Masanori OKANISHI (Project Assistant Professor, Misaki Marine Biological Station)

Hisanori KOUZUKA (Technical Specialist Staff, Misaki Marine Biological Station)

Key points of the presentation

• A new species of spider starfish was discovered at a depth of 90-140 m off the Miura Peninsula, Kanagawa Prefecture, Japan.
• Morphological study of this species revealed that it is a spider starfish of the genus Conidia, of which only one case has been recorded in the past 90 years, and that it has a juvenile-bearing ecology.
• This study once again demonstrated the high potential biodiversity of Sagami Bay, which has been studied for more than 100 years and is home to an unknown species.

Summary of Presentation

A research group led by Assistant Professor Masanori Okanishi and Technical Staff of the Misaki Marine Biological Station, Graduate School of Science, The University of Tokyo, collected organisms by bottom trawling in the 90-140 m depth zone of Sagami Bay, where the station is located. As a result, they recognized a new species of spider starfish, Ophiodelos okayoshitakai (standard Japanese name: Konjiki-komochi spider starfish), with a body length of approximately 1 cm.

The genus Ophiodelos has only been described in one case in the world, in 1930, when Ophiodelos insignis, collected from a depth of 300 m in Indonesia, was described as a new species. Ophiodelos insignis is a species that had not been studied taxonomically because it was less than 2 cm in length and had only been recorded from deep water. Ophiodelos insignis is the first report of the genus Ophiodelos in 90 years.

The results of this study are the fruit of the laboratory's continuous sampling activities in Sagami Bay, with a focus on even the smallest organisms.

Contents of Presentation

Research Background
Spider starfishes (Spider Starfishes) are a group of the "Echinodermata" ^{(Note 1)}, the same as starfishes and sea urchins, with about 2100 species known worldwide and about 340 species from Japan (Figure 1).

Figure 1: Various spider starfish species obtained from Sagami Bay.

Although their basic structure is similar to that of starfish, they are distinguished from starfish because they do not have a groove on the mouth side of their arms called a gular groove, as is the case in starfish. They also have slender arms that are generally longer than those of starfish, and they can dexterously wiggle their flexible arms to inhabit a variety of marine environments, including reefs, sand, mud, and coral reefs.

Although Sagami Bay is a familiar sea easily accessible from the suburbs of Tokyo, it is little known that it is a sea area with greater biodiversity than other sea areas. In the center of Sagami Bay, there is a steep submarine trough, called the Sagami Bay Trough, which is more than 1,000 m deep. This makes it possible to reach deep-sea areas in Sagami Bay within a short distance from land. The topography of the Sagami Bay seafloor is also complex due to the collision of continental plates. In addition, the influence of cold currents from the north and warm currents from the south creates an environment that is home to an extremely rich variety of organisms. In particular, the organisms that live at depths of more than 100 m have attracted much attention since the 1870s, when a variety of species, including many new species, were discovered.

Taxonomic studies on spider starfish species in Sagami Bay have been actively conducted, starting with the work of Dr. Hikoshichiro Matsumoto in 1917, and following the publication of "Sagami Bay Starfish" by Dr. Seiichi Irimura in 1982, approximately 150 species have been documented to date. Considering that there are approximately 340 species of spider starfish known in Japan, this means that nearly half of the total number of spider starfish, or about 45%, live in a single bay. However, even in recent years, the diversity of spider starfish species remains to be elucidated, with Ophiacantha kokusai being described as a new species in 2015.

The genus of spider starfish treated in this study, Conidia conidia, is a rare genus with only one known species, Ophiodelos insignis, recorded in 1930 from a depth of 300 m in Indonesia. Because Ophiodelos insignis is less than 2 cm in length and is the only known specimen in the world, detailed morphological observations have not been conducted, and its taxonomic status remains unclear.

Details of Research
A research group led by Project Assistant Professor Masanori Okanishi and Technical Specialist Hisanori Kozuka at the Misaki Marine Biological Station, Graduate School of Science, The University of Tokyo, conducted a bottom trawl survey in Sagami Bay by the research vessel "Rinkai Maru" (Figure 2) and collected marine benthic organisms. As a result, an individual measuring approximately 1 cm in length was recognized as a member of the genus Conidia (Fig. 3).

Figure 2: Bottom trawl survey in Sagami Bay by Hisanori Kozuka (back center of screen) and Masanori Okanishi (right side of screen), authors of this paper.

Figure 3: Image of Ophiodelos okayoshitakai sp. nov. described as a new species in Research Student. A small juvenile is being released from the parental specimen.

Based on detailed observations of the morphology of the final two individuals, we determined that this species is a new species that is morphologically distinct from the known Ophiodelos insignis and named it Ophiodelos okayoshitakai. The name " okayoshitakai," which is part of the species name, is named after Professor Yoshitaka Oka, who served as Director of the Misaki Marine Biological Station from 2017 to 2021. The partial DNA sequence of this species was decoded and compared with other spider starfish species, and it was found that this species belongs to at least the suborder Ophiacanthina. However, the family affiliation, which is a lower level of the suborder, is still unclear, and further analysis based on more DNA sequences is needed. During our observations of this species, we observed that it released more than 10 small juveniles from its body (Fig. 3). This suggests that this species is a "nursery species" that retains its young inside its body. The standard Japanese name for this species is "Konjiki Komochikumochikumohitate," in reference to its bright yellow body color.

Social Significance and Future Plans
Research on spider starfish in Sagami Bay began in earnest in 1917, and to date, approximately 150 species have been recorded. Even in recent years, the discovery of a new species in 2015 indicates that there is much room for clarification in the study of their species diversity. In such a situation, there are only a few cases in which attention was paid even to individuals as small as 1 cm in length, as in this study, so there is a high possibility that further new species will be discovered in Sagami Bay in the future by focusing on spider starfish of this size as well. Spider starfish have a large number of species and individuals among echinoderms, and in recent years, Project Assistant Professor Okanishi's research team has reported a number of fossil records. Therefore, it is expected that focusing on spider starfish will bring a new perspective to the clarification of the environmental changes in Paleo-Sagami Bay. In the future, we believe that the diversity of spider starfish in Japan can be clarified in more detail by examining a wide range of spider starfish, including minute species that inhabit the seafloor not only in Sagami Bay, but also in other parts of Japan. This study provides the basic information for this purpose.

Journal

Journal name	Zoological Science
Title of paper	Description of a New Brooding Species of Ophiodelos (Echinodermata: Ophiuroidea) from Japan
Author(s)	Masanori Okanishi*, Hisanori Kohtsuka
DOI Number	10.2108/zs200101
URL	https://doi.org/10.2108/zs200101

Glossary

Note 1: Echinoderms

A group of marine animals that includes sea urchins, sea cucumbers, starfish, etc. As a rule, they have star-shaped bodies. ↑ (up)