Lake Biwa is the largest and oldest freshwater lake in Japan, with a history of about four million years.
The lake is home to a rich biodiversity of aquatic organisms, including many endemic species that have evolved in isolation from other regions.
Among them are the freshwater snails of the genus Semisulcospira, which have undergone adaptive radiation and diversification in response to different environmental conditions in the lake.
However, the taxonomy and systematics of these snails have been unclear and confusing, due to their complex morphological and genetic variation.
A recent study by researchers from Kyoto University and other institutions has resolved some of these problems by revising the classification and description of seven species of Semisulcospira in Lake Biwa and discovering two new species that were previously unrecognized.
The Challenges of Identifying Semisulcospira Snails
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Semisulcospira snails are small (about one to two centimeters in shell length) and have a conical or cylindrical shell with various patterns of coloration, banding, and sculpture, as per Phys.org.
They inhabit different habitats in the lake, such as rocky shores, sandy bottoms, submerged vegetation, and river mouths.
They feed on algae, detritus, and organic matter, and are preyed upon by fish, birds, and other animals.
The genus Semisulcospira was first described in 1865 by German malacologist Eduard von Martens, who named nine species from Japan based on their shell morphology.
However, subsequent studies revealed that some of these species were not monophyletic (i.e., they did not share a common ancestor) and that there were more species than originally recognized.
For example, Semisulcospira decipiens, which was named in 1883 by French zoologist Alphonse Milne-Edwards, was found to be a complex of four closely related species: S. decipiens sensu stricto, S. multigranosa, S. elongate, and S. cryptica.
The latter two were newly described in 2022 by Naoto Sawada and Yusuke Fuke from Kyoto University.
The identification of Semisulcospira snails is challenging because their shell morphology can vary considerably within and between populations, depending on environmental factors, such as temperature, water chemistry, substrate type, and predation pressure.
Moreover, their genetic variation can be incongruent with their morphological variation, due to hybridization, introgression, and incomplete lineage sorting.
Therefore, a comprehensive approach that combines morphological, molecular, and ecological data is needed to clarify the diversity and evolution of these snails.
The study was conducted by researchers from Kyoto University, University College London, the University of Innsbruck, the University of Helsinki, and the University of Quebec at Montreal.
The researchers collected specimens of Semisulcospira snails from Lake Biwa and its tributaries between 2016 and 2019.
They examined their shell morphology using scanning electron microscopy and geometric morphometrics.
They also analyzed their genetic variation using mitochondrial DNA sequences and microsatellite markers, then compared their findings with previous studies and historical specimens deposited in museums.
The results showed that there are seven species of Semisulcospira snails in Lake Biwa, belonging to two groups: the niponica group and the nakasekoae group.
The niponica group consists of four species: S. niponica (the type species of the genus), S. decipiens sensu stricto (the true S. decipiens), S. elongata (a new species) and S. cryptica (another new species).
The nakasekoae group consists of three species: S. nakasekoae (the most common species in the lake), S. multigranosa (a species that was misidentified as S. decipiens for 140 years) and S. reticulata (a rare species with a reticulated shell pattern).
The study also revealed the distribution patterns, habitat preferences, and ecological characteristics of each species.
For example, S. niponica is widely distributed in the lake but prefers rocky habitats with high water flow; S. decipiens sensu stricto is restricted to the northern part of the lake but can tolerate various substrates; S. elongata is endemic to the southern part of the lake and prefers sandy habitats with low water flow; S. cryptica is also endemic to the southern part of the lake but prefers vegetated habitats with moderate water flow; S. nakasekoae is widely distributed in the lake and can adapt to different habitats; S. multigranosa is mainly found in the eastern part of the lake and prefers sandy habitats with high water flow; and S. reticulata is rare and patchily distributed in the lake and prefers rocky habitats with low water flow.
The study also provided insights into the evolutionary history and relationships of these snails.
The researchers estimated that the genus Semisulcospira diverged from its closest relatives about 10 million years ago and that the two groups of Lake Biwa snails diverged from each other about four million years ago, coinciding with the formation of the lake.
The researchers also inferred that the species within each group diverged from each other about one million years ago and that they have undergone adaptive radiation and ecological divergence in response to different environmental conditions in the lake.
The study concluded that there are two new snail species in Lake Biwa that have not been described or named before and that there are seven species of Semisulcospira snails in total in the lake.
The study also revised the classification and description of these snails based on morphological, molecular, and ecological data, and provided a key for their identification.
The study also updated our knowledge of the diversity, distribution, ecology, and evolution of these snails, and highlighted their importance as model organisms for studying adaptive radiation and speciation in freshwater systems.
