Kyoto University led group announce discovery of a new Semisulcospira species in Lake Biwa

A research group led by Doctoral Student Naoto Sawada (at the time of research) and Associate Professor Takafumi Nakano of the Graduate School of Science at Kyoto University, JSPS Postdoctoral Fellow Yusuke Fuke of the National Institute of Genetics, Associate Professor Osamu Miura of the Faculty of Agriculture and Marine Science at Kochi University and Professor Haruhiko Toyohara (at the time of research) of the Faculty of Agriculture at Setsunan University has discovered a new species, Semisulcospira nishimurai, from the sandy mud bottoms of Lake Biwa. The new species was found through genetic and morphological analysis of the genus Semisulcospira to redefine the taxonomic position of S. reticulata. The work was published online in Evolutionary Systematics.

Semisulcospira, an ovoviviparous freshwater snail genus, has undergone extensive species diversification in Lake Biwa, and 18 of the 21 species found in Japan are endemic to the Lake Biwa water system. These endemic species consist of two closely related species groups that have diversified independently within the lake. A recent review of classification has revealed that both species groups show similar diversification aspects in the rocky and sandy shores of Lake Biwa. S. elongata and S. davisi in the S. nakasekoae-group are known to have expanded their habitats to offshore sandy mud and mud bottoms, respectively. Meanwhile, the species diversity of the S. niponica-group in sandy mud and mud bottoms has not been fully evaluated, while S. reticulata has been known to occur in deep offshore areas. Moreover, the two species groups have not been compared in terms of diversification.

As a result, the research group investigated the taxonomic accounts of S. reticulata and overlooked offshore species through genetic analysis combined with morphological comparisons with a focus on the S. niponica-group in sandy mud and mud bottoms. S. niponica-group samples were collected from seven sites in Lake Biwa. Genetic analysis based on single nucleotide polymorphisms obtained using the MIG-seq method was performed in combination with data from previous studies. As a result, S. reticulata was clearly distinguished from other species in the S. niponica-group, and a previously unknown population was recognized from specimens collected from the sandy mud bottom.

Next, to identify morphological differences among this population, S. reticulata, and their relatives, the research group conducted a discriminant analysis using machine learning (Random Forest algorithm). They found that the angle and sculpture morphology of the teleoconch and the size of the protoconch are useful for species identification. Further comparison of radula morphology revealed that one population identified by genetic analysis and S. reticulata are characterized by more pointed tip of radulae compared to closely related species. Because the sandy mud population found was genetically and morphologically distinct from other species, they considered it a previously undescribed species with no scientific name and described it as a new species, S. nishimurai.

S. nishimurai prefers the second deepest areas in the S. niponica-group after S. reticulata and is characterized by a medium-sized narrow (small angle) teleoconch with a low elongation rate, about 20 granulated longitudinal ribs on the teleoconch, a small (about 2 mm) granulated protoconch in beige, and a pointed tip of radulae. S. nishimurai (Azai Kawanina in Japanese) was named after the Azai clan, a line of feudal lords (daimyo) in northern Omi (northeastern part of current Shiga Prefecture) during the Sengoku period because the known distribution area of Semisulcospira species roughly corresponds to the territory of the Azai clan at its peak. The findings of this study confirm that S. reticulata is an independent species of the S. niponica-group distributed in the offshore muddy bottoms of Lake Biwa and reveal that the species diversity of the genus Semisulcospira in the sandy mud bottoms has been underestimated.

Sawada said, "Lake Biwa is a rare place in the world where many Semisulcospira species can be observed within a small area. The genus Semisulcospira, which has undergone extensive species diversification, presents interesting questions regarding species differentiation and interspecies interactions. Based on the results of this study, I would like to continue to challenge the clarification of the evolutionary history of the genus Semisulcospira from various perspectives."

Journal Information
Publication: Evolutionary Systematics
Title: Redescription of Semisulcospira reticulata (Mollusca, Semisulcospiridae) with description of a new species from Lake Biwa, Japan
DOI: 10.3897/evolsyst.8.124491