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Sexual and size dimorphism in two deep-water hermit crabs (Decapoda: Parapaguridae) from the Western Atlantic Ocean

Abstract

The Parapaguridae comprises hermit crabs that inhabit deep-water environments. In these environments, shell availability can be limited, mostly consisting of small and fragile-shelled gastropods. Thus, different strategies have evolved to mitigate this limited shell supply. Sympagurus dimorphus (Studer, 1883) lives in association with a zoanthid cnidarian that creates a pseudo-shell that grows with the hermit crab. In contrast, Oncopagurus gracilis (Henderson, 1888) inhabits small, calcified gastropod shells. Therefore, we selected these two species as models to test sexual dimorphism and shape patterns of their chelipeds and cephalothoracic shield, due to their different shelter acquisition methods. We photographed the animals and digitized the images to employ comparative geometric morphometric techniques. We tested the differences in shape between the sexes within each species, and also tested sexual size dimorphism based on centroid size. For O. gracilis, we found shape differences for the chelipeds and cephalothoracic shield, however, we only observed sexual size dimorphism for the chelipeds. For S. dimorphus, an inverse pattern was found, in which females presented more robust chelipeds, and sexual size dimorphism was present in which males were larger. These differences can be reasonably explained by their shelter acquisition methods, in which O. gracilis depends on small shells that limit growth, while S. dimorphus grows with its cnidarian pseudo-shell. The robustness found in the shape patterns may also be related to their behaviors, e.g., in addition to competition for shells, they also fight during mating. However, we emphasize that future studies with other populations of these species are needed for comparative purposes.

Keywords:
Geometric morphometrics; Oncopagurus gracilis; reproduction; Sympagurus dimorphus

INTRODUCTION

The Parapaguridae comprises hermit crabs found in deep waters ranging from 100 meters to over 4000 meters, with a cosmopolitan distribution (Schembri and McLay, 1983Schembri PJ and McLay CL 1983. An annotated key to the hermit crabs (Crustacea: Decapoda: Anomura) of the Otago region (southeastern New Zealand). New Zealand Journal of Marine and Refreshwater Research, 17(1): 27-35. https://doi.org/10.1080/00288330.1983.9515984
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). Currently, there are ten genera described for this family (Lemaitre and McLaughlin, 2023Lemaitre R and McLaughlin P 2023. World Paguroidea & Lomisoidea Database. Parapaguridae Smith, 1882. Accessed through: World Register of Marine Species; Accessed through: World Register of Marine Species; https://www.marinespecies.org/aphia.php?p=taxdetails&id=106739 . Accessed on 28 Feburary 2023.
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). Gathering information on the biology and ecology of fauna inhabiting deep waters is an expensive and challenging task.

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Hermit crabs, with their unique non-calcified abdomen, rely on external structures for cover and shelter (Williams and McDermott, 2004Williams JD and McDermott JJ 2004. Hermit crab biocoenoses: a worldwide review of the diversity and natural history of hermit crab associates. Journal of Experimental Marine Biology and Ecology , 305(1): 1-128. https://doi.org/10.1016/j.jembe.2004.02.020
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). Within the Parapaguridae, Sympagurus dimorphus (Studer, 1883) is a widely distributed species in the southern hemisphere, inhabiting depths ranging from around 90 to 1995 meters (Lemaitre, 2004Lemaitre R 2004. A worldwide review of hermit crab species of the genus Sympagurus Smith, 1883 (Crustacea: Decapoda: Parapaguridae). In: Marshall B, Richer de Forges B, editors. Tropical deep-sea benthos. Vol. 23. Mémoires du Museum National d’Histoire Naturelle, 191: 85-149.; Landschoff and Lemaitre, 2017Landschoff J and Lemaitre R 2017. Differentiation of three common deep-water hermit crabs (Crustacea, Decapoda, Anomura, Parapaguridae) from the South African demersal abundance surveys, including the description of a new species of Paragiopagurus Lemaitre, 1996. Zookeys, ( 676 ): 21-45. https://doi.org/10.3897/zookeys.676.12987
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), and frequently associated with zoanthid cnidarians that build a pseudo-shell, rather than inhabiting empty gastropod shells (Schejter and Mantelatto, 2011Schejter L and Mantelatto FL 2011. Shelter association between the hermit crab Sympagurus dimorphus and the zoanthid Epizoanthus paguricola in the southwestern Atlantic Ocean. Acta Zoologica, 92(2): 141-149. https://doi.org/10.1111/j.1463-6395.2009.00440.x
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). Linear morphometrics and population studies have been recorded for S. dimorphus on both margins of the South Atlantic Ocean (Schejter and Mantelatto, 2015Schejter L and Mantelatto FL 2015. The hermit crab Sympagurus dimorphus (Anomura: Parapaguridae) at the edge of its range in the south-western Atlantic Ocean: population and morphometry features. Journal of Natural History, 49(33-34): 2055-2066. https://doi.org/10.1080/00222933.2015.1009406
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). Oncopagurus gracilis (Henderson, 1888) is another parapagurid with a wide distribution, inhabiting depths ranging from 146 to 900m in the Western and Eastern Atlantic (Merchán-Cepeda et al., 2009Merchán-Cepeda A, Hernando-Campos N, Franco A and Bermúdez A 2009. Distribución y datos biológicos de los cangrejos ermitaños (Decapoda: Anomura) del mar Caribe colombiano colectados por la expedición Macrofauna II. Boletin de Investigaciones Marinas y Costeras, 28: 121-142.; Lemaitre and Tavares, 2015Lemaitre R and Tavares M 2015. New taxonomic and distributional information on hermit crabs (Crustacea: Anomura: Paguroidea) from the Gulf of Mexico, Caribbean Sea, and Atlantic coast of South America. Zootaxa, 3994(4): 451-506. http://dx.doi.org/10.11646/zootaxa.3994.4.1
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). Unlike S. dimorphus, O. gracilis utilizes vacant gastropod shells as shelter (Lemaitre, 2014Lemaitre R 2014. A worldwide taxonomic and distributional synthesis of the genus Oncopagurus Lemaitre, 1996 (Crustacea: Decapoda: Anomura: Parapaguridae), with descriptions of nine new species. Raffles Bulletin of Zoology, 62: 210-301.). Despite its broad distribution, little is known about the biology of O. gracilis.

Decapod crustaceans exhibit a range of behaviors during resource disputes with their conspecifics. These agonistic events may involve the use of claws, which can serve as signals or weapons to injure opponents (Mariappan et al., 2000Mariappan P, Balasundaram C and Schmitz B 2000. Decapod crustacean chelipeds: an overview. Journal of Biosciences, 25(3): 301-313. https://doi.org/10.1007/BF02703939
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). Sexual dimorphism is often found in crustaceans, in which males are usually bigger than females (Oliveira and Custódio, 1998Oliveira RF and Custodio MR 1998. Claw size, waving display and female choice in the European fiddler crab, Uca tangeri. Ethology, Ecology andEvolution , 10(3): 241-251. https://doi.org/10.1080/08927014.1998.9522855
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), and when the sexual dimorphism is found in chelipeds, these structures are often bigger and stronger than those of females (Doake et al., 2010Doake S, Scantlebury M and Elwood RW. 2010 The costs of bearing arms and armour in the hermit crab Pagurus bernhardus. Animal Behavior, 80: 637-642. https://doi.org/10.1016/j.anbehav.2010.06.023
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); as seen for several anomuran (Turra and Leite, 1999Turra A and Leite FPP 1999. Population structure and fecundity of the hermit crab Clibanarius antillensis Stimpson, 1862 (Anomura, Diogenidae) in southeastern Brazil. Bulletin of Marine Science 64: 281-289. https://doi.org/10.1016/j.jembe.2004.04.008
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), and brachyuran species (Abelló et al., 1990Abelló P, Pertierra JP and Reid DG. 1990 Sexual size dimorphism, relative growth and handedness in Liocarcinus depurator and Macropipus tuberculatus (Brachyura: Portunidae). Scientia Marina, 54(2): 195-202.; Tsuchida and Fujikura, 2000Tsuchida S and Fujikura K 2000. Heterochely, relative growth, and gonopod morphology in the bythograeid crab, Austinograea williamsi (Decapoda, Brachyura). Journal of Crustacean Biology , 20: 407-414. https://doi.org/10.1163/20021975-99990052
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). However, crayfish that live in constant intersexuality, such as representatives of the genus Parastacus Huxley, 1879, do not exhibit sexual dimorphism (Almeida and Buckup, 2000Almeida AO and Buckup L 2000. Occurrence of protandric hermaphroditism in a population of the neotropical freshwater crayfish Parastacus brasiliensis (PARASTACIDAE). Journal of Crustacean Biology, 20: 224-230. https://doi.org/10.1163/20021975-99990034
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).

Despite having smaller claws, females also engage in resource competition using these structures (Briffa and Dallaway, 2007Briffa M and Dallaway D 2007. Inter-sexual contests in the hermit crab Pagurus bernhardus: females fight harder but males win more encounters. Behavior Ecology and Sociobiology, 61(11): 1781-1787. https://doi.org/10.1007/s00265-007-0411-5
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; Dalosto et al., 2019Dalosto MM, Ayres-Peres L, Araujo PB, Santos S and Palaoro AV 2019. Pay attention to the ladies: female aggressive behavior and weapon allometry provide clues for sexual selection in freshwater anomurans (Decapoda: Aeglidae). Behavioral Ecology and Sociobiology, 73(9): 1-11. https://doi.org/10.1007/s00265-019-2741-5
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; Rappaport and Lord, 2021Rappaport SD and Lord JP 2021. Linear Dominance Hierarchies in Female Grass Shrimp Palaemon pugio. Biology Bulletin, 241(2): 208-216. https://doi.org/10.1086/716227
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). Dimorphic traits have been analyzed in hermit crabs from the families Paguridae and Diogenidae, and it has been observed that males tend to have larger body sizes and weapons (claws) than females (Briffa and Dallaway, 2007Briffa M and Dallaway D 2007. Inter-sexual contests in the hermit crab Pagurus bernhardus: females fight harder but males win more encounters. Behavior Ecology and Sociobiology, 61(11): 1781-1787. https://doi.org/10.1007/s00265-007-0411-5
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; Yasuda et al., 2017Yasuda CI, Otoda M, Nakano R, Takiya Y and Koga T 2017. Seasonal change in sexual size dimorphism of the major cheliped in the hermit crab Pagurus minutus. Ecology Research 32(3): 347-357. https://doi.org/10.1007/s11284-017-1438-3
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; Nirmal et al., 2020Nirmal T, da Silva AR, Kumar AP, Jaiswar AK and Kumawat T 2020. Ontogenic allometry and sexual maturity of the hermit crab, Diogenes alias McLaughlin & Holthuis, 2001 (Decapoda, Anomura). Crustaceana , 93(1): 1-15. https://doi.org/10.1163/15685403-00003963
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). Despite this, information on the biology, morphological traits, and sexual dimorphism of the Parapaguridae remains scarce. Acquiring knowledge about the shape and biomechanics of weapons in hermit crabs is crucial since their claws are essential for competing for various resources, including food, sexual partners, and shells (Dowds and Elwood, 1983Dowds BM and Elwood RW 1983. Shell wars: assessment strategies and the timing of decisions in hermit crab shell fights. Behaviour, 85(1-2): 1-24.). During fights, especially for shells, hermit crabs use chelipeds to hold and hit an opponent’s shell in a series of bouts (Lane and Briffa, 2020Lane SM and Briffa M 2020. The role of spatial accuracy and precision in hermit crab contests. Animal Behavior , 167: 111-118. https://doi.org/10.1016/j.anbehav.2020.07.013
https://doi.org/10.1016/j.anbehav.2020.0...
; Lane et al., 2022Lane SM, Cornwell TO and Briffa M 2022. The angle of attack: rapping technique predicts skill in hermit crab contests. Animal Behavior , 187: 55-61. https://doi.org/10.1016/j.anbehav.2022.02.017
https://doi.org/10.1016/j.anbehav.2022.0...
), thus a biomechanically efficient claw assures a firmer grip in order to perform these actions with proper accuracy. However, the morphology of the claws in S. dimorphus may deviate from the general pattern observed in other hermit crab species. This is because these crabs have adapted to using a different type of living shelter, which is a structure called a carcinoecium. The carcinoecium is constructed from a living colony of the cnidarian Epizoanthus paguricola Roule, 1900, that mimics the internal structure of a gastropod shell (Schejter and Mantelatto, 2011Schejter L and Mantelatto FL 2011. Shelter association between the hermit crab Sympagurus dimorphus and the zoanthid Epizoanthus paguricola in the southwestern Atlantic Ocean. Acta Zoologica, 92(2): 141-149. https://doi.org/10.1111/j.1463-6395.2009.00440.x
https://doi.org/10.1111/j.1463-6395.2009...
). Schejter and Mantelatto (2011Schejter L and Mantelatto FL 2011. Shelter association between the hermit crab Sympagurus dimorphus and the zoanthid Epizoanthus paguricola in the southwestern Atlantic Ocean. Acta Zoologica, 92(2): 141-149. https://doi.org/10.1111/j.1463-6395.2009.00440.x
https://doi.org/10.1111/j.1463-6395.2009...
) hypothesized that the major cheliped, which closes the aperture of this pseudo-shell, much like a gastropod operculum, may serve as a template for the configuration of the growing aperture edge. Thus, this species does not need to engage in constant fights for shells, since the zoanthid grows with the hermit crab, so its claws could be selected to optimize other behaviors rather than fighting. In other hermit crab species, shell fighting behavior can select certain morphological traits (Yasuda et al., 2011Yasuda C, Suzuki Y and Wada S 2011. Function of the major cheliped in male-male competition in the hermit crab Pagurus nigrofascia. Marine Biology, 158(10): 2327-2334. https://doi.org/10.1007/s00227-011-1736-1
https://doi.org/10.1007/s00227-011-1736-...
; Yoshino et al., 2011Yoshino K, Koga T and Oki S 2011. Chelipeds are the real weapon: cheliped size is a more effective determinant than body size in male-male competition for mates in a hermit crab. Behavioral Ecology and Sociobiology , 65(9): 1825-1832. https://doi.org/10.1007/s00265-011-1190-6
https://doi.org/10.1007/s00265-011-1190-...
). On the other hand, O. gracilis occupies gastropod shells of the genus Turris Batsch, 1789 (Henderson, 1888). Unlike S. dimorphus, it is expected that O. gracilis engages more frequently in agonistic behaviors to acquire better-fitting shells as it grows, therefore its claws could be selected for traits to increase the chances of acquiring best fitting shells.

Given the potential differences in resource acquisition behaviors between these two species (i.e., S. dimorphus and O. gracilis), variation in the development of sexually selected traits may occur. When occupying poorer-fitting shells, hermit crabs may not achieve bigger sizes compared to those using best-fitting shells (da Silva et al., 2019da Silva AR, Galli GM, Stanski G, De Biasi JB, Davanso TM, Cobo VJ and Castilho AL 2019. Shell occupation as a limiting factor for Pagurus brevidactylus (Stimpson, 1859) in the Marine State Park of Laje de Santos, Brazil. Invertebrate Reproduction and Development , 63(1): 1-10. https://doi.org/10.1080/07924259.2018.1513087
https://doi.org/10.1080/07924259.2018.15...
); also when using non-dextral shells, the morphology of chelipeds, uropods and pleopods may change (Imafuku and Ikeda, 2014Imafuku M and Ikeda H 2014. Asymmetrical morphology and growth of the hermit crab Pagurus filholi (Decapoda, Anomura, Paguridae) reared in non-dextral shell conditions. Crustaceana , 87: 476-488. https://doi.org/10.1163/15685403-00003295
https://doi.org/10.1163/15685403-0000329...
). Consequently, we have selected these two species to investigate and compare the patterns of sexual dimorphism and shape variation in two crucial structures for hermit crabs: the cephalothoracic shield and claws. In S. dimorphus the shell competition is low due to the use of a constant-growing shelter (zoanthid carcinoecium), whilst, in O. gracilis the shell competition is higher and shell availability in deep sea environments may be limiting (Absalão et al., 2005Absalão RS, Pimenta AD and Caetano CHS 2005. Turridae (Mollusca, Neogastropoda, Conoidea) coletados no litoral sudeste do Brasil, programa Revizee “score” central. Biociências, 13(1): 19-47.; Figueira and Absalão, 2010Figueira RMA and Absalão RS 2010. Deep-water Drilliinae, Cochlespirinae and Oenopotinae (Mollusca: Gastropoda: Turridae) from the Campos Basin, southeast Brazil. Scientia Marina , 74(3): 471-481. https://doi.org/10.3989/scimar.2010.74n3471
https://doi.org/10.3989/scimar.2010.74n3...
; Abbate et al., 2022Abbate D, Lima POV and Simone LRL 2022. The genera Famelica Bouchet & Warén, 1980 and Aliceia Dautzenberg & Fischer, 1897 (Conoidea, Raphitomidae) collected by the MD55 expedition in the Brazilian coast, with descriptions of two new species. Zoosystema, 44(23): 565-574. https://doi.org/10.5252/zoosystema2022v44a23
https://doi.org/10.5252/zoosystema2022v4...
).

Thus, our hypothesis is that there will be differences in the size and shape of these selected traits between the two species. It is expected that more pronounced sexual dimorphism will be found in species that frequently engage in competitions for shell resources, leading to more significant selective pressures. Conversely, in species with reduced shell competition, selective pressures may be lower, resulting in similar sizes and weaponry between males and females.

MATERIAL AND METHODS

Data Acquisition

Specimens of the hermit crab O. gracilis (Fig. 1 A ) that were used in this study are part of the Carcinology collection at the Zoology Museum of the University of São Paulo (Lots - MZUSP 16810 to MZUSP 16812, MZUSP 16817, MZUSP 16819, MZUSP 16825, MZUSP 16828, MZUSP 16829, and MZUSP 16831). According to the collection information, the animals were all collected in 1987 in the southeastern region of Brazil, between approximately 19°36'S 38°53'W to 23°46'S 42°09'W, at depths ranging from 500 m to 785 m. The specimens of the hermit crab S. dimorphus (Fig. 1 B ) were collected in 2017 in Argentina, specifically in the shelf-break front area, at approximately 39°S and 100 m deep during scientific expeditions that were focused on the stock assessment of the Patagonian scallop Zygochlamys patagonica (P. P. King, 1832). Voucher specimens used in this study were deposited in the Carcinology collection at the Zoology Museum of the University of São Paulo (MZUSP 42442).

Figure 1.
Specimens of (A) Oncopagurus gracilis (Henderson, 1888) and (B) Sympagurus dimorphus (Studer, 1883). (C) Cephalothoracic shield, (D) Major propodus, and (E) Minor propodus of S. dimorphus. (F) Cephalothoracic shield, (G) Major propodus, and (H) Minor propodus of O. gracilis. The blue and red dots represent the digitized landmarks and semi-landmarks in each structure, respectively.

For the geometric morphometric analyses, 81 individuals of O. gracilis (41 males and 40 females) and 47 individuals of S. dimorphus (17 males and 30 females) were used.

Geometric morphometrics

We analyzed three different structures, namely the cephalothoracic shield (CPS), and the major (MaP) and minor propodus (MiP) of the chelipeds. These structures were selected because they exhibit variation in size and shape between males and females of several hermit crab species (Briffa and Dallaway, 2007Briffa M and Dallaway D 2007. Inter-sexual contests in the hermit crab Pagurus bernhardus: females fight harder but males win more encounters. Behavior Ecology and Sociobiology, 61(11): 1781-1787. https://doi.org/10.1007/s00265-007-0411-5
https://doi.org/10.1007/s00265-007-0411-...
; Yasuda et al., 2017Yasuda CI, Otoda M, Nakano R, Takiya Y and Koga T 2017. Seasonal change in sexual size dimorphism of the major cheliped in the hermit crab Pagurus minutus. Ecology Research 32(3): 347-357. https://doi.org/10.1007/s11284-017-1438-3
https://doi.org/10.1007/s11284-017-1438-...
; Nirmal et al., 2020Nirmal T, da Silva AR, Kumar AP, Jaiswar AK and Kumawat T 2020. Ontogenic allometry and sexual maturity of the hermit crab, Diogenes alias McLaughlin & Holthuis, 2001 (Decapoda, Anomura). Crustaceana , 93(1): 1-15. https://doi.org/10.1163/15685403-00003963
https://doi.org/10.1163/15685403-0000396...
). Oncopagurus gracilis and S. dimorphus exhibit patterns of heterochely and handedness, wherein one claw is larger than the other (heterochely), and the larger claw is always on the same side of the body (handedness). Accordingly, MaP and MiP correspond to the right and left propodi, respectively.

The photographs employed in the geometric morphometric analysis were captured using a Canon T6 camera fixed on a tripod, carrying an 18-55 mm lens. All photographs were taken by the same person. In addition, the distance between the lens and structures, the zoom constancy, and the position of the structures were standardized in all photos (sensu Viscosi and Cardini, 2011Viscosi V and Cardini A 2011. Leaf morphology, taxonomy and geometric morphometrics: a simplified protocol for beginners. PloS one, 6(10): e25630. https://doi.org/10.1371/annotation/bc347abe-8d03-4553-8754-83f41a9d51ae
https://doi.org/10.1371/annotation/bc347...
).

Landmarks and semi-landmarks were digitized to characterize the CPS, MaP, and MiP shapes. The number of digitized landmarks and semi-landmarks between species was different due to interspecific morphological variation. In S. dimorphus, 9 landmarks and 12 semi-landmarks were digitalized in CPS, 4 landmarks and 10 semi-landmarks in MaP, and 5 landmarks and 14 semi-landmarks in MiP (Figs. 1C, 1D and 1E). In O. gracilis, 7 landmarks and 12 semi-landmarks were digitized on the CPS, 4 landmarks and 14 semi-landmarks on MaP and 5 landmarks and 14 semi-landmarks on MiP (Figs. 1F, 1G and 1H). All landmarks and semi-landmarks were digitized using tpsDig 2.1 and tpsUtil 1.26 software (Rohlf, 2004Rohlf FJ 2004. tpsUtil, version 1.26. Department of Ecology and Evolution , State University of New York at Stony Brook, Stony Brook, NY.; 2006Rohlf FJ 2006. tpsDig, version 2.10. Department of Ecology and Evolution , State University of New York at Stony Brook, Stony Brook, NY .). The semi-landmarks were slid to reduce the bending energy and avoid unreal deformations of the structures, due to the non-homologous nature of the semi-landmarks. This procedure was performed using the tpsUtil 1.6 software (Perez et al., 2006Perez SI, Bernal V and Gonzalez PN 2006. Differences between sliding semi‐landmark methods in geometric morphometrics, with an application to human craniofacial and dental variation. Journal of Anatomy, 208(6): 769-784. https://doi.org/10.1111/j.1469-7580.2006.00576.x
https://doi.org/10.1111/j.1469-7580.2006...
; Gunz and Mitteroecker, 2013Gunz P and Mitteroecker P 2013. Semilandmarks: a method for quantifying curves and surfaces. Hystrix, 24(1): 103-109. https://doi.org/10.4404/hystrix-24.1-6292
https://doi.org/10.4404/hystrix-24.1-629...
).

The variables that describe the CPS, MaP, and MiP shapes (weight matrix) and the centroid size (CS) were obtained using the tpsRelw 1.49 software (Rohlf, 2010Rohlf FJ 2010. tpsRelw, version 1.49. Department of Ecology and Evolution , State University of New York at Stony Brook, Stony Brook, NY .) after Generalized Procrustes Analysis (GPA). The CS is a multivariate measure that describes the size of the analyzed structure and is defined as the square root of the sum of the squared distances of each anatomical landmark and the center of mass of the structure (Bookstein, 1997Bookstein FL 1997. Landmark methods for forms without landmarks: morphometrics of group differences in outline shape. Medical Image Analysis, 1(3): 225-243. https://doi.org/10.1016/S1361-8415(97)85012-8
https://doi.org/10.1016/S1361-8415(97)85...
).

Data analysis

A principal component analysis (PCA) was performed with the weight matrix variables to check how many and which components explained more than 99 % of the data variation. Subsequently, only those components were utilized in the analysis of shape variation between males and females. This method was used to reduce the dimensionality of the data matrix and increase the power of the statistical test without losing the representativeness of the shape variation of the structures (Mitteroecker and Gunz, 2009Mitteroecker P and Gunz P 2009. Advances in geometric morphometrics. Evolution ary Biology, 36: 235-247. https://doi.org/10.1007/s11692-009-9055-x
https://doi.org/10.1007/s11692-009-9055-...
).

To evaluate the degree of sexual shape dimorphism between males and females of both species, Hotelling T 2 tests were performed, followed by discriminant analyses to determine whether the sexes could be differentiated based on the shape of the structures. The software tpsRegr 1.31 was utilized to represent the shape of the structures associated with the axes of the discriminant analyses (canonical variables) (Rohlf, 2009Rohlf FJ 2009. TpsRegr, version 1.31. Department of Ecology and Evolution , State University of New York at Stony Brook, Stony Brook, NY .). Sexual size dimorphism was assessed using the centroid size as a proxy for size. The centroid size of males and females of both species was compared using a t-test (α = 0.05), prior to this step the data were checked for normality and homocedascity to account for the statistical premises. All analyzes were performed in R (R-Core Team, 2023R Core Team 2023. _R: A Language and Environment for Statistical Computing_. R Foundation for Statistical Computing, Vienna, Austria. <https://www.R-project.org/>.
https://www.R-project.org/...
).

RESULTS

Oncopagurus gracilis

The shape of CPS (Hotelling T 2 = 98.62, F = 4.28, p < 0.001), MaP (Hotelling T 2 = 68.77, F = 6.83, p < 0.001), and MiP (Hotelling T 2 = 79.43, F = 5.7, p < 0.001) were different between male and female individuals. The discriminant analysis accurately classified 90 %, 82.05 %, and 82.72 % of individuals based on their CPS, MaP, and MiP shape, respectively.

The shape of the CPS of females was observed to be wider anteriorly than that of males (Fig. 2 A ), while the posterior margin of this structure was wider in males than in females (Fig. 2 A ). The variation of the MaP and MiP shapes exhibited a similar pattern. The manus region of MaP and MiP was more robust in males than in females (Fig. 2 B ). Furthermore, the fixed finger of the MiP was also found to be more robust in males than in females (Fig. 2 C ).

Figure 2.
Oncopagurus gracilis (Henderson, 1888). Histograms and graphical representations of the shape of the structures originated from the discriminant analysis. There was low overlap between males and females in all analyzed structures.

Sexual size dimorphism between males and females was detected in almost all structures, except CPS (t-test; t = 1.53, p = 0.18). In contrast, the size of MaP (t-test; t = -5.99, p < 0.001) and MiP (t-test; t = -4.91, p < 0.001) differed significantly between the sexes (Fig. 3 A ).

Figure 3.
Comparison of the centroid size of the cephalothoracic shield, major propodus, and minor propodus between males and females of (A) Oncopagurus gracilis (Henderson, 1888) and (B) Sympagurus dimorphus (Studer, 1883). Overall, males of both species are larger than females. The size of the cephalothoracic shield of males and females of O. gracilis was not statistically different (t-test; t = 1.53, p = 0.18).

Sympagurus dimorphus

The shape of CPS (Hotelling T 2 = 174.1, F = 6.59, p < 0.001), MaP (Hotelling T 2 = 101.27, F = 8.25, p < 0.001), and MiP (Hotelling T 2 = 70.76, F = 3.85, p < 0.001) were different between males and females. The discriminant analysis accurately distinguished all individuals in the comparison between CPS and MaP, and about 86 % for MiP.

The frontal region of the CPS exhibited a similar shape between males and females. However, the posterior region of the CPS was wider in females than in males (Fig. 4 A ). In contrast, MaP was more robust in females than in males. Additionally, males displayed a slight projection of the fixed finger of the MaP, while females did not exhibit this characteristic (Fig. 4 B ). The differences in the shape of MiP between males and females were smaller compared to the other structures. This structure was slightly more robust in females, whereas the fixed finger of males was more robust than that of females (Fig. 4 C ).

Figure 4.
Sympagurus dimorphus (Studer, 1883). Histograms and graphical representations of the shape of the structures originated from the discriminant analysis. There was low overlap between males and females only in the shape of minor propodus.

Sexual size dimorphism was evident between males and females in all structures analyzed, CPS (t-test; t = -9.69, p < 0.001), MaP (t-test; t = -9.07, p < 0.001), and MiP (t-test; t = -11.04, p < 0.001) (Fig. 3 B ).

DISCUSSION

Differences in the pattern of sexual dimorphism were observed in the two species studied, in line with our initial hypotheses. Sexual size dimorphism is a common characteristic among crustaceans, where males are typically larger than females. However, deviations from this pattern have been observed in some crustaceans, which have been attributed to specific types of reproductive behaviors and mating systems (Hartnoll, 2001Hartnoll RG 2001. Growth in Crustacea-twenty years on. p. 111-122. In: Paula JPM, Flores AAV, Fransen CHJM (Eds.). Advances in Decapod Crustacean Research, Dordrecht (NLD), Springer. https://doi.org/10.1007/978-94-017-0645-2_11
https://doi.org/10.1007/978-94-017-0645-...
; Cothran and Thiel, 2020Cothran R and Thiel M 2020. The natural history of the Crustacea: reproductive biology. New York (NY), Oxford University Press. 584p.; da Silva et al., 2021da Silva AR, Lemes LGM, Nogueira CS, Bispo PC and Castilho AL 2021. Heteroquely, laterality, maturity body size and shape variation of males and females of the endemic South American anomuran Aegla quilombola Moraes, Tavares & Bueno, 2017. Invertebrate Reproduction and Development , 65(1): 12-23. https://doi.org/10.1080/07924259.2020.1821799
https://doi.org/10.1080/07924259.2020.18...
). In the case of S. dimorphus, males were larger than females, consistent with the typical size dimorphism pattern. However, in O. gracilis, no size dimorphism was observed for the cephalothoracic shield. This could be related to the availability of shells in deep-sea environments, where gastropods, although abundant, are relatively small and fragile, which may limit the growth of hermit crabs (Absalão et al., 2005Absalão RS, Pimenta AD and Caetano CHS 2005. Turridae (Mollusca, Neogastropoda, Conoidea) coletados no litoral sudeste do Brasil, programa Revizee “score” central. Biociências, 13(1): 19-47.; Figueira and Absalão, 2010Figueira RMA and Absalão RS 2010. Deep-water Drilliinae, Cochlespirinae and Oenopotinae (Mollusca: Gastropoda: Turridae) from the Campos Basin, southeast Brazil. Scientia Marina , 74(3): 471-481. https://doi.org/10.3989/scimar.2010.74n3471
https://doi.org/10.3989/scimar.2010.74n3...
; Abbate et al., 2022Abbate D, Lima POV and Simone LRL 2022. The genera Famelica Bouchet & Warén, 1980 and Aliceia Dautzenberg & Fischer, 1897 (Conoidea, Raphitomidae) collected by the MD55 expedition in the Brazilian coast, with descriptions of two new species. Zoosystema, 44(23): 565-574. https://doi.org/10.5252/zoosystema2022v44a23
https://doi.org/10.5252/zoosystema2022v4...
). It is known that shell availability can influence hermit crab growth (Angel, 2000Angel JE 2000. Effects of shell fit on the biology of the hermit crab Pagurus longicarpus (Say). Journal of Experimental Marine Biology and Ecology, 243(2): 169-184. https://doi.org/10.1016/S0022-0981(99)00119-7
https://doi.org/10.1016/S0022-0981(99)00...
; da Silva et al., 2019da Silva AR, Galli GM, Stanski G, De Biasi JB, Davanso TM, Cobo VJ and Castilho AL 2019. Shell occupation as a limiting factor for Pagurus brevidactylus (Stimpson, 1859) in the Marine State Park of Laje de Santos, Brazil. Invertebrate Reproduction and Development , 63(1): 1-10. https://doi.org/10.1080/07924259.2018.1513087
https://doi.org/10.1080/07924259.2018.15...
). Therefore, if shells are a limiting resource, male growth could be restricted to the same size as females. Some studies on sexual size dimorphism in hermit crabs suggest that male size has a low impact on mating and female choice (Contreras-Garduño and Córdoba-Aguilar, 2006Contreras‐Garduño J and Córdoba‐Aguilar A 2006. Sexual selection in hermit crabs: a review and outlines of future research. Jourrnal of Zoology, 270(4): 595-605. https://doi.org/10.1111/j.1469-7998.2006.00182.x
https://doi.org/10.1111/j.1469-7998.2006...
) but a high impact on male-male competition (Contreras-Garduño and Córdoba-Aguilar, 2006Contreras‐Garduño J and Córdoba‐Aguilar A 2006. Sexual selection in hermit crabs: a review and outlines of future research. Jourrnal of Zoology, 270(4): 595-605. https://doi.org/10.1111/j.1469-7998.2006.00182.x
https://doi.org/10.1111/j.1469-7998.2006...
; Yoshino et al., 2011Yoshino K, Koga T and Oki S 2011. Chelipeds are the real weapon: cheliped size is a more effective determinant than body size in male-male competition for mates in a hermit crab. Behavioral Ecology and Sociobiology , 65(9): 1825-1832. https://doi.org/10.1007/s00265-011-1190-6
https://doi.org/10.1007/s00265-011-1190-...
).

The CPS of both species exhibited shape differences between males and females, with an inverse pattern observed. In females of S. dimorphus, the structure displayed a more rounded posterior margin than that seen in males, whereas in males of O. gracilis, this structure exhibited a posterior margin that was more rounded than that seen in females. In crustaceans with an advanced carcinization process, it is common to observe females carrying a carapace with a more robust shape than males due to the repositioning of the abdomen in relation to the body and the larger abdomen size in females (Trevisan et al., 2012Trevisan A, Marochi MZ, Costa M, Santos S and Masunari S 2012. Sexual dimorphism in Aegla marginata (Decapoda: Anomura). Nauplius , 20(1): 75-86.; Keiler et al., 2017Keiler J, Wirkner CS and Richter S 2017. One hundred years of carcinization-the evolution of the crab-like habitus in Anomura (Arthropoda: Crustacea). Biological Journal of Linnean Society, 121(1): 200-222. https://doi.org/10.1093/biolinnean/blw031
https://doi.org/10.1093/biolinnean/blw03...
; Marochi et al., 2019Marochi MZ, Costa M, Leite RD, Cruz IDC and Masunari S 2019. To grow or to reproduce? Sexual dimorphism and ontogenetic allometry in two Sesarmidae species (Crustacea: Brachyura). Journal of Marine Biological Association of UK , 99(2): 473-486. https://doi.org/10.1017/S0025315418000048
https://doi.org/10.1017/S002531541800004...
; da Silva et al., 2021da Silva AR, Lemes LGM, Nogueira CS, Bispo PC and Castilho AL 2021. Heteroquely, laterality, maturity body size and shape variation of males and females of the endemic South American anomuran Aegla quilombola Moraes, Tavares & Bueno, 2017. Invertebrate Reproduction and Development , 65(1): 12-23. https://doi.org/10.1080/07924259.2020.1821799
https://doi.org/10.1080/07924259.2020.18...
; Wolfe et al., 2021Wolfe JM, Luque J and Bracken‐Grissom HD 2021. How to become a crab: Phenotypic constraints on a recurring body plan. Bioessay, 43(5): 2100020. https://doi.org/10.1002/bies.202100020
https://doi.org/10.1002/bies.202100020...
). Therefore, due to the ventral location of the abdomen in the animal's body, which is connected to the posterior region of the carapace, modifications occur in the carapace to support the weight of the robust abdomen (Marochi et al., 2019Marochi MZ, Costa M, Leite RD, Cruz IDC and Masunari S 2019. To grow or to reproduce? Sexual dimorphism and ontogenetic allometry in two Sesarmidae species (Crustacea: Brachyura). Journal of Marine Biological Association of UK , 99(2): 473-486. https://doi.org/10.1017/S0025315418000048
https://doi.org/10.1017/S002531541800004...
; Wolfe et al., 2021Wolfe JM, Luque J and Bracken‐Grissom HD 2021. How to become a crab: Phenotypic constraints on a recurring body plan. Bioessay, 43(5): 2100020. https://doi.org/10.1002/bies.202100020
https://doi.org/10.1002/bies.202100020...
). Thus, hermit crabs, which lack advanced carcinization and possess a non-calcified abdomen, are not expected to exhibit this type of variation (Keiler et al., 2017Keiler J, Wirkner CS and Richter S 2017. One hundred years of carcinization-the evolution of the crab-like habitus in Anomura (Arthropoda: Crustacea). Biological Journal of Linnean Society, 121(1): 200-222. https://doi.org/10.1093/biolinnean/blw031
https://doi.org/10.1093/biolinnean/blw03...
). The observed variation in the CPS shape in S. dimorphus and O. gracilis may be related to the specific allometric constants that determine the size and shape of the CPS of males and females of each species, as highlighted for the hermit crab Clibanarius signatus Heller, 1861 (Ismail, 2018Ismail TG 2018. Effect of geographic location and sexual dimorphism on shield shape of the Red Sea hermit crab Clibanarius signatus using the geometric morphometric approach. Canadian Journal of Zoology, 96(7): 667-679. https://doi.org/10.1139/cjz-2017-0050
https://doi.org/10.1139/cjz-2017-0050...
).

The claws of both species, MaP and MiP, exhibited sexual size dimorphism, with males displaying larger claws than females. This phenomenon is commonly observed in crustaceans, where the larger claw is frequently linked to resource competition (Turra and Denadai, 2004Turra A and Denadai MR 2004. Interference and exploitation components in interespecific competition between sympatric intertidal hermit crabs. Journal of Experimental Marine Biology and Ecology , 310(2): 183-193. https://doi.org/10.1016/j.jembe.2004.04.008
https://doi.org/10.1016/j.jembe.2004.04....
; Almerão et al., 2010Almerão M, Bond-Buckup G and Mendonça MS 2010. Mating behavior of Aegla platensis (Crustacea, Anomura, Aeglidae) under laboratory conditions. Journal of Ethology, 28: 87-94. https://doi.org/10.1007/s10164-009-0159-7
https://doi.org/10.1007/s10164-009-0159-...
; Kruesi et al., 2022Kruesi K, Burciaga LM and Alcaraz G 2022. Coexistence of similar species: evidence of a resource and microhabitat sharing in two intertidal hermit crab species. Hydrobiologia , 849(6): 1531-1541. https://doi.org/10.1007/s10750-022-04800-4
https://doi.org/10.1007/s10750-022-04800...
; Nogueira et al., 2022Nogueira CS, da Silva AR and Palaoro AV 2022. Fighting does not influence the morphological integration of crustacean claws (Decapoda: Aeglidae). Biological Journal of Linnean Society , 136(1): 173-186. https://doi.org/10.1093/biolinnean/blac026
https://doi.org/10.1093/biolinnean/blac0...
). In hermit crabs, such contests can pertain to food, mates, and/or shell acquisition (Turra and Denadai, 2004Turra A and Denadai MR 2004. Interference and exploitation components in interespecific competition between sympatric intertidal hermit crabs. Journal of Experimental Marine Biology and Ecology , 310(2): 183-193. https://doi.org/10.1016/j.jembe.2004.04.008
https://doi.org/10.1016/j.jembe.2004.04....
; Yoshino et al., 2011Yoshino K, Koga T and Oki S 2011. Chelipeds are the real weapon: cheliped size is a more effective determinant than body size in male-male competition for mates in a hermit crab. Behavioral Ecology and Sociobiology , 65(9): 1825-1832. https://doi.org/10.1007/s00265-011-1190-6
https://doi.org/10.1007/s00265-011-1190-...
; Quinn, 2020Quinn BK 2020. Potential kleptoparasitism-scavenging interactions between crabs and shell-boring gastropods feeding on bivalve prey: a global survey. Plankton and Benthos Research , 15(2): 132-145. https://doi.org/10.3800/pbr.15.132
https://doi.org/10.3800/pbr.15.132...
). Hence, the selective pressures associated with these contests may influence claw size, thereby resulting in the observed differences between both species (Dennenmoser and Christy, 2013Dennenmoser S and Christy JH 2013. The design of a beautiful weapon: compensation for opposing sexual selection on a trait with two functions. Evolution 67(4): 1181-1188. https://doi.org/10.1111/evo.12018
https://doi.org/10.1111/evo.12018...
; Palaoro et al., 2020Palaoro AV, Peixoto PEC, Benso-Lopes F, Boligon DS and Santos S 2020. Fight intensity correlates with stronger and more mechanically efficient weapons in three species of Aegla crabs. Behavioral Ecology and Sociobiology , 74: 1-11. https://doi.org/10.1007/s00265-020-02834-z
https://doi.org/10.1007/s00265-020-02834...
; Levinton and Weissburg, 2021Levinton JS and Weissburg M 2021. Length of a sexually selected ornament-armament in fiddler crabs (Decapoda: Brachyura: Ocypodidae): One way, over deep time and space. Journal of Crustacean Biology , 41(4): ruab066. https://doi.org/10.1093/jcbiol/ruab066
https://doi.org/10.1093/jcbiol/ruab066...
; Nogueira et al., 2022Nogueira CS, da Silva AR and Palaoro AV 2022. Fighting does not influence the morphological integration of crustacean claws (Decapoda: Aeglidae). Biological Journal of Linnean Society , 136(1): 173-186. https://doi.org/10.1093/biolinnean/blac026
https://doi.org/10.1093/biolinnean/blac0...
). Given that males possess larger claws, they are likely to generate more force using their claws (Schenk and Wainwright, 2001Schenk SC and Wainwright PC. 2001. Dimorphism and the functional basis of claw strength in six brachyuran crabs. Journal of Zoology, 255(1): 105-119. https://doi.org/10.1017/S0952836901001157
https://doi.org/10.1017/S095283690100115...
; Claussen et al., 2008Claussen DL, Gerald GW, Kotcher JE and Miskell CA 2008. Pinching forces in crayfish and fiddler crabs, and comparisons with the closing forces of other animals. Journal of comparative Physiology, 178: 333-342. https://doi.org/10.1007/s00360-007-0226-8
https://doi.org/10.1007/s00360-007-0226-...
), which is an advantageous trait during competitions for resources or mates. Additionally, claws can also play a signaling role in a "pre-fight" scenario, intimidating potential opponents. In some instances, individuals tend to refrain from initiating a fight if their opponents possess large weapons. Prior to engaging in combat, a display phase may occur, where males showcase their claws to each other, and if one of the competitors assesses that the energy cost involved in the fight exceeds its potential gain, the individual may flee, thus avoiding combat (Parra et al., 2011Parra CA, Barria EM and Jara CG 2011. Behaviour al variation and competitive status in three taxa of Aegla (Decapoda: Anomura: Aeglidae) from two-community settings in Southern Chile. New Zealand Journal of Marine and Freshwater Research, 45: 249-262. https://doi.org/10.1080/00288330.2011.556651
https://doi.org/10.1080/00288330.2011.55...
; Palaoro et al., 2020Palaoro AV, Peixoto PEC, Benso-Lopes F, Boligon DS and Santos S 2020. Fight intensity correlates with stronger and more mechanically efficient weapons in three species of Aegla crabs. Behavioral Ecology and Sociobiology , 74: 1-11. https://doi.org/10.1007/s00265-020-02834-z
https://doi.org/10.1007/s00265-020-02834...
).

As mentioned above, competition for shells in S. dimorphus is low, hence the selective pressure on claws in this species is also expected to be low. Nevertheless, differences in size and shape of claws between males and females have been observed. Thus, we can propose an alternative hypothesis for the occurrence of this dimorphism, which may be related to some genetic factor involved in the development of the cheliped. This could be attributed to the claw development not being linked to sex genes, allowing this structure to develop and grow, attaining ideal shapes and sizes for both sexes (Bonduriansky, 2007Bonduriansky R 2007. Sexual selection and allometry: A critical reappraisal of the evidence and ideas. Evolution, 61: 838-849. https://doi.org/10.1111/j.1558-5646.2007.00081.x
https://doi.org/10.1111/j.1558-5646.2007...
; Bonduriansky and Chenoweth, 2009Bonduriansky R and Chenoweth SF. 2009. Intralocus sexual conflict. Trends in Ecology andEvolution , 24: 280-288. https://doi.org/10.1016/j.tree.2008.12.005
https://doi.org/10.1016/j.tree.2008.12.0...
).

The shape of the major cheliped (MaP) varied between males and females of both species, but there was an inverse pattern between the two species. In S. dimorphus, females presented a more robust MaP, whereas in O. gracilis, males exhibited a more robust MaP. Oncopagurus gracilis follows the typical pattern observed in crustaceans, where males possess more robust claws than females (Mariappan et al., 2000Mariappan P, Balasundaram C and Schmitz B 2000. Decapod crustacean chelipeds: an overview. Journal of Biosciences, 25(3): 301-313. https://doi.org/10.1007/BF02703939
https://doi.org/10.1007/BF02703939...
; Nirmal et al., 2020Nirmal T, da Silva AR, Kumar AP, Jaiswar AK and Kumawat T 2020. Ontogenic allometry and sexual maturity of the hermit crab, Diogenes alias McLaughlin & Holthuis, 2001 (Decapoda, Anomura). Crustaceana , 93(1): 1-15. https://doi.org/10.1163/15685403-00003963
https://doi.org/10.1163/15685403-0000396...
). In hermit crabs, claws serve multiple functions, including food acquisition and shell-fighting. In the latter scenario, hermit crabs use their claws to grab their opponents' shells and repeatedly hit them against their own shell, causing their opponent to exit their shell. This aggressive behavior is referred to as rapping (Lane and Briffa, 2020Lane SM and Briffa M 2020. The role of spatial accuracy and precision in hermit crab contests. Animal Behavior , 167: 111-118. https://doi.org/10.1016/j.anbehav.2020.07.013
https://doi.org/10.1016/j.anbehav.2020.0...
; Lane et al., 2022Lane SM, Cornwell TO and Briffa M 2022. The angle of attack: rapping technique predicts skill in hermit crab contests. Animal Behavior , 187: 55-61. https://doi.org/10.1016/j.anbehav.2022.02.017
https://doi.org/10.1016/j.anbehav.2022.0...
). The claw also plays a crucial role in mating-related behaviors. For instance, inverse rapping behavior (Kido and Wada, 2020Kido Y and Wada S 2020. Males display “inverse rapping” as a mating behavior to receptive females in the hermit crab Pagurus nigrofascia. Plankton and Benthos Research, 15(3): 279-288. https://doi.org/10.3800/pbr.15.279
https://doi.org/10.3800/pbr.15.279...
) and male hermit crabs protecting females during the pre-mating phase (Yoshino et al., 2004Yoshino K, Ozawa M and Goshima S 2004. Effects of shell size fit on the efficacy of mate guarding behaviour in male hermit crabs. Journal of Marine Biological Association of UK , 84(6): 1203-1208. https://doi.org/10.1017/S0025315404010653h
https://doi.org/10.1017/S002531540401065...
; Contreras-Garduño and Córdoba-Aguilar, 2006Contreras‐Garduño J and Córdoba‐Aguilar A 2006. Sexual selection in hermit crabs: a review and outlines of future research. Jourrnal of Zoology, 270(4): 595-605. https://doi.org/10.1111/j.1469-7998.2006.00182.x
https://doi.org/10.1111/j.1469-7998.2006...
) involve the use of the claw. Furthermore, males use their claws to reach females inside the shells and drag them out for forced copulation (Yoshino et al., 2004Yoshino K, Ozawa M and Goshima S 2004. Effects of shell size fit on the efficacy of mate guarding behaviour in male hermit crabs. Journal of Marine Biological Association of UK , 84(6): 1203-1208. https://doi.org/10.1017/S0025315404010653h
https://doi.org/10.1017/S002531540401065...
). Thus, a claw with a robust shape is likely advantageous for male hermit crabs, as it facilitates their performance in diverse tasks.

Sympagurus dimorphus is primarily found inhabiting zoanthid colonies, although occasionally larger males can be observed using gastropod shells as shelter (Schejter and Mantelatto, 2011Schejter L and Mantelatto FL 2011. Shelter association between the hermit crab Sympagurus dimorphus and the zoanthid Epizoanthus paguricola in the southwestern Atlantic Ocean. Acta Zoologica, 92(2): 141-149. https://doi.org/10.1111/j.1463-6395.2009.00440.x
https://doi.org/10.1111/j.1463-6395.2009...
). The frequent use of zoanthids as shelter implies that shell fighting may not be a crucial factor for this species, which has resulted in a dimorphic pattern where females possess a more robust and rounded claw while males have a spear-shaped claw. Males of S. dimorphus may exhibit unique behavioral traits that influence the shape of their MaP. However, it is essential to carry out experiments that detail the behavioral repertoire of these organisms to confirm this hypothesis. Nevertheless, conducting such experiments can be challenging due to the difficulty in collecting and rearing these animals in a controlled environment, given the pressure of their natural habitat.

The variation in shape of the MiP was similar between the two species, with males presenting a more robust MiP compared to females. Although this structure is less frequently used during agonistic events, it remains important, particularly for hermit crabs, due to their rapping and inverse rapping behaviors (Dowds and Elwood, 1983Dowds BM and Elwood RW 1983. Shell wars: assessment strategies and the timing of decisions in hermit crab shell fights. Behaviour, 85(1-2): 1-24.; Kido and Wada, 2020Kido Y and Wada S 2020. Males display “inverse rapping” as a mating behavior to receptive females in the hermit crab Pagurus nigrofascia. Plankton and Benthos Research, 15(3): 279-288. https://doi.org/10.3800/pbr.15.279
https://doi.org/10.3800/pbr.15.279...
). As previously mentioned, some male hermit crabs exhibit pre-copulatory behavior known as inverse rapping, whereby they hold the female's shell with both claws and hit it against their own shell (Kido and Wada, 2020Kido Y and Wada S 2020. Males display “inverse rapping” as a mating behavior to receptive females in the hermit crab Pagurus nigrofascia. Plankton and Benthos Research, 15(3): 279-288. https://doi.org/10.3800/pbr.15.279
https://doi.org/10.3800/pbr.15.279...
). Thus, possessing a more robust and stronger MiP can facilitate the execution of these movements, and ensure the reproductive success of these organisms. This highlights the critical importance of this structure in the life history of hermit crabs.

This study presents novel and complementary information on sexual dimorphism in two species of hermit crabs from the family Parapaguridae, which is less studied compared to other families such as Paguridae and Diogenidae. We observed that one of the studied species, S. dimorphus, exhibited shape variations that deviate from the general crustacean pattern, likely related to its habitat and the adaptive processes it underwent to thrive in an environment with limited shelter availability. Furthermore, our findings highlight the importance of claws for hermit crabs, as both propodi displayed variations in size and shape that follow a similar trend, potentially related to their use during agonistic events as observed in other hermit crab species (Lane and Briffa, 2020Lane SM and Briffa M 2020. The role of spatial accuracy and precision in hermit crab contests. Animal Behavior , 167: 111-118. https://doi.org/10.1016/j.anbehav.2020.07.013
https://doi.org/10.1016/j.anbehav.2020.0...
; Lane et al., 2022Lane SM, Cornwell TO and Briffa M 2022. The angle of attack: rapping technique predicts skill in hermit crab contests. Animal Behavior , 187: 55-61. https://doi.org/10.1016/j.anbehav.2022.02.017
https://doi.org/10.1016/j.anbehav.2022.0...
). Finally, we emphasize the need for further studies on other populations and species of Parapaguridae to facilitate comparative analyses.

ACKNOWLEDGMENT

This work was supported by the São Paulo State Research Support Foundation (FAPESP) [grant number 2019/00661-3; 2023/01445-8]; and the National Council for Scientific and Technological Development (CNPq) [grant number 151038/2022-8].

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  • Zoobank:

    http://zoobank.org/urn:lsid:zoobank.org:pub:EDB9672A-A469-4BDA-A342-A34B499821A4
  • Author contribution

    Conceptualization and Design: ARdS, LS. Performed research: AC, CSN, ARdS. Acquisition of data: AC, LS, ARdS. Analysis and interpretation of data: AC, ARdS, CSN. Preparation of figures: CSN. Writing original draft: AC, CSN. Writing - critical review & editing: ARdS, LS, CSN.
  • Consent for publication

    All authors declare that they have reviewed the content of the manuscript and gave their consent to submit the document.
  • Funding and grant disclosures This work was supported by the São Paulo State Research Support Foundation (FAPESP) [grant number 2019/00661-3; 2023/01445-8]; and the National Council for Scientific and Technological Development (CNPq) [grant number 151038/2022-8]
  • Disclosure statement

    The authors report there are no competing interests to declare.
  • Data availability statement

    The data that support the findings of this study are available from the corresponding author, ARdS, upon reasonable request.

Edited by

Associate Editor:

Dr. Kareen Schnabel

Publication Dates

  • Publication in this collection
    04 Dec 2023
  • Date of issue
    2023

History

  • Received
    30 Jan 2023
  • Accepted
    17 July 2023
Sociedade Brasileira de Carcinologia Instituto de Biociências, UNESP, Campus Botucatu, Rua Professor Doutor Antônio Celso Wagner Zanin, 250 , Botucatu, SP, 18618-689 - Botucatu - SP - Brazil
E-mail: editor.nauplius@gmail.com