Intraspecific Sensory Diversity and the Decapod Claw: Patterns of Sensillation Are Heterochelic and Sexually Dimorphic In Pagurus bernhardus

Abstract

Information detection affects physiological performance and behaviour and is vital to survival and fitness. Despite the recognised importance of sensory adaptations in information acquisition and manipulation, many forms of sensory variation-from within individuals to between species-remain underexplored. To better understand the role of information in evolution, it is important to examine sensory variation as part of a cohesive framework of sensory diversity. Using the decapod claw, a structure well-recognised for its morphological variation, we investigated sensory diversity at the intraspecific level by assessing heterochely and sexual dimorphism in the chelar morphologies of Pagurus bernhardus hermit crabs. We employed a novel methodology using scanning electron microscopy (SEM) to assess moulted chelar tissue from both the major and minor claws. The shape, size, and sensillation (i.e., the distribution and abundance of sensilla) of both chelipeds were examined by geometric morphometric landmark analysis (GMLA), generalised Procrustes analysis (GPA), and linear mixed effects models. Hermit crabs exhibited heterochely and sexual dimorphism in both gross and sensory chelar morphologies. Sexual dimorphism was greater in the sensory morphology of the major claw, suggesting sex-based sensory specialisations, likely due to differences in mating roles and behaviours. In contrast, the minor claw's sensory morphology lacked sexual dimorphism, suggesting the sensory role of this appendage is equally important for both sexes. Our results highlight sensory variation as a fundamental aspect of functional morphology and emphasise the need to consider sexual dimorphism and body asymmetry in information acquisition. These findings contribute to a broader framework for studying sensory diversity, underscoring the importance of integrating sensory morphology, function, and ecology to fully understand the evolutionary implications of sensory specialisations.

Keywords: chela; crustacea; hermit crab; scanning electron microscopy (SEM); sensilla; sensory morphology; sexual dimorphism.

Figure 1

(A) Scanning electron micrograph montage image of the dorsal surface of the major cheliped (MJC) of Pagurus bernhardus; scale bar = 500 μm with the localisation of assigned landmarks (B) LM1 to LM16 for the dorsal surface of the MJC and (C) LM1 to LM15 for the minor cheliped (MNC). Cropped and zoomed micrographs showing (D) single (S1), (E) double (S2), (F) triple (S3) and (G) clustered (S4) articulation sites; scale bar (white) = 50 μm.

Figure 2

Wireframe diagrams showing shape‐based (A) heterochely and (B) sexual dimorphism in the major (MJC) and minor (MNC) chelipeds of Pagurus bernhardus hermit crabs. In (A), the differences between the MJC (blue) and MNC (orange) are derived from the pooled data set, the results of which are presented for males and females combined and separately. The MNC has been reflected along the Y axis to show the orientation of landmarks in its correct anatomical orientation relative to the midline of the hermit crab. In (B), chelar sexual dimorphism is shown for female crabs (blue) and male crabs (orange), based on the individual datasets for the MJC and MNC.

Figure 3

A. Size‐based heterochely in terms of total chelar area (mm²) in male and female Pagurus bernhardus for both the major (MJC) and minor (MNC) chelipeds. B. Relationship between log crab mass (log(g)) and log chelar area (log(mm²)) for male and female hermit crabs for both claws.

Figure 4

A. Sensory investment in terms of log‐transformed absolute sensillar site density (sites/total chelar area in mm²) on the major (MJC) and minor (MNC) chelipeds of female and male Pagurus bernhardus. B. Relationship between crab mass (g) and sensory investment by coverage (i.e., log‐transformed weighted sensillar density) for both the MJC and MNC in male and female hermit crabs.

Figure 5

Frequency maps showing the absolute sensillar abundance (i.e., the number of sensillar articulation sites) on the dorsal chelar surface in female (A = MJC, B = MNC) and male (C = MJC, D = MNC) Pagurus bernhardus hermit crabs. Brighter colouration indicates a greater frequency of sensillar articulation.

Figure 6

Frequency maps showing the distribution of sensillar sites by type of articulation grouping for the major (MJC) and minor (MNC) chelipeds of Pagurus bernhardus. For the MJC, single (S1) sites (A) were most abundant proximally; double (S2) sites (B) were concentrated at the chelar margins; triple (S3) sites (C) were sparse and located distally; and clustered (S4) sites (D) were located at the tips of the dactylus and propodal extension. A combined frequency map (E) shows site distribution on the MJC dorsal chelar surface. In (F), this map is coded by colour with the frequency of S1 (magenta), S2 (cyan), S3 (lime), and S4 sites (gold) delineated. For the MNC, S1 sites (G) were most abundant proximally and inferiorly; S2 sites (H) were concentrated at the chelar margins, especially superiorly, and the midline; S3 sites (I) were more abundant than on the MJC and were found most frequently around the propodal‐dactyl joint; and S4 sites (J) were heavily featured at the tips of the dactylus and propodal extension, more so than on the MJC. The combined frequency map (K) shows the site distribution on the MNC dorsal chelar surface. In (L), this map is coded by colour with the frequency of S1 (magenta), S2 (cyan), S3 (lime), and S4 sites (gold) delineated. The arrow and dotted circle in (G) show the location of the unique single sensillar (S1) site present on all MNCs examined.