Here, we provide the first complete mitochondrial genomes for two higher taxa of Peracarida, Lophogastrida and Stygiomysida. We examined Lophogaster typicus as a representative of Lophogastrida and Spelaeomysis bottazzii as a representative of Stygiomysida. Both mitogenomes have all typical metazoan genes (13 protein‐coding genes, two ribosomal RNA genes and 22 transfer RNAs). The mitogenomes have a length of 15,076 bp in L. typicus and 14,806 bp in S. bottazzii. Gene order differs markedly from the hypothetical pancrustacean/malacostracan ground pattern in both species, and in L. typicus, all genes were encoded on the heavy strand. This is the first time this is described for a crustacean. We also reconstruct eumalacostracan phylogenies using a data set consisting of 98 species based on alignments comprising all protein‐encoding genes as well as the protein‐encoding genes and the two ribosomal RNAs. We find support for the monophyly of Mysidacea based on species from all three higher taxa (Mysida, Lophogastrida, Stygiomysida). Moreover, our analyses also support a monophyletic Peracarida with Amphipoda or Amphipoda + Mysidacea as the sister group to the remaining Peracarida.

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.

The iconic ‘mountain shrimps’ of the genus Anaspides Thomson, 1894, are endemic to Tasmania, inhabiting various freshwater habitats such as mountain tarns and creeks, as well as streams inside caves. They are often labelled as ‘living fossils’ because of their close resemblance to their Triassic relatives. Prior to 2015, only two species were recognised but recent studies have uncovered a total of at least seven species. The type species of Anaspides, A. tasmaniae (Thomson, 1893), was previously believed to occur throughout Tasmania, but following a review in 2016, this species was confirmed only from a small range on the east and south-east side of Mt Wellington, with Anaspides from other parts of Tasmania referable to other species. We herein provide a detailed assessment of the distribution and genetic structure of A. tasmaniae based on extensive field surveys throughout the ranges of all species of Anaspides. The distribution of A. tasmaniae is extended to include four separate localities in and around the Mt Field National Park, 50 km north-west of Mt Wellington. The recovered genetic structure of A. tasmaniae based on 48 specimens indicates that the disjunct distribution is unlikely to be the result of artificial translocation but, instead, probably reflects postglacial relictualisation of a formerly continuous range present during Pleistocene glacial maxima. Of particular interest is the record of syntopy in Anaspides, observed at the entrance of Khazad Dum cave, where both A. tasmaniae and A. swaini inhabit the inflow stream.

Species of the genus Anaspides, known as mountain shrimps, are endemic to Tasmania and inhabit a variety of freshwater habitats such as mountain tarns, pools, creeks and runnels, as well as caves. Until 2015 only two species of Tasmanian mountain shrimps were recognized, A. tasmaniae (Thomson, 1893), which was believed to be widespread all over the island and A. spinulae from Lake St. Clair. Revision of the genus by Ahyong in 2016 recognized 7 species, most having narrow geographic distributions. Only two widespread species remained: A. richardsoni, occurring mainly on the Central Plateau and its margins, and A. swaini, occurring largely in south-western Tasmania. Notably, within A. swaini, three geographically correlated morphological forms were observed. We re-evaluated all three forms of A. swaini and herein describe one of the forms as a new species, A. driesseni, on the basis of morphological and molecular data. Anaspides driesseni corresponds to A. swaini form 3 and occurs mainly in south-eastern Tasmania from the Hartz Mountains over the Snowy Mountains to the Wellington Range. Telson structure, spination and male secondary sexual characters proved taxonomically instrumental.

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.