Peracarida is a clade of malacostracan crustaceans united by two morphological characters, brooding of the young by the female in a marsupium or brood pouch, and the presence of a lacinia mobilis on the mandible in the adults. Within the basic malacostracan body plan of five head segments, eight thoracic segments, and six pleonal segments, peracarids have at least the first thoracic segment fused to the head and the first pair of thoracic appendages modified as maxillipeds (also called unguiped; Grams et al. 2023), with up to three thoracic segments fused to the head and three pairs of maxillipeds. There are approximately 26,000 described peracarid species (WoRMS Editorial Board 2025) distributed across one fossil (Pygocephalomorpha) and 12 extant orders. The extant groups generally considered as comprising Peracarida are Amphipoda, Bochusacea, Cumacea, Ingolfiellida, Isopoda, Lophogastrida, Mictacea, Mysida, Spelaeogriphacea, Stygiomysida, Tanaidacea and Thermosbaenacea. Morphologically, peracarids are spectacularly diverse. Table 1 summarizes the currently accepted orders.

In the broader context, Pancrustacea (crustaceans and hexapods) account for 80% of described animal diversity on Earth (Roskov et al. 2022). Within Pancrustacea, peracarids represent about 39% of all non-hexapod pancrustacean diversity and about 65% of malacostracan diversity. The majority of peracarids are found in marine environments, but there are representatives in every environment on Earth, from terrestrial deserts to oceanic trenches. Peracarids occupy many ecological roles. They can be parasites of hosts from decapods to cnidarians, predators on smaller organisms, scavengers responsible for rapid recycling of benthic food falls, as well as filter feeders and deposit feeders cycling organic carbon back into the food web. Economic damage from peracarids can be extensive. Wood boring isopods known as gribbles (Limnoria) cause extensive damage to wooden docks, piers and boats. Parasitic bopyrid isopods afflict commercially harvested decapods, both wild caught and aquacultured, including Macrobrachium species (Gopalakrishnan et al. 2017). Peracarid bodies are remarkably morphologically plastic. This is especially true within Isopoda, which have body plans ranging from a standard roly-poly (Hornung 2011) to essentially formless parasites (Williams and Boyko 2012). Evolutionarily, peracarids are fascinating, with rapid molecular evolution and highly variable genome size (Rees et al. 2007; Hessen and Persson 2009; Jeffery 2015). There is evidence for multiple habitat transitions through time, from marine to terrestrial and freshwater environments (Wägele et al. 2003; Wilson 2009; Raupach et al. 2009; Riehl et al. 2014; Lins et al. 2017; Wetzer et al. 2018) and transitions from shallow marine waters to the deep sea (Raupach et al. 2009). The mode and tempo of these transitions remain incompletely understood. For example, some studies cast doubt on the monophyly of terrestrial isopods (Oniscidea; e.g., Dimitriou et al. 2019), yet two recent phylogenomic studies by Thomas Thorpe (2024) and Iwasa-Arai et al. (2025) recovered the group monophyletic and inferred a single transition to land in the Carboniferous–Permian.

Despite being generally accepted as a taxon for over 100 years, monophyly of Peracarida is not entirely clear, and neither are relationships within the group, as summarized in Fig. 1 and Table SS1 (Boas 1883; Grobben 1892; Siewing 1963; Watling 1981, 1999; Hessler 1983; Schram 1984; Pires 1987; Wagner 1994; Mayrat and Saint Laurent 1996; Schram and Hof 1998; Wheeler 1998; Wills 1998; Jarman et al. 2000; Richter and Scholtz 2001; Poore 2005; Spears et al. 2005; Meland and Willassen 2007; Wilson 2009; Wirkner and Richter 2010; Schwentner et al. 2018; Lozano-Fernandez et al. 2019; Höpel et al. 2022; Bernot et al. 2023; Yu et al. 2024; Cannizzaro and Berg 2025; Iwasa-Arai et al. 2025). Recent studies that support peracarid monophyly had limited sampling of the extant orders (Höpel et al. 2022), mitochondrial genomes – 58%; (Bernot et al. 2023), nuclear protein-coding genes from transcriptomes and genomes – 50%; (Yu et al. 2024), genomes, transcriptomes, and raw RNA-seq reads – 41%, and ultraconserved elements (UCEs) – 45% (Cannizzaro and Berg 2025) and 63% (Iwasa-Arai et al. 2025).

Earlier molecular studies based on one or few PCR-amplified gene fragments that included more orders are not in agreement about peracarid monophyly (i.e., Mysida placed within Peracarida or not; summarized in Meland et al. 2015). There is no consensus on relationships within the group, regardless of the analysis methodology used (the various hypotheses are summarized in Fig. 1 and Table SS1). The first-branching group within Peracarida has been suggested to be Amphipoda (Mayrat & de Saint Laurent 1996; Wirkner and Richter 2010; Bernot et al. 2023; Yu et al. 2024), or Mysida (Schram 1984; Schwentner et al. 2018), or Mysidacea + Amphipoda (Höpel et al. 2022), or Thermosbaenacea (Siewing 1963; Watling 1983; Pires 1987; Iwasa-Arai et al. 2025). Mancoida has been supported as monophyletic in both morphological studies (Richter and Scholtz 2001; Wilson 2009) and molecular studies (Schwentner et al. 2018; Höpel et al. 2022; Bernot et al. 2023), but taxon sampling has been very unbalanced, as these studies included multiple isopods but very few tanaids and cumaceans, prompting concerns about long-branch attraction. Depending on the molecular marker(s) used, peracarids exhibit moderate to severe branch length heterogeneity in molecular phylogenetic analyses (Spears et al. 2005; Bybee et al. 2011; Schwentner et al. 2018; Höpel et al. 2022; Bernot et al. 2023; Barta et al. 2025; Iwasa-Arai et al. 2025). The various hypotheses are summarized in Fig. 1 and Table SS1.

All attempts to resolve peracarid phylogeny, whether based on morphological or molecular data, suffer from the same fundamental problems, limited character sampling on the one hand, limited taxon sampling on the other hand. Here, we review the current understanding of peracarid diversity and phylogeny based on both morphological and molecular approaches. We highlight open questions about peracarid evolution, describe work in progress, and opportunities for improving understanding of this fascinating group.

After many studies utilizing a variety of approaches and data sources, we are essentially in the same place where we started. The natural grouping of the taxon Peracarida is generally accepted, but the exact composition of the group, monophyly of the group, and relationships within the group are not well resolved, with conflicting answers depending on the data and analytical approach. Persistent questions revolve around inclusion of Thermosbaenacea and Mysida, monophyly or paraphyly of Mysidacea, the identity of the earliest branching peracarid lineage, and the overall topology of the tree.

Thermosbaenacea is distinguished from other members of this superorder by being dorsal brooders, the pouch formed by the carapace. All other peracarids sensu lato are ventral brooders, with brood plates or oostegites derived from the coxa of the thoracopods. Their current inclusion in the Peracarida stems from morphological analyses, the most obvious shared synapomorphy is the presence of a lacinia mobilis in adults (Wagner 1994; Schram and Hof 1998; Wills 1998; Wirkner and Richter 2010) and single gene studies (Spears et al. 2005; Meland and Willassen 2007), as well as tradition.

The historical Mysidacea has been split into the extant orders Mysida, Lophogastrida, Stygiomysida and the extinct order Pygocephalomorpha. The relationships between the extant orders, and inclusion or exclusion of Mysida from Peracarida are all quite unclear. Various analyses suggest Mysidacea could be paraphyletic or monophyletic (see Höpel et al. 2022) for discussion), that the Lophogastrida are the sister group to the Mysida, or not. Stygiomysida has rarely been included in analyses separately from the Mysida, as they were only recently recognized as a separate clade (Meland et al. 2015). Mysida has been variously suggested to be within Peracarida (as part of Mysidacea) or the sister taxon to Peracarida (Schwentner et al. 2018; Iwasa-Arai et al. 2025).

There is no robust or stable topology for relationships within Peracarida. Topologies are summarized in Fig. 1 and their tree files are offered in Table SS1. Bernot et al. (2023) demonstrated comprehensively that small differences in taxon sampling had outsize impacts on phylogenetic reconstruction across Malacostraca, and especially within Peracarida. The recent analysis by Yu et al. (2024) points to long branch attraction as a major problem across Pancrustacea. There is one consistent grouping recovered within Peracarida in most analyses, Mancoida, consisting of Cumacea, Isopoda, and Tanaidacea, bearing in mind that not all peracarid taxa were included in these analyses. However, within Mancoida, all possible topologies have been recovered. Similarly, across Peracarida, a wide variety of topologies have been recovered. It is notable that of the 15 crucial taxa identified by Bernot et al. (2023) for resolving Pancrustacea phylogeny, 40% are peracarids, suggesting that instability in the peracarid phylogeny disproportionately destabilizes efforts to resolve the phylogeny of the entire Pancrustacea.

Serendipitously, four projects across four countries have been funded to address peracarid phylogeny and evolution. At the same time, the ability to acquire data, both morphologically and molecularly, has increased exponentially with micro-CT and other advanced imaging techniques alongside the vast expansion and declining cost of next generation sequencing.

“A Backbone for the Peracarida” is funded by the United States National Science Foundation, concentrating on generating a phylogeny with family-level coverage across Peracarida using a combination of transcriptomes, genomes, and target capture. Simultaneously, “Transformations in the Evolution of Peracarida (Crustacea)” is funded by the Deutsche Forschungsgemeinschaft and Österreichischer Wissenschaftsfond, concentrating on studying higher-level phylogeny and evolutionary rates across the peracarids in comparison with reproductive strategy and habitat, using a combination of transcriptomes, genomic, and morphological analyses, with emphasis on Isopoda. Two projects funded by the National Science Centre in Poland aim to delineate the evolutionary arenas of shallow- and deep-sea Tanaidacea by reconstructing their phylogenetic history, assessing colonization pathways into the deep-sea and polar regions and identifying the morphological traits that are associated with diversification across marine habitats. All four of these projects are working hand-in-hand to generate a stable and well supported phylogenetic hypothesis for Peracarida.

Efforts to resolve the phylogeny of Peracarida have been impacted by both lack of taxon sampling (breadth) and lack of sufficient data in the taxa sampled (depth). New morphological and molecular tools can address the issues of limited data in phylogenetic studies, with advances in both morphological and molecular technology. Advanced imaging techniques like confocal laser microscopy (cLSM) and micro-computed tomography (micro-CT) have exponentially increased morphological data acquisition. Genomes and transcriptomes are becoming more cost effective, although these require material to be freshly collected, preserved in specific ways, and kept at very cold temperatures. Given the range of environments where peracarids are found (terrestrial deserts to deep-sea trenches), many taxa cannot easily be freshly collected, thus taxon sampling is still a problem even when using -omics tools that require frozen material. However, target capture techniques can be used to generate large amounts of data from ethanol-preserved specimens stored at room temperature, making museum specimens of rare taxa potentially available for expanding taxon sampling (e.g., Cannizzaro and Berg 2025; Iwasa-Arai et al. 2025).

Across the projects, we will be sequencing genomes from hitherto understudied orders (e.g., Cumacea, Mysida, Lophogastrida) and approximately 200 new peracarid transcriptomes from across the group. These reference datasets will be leveraged to design probes for conserved loci, and an additional 2,000 taxa will be sequenced using a target capture approach for a minimum of 250 loci. For samples that aren’t suitable for target capture (e.g., due to their extremely small size), libraries will be sequenced at 30x without target capture. These genome skimming data will be de novo assembled, and target loci will be extracted bioinformatically. Overall, this approach is expected to address both of the current problems in resolving peracarid phylogeny, with a minimum of 250 genes per taxon sampled, and over 2,200 taxa sampled (more than 20x the taxa in the analyses with the greatest coverage to date). Coverage of taxa is expected to include the vast majority of recognized families. The anticipated stable, robust, and well-resolved phylogenetic hypothesis for Peracarida will allow research across the group to expand and grow.

The community of peracarid workers has been committed to trying to get a comprehensive phylogeny funded since 2010, and we are excited it is finally happening. These projects will only be successful with the participation of the global community, from providing specimens, to collaborating on workshops, to participating in symposia and discussions.

The first gathering in Rostock (hosted by Dr. Stefan Richter) in May 2024 brought together representatives from all the teams and established the cooperation and sharing of specimens and data. The July 2025 Crustacean Society Conference held in Paris brought together more than 30 global peracarid workers all participating in this effort and solidified our unified and supportive approach. A gathering will be held at the Natural History Museum of Los Angeles County in 2027 to synthesize the phylogenetic and genomic work and integrate vetted fossil calibrations for divergence-time estimation. We are also planning future symposia on Peracarida, including the joint SICB/TCS meeting in 2027 and International Crustacean Congress XI.

Field-based taxonomy workshops provide rare, high-impact training for early-career researchers. By providing training in specimen collection, preservation, and identification with modern molecular and imaging approaches, these courses empower participants with a comprehensive skillset for twenty-first-century systematics research. The first “Confusing Crustaceans” peracarid workshop was a field-based workshop, held at the Smithsonian Tropical Research Institute in Bocas del Toro, Panama in August of 2024, where 26 established and early-career peracarid researchers learned and shared tropical sampling, sorting, identification, and preservation techniques with each other.

Taxonomy training workshops provide rare access to the collective expertise of multiple specialists from multiple parts of the world. During taxonomy workshops, trainees work side-by-side with expert taxonomists, gaining practical experience that is difficult to acquire through standard coursework, while also building professional networks and collaborative ties. Such immersive programs help sustain critical taxonomic expertise, foster a new generation of systematists, and strengthen the foundation for future biodiversity research and conservation efforts. “Confusing Crustaceans II,” a taxonomy focused workshop held October 2025 in Wilhelmshaven, Germany trained 30 established and early career workers in species-level identification and description, and increased morphological capacity in the community. We are interested in expanding our program of workshops to other parts of the world, encouraging more peracarid research in collaboration with local hosts.

Outcomes of these projects will include a probe set targeting carefully-selected, conserved exons across all orders of Peracarida, several genomes, many transcriptomes, an expanded morphological character set, and new tools and techniques specific to Peracarida. Probes, laboratory workflows, sequence data, and bioinformatic scripts will be made publicly available in open repositories to enable immediate adoption by other peracarid researchers. Further, our field work will result in the collection of thousands of specimens that were carefully preserved with genetic and genomic work in mind. Vouchers and extra or unused specimens will be deposited in natural history museums (e.g., Natural History Museum of Los Angeles County, Senckenberg, and Alabama Museum of Natural History) where they will be available for loan requests by other researchers. We welcome collaboration with all researchers working on Peracarida and are especially interested in partnerships aimed at broadening taxon sampling, expanding geographic and habitat coverage, and providing opportunities and training for the next generation of peracarid researchers. We especially welcome help securing specimens of rare and hard-to-find taxa.

We are grateful to Jørgen Olesen, Kenneth Meland, and Gonzalo Giribet for their comments and suggestions for improving the manuscript. We thank the Encyclopedia of Life for funding the 2009 workshop held on Santa Catalina Island, California which was the impetus for seeking funding to build a molecular-based backbone phylogeny of Peracarida. This initial gathering of colleagues included: Jim Lowry, Magda Błażewicz, John Markham, Chris Boyko, Stefano Taiti, Kenneth Meland, Gary Anderson, Sarah Gerken, Daniel Roccatagliata, Dean Pentcheff, Gary Poore, Marilyn Schotte, Adam Wall, and Regina Wetzer (left-to-right, Fig. 5). Over the ensuing 15 years and five NSF funding attempts, this aspiration is now a reality. We thank Keith Crandall for his tireless support on four of these attempts, and Dean Pentcheff for his continual technical, editorial, and moral support. Ethan Kahn, thank you for help with the citations. This project is now funded by the following grants: United States National Science Foundation (DEB-2321306, DEB-2321307, DEB-2321308, OPP-2138993, OPP-2138934) awarded to Sarah Gerken, Regina Wetzer, and Kevin Kocot, by the Deutsche Forschungsgemeinschaft (DFG RI 837 29-1) awarded to Stefan Richter, and by the Österreichischer Wissenschaftsfond (FWF I6550) awarded to Martin Schwentner.

The projects are funded by the United States National Science Foundation (DEB-2321306, DEB-2321307, DEB-2321308), Deutsche Forschungsgemeinschaft (DFG RI 837 29-1), Österreichischer Wissenschaftsfond (FWF I6550), National Science Centre, Poland (2023/49/B/NZ8/03547; 2023/49/B/NZ8/04237).