Pseudotachidiidae (SOO‑doh‑TAK‑ih‑DY‑ee‑dee) is a family of crustaceans in the order Harpacticoida.. Members of this family are primarily benthic copepods, inhabiting marine and occasionally brackish sediments. They are generally small, ranging from 0.2 to 1.5 mm in length, and are dorsoventrally flattened to facilitate movement through interstitial spaces in sediment. Pseudotachidiidae play a significant role in sedimentary food webs, functioning as deposit feeders that consume detritus, microalgae, and bacterial biofilms, and serving as prey for nematodes, small fish, and other benthic predators. The family is cosmopolitan in distribution, with species recorded from tropical, temperate, and polar regions, inhabiting habitats ranging from shallow coastal sediments and mudflats to continental shelves, submarine canyons, and abyssal plains. Certain species also colonize structured habitats such as coral rubble and sponge skeletons, where they often dominate the meiofaunal community. Pseudotachidiidae exhibit a high degree of morphological diversity. Key diagnostic features include antennule and antenna segmentation, setation patterns of thoracic swimming legs (P1–P4), morphology of caudal rami, and sexual dimorphism, particularly in male P5 and antennule structures. These traits are critical for species identification and taxonomic classification. Ecologically, Pseudotachidiidae contribute to sediment nutrient cycling and are often used as bioindicators for environmental monitoring, given their sensitivity to changes in sediment composition, oxygen levels, and organic content. Their abundance and diversity make them a key focus of meiofaunal studies, particularly in deep-sea and coastal ecosystems where they influence benthic community structure and trophic dynamics.

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