Into the Host Nervous System: Bulb-Shaped Organs of Heterosaccus (Cirripedia: Rhizocephala), a New Type of Rhizocephalan Invasive Rootlet

Rhizocephalan barnacles induce profound changes in their crustacean hosts, yet the mechanisms underlying host manipulation remain poorly understood. Specialized invasive rootlet of the parasite interna penetrate the host ventral nerve cord and may mediate interactions with host nervous tissue. Here, we provide the first morphological and ultrastructural characterization of invasive rootlets in two species of Heterosaccus: H. papillosus parasitizing Charybdis anisodon and H. ruginosus infecting Thranita crenata in Vietnam. We describe a previously unrecognized type of invasive rootlet in Rhizocephala: bulb-shaped organs located at the distal tips of long afferent rootlets. These organs consist of columnar cells rich in vesicles and mitochondria, suggesting high cellular activity associated with transport or secretion. Afferent rootlets show progressive structural changes toward the bulb-shaped organs, including modifications of the cuticle, axial cells, and lacunar system. Electron-dense granules in adjacent host tissue may indicate a local response at the parasite–nerve interface. Similarities between bulb-shaped organs and follicle-like structures described in Peltogasterella gracilis raise questions about the broader distribution of this type of invasive rootlets in Rhizocephala. Overall, our findings expand the known diversity of rhizocephalan invasive rootlets and provide new insights into their functional and evolutionary divergence.