A complex digestive system, similar to that of more complex organisms, discovered in the jellyfish Cotylorhiza tuberculata

An international research team, led by the University of Trieste and the National Institute of Oceanography and Applied Geophysics - OGS, has recently discovered a complex digestive system in the jellyfish Cotylorhiza tuberculata (also known as the "fried egg jellyfish", "egg-yolk jellyfish" or "Mediterranean jellyfish"). This system is similar to that found in more advanced organisms.

Among the partners involved in the study, which was published in the scientific journal PLOS One, there are Elettra-Sincrotrone Trieste S.C.p.A., the University of Milan, the Slovenian National Building and Civil Engineering Institute of Ljubljana, the University of Primorska, and the Piran Aquarium. The study was also made possible thanks to the PNRR National Biodiversity Future Centre (NBFC) project.

A sophisticated technique was adopted to analyse the anatomical structure of the jellyfish: a special resin was injected into the invertebrate's gastrovascular system. Once hardened, it produced a precise three-dimensional cast of all the internal channels. The resulting replica was then analysed using X-ray microtomography.

This methodology was instrumental in identifying proper channels that branch into the oral arms of the jellyfish, with each of these channels exhibiting a central constriction that functionally divides it into two semi-channels.

Further experimental evidence for this claim was provided by the injection of non-toxic dyes into the stomachs of live jellyfish to observe their internal flows. The results showed that these channels have a bidirectional circulation. Seawater, which is rich in prey, is "ingested" through the innermost openings of the arms. After passing through the stomach cavities for digestion, it flows down through the second (outer) semi-channel and is finally expelled through the more peripheral and distal openings of the oral arms.

It has long been held that the same opening serves as both mouth and anus in jellyfish. However, these new findings reveal the opposite: a continuous (through-gut) and specialised digestive tract in Cotylorhiza tuberculata, similar to that seen in more evolved animals. Some of the co-authors of this work had previously identified such features in another jellyfish species, Rhizostoma pulmo. This suggests that the digestive apparatus of all jellyfish species may be more complex than previously thought.