Evolutionary morphology of hermit crab claws (Decapoda, Paguroidea) – what determines shape?

• Applicant: Professor Dr. Stefan Richter
• Subject Area: Systematics and Morphology (Zoology)
• Term: since 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 428617143

Hermit crabs (Paguroidea), numbering over 1000 species, represent one of the most successful groups within decapod crustaceans. The groups’ most conspicuous feature is the soft-shelled pleon, which is hidden within a snail shell in most of the species. Phylogenetically, king crabs and the terrestrial coenobitids, also belong to the hermit crabs. Not less interesting are the first walking legs, which are formed as chelipeds and can reach astonishing sizes in relation to body length. There are hermit crabs with a prominent right (Paguridae, Lithodoidea) respectively a prominent left (Diogenidae, Coenobitidae) claw (chela) however some species display no discrepancy in claw size (Pylochelidae). Various tasks (i.e. biological roles) are being carried out by these claws, e.g. food manipulation and consumption. They also serve as closure for inhabited snail shells or are used as weapons to fight potential predators. Depending on the taxon, closure of the shell can be ensured by the right, the left or both chelipeds. For food consumption, the claws overall shape is apparently less relevant than the cutting edge between the immovable and movable finger. Characteristics of the dentition of the cutting edges apparently resemble different types of pliers regarding their function.The proposed project aims for understanding the evolution of the chelipeds’ form, function and biological role in hermit crabs. For this purpose, three different approaches tackling different levels of evolutionary transformations are pursued. First, a variety of species of every paguroid family shall be examined and evolutionary transformations studied, based on recent phylogenetic hypotheses. The overall claw shape will be determined using new techniques such as μCT, digital 3D reconstruction and geometric morphometrics. Furthermore, a functional classification based on the dentition of the cutting edges is planned.Second, for understanding the basis of evolutionary transformations leading to such a disparity, the intraspecific phenotypic variability of the claw shall be examined in two abundant hermit crab species: the common hermit crab (Pagurus bernhardus) and the small hermit crab (Diogenes pugilator). Both species are well known for using their dominant claws as closure for the snail shells they inhabit. In addition to differences in claw shape between populations, the correlation between claw shape and the entrance of the respective snail shells will be studied. The third approach is based on the phenomenon, that freshly molted hermit crabs can adjust their claw shape to the entrance of the snail shells. In keeping experiments, freshly molted common and small hermit crabs will be confronted with 3D-printed snail shells, to determine the degree of phenotypic plasticity in claw shape and to separate phenotypic plasticity from genetic variability. The latter one is important for understanding variability as starting point in evolutionary transformations.

• DFG Programme: Research Grants