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The morphological traits and mitochondrial genes of sympatric sea slaters from the genus Ligia were analysed from samples collected along northeastern coastal China and adjacent coastal East Asia to assign the species and to obtain interpretations of their distributions. Using 338 sequences and 199 diagnoses of 16S polymerase chain reaction-restriction fragment length polymorphism ( PCR-RFLP) from 331 specimens sampled from 26 localities, and homological sequences from counterparts from South Korea and the west coast of the USA, we assigned these taxa into two distinct species, Ligia exotica and Ligia cinerascens. Both species are recovered as two monophyletic clades in the robust phylogenetic hypothesis presented here. The northern boundaries of these species in China and the Korean peninsula are more southern than those of counterparts in Japan. Ligia exotica in northeastern coastal China is further split into two geographical subclades ( B1 and B2), coexisting near 35°N. The distribution range of L. cinerascens covers the whole range of L. exotica subclade B1. Counterparts in Japan show a similar pattern. Oceanic rafting, shipping, damming and planting of seaweed are proposed as the media for the contemporary connectivity among populations of Ligia species inhabiting coastal China. © 2013 The Linnean Society of London [ABSTRACT FROM AUTHOR]

• Mitogenome order data are expanded to 56 species in 32 genera of freshwater crabs. • Nine gene order patterns were discovered in these freshwater crab mitogenomes. • Mitogenome gene order provides a powerful means for phylogenetics of Potamiscinae. We present here a combined mitogenome gene order dataset totalling 62% of the known genera of East Asian potamiscine freshwater crabs that includes first-time mitogenome data for 14 species and gene order data from 37 other species of potamiscines. A surprisingly high number of mitogenome gene order rearrangements were found in the taxa studied (comprising nine different rearrangements and seven major patterns, one of which has two sub-arrangements). Our phylogenetic reconstructions indicate that the mitogenome gene order rearrangements are associated with the evolutionary history of potamiscine lineages. We also used a new Event-based Maximum Parsimony method to reconstruct ancestral gene orders, which takes into consideration gene duplication, pseudogeneticization, and tandem duplication random loss. Furthermore, shared mitogenome gene order patterns were used to inform the taxonomic placement of Sinopotamon parvum , and the cryptic diversity in Potamiscus. The remarkably frequent mitogenome rearrangements in potamiscine freshwater crabs have great potential to contribute to our understanding of the evolutionary history of these highly diverse decapods in East Asia. [ABSTRACT FROM AUTHOR]