Establishment of a new subfamily for the parasitic isopod genus PleurocryptellaBonnier, 1900 (Isopoda: Bopyridae), including recognition of epicaridium larval yolk sacs and description of a new genus and species of hyperparasite.

This article focuses on the discovery and description of a new species, *Pleurocryptella poseidon* Williams & Boyko sp. nov., a bopyrid isopod ectoparasite found in the branchial chambers of the squat lobster *Galacantha bellis* from the Arabian Sea. Notably, the first larval stage (epicaridium) of this genus exhibits a rare posterior external yolk sac connected to the alimentary canal, a feature confirmed here and unique among Bopyridae. Morphological and molecular (COI gene) analyses distinguish *Pleurocryptella* from other bopyrids, leading to the establishment of a new subfamily, Pleurocryptellinae. Additionally, a new genus and species of hyperparasitic isopod, *Pleuroniscus bellisamans* gen. nov., sp. nov., is described from the cryptoniscus larval stage found within *P. poseidon*. The study provides detailed morphological descriptions, larval keys, and comparative analyses of larval mouthparts and yolk sacs to aid future research on epicaridean life histories and phylogeny.