New insights into the evolution of portunoid swimming crabs (Portunoidea, Heterotremeta, Brachyura) and the brachyuran axial skeleton

Abstract Portunoidea (Heterotremata) represents a morphologically disparate taxon of true crabs (Brachyura), best-known for many of its representatives being considered as “swimming crabs”. This term, however, sometimes refers to a distinct taxon (traditionally to Portunidae within Portunoidea), but sometimes also to a certain morphotype, with the 5th pereiopod (P5) having the shape of a specific swimming leg. We herein use the term “P5-swimming crab” or “P5-swimmer”, not only to restrict it to the morphotype, but also to distinguish it from other kinds of swimming in Brachyura. The evolution of P5-swimming crabs is still poorly investigated. For example, it is not known, whether the morphotype evolved several times independently in different lineages of Portunoidea or if it evolved only once and had been lost in several lineages. We here present the first approach combining molecular with morphological data resulting in a new phylogenetic hypothesis of Portunoidea. For the first time, the latter involve data from the axial skeleton and extrinsic musculature. Morphological examinations revealed that axial skeleton and extrinsic musculature in P5-swimming crabs were more diverse than previously thought, except for the origin of the P5 anterior coxa muscle origin at the median plate, which was present in all P5-swimmers. Ancestral state reconstructions based on parsimony revealed that the stem species of Portunoidea already showed the morphotype of a P5-swimming crab, but with a long merus which probably resulted in a less effective P5-swimming ability compared to that of extant P5-swimming crab species with short merus. Several other extant taxa represent a reversal of the P5-swimmer morphotype with variable degree, with some extant species showing a complete reversal of unambiguous P5-swimming crab character states, like for example the common shore crab Carcinus maenas. Finally, a missing connection of interosternite 7/8 to the sella turcica (the secondary loss of the “brachyuran sella turcica”) in the stem species of Heterotremata, resulting in a junction plate forming a cavity that offered room and attachment sites for the P5 extrinsic musculature is uncovered as preadaptation to the P5-swimmer morphotype in Heterotremata, which is missing in “Podotremata" and Thoracotremata, the other two traditional main taxa of Brachyura.