Researchers from a prominent Russian university examined the muscular and abdominal systems of the external reproductive structure in rhizocephalans, parasites that transform certain crustaceans into living hosts. They infect the host and eventually hijack its physiology, forming an externa on the host’s surface, a framework where parasite larvae mature. While scientists worldwide explore these remarkable animals, the physiology of the ectopic region remains only partly understood. The study was discussed in St. Petersburg, and findings were clarified by faculty at St. Petersburg State University.
Rhizocephalans are a family of unique parasitic crustaceans found in oceans across the globe. In these species, the female injects a small cluster of cells into the host, which then grow into the interna, a network of processes inside the host’s body cavity. Through invasion of multiple host systems, the parasite takes control, and rhizocephalans can induce parasitic castration by altering host behavior and morphology and even shifting metabolic processes. Notably, the stem of the adult rhizome regenerates without passing on any material from the larva.
Following interna formation, the externa begins to form—this second component of the female rhizocephalan’s body sits above the host’s integument. The externa houses the site where the male rhizocephalus completes the larval deposition process. As a result, neither sex retains the usual morphological features of the host organism.
Researchers still seek answers on how nutrients move from the interna to developing larvae. To investigate, the team employed cutting-edge imaging technologies to observe internal exchange processes in real time.
One leading hypothesis proposes that circular muscles drive fluid movement: contraction narrows the lumen in the outer chamber, pushing fluid from the outer lacunae into the interior. When the muscles relax, the outer region expands and the cavities widen, allowing the mixed fluid to flow passively into the interna and then into the externa. This mechanism would also connect the externa with the interna. The team identified circular muscles at the base of the externa and along the supporting stalk, suggesting they may regulate fluid flow, either confining movement within the externa or preventing it from entering from the interna so the fluid can mix properly inside the interna first, according to Natalya Arbuzova of the university.
Earlier research in Russia explored themes around host-parasite dynamics and life cycle complexity, with implications for understanding how these crustaceans manipulate their hosts. The ongoing work in St. Petersburg highlights the importance of advanced imaging to reveal the hidden physiology of these fascinating organisms and to illuminate how their internal systems synchronize with external development to support larval growth.