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

Abstract

Portunoidea (Heterotremata) is a morphologically disparate taxon of true crabs (Brachyura) best-known for many of its representatives being considered “swimming crabs”. The term “swimming crab”, however, sometimes refers to a distinct taxon (traditionally to Portunidae within Portunoidea), and sometimes to a certain morphotype in which the 5th pereiopod (P5) has a specific shape that facilitates swimming. We use the term “P5-swimming crab” or “P5-swimmer” herein, not only to restrict it to the morphotype, but also to distinguish the swimming in question from other kinds of swimming in Brachyura. The evolution of P5-swimming crabs has not yet been satisfactorily investigated. In particular, it is not known whether the morphotype evolved several times independently in different lineages of Portunoidea or whether it evolved only once and was lost in several lineages. Ours is the first approach combining molecular with morphological data to result in a new phylogenetic positioning of some members of Portunoidea. For the first time, data from the axial skeleton and extrinsic musculature are used. Morphological examinations reveal that the axial skeleton and extrinsic musculature in P5-swimming crabs are more diverse than previously thought, with the exception of the P5 anterior coxa muscle, which originates at the median plate in all P5-swimmers. Ancestral state reconstructions based on parsimony reveal that the stem species of Portunoidea already showed the morphotype of a P5-swimming crab, but with a long merus which probably resulted in less effective P5-swimming than in extant P5-swimming crab species with a short merus. Several other extant taxa represent a reversal of the P5-swimmer morphotype to varying degrees, with some extant species showing a complete reversal of unambiguous P5-swimming crab character states—one example being the well-known common shore crab Carcinus maenas. The absence of a connection between interosternite 7/8 and the sella turcica (the secondary loss of the “brachyuran sella turcica”) in the stem species of Heterotremata, resulting in a junction plate which forms a cavity that offers room and attachment sites for the P5 extrinsic musculature is uncovered as preadaptation to the P5-swimmer morphotype in Heterotremata. This preadaptation is missing in Podotremata and Thoracotremata, the other two traditional main taxa of Brachyura.

A new subfamily classification of the highly diversified Dorippidae H. Milne Edwards, 1837 (Crustacea, Decapoda, Brachyura, Dorippoidea), using morphological, molecular and palaeotonlogical data, with special emphasis on its unique female reproductive system

“…The absence of a median plate (see Fig. 7A) has been recently reported in both sexes (Hazerli et al 2022: 44), who consider that, with respect to the axial skeleton, Dorippidae (exemplified by Medorippe lanata ) is “positioned as sister to a clade comprising Heterotremata and Thoracotremata”.…”

“…1A) give an interesting interpretation of the raised laminae and infoldings issued from the sternal floor, present only in males: they are not homologous to the median plate found in other hetrotreme taxa: we agree that a median plate, similar to that of other Eubrachyura, is effectively absent in Medorippe lanata . Hazerli et al (2022) concur with Guinot et al (2013: table 8) that the interosternite 7/8 is directly connected to the sella turcica and the intertagmal phragma is connected to the interopleurite 7/8, with these authors adding that the latter is confluent with the sella turcica. Hazerli et al (2022: 44, fig.…”

“…Hazerli et al (2022) concur with Guinot et al (2013: table 8) that the interosternite 7/8 is directly connected to the sella turcica and the intertagmal phragma is connected to the interopleurite 7/8, with these authors adding that the latter is confluent with the sella turcica. Hazerli et al (2022: 44, fig. 33) conclude in their phylogenetic analysis that, regarding this character, Medorippe lanata is “positioned as sister to a clade comprising Heterotremata and Thoracotremata (not as a heterotrematan taxon as suggested by Jamieson & Tudge, 1990)”.…”

“…Hazerli et al (2022: 9, fig. 1A) give an interesting interpretation of the raised laminae and infoldings issued from the sternal floor, present only in males: they are not homologous to the median plate found in other hetrotreme taxa: we agree that a median plate, similar to that of other Eubrachyura, is effectively absent in Medorippe lanata .…”