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Five species of Platorchestia Bousfield, 1982, are described and figured from Atlantic Ocean shores (including the Caribbean, Baltic, and Mediterranean seas). Four of these are new to science. All five species had previously been illustrated in the literature but four of them had incorrectly been allocated to either Orchestia platensis Krøyer, 1945 or O. monodi Mateus, Mateus & Afonso, 1986.

Seventy-three male specimens of “Platorchestia platensis” from Japan were inspected on 13 morphological characters. Most characters revealed high variation. The coxa and propodus of gnathopod 2 and the carpus of pereopod 7 indicated that the specimens comprised two species: Platorchestia pacifica Miyamoto and Morino, 2004 and Demaorchestia joi (Stock and Biernbaum, 1994) sensu lato. Both species were rediagnosed. Demaorchestia hatakejima Lowry and Myers, 2022 was synonymized to P. pacifica. A key to allied species in Platorchestia and Demaorchestia from Japan and the surrounding countries was given.

In this paper, we determined the complete mitochondrial genome (mitogenome) sequences of two talitrid amphipods, Platorchestia japonica and P. parapacifica. The complete mitogenomes of P. japonica and P. parapacifica were 14,780 and 14,787 bp in length, respectively, with the typical 13 protein coding genes (PCGs), 22 transfer RNAs (tRNAs), two ribosomal RNAs (rRNAs) and a control region (CR). In the gene order analysis, two PCGs (nad3 and nad6) were rearranged in comparison to the typical pan-crustacean ground pattern. A maximum-likelihood tree, constructed based on 31 eumalacostracan mitogenomes, confirmed that P. japonica and P. parapacifica (Talitridae) were closely related to Parhyale hawaiensis (Hyalidae), and supports the monophyly of the superfamily Talitroidea.