1,2**, Kumthorn Thirakhupt




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Shell utilization by the land hermit crab Coenobita rugosus (Anomura, Coenobitidae) with noteS on the first record of bivalve shell use

Thanakhom Bundhitwongrut1,2**, Kumthorn Thirakhupt2, and Art-ong Pradatsundarasar2*
Abstract
The study of the use of shells as an indispensible resource for land hermit crabs adds more understanding of their life history. We investigated shell utilization of the land hermit crab Coenobita rugosus by multiple quadrat sampling from April 2011 to March 2012 on Phuket Island in the Andaman Coast of Thailand. A total of 1,322 individuals of C. rugosus were collected (711 males, 507 non-ovigerous females and 104 ovigerous females), and were found to use 63 molluscan shell species, including 62 gastropod shell species from 20 families and one bivalve shell. The diversity of shells used increased with body size from small to medium sized crabs, but decreased in larger crabs. The most commonly occupied shell species was N. albicilla (19.6 % of crabs). However, N. albicilla used by C. rugosus was not the lightest shell species in relation to the ratio between internal volume and weight according to the energy saving hypothesis. Globose shells and those with ovate apertures were the most commonly used shell types. The shell utilization patterns of C. rugosus at the study site were different between sexes and among reproductive stages.. Different patterns of shell use by C. rugosus were recognized in relation to different sizes and among reproductive stages. Furthermore, strong correlations between internal volume and aperture size of occupied shells and hermit crab characters suggest that the shell internal volume and size of aperture these shell characteristics are the main determinants for shell utilization of C. rugosus. Consequently, the pattern of shell utilization of C. rugosus is seemingly similar to those of other coenobitid species based on the frequent occupation of certain shell species and shapes. Additionally, the great shell diversity used by this population of C. rugosus compared with other conspecific populations and congeneric species may reflect more plasticity in shell utilization due to the high diversity of shell resources in the tropical habitats in the Indo-Pacific region.
Keywords: Coenobitidae, terrestrial hermit crab, shell use patterns, shell quality, bivalve shell use
1Biological Science Program, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand

2Department of Biology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand

*Corresponding author; e-mail: artong.biol@gmail.com

**Co-corresponding author; e-mail: thanakhom@hotmail.com

Introduction

Sixteen species of land hermit crabs of the genus Coenobita, family Coenobitidae, among 1,106 currently recognized species of hermit crabs have been reported in tropical and subtropical coastal areas around the world (McLaughlin et al., 2007; De Grave et al., 2009; McLaughlin et al., 2010). Land hermit crabs are the most common crustaceans in some tropical islands (Page & Willason, 1982; Morrison, 2005). These crabs are also an important component in the marine-land interface of the supralittoral zones of insular and coastal areas (Morrison & Spiller, 2006) as generalist scavengers accelerating the rate of recycling of nutrients and energy in food webs (Laidre, 2013).

In order to protect their soft and vulnerable abdomens, all land hermit crab species use empty gastropod shells as portable homes (Burggren & McMahon, 1988). The unique shell-carrying habit of coenobitid crabs provides many types of benefits, including protection against predators and from desiccation (Burggren & McMahon, 1988; Greenaway, 2003). The space inside the occupied shell is available for storing water to maintain crab body moisture, allowing them to forage farther inland (Wilde, 1973). Several shell characteristics have been reported to be correlated with hermit crab morphological characters (i.e., shell size and weight, aperture size, internal volume) (Hazlett, 1981). Shells probably play a role as a limiting resource for certain hermit crab populations (Fotheringham, 1976; Kellogg, 1976; Laidre, 2012). In addition, inhabited shells possibly affect growth, reproduction and risk of predation (Blackstone, 1985; Osorno et al., 2005; Sallam et al., 2008; Contreras-Garduno et al., 2009; Sallam, 2012). Furthermore, shell resources for hermit crabs in different areas have effects on population characteristics such as abundance, maximum size and reproduction (Fotheringham, 1976; Sallam et al., 2008).

The shell utilization pattern of land hermit crabs has been studied in several areas of the world (e.g. Western Atlantic by Morrison & Spiller (2006); Eastern Pacific by Abram (1978), Guillen & Osorno (1993) and Laidre & Vermeij (2012); North Pacific by Willason & Page (1983) and Szabo (2012); Western Pacific by Boneka et al. (1995); Red Sea by Volker (1967), Sallam et al. (2008) and Sallam (2012); Western Indian Ocean by Barnes (1999, 2001, 2002)). Nevertheless, information on shell use by land hermit crabs of the Andaman Sea east of the Indian Ocean, an area of high gastropod diversity, (Tantanasiriwong, 1978; Middelfart, 1997), is scant.

C. rugosus has been recorded as a widely distributed and common species in the Indo-Pacific region (McLaughlin et al., 2007). Among three species of land hermit crabs recorded in Thailand, C. rugosus is the most common (McLaughlin, 2002). This species is usually found living on sandy beaches and beach forests in the supralittoral zone (Frith & Alexander, 1978; Boneka et al., 1995; Nakasone, 2001; Barnes, 2002). At Cape Panwa, Phuket Island, Thailand, C. rugosus was recently investigated in relation to population ecology (Bundhitwongrut et al., 2014). The present study is an initial attempt to investigate the life history and shell utilization of C. rugosus. The objectives of the present study are to examine shell utilization patterns and the relationship between characters of shells used and crab characteristics of the population of C. rugosus at Cape Panwa, Phuket Island, Andaman coast of Thailand.

Materials and methods


Study Area
This study was conducted at Cape Panwa (7°48′26″N, 98°24′35″E), which is situated on the southeast side of Phuket Island on the Andaman coast of Thailand approximately 10 km south of the town of Phuket. The climate is wet tropical and is influenced by the wet southwesterly monsoon from May to October and the dry northeasterly monsoon from November to April (Khokiattiwong et al., 1991). This study was conducted at the beach in the supralittoral zone in the area of the Phuket Marine Biological Center (PMBC) at Cape Panwa, Phuket. The beach of Cape Panwa is an open sand scrub beach, comprised of rather coarse sand patches of shale (phylitte) (Nielsen, 1976a). The inland edge is covered with sparse vegetation alternating with dense vegetation before cliffs. The study beach is located behind the office of PMBC. This beach is about 50 m wide and the distance between the mean sea level of the study site and the office of PMBC is about 45 m. The study area is exposed to the semidiurnal tide with an amplitude of 2.15–2.27 m at spring tide to 0.85–1.15 m at neap tide (Limpsaichol, 1981). A map of the study area and environmental conditions during the study period are given in Bundhitwongrut et al. (2014).
Specimen Sampling and Analysis
The current study was conducted during April 2011 to March 2012, simultaneously with an investigation of the population ecology of C. rugosus (Bundhitwongrut et al., 2014). C. rugosus were collected on three days per month. The hermit crabs were collected by hand at low tide from the supralittoral zone by the same person (the first author) in the early morning (Sallam et al., 2008). The multiple sampling quadrat technique (Barnes, 1999) was used to collect C. rugosus. Four temporary line transects at 15-m intervals were randomly drawn perpendicular to the shoreline from the supralittoral zone to the inland area. Sixteen temporary quadrats of area 1 m2 were placed every 5 m on transects from 5 m above the mean sea level to 45 m further inland between 5 m and 45 m above the mean sea level. The quadrats had walls 10 cm high to prevent crabs from escaping, as land hermit crabs are agile and can move quickly.

All crabs sampled were brought to the laboratory in the office of PMBC. Each crab was carefully pulled out of its shell while holding the crab in the air and waiting until most of its body extended from the shell in order to investigate crab characters. According to the policy of the PMBC, removal of crabs from the population at the study site was not permitted to avoid negative impacts on native animals. Additionally, all authors agreed to the current sampling method without unnecessary crab killing in order to maintain and preserve this population of C. rugosus. Thus, after the investigation, all crabs were allowed to reinhabit their previously occupied shells and were maintained in several aquaria with food and water until the end of the investigations in each sampling month.

An additional marking method was conducted in this study. After surveys in each sampling month, all sampled crabs were marked before being released into the natural habitat at the same point from where they were collected. The markings were made with a waterproof pen and then coated with nail vanish on both crabs (on the outer surface of palm of the major cheliped) and their occupied shells (on the surface of the body whorl near the outer lip). From the results in preliminary trials, the markings were durable at least one month in the natural habitat of crabs. Additionally, all recaptured crabs in every month were marked again, if encountered in the sampling quadrats.

Several characteristics of C. rugosus were investigated and measured, including cephalothoracic shield length (CSL; from the tip of the rostrum to the midpoint of the posterior edge of the cervical groove) and width (CSW; the greatest width of the cephalothoracic shield perpendicular to CSL), weight (CW), sex and reproductive stage (males, non-ovigerous females, and ovigerous females), major chela length (MCL; from the articulation between carpus and propodus to the tip of fixed finger of the left cheliped) and major chela width (MCW; the greatest distance from the dorsal margin to the ventral margin of propodus of the left cheliped perpendicular to MCL).

Species of occupied and unoccupied shells were identified using several references (Brandt, 1974; Nielsen, 1976a, b; Wium-Andersen, 1977; Tantanasiriwong, 1978; Middelfart, 1997; Poutiers, 1998; Tan & Clements, 2008). In addition, shells were compared with specimens deposited in the reference collection of PMBC to confirm their identities. The quantitative characteristics included shell length (SL), width (SW), weight (WW), internal volume (SIV), aperture length (SAL), and aperture width (SAW). We measured the internal volume of shells by gradually adding water from a graduated syringe (Floeter et al., 2000). If a shell was damaged, holes were closed with UHU® patafix glue pads (UHU GmbH & Co., Germany) before they were filled with water. We investigated all quantitative measurements to the nearest 0.01 mm for size using digital vernier calipers, 0.01 g for weight using digital weighing scales, and 0.1 ml for volume using graduated syringes.

Qualitative shell characteristics, including shell shape, aperture (shell opening) shape and shell quality, were categorized and recorded. The shell and aperture shapes were classified according to Springsteen & Leobrera (1986) and Poutier (1998). Tables 1 and 2 give descriptions and schematic drawings of representative categories of shell and aperture shapes. The categories of shell shapes included biconical, conical, elongately conical, fusiform, globose, oval, pyramidal, pyriform, turban and vermiform. The categories of shell aperture shapes were classified as elongately ovate, irregular, ovate, round and semicircular. The shell quality categories were undamaged and damaged shells. Damaged shells were those with a broken apex, a hole, damaged inner lip, broken outer lip of the last whorl, or greater damage in a large portion of the shell (Barnes, 1999). Finally, the ratio between the shell internal volume and weight (SIV/W ratio) was calculated for all shell species occupied by C. rugosus as a predictor of shell quality (Osorno et al., 1998).


Statistical Analysis
The data of all recaptured individuals of C. rugosus were excluded to avoid possible pseudosamples. Crabs were classified into groups according to reproductive stage as either male, female, non-ovigerous female, or ovigerous female. We used chi-square (χ2) tests to compare the frequencies of occupation of different shell species, shell and aperture shapes, and the rate of occupancy of the undamaged and damaged shells between sexes and among crab reproductive groups (Zar, 2010). Small samples (n < 5) were pooled before using chi-square tests. The 0.5-mm size classes (CSL) intervals were applied to facilitate the comparison in shell use as a function of hermit crab size, following Nakasone (2001), Sallam et al. (2008) and Bundhitwongrut et al. (2014). To determine relationships between characters of hermit crabs and occupied shells, regression analyses were performed using the power function equation (Y = aXb) (Sallam et al., 2008). In all statistical tests, the critical significance level adopted was p < 0.05. All statistical analyses were performed using SPSS Statistics 17.0 (SPSS Inc., 2008).

Results
Shell Utilization Patterns of Coenobita rugosus


A total of 1,322 individuals of C. rugosus were collected, including 711 males and 611 females (507 non-ovigerous females and 104 ovigerous females) and were used for data analysis.
Diversity and groups of shells used
Coenobita rugosus was found occupying at least 63 species of molluscan shells (Table 3), including 62 gastropod shell species of 20 families. Interestingly, a valve of one marine bivalve species, Chama sp., was occupied by one individual of C. rugosus (Figure 1).

Coenobita rugosus occupied the shells of 59 species of marine gastropods and only two species of freshwater gastropod and one species of terrestrial gastropod. The gastropod family with the highest number of species utilized by C. rugosus was Muricidae (27.0%; 17 species), followed by Neritidae (12.7%; 8 species) and Turbinidae (9.5%; 6 species) (Table 3).
Unused shells found in sampling quadrats
In sampling quadrats, we found 132 shells of 18 gastropod species that were not used by hermit crabs during the study period. Most of unoccupied shells were damaged or plugged with gravel at the aperture. These shells were apparently unable to be used by crabs. One species of gastropod shell, Trochus maculatus (n = 2), was unoccupied by land hermit crabs at the study site.
Shell species used in relation to crab reproductive groups
The shell utilization pattern of C. rugosus varied in relation to shell species (Table 4). The most-used shell species was Nerita albicilla (19.6%, n = 259), followed by N. chamaeleon (11.6%, n = 153) and N. polita (11.3%, n = 149).

Male and female C. rugosus utilized the same number of shell species (53 species) with 43 species (81%) used by both sexes. Ten shell species were occupied only by males (Cerithidea cingulata (n = 1), Chama sp. (n = 1), Murex pecten (n = 1), Nerita articulata (n = 3), Polinices didyma (n = 1), Pomacea canaliculata (n = 4), Pugilina cochlidium (n = 3), Pugilina colosseus (n = 1), Purpura panama (n = 1) and Tenagodus cumingii (n = 1)). In addition, ten other shell species were occupied only by females (Cyclophorus pfeifferi (n = 1), Cymatium muricinum (n = 1), Cymatium pileare (n = 1), Cymatium succinctum (n = 1), Cymatium sp. (n = 2), Drupa rubusidaeus (n = 1), Fusinus nicobaricus (n = 1), Littorina scabra (n = 1), Rapana rapiformis (n = 1) and Turritella sp. (n = 3)). Males used shells of N. albicilla in highest proportion (20.8%, n = 148), followed by N. polita (13.2%, n = 94) and N. chamaeleon (12.5%, n = 89). Females also occupied N. albicilla shells the most (18.2%, n = 111), followed by N. chamaeleon (10.5%, n = 64) and Drupella rugosa (9.3%, n = 57). There was significant difference in shell species occupation between males and females (χ2 = 34.125, d.f. = 18, p = 0.012).

Non-ovigerous females utilized more diverse shell species (51 species) than ovigerous females (22 species). There were significant differences in shell species occupation between non-ovigerous females and ovigerous females (χ2 = 39.494, d.f. = 3, p < 0.001). Non-ovigerous females mostly used N. albicilla (20.3%, n = 103), followed by D. rugosa (11.2%, n = 57) and N. chamaeleon (9.5%, n = 48). Nevertheless, the most occupied shell species by ovigerous females of C. rugosus were N. polita (23.1%, n = 24) and N. chamaeleon (15.4%, n = 16) and followed by N. costata (10.6%, n = 11). There were also significant differences in shell species occupation between males and non-ovigerous females (χ2 = 44.168, d.f. = 16, p < 0.001) and between males and ovigerous females (χ2 = 14.832, d.f. = 3, p = 0.002).
Shell species used in relation to crab size
Shell utilization pattern of C. rugosus varied in relation to crab size (Figures 2 and 3). The diversity of shells used by C. rugosus increased with body size from small to medium-sized crabs, but decreased in larger size classes (Figure 2). Medium-sized crabs (3.5–9.5 mm) utilized more diverse shell species (17–27 species) than smaller (<3.5 mm CSL, 7–11 species) and larger (>9.5 mm CSL, 1–11 species) crabs.

The three most-occupied shell species in the genus Nerita were inhabited by small to medium crabs (2.5–11.5 mm CSL) (Figure 3). Nerita albicilla was used by crabs 2.5 to 10.5 mm in size (n = 259), while N. chamaeleon was occupied by crabs of sizes 3.0–11.0 mm (n = 153) and N. polita was utilized by crabs of size 3.0–11.5 mm (n = 149). Most small crabs (2.5–6.5 mm) occupied shells of D. rugosa (10.4%, n = 138). Shells used in the genus Turbo, which were mainly T. cinereus (4.6%, n = 61) and T. petholatus (2.3%, n = 30), were inhabited by a wide range of size classes of crabs (3.5–16.0 mm).


Shell use in relation to shell shape
Shell utilization patterns of C. rugosus varied in relation to shell shape (Figure 4). Globose shells (53.9%, n = 712) were the most-used shell shape by all C. rugosus, followed by biconical shells (18.4%, n = 243) and shells with turban shape (11.5%, n = 152). Males were found occupying more categories of shell shape (10 shapes) than non-ovigerous females (9 shapes) and ovigerous females (5 shapes). There were significant differences in shell shape occupation between males and non-ovigerous females (χ2 = 34.335, d.f. = 9, p < 0.001), between males and ovigerous females (χ2 = 18.756, d.f. = 9, p = 0.027) and between non-ovigerous females and ovigerous females (χ2 = 36.612, d.f. = 8, p < 0.001).
Shell use in relation to shell aperture shape
Shell utilization patterns of C. rugosus varied in relation to shape of shell aperture (Figure 5). Shells with ovate apertures (75.0%, n = 992) were most used by all C. rugosus, followed by the shells with round (13.5%, n = 179) and semicircular apertures (4.8%, n = 64).

Males and non-ovigerous females of C. rugosus were found occupying shells in all five categories of aperture shape, while ovigerous females were found using only four categories. Shells with elongately ovate apertures were unoccupied by ovigerous females. There were significant differences in shell aperture shape occupation between males and non-ovigerous females (χ2 = 23.139, d.f. = 4, p < 0.001), between males and ovigerous females (χ2 = 9.622, d.f. = 4, p = 0.047) and between non-ovigerous females and ovigerous females (χ2 = 24.113, d.f. = 4, p < 0.001).


Shell use in relation to shell damage
All crab groups used both damaged and undamaged shells. Crabs used undamaged shells (50.5%, n = 668) slightly more than damaged shells (49.5%, n = 654). There was no significant difference in the rate of occupancy of undamaged and damaged shells between males and non-ovigerous females (χ2 = 3.376, d.f. = 1, p = 0.066). Nevertheless, there were significant differences in the rate of occupancy of the undamaged and damaged shells between males and ovigerous females (χ2 = 21.543, d.f. = 1, p < 0.001) and between non-ovigerous females and ovigerous females (χ2 = 30.318, d.f. = 1, p < 0.001). Ovigerous females occupied undamaged shells (75.0%, n = 78) obviously more than damaged shells (25.0%, n = 26).
Shell use in relation to SIV/W ratio
The values of SIV/W ratio of shells used by C. rugosus at the study area ranged from 0.115 to 4.650. The most used shell species by C. rugosus at this site was not the lightest shell species as reflected by the ratio of SIV/W. N. albicilla, the most-occupied shell species, had a SIV/W ratio (mean ± SD) equal to 0.364 ± 0.114 (n = 259) (Table 3), but had a very low ranking SIV/W ratio (52nd out of 63 shell species). However, the lightest shell species was P. canaliculata with a SIV/W equal to 3.589 ± 0.985 (n = 4), but ranked 40th in terms of use by C. rugosus.
Relationship between Crab and Shell Characteristics
The relationship between crab characters and occupied shells are shown in Table 5. The values of determination coefficient (r2) from regression equations ranged between 0.32 and 0.94. Strong correlations were observed in the equations between characters of crabs and internal volume, aperture width and length of utilized shells (r > 0.90). Shell aperture width was the most correlated shell character with crab characters (r > 0.95) whereas shell length was the shell character with least correlation with characters of crabs (r < 0.65).The relationship between crab characters and occupied shells are shown in Table 5. Strong correlations were observed in the equations between characters of crabs and internal volume, aperture width and length of utilized shells (r > 0.90).

Discussion


At Cape Panwa, Phuket Island, the shell utilization pattern of C. rugosus appears to be similar to those of other congeneric species. The particular shell species occupied varied with the size of the crab. Shell utilization patterns of C. rugosus also varied in relation to shell and aperture shape. The body size of C. rugosus was most correlated with shell internal volume and aperture size. The plasticity of use of shell resources by C. rugosus is inferred by the greatest shell diversity used by this population at the study site compared with other land hermit crab species and populations. Last but not least, the first record on bivalve shell used by land hermit crab was noted.

Coenobita rugosus at the study site was found using the highest number of shell species (63 species) compared with other reported coenobitid species (C. scaevola (29 shell species) by Volker (1967); C. compressus (28 shell species) by Abram (1978); C. compressus (11 shell species) by Guillen & Osorno (1993); C. clypeatus (4 shell species) by Walker (1994); C. cavipes (21 shell species) and C. rugosus (20 shell species) by Barnes (1999); C. clypeatus (14 shell species) by Morrison & Spiller (2006); C. scaevola (10 shell species) by Sallam et al. (2008); C. compressus (41 shell species) by Laidre & Vermeij (2012)). Disparity of shell utilization pattern (i.e., shell species used) is probably a function of the different areas of occurrence of the hermit crabs (Garcia & Mantelatto, 2000; Mantelatto & Garcia, 2000). This is probably influenced by gastropod life cycle, abiotic environmental factors, and pressure from predation (Sallam et al., 2008). Moreover, Andaman Coast of Thailand including Cape Panwa, Phuket Island has a high species number of gastropod molluscs (382 species) (Tantanasiriwong, 1978) that probably supplies shell resources for hermit crab fauna living in this area.

The first record of the unusual occupation of a valve of marine bivalve by land hermit crab was observed at this study site. The shell of bivalves used as shelter was previously recorded only in marine hermit crabs in the genera Alainopagurus, Bivalvopagurus, Patagurus, Porcellanopagurus, Solitariopagurus (Lemaitre, 1993; Anker & Paulay, 2013) and Dardanus venosus (Garcia et al., 2003). Coenobita rugosus inhabiting a bivalve shell in this study possessed poor physical appearance with a short abdomen compared to the same sized crab (personal observation). This individual may be the defender (defined by Osorno et al. (1998)) whose shell is lost to an attacking crab during shell exchange. The large opening and small internal space of the bivalve shell of this crab were inappropriate for living because it was unable to withdraw completely into the shell. Most parts of the crab, including chelipeds, ambulatory legs and anterior part of cephalothorax, were beyond the shell opening when the crab was fully retracted and the crab could be easily be pulled out by predators. The shells of this bivalve, Chama sp., were sporadically found during the study period (personal observation) although its abundance was not evaluated.



Coenobita rugosus at the study site showed occupation of one species of gastropod shell over others as previously recorded (Abram, 1978; Achituv & Ziskind, 1985; Guillen & Osorno, 1993; Walker 1994; Barnes, 1999; Morrison & Spiller, 2006; Sallam et al., 2008; Laidre & Vermeij, 2012). Although shell availability was not evaluated, this different proportions of shell species occupied by C. rugosus may indicate active behavior in shell selection (Sallam et al., 2008).

The shell utilization patterns of C. rugosus at the study site were different between sexes and among reproductive stages. This finding is similar to those of a previous study of C. scaevola (Sallam et al., 2008). This result may be attributed to the fact that crab individuals of each sex and/or reproductive stage compete for shells and allot shell resources according to their appropriateness (Sallam et al., 2008). Furthermore, the discrepancy of shell use between sexes may result from intraspecific competition, behavior, reproductive strategies and different sizes (Imazu & Asakura, 1994; Asakura, 1995; Garcia & Mantelatto, 2000).

Differences in shell use among different sized crabs were noted in this population of C. rugosus. It is possible that C. rugosus at the study site utilize the shells of at least two different gastropod species as they grow. For instance, the most utilized shell species in the genus Nerita were inhabited by a wide size range of small to medium crabs (2.5–11.5 mm). Nevertheless, larger crabs (>11.5 mm) used other larger shell species rather than nerite shells. In another case, Turbo shell species were also commonly used by a wide range of crab sizes (3.5–16.0 mm). However, smaller individuals (<3.5 mm) needed to use other smaller-shell species before reaching the size allowing crabs to occupy Turbo shells. This inference is similar to the study by Morrison & Spiller (2006) who pointed out that C. clypeatus probably uses the shells of two or three different gastropod species during their growth. Therefore, the conservation of shell diversity is required to preserve hermit crabs because these crabs need different types and sizes of shells to complete their life cycle.

Coenobita rugosus showed occupation of certain types of shell and aperture shapes. Most shell shapes used by C. rugosus at the study site were low-spired shells frequently occupied by this coenobitid species in other areas as previously recorded by Willason & Page (1983), Barnes (1999) and Szabo (2012). Coenobita rugosus is considered as a burrowing species according to its behavioral ecology, in which selection of low-spired shells probably facilitates burrowing to avoid desiccation during the day (Barnes, 1999). Additionally, shells with ovate, round and semicircular or D-shape apertures were the most occupied aperture shapes by C. rugosus at the study area. Coenobita rugosus mostly occupied shells with round to circular and D-shape apertures probably because they enable the crabs to avoid desiccation by fully sealing the shell aperture with the major chela (Barnes, 1999; Szabo, 2012). Nevertheless, further studies on shell availability are needed to better understand shell choice in this hermit crab species.

Although there was no significant discrepancy between utilization of damaged and undamaged shells by all individuals of C. rugosus, most shells occupied by crabs were in worn and old condition (unpublished data), which probably had been used previously by other crabs over a period of many years (Ball, 1972; Abram, 1978; Boneka et al., 1995). Moreover, the columella of most shells used was missing (unpublished data) as previously recorded by Kinosita & Okajima (1968), Ball (1972), Laidre (2012) and Szabo (2012). Additionally, unoccupied shells in good condition were scarce at the study site, as formerly reported (Ball, 1972; Morrison & Spiller, 2006; Laidre & Vermeij, 2012). Therefore, shell supply in this population of C. rugosus probably circulates through these old and worn shells that are still suitable, especially for adult crabs, as shell facilitationcirculation rather than competition according to Abram (1978). Nevertheless, further investigations of other shell conditions as well as shell exchange of C. rugosus in natural habitat may be required to test this hypothesis.

The carrying of the lightest shell available, that of P. canaliculata, ought to be advantageous for C. rugosus because it would save energy, but its thin shell wall may render it more vulnerable to predators such as the rough red-eyed crab, Eriphia smithii, that was frequently encountered during the study (Bundhitwongrut et al., 2014). Although the most occupied shell species by C. rugosus in this study was not the lightest species according to the energy saving hypothesis proposed by Osorno et al. (1998), it is possible that in fact crabs may try to search and occupy the lightest shells in each shell exchange. Although the availability of the lightest shells in natural habitat at the study site was not evaluated, it may be a limiting factor. Consequently, it is possible that crabs use the remaining shells, which are subsequently inferior to the lightest ones, with higher SIV/W ratio compared to their previously occupied shells. Further investigation of shell exchange in the natural habitat and shell selection in laboratory condition would help answer this question.

There is a possibility that ovigerous female C. rugosus in the present study show selective greater tendency to select shells for use. These females used fewer shell species and shell and aperture shapes than other crab groups. This result suggests that ovigerous females more specifically selected shells, probably due to their reproductive condition that requires more protection during the vulnerable egg-carrying period. Moreover, the shell species most occupied by ovigerous females had a higher SIV/W ratio than those mainly used by other crab groups. The lighter shells (higher SIV/W) probably help egg-carrying females save energy for reproductive activity (Osorno et al., 1998). Ovigerous female C. rugosus in this study used more shell species (22 species) than egg-carrying female C. scaevola in the Red Sea (8 species) studied by Sallam (2012). This is probably associated with shell availability that is different between the areas (Garcia & Mantelatto, 2000; Mantelatto & Garcia, 2000).

The significant relationships were detected between characters of utilized shells and C. rugosus at the study area as previously reported by Boneka et al. (1995) and Sallam et al. (2008). The results appear to indicate that these shell characteristics are the main determinants of shell “selection” of C. rugosus at the study site. The intense degree of relationships between shell internal volume and crab characters possibly indicates that internal volume is important in providing ample space for C. rugosus to store water inside to maintain body moisture, which is crucial for terrestrial life (Wilde, 1973; Greenaway, 2003). In addition, more space in occupied shells may allow crabs to grow rapidly or retain more fertilized eggs during reproduction (Osorno et al., 1998). Furthermore, the strong correlations between shell aperture size and crab morphology allow C. rugosus to effectively seal the aperture firmly when retreating into the shell, thereby resulting in more protection against predators and from desiccation (Ball, 1972; Abram, 1978; Sanvicente-Anorve & Hermoso-Salazar, 2011).

In conclusion, the present study has portrayed the patterns of shell utilization by C. rugosus at Cape Panwa, Phuket Island, as well as the relationship between crab and shell characters. The shell utilization patterns of C. rugosus at the study site are seemingly similar to those of other coenobitid species. The great shell diversity used by this population suggests more plasticity on use of shell resources by C. rugosus. Thus, the information from this study provides comparative knowledge on shell use of C. rugosus and other coenobitid crabs.


Acknowledgements
This study was financially supported by a Thai Government Science and Technology Scholarship (NSTDA). The first author wishes to express his gratitude to PMBC and the staff for allowing use of their facilities for research throughout the study period. The first author would like to thank Miss Vararin Vongpanich (PMBC) for her help in identification and confirmation of some shell species. The authors give special thanks to Dr. George A. Gale (King Mongkut’s University of Technology, Thonburi), Dr. Warren Y. Brockelman (Mahidol University) and Dr. Prachya Musikasinthorn (Kasetsart University) as well as anonymous reviewers for constructive and invaluable suggestions on the manuscript.
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