A new species of mysterious jellyfish belonging to a new subfamily discovered in a submarine volcanic caldera. -Environmental impact assessments for seafloor resource development yields new discoveries-

1. Key points of the presentation

• A new species of jellyfish was discovered and named 'St. George's Cross Medusa (scientific name: Santjordia pagesi)' and described as a new species. Based on the results of morphological and genetic analyses, it was classified as a new subfamily^※1 , a higher taxonomic group than genus, as it has many different characteristics from jellyfish species considered related to it.

• While hundreds of hours of research have been conducted in the ocean twilight zone in various areas, this species has only been found inside the Sumisu Caldera in the Ogasawara Islands, where submarine hydrothermal deposits are known to exist, requiring further research to investigate its life history and possible endemicity.

Fig. 1: 'St. George's Cross Medusa', a new species, new genus and new subfamily of jellyfish (photographed in an aquarium after collection).

2. Overview

A research team headed by Dhugal Lindsay from the Institute for Extra-cutting-edge Science and Technology Avant-garde Research (X-star), Japan Agency for Marine-Earth Science and Technology (President: Hiroyuki Yamato, hereafter referred to as 'JAMSTEC'), discovered and described a new species, the 'St. George's Cross Medusa' (scientific name: Santjordia pagesi) (Fig. 1), which is classified as a new subfamily, a higher taxonomic group than that of a new genus.

The jellyfish was first discovered and collected by the Remotely Operated Vehicle (ROV) Hyperdolphin during a dive in the Sumisu Caldera, a submarine caldera in the Ogasawara Islands, in 2002 but was not encountered again until 2020, when another individual of the same species was observed and described during a research dive in the Sumisu Caldera by the ROV KM-ROV.

Genetic and morphological analyses determined that it differed from all presently described species, being assignable to the same family as the common moon jellyfish (Ulmaridae), but also found that the Ulmaridae needs to be divided into several distinct and different families. However, since many species in this family have not yet been sampled for genetic analyses, it was decided not to undertake this taxonomic reassignment at the present time. For this reason, a conservative approach was taken and the new medusa Santjordia pagesi was classified into a new subfamily, a higher taxonomic rank than that of a new genus.

The 'St. George's Cross Medusa' can be considered a rare species, as it has only been found inside the Sumisu caldera in the Ogasawara Islands, where submarine hydrothermal polymetallic sulphide deposits are known to exist. It is likely that there is a polyp^※2 life-history stage that attaches to the seabed, and it is possible that the polyps attach to rocks specifically associated with exposed seafloor mineral resources. Further research on its life history and possible endemicity could contribute to environmental impact assessments in the development of seabed resources in the Sumisu caldera. Considering that this species differs at such a high taxonomic level from presently described species, it may possess novel venoms and be a useful genetic resource. Since it has only been observed in the Sumisu Caldera, further research and studies are warranted.

The results will be published in 'Zootaxa' on 20t of November(JST).

3. Background

In the face of global climate change, minerals such as rare earths, nickel and cobalt are essential for harnessing and storing green energy generated by wind and solar power. Seafloor mineral resources are attracting attention as a means of securing these useful materials. JAMSTEC has been working on various projects with the aim of establishing environmental impact assessment and environmental monitoring technologies necessary for the sustainable use of seabed mineral resources. Focusing on ecosystems in submarine volcanic calderas, which are closed environments that are considered to be particularly vulnerable to the mining of seabed resources, JAMSTEC has been conducting a series of surveys in various calderas.

One of the target areas of these surveys is the submarine caldera known as the Sumisu Caldera (Fig. 2). The Sumisu caldera is located on a volcanic front in the central Izu-Ogasawara area (approximately 460km south of Tokyo) and has a diameter of approximately 10 km (in the east-west direction). JAMSTEC has been attempting to conduct comparative studies inside and outside the caldera to contribute to the establishment of environmental impact assessment and environmental monitoring techniques.

Fig. 2: Sumisu Caldera, located on a volcanic front in the central Izu-Ogasawara area (about 460km south of Tokyo). Environmental impact assessment comparison studies have been carried out inside and outside the caldera.

4. Results

In 2002, when the ROV Hyperdolphin expedition conducted a dive survey inside the Sumisu Caldera, a submarine caldera located in the Ogasawara Islands, a curious jellyfish was discovered and collected. A morphotaxonomic studies suggested that this jellyfish was a completely new and unique species that had never before been reported, but after this specimen was collected, it was never encountered again for almost 20 years. The description of a new species based on only one individual can be problematic due to the possibility that the specimen could be a mutant or otherwise non-representative of the majority of individuals of that species. Its formal description was therefore postponed until a second individual of this same jellyfish species was successfully observed and collected in 2020, during a research expedition to the same Sumisu caldera by the ROV KM-ROV.

Fig. 3 Deep-sea in situ image of the new species, new genus and new subfamily of jellyfish, the 'St. George's Cross Medusa' (ROV Hyperdolphin dive No. 84).

Genetic analysis revealed the no closely related species had been sequenced with the sequences archived in the global public sequence database called Genbank. During the almost 20 years since the species' first discovery, we have collected and genetically analysed possible relatives of this jellyfish species in many parts of the world's oceans, including the Antarctic Ocean, the Caribbean and the Japan Trench. This allowed us to determine that its closest relatives are the large deep-sea jellyfish species Tiburonia granrojo (Big red), Deepstaria enigmatica (the Deepstar medusa) and the giant viviparous medusa Stygiomedusa gigantea, all of which are characterised by having only mouth-arms surrounding the mouth, without tentacles fringing the umbrella. The new jellyfish species discovered in this study does not have tentacles on the umbrella margin, but has tentacles under the umbrella between the umbrella margin and the mouth-arms. The next most closely related jellyfish taxon cannot be conclusively determined from genetic analyses of the jellyfish species collected so far – only that it is a member of the family Ulmaridae. Our study also found that the family Ulmaridae, as currently defined, needs to be newly divided into several different families, but this taxonomic revision should be postponed because many species have not yet been sampled for genetic analyses. Santjordia pagesi should therefore, for the time being, be classified as a new species and new genus, and be placed into a new subfamily.

Fig. 4 Phylogeny based on genetic analyses. The new genus and subfamily of jellyfish, 'St. George's Cross Medusa' (red) and closely related species (blue).

This species has a bright red stomach, and when viewed from the top of the umbrella, the stomach resembles the shape of a cross, hence the Latin genus name Santjordia^※3, which is based on the image of the red cross - the St. George's Cross. The Japanese name is "sekijuujikurage" (Red Cross Medusa). The species ephiphet in Latin is "pagesi", in honour of Dr. Francesc Pagès, a jellyfish taxonomist from Barcelona who was a mentor of Dr. Lindsay during his stay there, but who passed away while still young.

Glossary

※3

Saint George
Christian saint said to have been a dragon-slayer and the patron saint of Catalonia, a region including the city of Barcelona. In the Catalan language, he is known as Saint Jordi.

（Fig：St. George's Cross）

5. Future prospects

While hundreds of hours of midwater surveys have been conducted by ROVs and submersibles in various areas of the ocean, this species has only been found inside the Sumisu caldera in the Ogasawara Islands, where submarine hydrothermal deposits are known to exist, thereby making it a truly rare species. It is thought that this species probably has a polyp generation that attaches to the seabed. It is possible that the polyps live attached to rocks covered by volcanic submarine hydrothermal mineral deposits (polymetallic sulphides). If the polyp generation can be found, experiments can be carried out to determine whether the attachment substrate is specifically selected. This is necessary baseline data for considering the sustainable development of submarine resources in the Sumisu caldera in the future to assess the environmental impacts of such development. In addition, the 'St. George's Cross Medusa' is highly likely to possess a new cnidarian venom, considering that it is so different from other species, and both further research on this species as a genetic resource, and research on the midwater communities of submarine calderas is necessary.

※Reference video
　https://youtu.be/4ij1tiXez9E