The single, very large antipatharian colony was observed at a depth of 525 m and was initially identified as a Leiopathes sp. based on external appearance including coloration. The colony was growing on the ridge of the seamount. Exact location and other detailed information are available upon reasonable request from the corresponding author. From our video analyses, the dimensions of the colony were estimated as a height of approximately 308 cm, a width of approximately 441 cm, and a basal trunk diameter of approximately 27.9 cm. On this Leiopathes colony we observed the spikefish Triacanthodes anomalus, Physiculus rhodopinnis, and the abyssal cutthroat eel Meadia abyssalis. We also confirmed the presence of numerous invertebrate species, including epibionts such as the king crab Paralomis kyushupalauensis, spider crab Samadinia sp., basket star Gorgonocephalus eucnemis, and the brittle star Ophiacanthidae gen. sp. We also observed sponge Hexactinellida spp., unidentified Cnidaria spp., squat lobster Eumunida spp., spiny deep-sea spider crab Cyrtomaia micronesica, and sea urchin Echinoidea spp. in the immediate area around the colony. Additionally, the channeled rockfish Setarches guentheri, roughy Hoplostethus sp., and deepwater cardinalfish Epigonus sp. were noted in the vicinity. While we observed other antipatharian colonies of similar branching pattern and form that were likely conspecific Leiopathes, we did not observe any other colony of similar extraordinarily large size during the KM20-10 research cruise.
Based on our molecular sequencing results (GenBank accession number XXXX for IgrN, XXXX for IgrW), we confirmed the identity of the colony as Leiopathes cf. glaberrima, as the sequences of both markers matched 100% with previously reported sequences of Leiopathes glaberrima and other Leiopathes species [GenBank Accession Numbers on ND5 sequences: KF013041 (Ruiz-Ramos et al. 2015), MT318846-8 (Barret et al. 2020), KF054669 (Brugler et al. 2013)].
Large anthozoan colonies often host a diverse assemblage of associated organisms, and the Leiopathes cf. glaberrima we observed is no exception, and can be considered an important ecosystem engineer on this seamount. As the Ministry of Environment of Japan has designated this oceanic region as a marine protected area from 2020, and given that data on deep-sea cnidarians in the northwestern Pacific can be considered very scarce (Reimer et al. 2021), it is clear that more data are needed to properly inform conservation efforts.
Growth rates of deep-sea anthozoans such as Leiopathes have been the focus of much research, with wide ranges reported based on the methodologies employed (Roark et al. 2006; 2009). Based on work using stable isotope analyses, Roark et al. (2009) estimated radial growth rates as between < 5 to 13 µm per year in Leiopathes glaberrima from Hawaii, while noting that their analyses only examined branches and the upper range, stating that lifespans may be much higher. Similar work for Leiopathes sp. from the Gulf of Mexico estimated growth rates of 8 to 22 µm per year (Prouty et al. 2011). Even with a relatively conservatively fast growth rate estimate of 20 µm per year, and if this large L. cf. glaberrima colony we observed has grown in radial fashion, the age of our observed colony can be estimated at approximately 7,000 years old (diameter = 27.9 cm, radius = 13.95 cm/20 µm per year). This is far older than previous longevity estimates of anthozoans, such as 2,100 years for Leiopathes in the Gulf of Mexico (Prouty et al. 2011), and 2,742 years for the Hawaiian gold coral Kulamanamana haumeaae and 4,265 years for L. glaberrima in Hawaii (both Roark et al. 2009). Among other marine organisms, clonal organism such as some seagrass species may have lifespans of hundreds to thousands of years (Reusch et al. 1999; Arnaud-Haond et al. 2007). Unfortunately, confirmation of the age of the current L. cf. glaberrima colony would require invasive sampling, but at the least given its extreme size we can consider it possible that this colony may be among the oldest living organisms on Earth. Such extreme lifespans counter the argument that these colonies represent renewable bioresources (Roark et al. 2009), and it is hoped this report spurs concerted conservation efforts for this seamount.