Phylogenomics supports monophyly of marsupial crustaceans: a journey to direct development.

Peracarida (marsupial crustaceans) represent one of the most diverse and ecologically important crustacean groups, yet their evolutionary relationships have long been debated. Here, we present the most comprehensive phylogenomic analysis of Peracarida to date, incorporating the relict taxa Thermosbaenacea, Mictacea, Ingolfiellida, and Spelaeogriphacea in a phylogenomic framework. Our results robustly confirm peracarid monophyly and recover a well-supported clade uniting Mancoida (Isopoda, Tanaidaca, Cumacea, Spelaeogriphacea) and Mictacea. We propose for Mictacea to be classified as part of the Mancoida, which is defined by shared developmental and morphological traits. Thermosbaenacea is suggested as sister group to all other Peracarida, although this position was not recovered in all analyses. The inferred phylogeny further suggests that evolution of Peracarida involved a transition from an intermediate "pseudodirect" developmental mode to the direct development seen in most lineages. We further show that the shift to extensive brood care within the marsupium, accompanied by the loss of a free-swimming larval stage, may have accelerated rates of molecular evolution across lineages. Together, these findings provide a robust evolutionary framework for this major malacostracan lineage and highlight how key reproductive innovations reshaped the genomic and life-history trajectories of the marsupial crustaceans.