On Shell Closure and Beyond: Classification and Functional Interpretation of Chela Types in Paguroidea (Decapoda)

In Paguroidea, the chelae of the first thoracopods (chelipeds) encompass heterochelate (unequal), homoiochelate (subequal, weakly differentiated), and homochelate (equal, mirrored) conditions, reflecting functional differentiation and evolutionary adaptation across taxa. For 30 species from seven hermit crab families detailed 3D models of chelae were generated using microCT imaging. For these, 3D shape analysis identified six distinct shape types (I-compact, II- sturdy, III-elongate, IV-scutiform, V-discoid, VI-semidiscoid) across the investigated taxa. These shape types correspond closely with key biological roles. Paired semidiscoid chelae enable symmetrical shelter closure, while scutiform and discoid chelae function in one-sided shelter closure and defense and are associated with the utilization of gastropod shells. Compact and elongate chelae serve in food manipulation and grooming. Sturdy chelae, which include most homoiochelate forms, are associated with intermediate conditions related to alternative defensive strategies, such as complete (deep) withdrawal into shells, preference for shells with narrow apertures, or symbiotic relationships with anthozoans. Phylogenetic patterns might indicate that potentially basal taxa like Pylochelidae retain plesiomorphic, symmetrical semidiscoid chelae, but it cannot be excluded that these homochelate chelae represent an apomorphic condition for the taxon. In any case, the asymmetric hermit crabs (Parapaguridae, Paguridae, Diogenidae, Coenobitidae, Lithodidae) evolved pronounced heterochely, with the dominant chela adapted (primarily) for shelter closure and the subdominant one for feeding and grooming. Homoiochelate forms occur repeatedly across Diogenidae, suggesting multiple evolutionary transitions related to shifts in defensive strategy. The association between chela shape, biological role, and phylogeny highlights the dynamic interplay of evolutionary history, functional demands, and ecological adaptation in shaping hermit crab morphology.