A new species of annelid discovered by the University of Tokyo in the waters of Sado Island: Ramisyllis kingghidorahi

An international research team led by Professor Toru Miura of the Misaki Marine Biological Station of the Graduate School of Science, the University of Tokyo, and Professor Teresa Aguado of the University of Göttingen in Germany discovered a new species of annelid through a diving survey on Sado Island. It coexists within sponges and has a unique morphology with a branched trunk. Since the trunk branches, it was named Ramisyllis kingghidorahi after the fictional monster "King Ghidorah" with three heads that appears in the Godzilla movies. These findings were published in Organisms Diversity & Evolution.

Unique ecology

Bilaterian animals are bilateral on the cranio-caudal axis (body has two symmetrical sides), and it is extremely rare for the cranio-caudal axis to diverge during normal development. However, in the annelids of the Syllidae family, the body axis is bifurcated (branched). The most prominent is Syllis ramosa, which was reported in the waters of Asia around the end of the 19th century, and also within deep-sea sponges near Misaki (Miura City, Kanagawa Prefecture, Japan). In addition, in recent years, Ramisyllis multicaudata, a syllid with bifurcated body axes, was reported in the shallow waters near Darwin in northern Australia. This species differs from Syllis ramosa with regard to the host sponge species, habitat, and morphology. It shows a peculiar ecology, such as free-swimming breeding individuals (stolons) which detach from the organisms' multiple tails. Only these two species of syllids with bifurcated body axes have been reported. However, a new species of the genus Ramisyllis was discovered in the shallow waters around Sado Island in the Sea of Japan.

An international research team comprising the Misaki Marine Biological Station, the Niigata University Sado Marine Biological Station, the University of Göttingen, and the Autonomous University of Madrid conducted a diving survey in Shunegi, southern Sado Island, in October 2019, to collect the sponges inhabited by the annelids. The collected individuals were subjected to observation, histological observation, and in vivo immunostaining. DNA was extracted, several gene sequences were determined, and molecular phylogenetic analysis was performed. The entire mitochondrial genome sequence was determined. Based on these results, the species from Sado Island is closely related to R. multicaudata from Australia. However, it was strongly suggested to be another species and was described and named R. kingghidorahi. Numerous cilia were found in the gastrointestinal tract of the new species, especially at the distal branches and anus, suggesting the prompting of seawater influx by the tail. The remarkably divergent body of the new species has the breeding advantage of releasing breeding individuals from multiple tails and the possible role of incorporating seawater from the tails to increase nutrient uptake efficiency.

New knowledge

Animals with branching body patterns have been of great developmental interest. However, S. ramosa, which was discovered off the coast of Japan, has not been seen recently, and thus, its study stagnated. The fact that R. kingghidorahi was collected from relatively accessible waters near Japan, and their morphology was described in detail is vital for clarifying the morphologies and ecology of various animal species in the future. R. kingghidorahi has a very paradoxical morphology compared with common animals. Unraveling its ecology and physiological functions is likely to overturn a good deal of previously accepted biological knowledge. The biological characteristics of the new species remain unclear, and there are high expectations for new biological knowledge to be clarified in the future.