Although the 2018 symposium of the Japanese Society of Systematic Zoology, entitled ‘Recent progress and future potential in faunal studies of the submarine caves in the Ryukyu Islands’ was scheduled to be held on the occasion of the 89th annual meeting of the Zoological Society of Japan in Hokkaido, it was unfortunately canceled due to the earthquake ‘2018 Hokkaido Eastern Iburi Earthquake’. However, the five interesting articles were given by the special invited authors: 1) Yoshihisa Fujita ‘The fauna of decapod crustaceans in submarine caves of the Ryukyu Islands, with special reference to environmental factors in the caves’, 2) Yuji Ise ‘Preliminary report of submarine cave sponges in Shimoji Island, Miyako Islands, Okinawa’, 3) Mitchitaka Shimomura ‘Review of recent taxonomic studies of peracarid crustaceans in submarine and anchialine caves of the Ryukyu Islands’, 4) Masanori Okanishi and Yoshihisa Fujita ‘Ophiuroids from submarine caves of the Ryukyu Islands, Japan’, and 5) Akira Iguchi, Masaru Mizuyama, Takefumi Yorisue and Yoshihisa Fujita ‘Current situation and future issues of DNA studies of submarine caves of the Ryukyu Islands’. In this paper, we introduce the purpose and the contents of this special issue.

This paper provides a review of the past progress and future potential for research on the decapod crustacean fauna in submarine caves of the Ryukyu Islands. Until now, approximately 66 decapod crustacean species (shrimps, lobsters, and anomuran and brachyuran crabs) have been recorded from such caves at six islands in this chain, but recent surveys suggest that up to 100 species may be present. Many of them, however, appear to be accidental intruders or nocturnally emergent species, whereas “cavernicolous” species (animals that have characteristic morphological features such as reduced eyes and/or elongate thoracic limbs and spend their entire adult lives in the caves) are not so common. In addition, recent surveys have shown that the factors determining the distributional pattern of cavernicolous decapods among caves and their ecological partitioning within caves may be influenced by cave morphology and various aspects of the interior environment (light conditions, salinity, substratum, etc.). The urgency of preserving submarine cave environments in the Ryukyu Islands in the face of various threats is emphasized. In particular, 16 species of cavernicolous decapods found in submarine caves are already currently listed in the “Red List of Threatened Marine Species” issued by the Ministry of the Environment, Government of Japan.

Submarine caves are widely known for their unique biodiversity and peculiar ecosystem. In the submarine caves around the Ryukyu Islands, sponges are the most abundant and diverse organisms. These sponges are considered to play an important role for the coral reef ecosystem in terms of supplying food resources to other organisms. An overview of the features of submarine cave sponge fauna especially at the “Akuma-no-Yakata” cave in Shimoji Island, Miyako Islands, Okinawa is presented herein. Sponges in “Akuma-no-Yakata” are roughly categorized into three types based on their distribution pattern inside the cave. The first category is sponges living around entrance of the cave, which are conspecific with species attached on underside of overhangs and interstices of coral reef. These species are considered as species just favoring cryptic habitats. The second category is sponges living around the mid part of the cave, some of which are closely related to species found in the deep-sea fauna. Lastly, the third category is sponges living in innermost part of the cave, which possibly have strong tolerance to low salinity water. The environment of second and third category is affected by fresh water infiltrated through calcareous ground of the island, and can be regarded as anchialine habitat, where the sponge fauna is very unique.

Recent taxonomic studies on Peracarida in submarine and anchialine caves of the Ryukyu Islands were reviewed. Peracarida is a large suborder of malacostracan crustaceans including more than 24,000 species known form marine, brackish, fresh water and terrestrial habitats. They are defined by the presence of a brood pouch. A total of 9 species, belonging to 7 genera of 7 families have been recorded in submarine and anchialine caves of the Ryukyu Islands.

Ophiuroidea is the largest class of echinoderms and can be found in a great variety of marine habitats such as: in interstices in sponges and hard corals; on muddy ground; infaunally buried in sediments; under rocks; and on the surfaces of various animals such as soft corals. Very few species, however, have been recorded from submarine caves and only one anchialine endemic species, Amphicutis stygobita Pomory, Carpenter & Winter, 2011 is known from Bernier Cave, Bahamas, and three undescribed species were inferred by molecular phylogenetic analysis based on the specimens collected from Mexico. During the course of benthic faunal surveys in three submarine caves of the Ryukyu Islands, southwestern Japan, at 2016 to 2018, 19 ophiuroid species in of 13 genera from 8 families were recorded.

In the submarine caves of the Ryukyu Islands, the existence of many non-described species has been indicated in field surveys. To estimate the magnitude of biodiversity in the submarine caves, application of genetic analyses would provide a promising approach. Genetic analysis by DNA barcoding based on mitochondrial cytochrome c oxidase subunit I (COI) is informative for the study of submarine caves because the data of COI region have been accumulated in many marine animals. Also, in order to understand the formation process of communities of submarine caves, population genetic approaches would be useful to evaluate the connectivity patterns among submarine caves which are not continuously distributed along the Ryukyu Islands. In addition, utilization of high-throughput sequencing technology (e.g., environmental DNA (eDNA) analysis) would be also powerful for the ecological studies of submarine caves of the Ryukyu Islands. We discuss the current situation of genetic analyses in the submarine caves of the Ryukyu Islands and propose the prospects of genetic analyses for future studies.

We report parasitic mites from six species of land snails, which are listed in Prefectural Red Lists and/or Red Data Books authorized by Ministry of the Environment Government of Japan and Prefectural Governments in Japan; Riccardoella reaumuri Fain and van Goethem, 1986 were found from Euhadra brandtii sapporo Ijima, 1891 in Hokkaido, Oxyloma hirasei Pilsbry, 1901 in Ibaraki, and Phaeohelix submandarina Pilsbry, 1890 in Kagoshima. Riccardoella tokyoensis Waki and Shimano, 2018 were sampled from Tauphaedusa tau Boettger, 1877 in Ibaraki, Megalophaedusa surugensis Kuroda, 1936 in Kanagawa, and Megalophaedusa yagurai Kuroda, 1936 in Tokyo. Extinction risks of the two mites should be low because they are found in land snails which are currently common and do not appear in any categories of Red List and Red Data Book.

Meiobenthos is a term usually referring to microscopic benthic organisms which pass through a 1 mm mesh sieve and are retained on a 32–63 μm one. Meiobenthos occurs in any aquatic environment, shows high species diversity as well as high biomass, and often plays an important role in ecological and evolutionary studies. However, the species diversity of these animals in Japanese waters has been insufficiently investigated. Here we review several methodologies for collecting extant meiobenthos from the marine environment, including the method for sampling sediments in various environments, extracting meiobenthos from the sediment samples, and some tips for sorting, fixation, and observation of them.

Until recently, Actiniaria had been classified by the classification system organized by Carlgren in 1949. This classification was based on several traditional morphological features such as mesenterial arrangement, basal disc, acontium, and sphincter muscle. Although this classification system had been used for around 60 years, it was recently called into question by several studies utilizing molecular phylogenetic analyses. The most comprehensive phylogenetic analyses clarified that the Carlgren classification system did not reflect the actual phylogenetic relationships, and thus a new classification system from suborder to superfamily was established. At the same time, new common morphological features were proposed as the traditional morphological features of sea anemones in Carlgren’s classification system were proven to not be monophyletic. Here, we introduce the new classification system and propose Japanese names for all taxa higher than family.