The 2025 Symposium of the Japanese Society of Systematic Zoology was held in Nagoya on the evening of 5 September 2025 as a satellite meeting of the 96th Annual Meeting of the Zoological Society of Japan. The symposium aimed to share recent case studies that highlight the challenges of exploring of deep-sea ecosystems that remain poorly surveyed due to extreme pressure, darkness, complex topography, and logistical constraints. Because approximately 98% of the ocean lies deeper than 200 m, much of Earth’s biosphere is effectively deep sea, where novel habitats and biota continue to be discovered as access technologies advance, including crewed and uncrewed submersible platforms.

This paper provides an overview of current knowledge on meiobenthic communities on seamounts and presents taxonomic findings from kinorhynch samples collected at eight seamounts and adjacent deep-sea sites in the Arctic, Atlantic, and Mediterranean. Meiobenthos represent important but still insufficiently explored members of marine biodiversity, and seamounts present similar research gaps despite their ecological importance. From the examined kinorhynch samples, at least 23 species were identified, including 21 species that represent new taxa, eight of which have already been formally described. Of the 23 species identified, more than half were restricted to a single seamount, suggesting strong local endemism and limited exchange among sites. Nevertheless, several species occurred across multiple seamounts or between seamounts and coastal or deep-sea habitats, suggesting that seamounts may act as occasional stepping points for dispersal. One species displayed an exceptionally broad distribution across deep-sea regions, raising questions about long-distance connectivity and/or the potential presence of cryptic diversity. Taken together, these findings highlight the need for broader, integrative surveys across seamount systems to clarify how these geomorphological features shape meiobenthic diversity patterns and dispersal dynamics.

The hadal zone (below 6,000 m or 6,500 m) is a poorly understood, extreme deep-sea environment. Knowledge of anthozoans (Cnidaria) fauna in hadal zone is extremely limited. Only species from the Hexacorallia, primarily Actiniaria (sea anemones) and Antipatharia (black corals), have been reported exceeding the depth of 6,500 m. Actiniaria has the most records, including endemic species. Research has historically been challenging, but recent, modern expeditions utilizing various sampling techniques are now accumulating new specimens, be expected to uncover the true diversity of hadal anthozoans.

This paper introduces the development and field applications of the mini deep-sea ROV “TripodFinder2 (TPF2),” designed to achieve 1,000 m-class dives while remaining lightweight and operable by hand from small vessels. Conventional ROV surveys often require large, high-cost systems, limiting opportunities for scientific exploration. TPF2 adopts a simplified, mission-focused design with onboard batteries, enabling low-cost deployment and efficient operation across diverse environments. Using TPF2, independent deep-sea surveys with aquariums, midwater biological observations, and adventurous missions such as deep-sea cave investigations were conducted. These results demonstrate that the Mini ROV can reduce psychological and economic barriers to deep-sea research and expand access to previously unexplored regions. Future developments will focus on higher-resolution imaging, new sampling tools, and the integration of measurement instruments to further enhance the capabilities of Mini ROV.

Caves, whether terrestrial or submerged, are widely regarded as potential refugia for relict lineages often referred to as “living fossils.” In contrast, deep-sea caves have remained almost entirely unexplored, owing to the absence of established survey methods and limited knowledge of their distribution. To address these challenges, the Ocean Shot Research Grant Program “D-ARK” (Deep-sea Archaic Refugia in Karst) initiated a project to develop specialized technologies for deep-sea cave exploration and to document their biodiversity. Surveys conducted around the Daito Islands have led to the identification of several deep-sea caves and the development of new observational tools, including a deep-sea endoscope and a miniature underwater drone deployable from a large ROV. These instruments enabled the first direct investigations of deep-sea cave interiors, resulting in the discovery of several new species, as well as the documentation of additional rare taxa. These findings provide the first biological baseline for deep-sea cave ecosystems and highlight their potential to harbor previously unrecognized biodiversity. Further surveys planned for the coming year are expected to refine our understanding of these ecosystems and to expand knowledge of their fauna.

This article presents a Japanese translation of the amendment of Article 8 of the 4th edition of the International Code of Zoological Nomenclature by Declaration 46, regarding the nomenclatural status of retracted works. It also presents the background of the amendment, which was not fully mentioned in the Declaration. We discuss the following issues: retraction for scientific and ethical reasons, new scientific names based on fabricated specimens, self-organization by academic communities in terms of name usage, and interpretation of the consequences of retraction.

Larvae of the cartilaginous fish cestode Yamaguticestus Caira, Bueno & Jensen, 2021 were detected in the intestine of cirrate octopod Exsuperoteuthis depressa (Ijima & Ikeda, 1895) sampled in Suruga Bay, Japan. The larvae were identified to genus level based on morphological and molecular analyses. This is the first report of helminth parasites from the host octopus E. depressa. Since the larvae were found in the intestine, the host octopus may have harbored these larvae via predation.

Five specimens (three males and two females) of an argulid branchiuran were collected from the body surface of bitterling Rhodeus albomarginatus Li and Arai, 2014 (Cypriniformes: Acheilognathidae) in an irrigation pond in Hyogo Prefecture, Japan, on 4 September 2024. These specimens are identified as Argulus japonicus Thiele, 1900 and briefly described. They are characterized by a single plumose seta on the posterior margin of each coxa of the first legs and 43–56 supporting rods per first maxilla. The second leg coxa of the male specimen has two protrusions adorned with small spines on the ventroposterior margin, but that coxa of the female specimen bears no such protuberances. Rhodeus albomarginatus was originally described in Anhui Province, China, in 2014 and very recently reported as an invasive species from the irrigation pond surveyed in this study. In Hyogo Prefecture, A. japonicus has been reported from wild fishes, such as common carp Cyprinus carpio Linnaeus, 1758, and it is inferred that this parasite utilizes those fishes as its hosts and infected individuals of R. albomarginatus in the irrigation pond. This study also suggests that A. japonicus was not introduced along with R. albomarginatus from China into Japan.

A single large specimen [505 mm standard length (SL)] of the Mud Grouper Epinephelus randalli Hoshino, Senou and Nguyen, 2024 (Perciformes: Epinephelidae), previously recorded only from the South China Sea, was obtained from western Kyushu, Japan. The specimen represents the first record of E. randalli from Japanese waters and the northernmost record for the species. A detailed description of the specimen and comparison with large specimens of the Longtooth Grouper Epinephelus bruneus Bloch, 1793, previously confused with E. randalli, are provided. Although E. randalli was characterized by having six transverse brown bars on the body and four brown bands on the head, our study revealed that the markings become indistinct in larger individuals. Additionally, a single old specimen (203.2 mm SL) of E. randalli from the East China Sea was also confirmed, during this study.