The evolution of the swimming crab morphotype – an extraordinary case of convergence?

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The term "swimming crab" usually refers to a specific morphological manifestation (morphotype) within the phylogenetically well-established taxon true crabs (Brachyura). Swimming crabs differ from all other members of Brachyura in displaying a range of morpho-functional features that together enable them to perform a unique mode of swimming behavior using their 5th pereiopods (P5). Swimming crabs (together with some morphologically similar non-swimming taxa, such as the well-known green crab (Strandkrabbe) Carcinus meaenas) have been assigned to the superfamily Portunoidea. The assumption that swimming crabs are a monophyletic group has recently been challenged, and it seems that their specific morphological adaptations could have multiple origins and possibly have evolved multiple times. The main questions explored in the proposed project are: (1) Are the swimming legs (and functionally related structures of the axial skeleton) of different Portunoidea homologous or did the swimming capacity of the 5th pereiopods evolve independently? (2) Which combination of features enables the 5th pereiopods to act as swimming legs? (3) What prerequisites (in terms of body shape, axial skeleton, structure of the pereiopods) which led to the evolution of swimming crabs were present in heterotrematan brachyurans and not in podotrematan and thoracotrematan crabs (the two other major taxa within Brachyura)? To answer these questions, the structure of the locomotor apparatus of various brachyuran crabs will be studied using μCT and digital 3D reconstruction and visualization. The detailed descriptions thus obtained will be used to conceptualize characters and character states, and the transformation of character states will be studied on the basis of current phylogenetic hypotheses. Locomotory capability will be studied ex vivo using modeling (Maya 2018) by virtually reconstructing the maximum range of mobility (ROM) of all P5 limb joints together, and in vivo via video reconstruction of moving morphology (VROMM) in species where living specimens can be studied. Both methods will shed light on the significance of individual features of the P5 in terms of swimming behavior.