Morphology and Phylogeny of Thylacoplethus squillae (Rhizocephala: Thompsoniidae), a Parasite of the Mantis Shrimp Harpiosquilla harpax.

Parasitic barnacles (Cirripedia: Rhizocephala) are among the most highly specialized parasitic groups, exhibiting profound morphological modifications and a remarkable capacity to manipulate crustacean hosts. Thompsoniidae, a family of highly derived rhizocephalans parasitizing a wide range of hosts (stomatopods, anomurans, and brachyurans), remains poorly studied. Here, we reconstructed the phylogeny of Thompsoniidae and investigated the morphology of Thylacoplethus squillae, a parasite of the mantis shrimp Harpiosquilla harpax, with particular emphasis on the interna. Our phylogenetic analyses recovered the stomatopod parasite Jensia serenei as the basal lineage within Thompsoniidae. Moreover, we provide the first description of invasive rootlets in a rhizocephalan parasitizing a non-decapod host. These rootlets belong to the neuropil type and invade the entire ventral nerve cord of the host, including both pereonal and pleonal ganglia. Similar to other rhizocephalans, invasive rootlets differ from trophic rootlets in the organization of their cuticle and cellular layers. Observed extensive subcuticular spaces may represent a distinctive feature of T. squillae or Thompsoniidae in general. We hypothesize that the broad distribution of invasive rootlets throughout the host nervous system may reflect an adaptation to parasitism in stomatopods. Together, these findings provide new insights into the evolution of Thompsoniidae and rhizocephalan interactions with the host nervous system.