and was the most available species on the coast of São Paulo state, especially in the Ubatuba region (Ayres-Peres et al., 2012).The availability of this gastropod in the study area is probably similarly high, as also observed on the coast of Paraná (southern Brazil) (Masunari et al., 2008).Shells of O. urceus possessed a smaller aperture than the other gastropod shells.During intra or inter-specific fighting for the shell or predator attack, hermit crabs are removed from their shells through the aperture (Turra et al., 2005).Therefore, a smaller aperture hinders their removal and provides greater protection (Bertini and Fransozo, 2000).Another feature of this shell is its statistically higher weight and thicker wall than similar-sized shells of other species (Ayres-Peres et al., 2012), which might provide better protection against predators that break shells, and be more robust against adverse environmental conditions, such as wave action (Reese, 1962;Bertness, 1981b).The weight of this shell might also contribute to the improved stability of hermit crabs on an unconsolidated substrate, thus preventing the animal being dragged along by the tide or water current (Masunari et al., 2008).
The disadvantage of the use of a heavy shell by the hermit crab is that it requires higher energy expenditure in movement (Siu and Lee, 1992); however, I. sawayai remains half-buried, with only its antennae protruding from the substrate to obtain food particles in suspension (Melo, 1999), which potentially requires a low energy expenditure.Babitonga Bay forms the largest estuarine complex on the Santa Catarina coast, that provides a widespread and constant circulation of materials in suspension for the coastal region and abundant food for benthic suspension feeder species (D' Aquino et al., 2011), conditions that are favorable for the establishment of I. sawayai in the region.

Loxopagurus loxochelis
Loxopagurus loxochelis occupied a considerable number of shells of gastropod species (12), similar to the numbers registered by Ayres-Peres et al. (2012) (14 species) and Frameschi et al. (2013) (10 species), also on the North coast of São Paulo state.Olivancillaria urceus was one of main resources used by this species in this study (37.5%), compared with a frequency in previous studies of 77.7% (Ayres-Peres et al., 2012) and 31.3%(Frameschi et al., 2013).According to Wait and Schoeman (2012), shell selection is a process that involves individual and sexual preferences in different dimensions, with the aim of providing the best protection for the hermit crab and sufficient space for the development of embryos and their oxygenation, because females that inhabit very small shells show difficulties in reproducing or reduced rates of reproduction (Fantucci et al., 2008).Thus, the observed highlighted differential shell occupation by L. loxochelis is potentially explained by the physiological demands of each sex during the life cycle, i.e., most males occupied O. urceus that were heavy, with thick walls, to obtain protection against mechanical abrasion (Reese, 1969).On the other hand, the higher occupancy of Semicassis granulata shells by females suggests an intimate relationship with the weight of the shell (which are on average lighter than those of O. urceus), which facilitates transport, especially during the breeding season when the females carry embryos in the abdomen (Elwood et al., 1995).
Shells of the other species used by L. loxochelis are probably occupied sporadically, due to the immediate need of the animal to find a shelter when other more appropriate resources are not available for occupation.
It has been demonstrated here that specific preferences exist in the species of shell occupied; some shells are selected according to type, shape or weight, emphasizing that success in selecting this feature is related not only to the availability of the resource in the environment, but also to variation in shape and size.In many cases, the relationship between the morphology of the animal and the shell dimensions is not sufficient alone to explain the choice of a particular shell by the hermit crab species in the natural environment, but it is necessary to take into account potential ecological interactions, such as inter and intra-specific competition that might occur (Biagi et al., 2006).Thus, the study of the gastropod community and hermit crabs becomes essential to draw more accurate conclusions concerning the fascinating mechanisms of availability and shell occupation, as well as the inter or intra-specific ecological processes that determine shell occupation patterns in nature.We thank many colleagues from the NEBECC group who helped with sampling and laboratory analyses; and the "Instituto Brasileiro do Meio Ambiente e dos Recursos Naturais Renováveis" (IBAMA) for granting permission to collect the animals.

Figure 1 .
Figure 1.Location of the study area, highlighting the sampled stations in an area adjacent to Babitonga Bay, Santa Catarina, Brazil (source: Grabowski et al., 2014).

Figure 2 .
Figure 2. Mean values and standard deviation of Shell Wet Weight and Shell Aperture Width of the four most abundant gastropod species (Olivancillaria urceus, Stramonita haemastoma, Polinices hepaticus and Semicassis granulata) during the study period in an area adjacent to Babitonga Bay, Santa Catarina, Brazil.

Figure 4 .
Figure 4. Linear Regression, between Hermit Wet Weight (HWW) vs. Shell Wet Weight (SWW) of gastropod species more occupied by Isocheles sawayai during the study in an area adjacent to Babitonga Bay, Santa Catarina, Brazil.

Figure 5 .
Figure 5. Loxopagurus loxochelis: Linear Regressions between Hermit Wet Weight (HWW) vs. Shell Wet Weight (SWW) of Olivancillaria urceus, and HWW vs. Shell Aperture Width (SAW) of Semicassis granulata during the study period in the area adjacent to Babitonga Bay, Santa Catarina, Brazil.

Table 1 .
Overview in the shell occupation by interest group: females (F), ovigerous females (OF) and males (M) of the hermit crab Isocheles sawayai in an area adjacent to Babitonga Bay, Santa Catarina, Brazil.

Table 2 .
Overview in the shell occupation by interest group: females (F), ovigerous females (OF) and males (M) of the hermit crab Loxopagurus loxochelis in an area adjacent to Babitonga Bay, Santa Catarina, Brazil.