A research team including Associate Professor Yuuki Kawabata, Graduate Students Junya Taniguchi, Tsubasa Inoue, and Kano Kohara from the Graduate School of Integrated Science and Technology (Faculty of Fisheries) at Nagasaki University has revealed that the "sideways walking" locomotion observed in many crabs most likely originated from a common ancestor approximately 200 million years ago. This insight is crucial for understanding how animal locomotion evolved and contributed to species diversification. The findings were published in eLife.
The research team acquired movement data from a total of 50 crab species. Choosing one individual from each species, they video-recorded their behavior for 10 minutes inside a circular tank that mimicked their natural habitat.
Based on the classification of locomotion from this data, 35 species were determined to walk sideways, while 15 species walked forward. The data was integrated with the latest phylogenetic analysis based on 10 gene sequences from 344 species. After organizing the analysis units into genus, family, and superfamily levels, the researchers reconstructed their evolutionary history.
The results suggested that "sideways walking" evolved only once from a single ancestor that walked forward, and has since been maintained without being significantly lost.
The origin of sideways walking is estimated to be approximately 200 million years ago, placing it in the Early Jurassic period following the end-Triassic mass extinction. Sideways walking is thought to offer advantages related to escaping from predators, such as the ability to move quickly to either the left or right and making the direction of escape difficult to predict.
On the other hand, during this era, the breakup of the supercontinent Pangaea and the expansion of shallow seas created new environments available to crabs, potentially setting up conditions that favored diversification. Both this evolution of locomotion and the environmental changes of the time may have been related to the diversification of crabs.
This study provides an important clue to clarifying how behavioral traits were acquired and retained during the evolutionary process, contributing to the diversification and prosperity of organisms. In the future, it is expected that the relationship between sideways walking and diversification will be further verified by combining fossil data with experiments investigating the specific advantages of sideways walking.
Kawabata stated: "This study began in 2020, and the three co-first authors, Junya Taniguchi, Tsubasa Inoue, and Kano Kohara, took the lead and spent a truly long time acquiring and analyzing the data. Including not only the data finally used in the paper but also the data used for preliminary experiments and those that couldn't be used for analysis, they recorded and analyzed the behavior of at least 100 or more individual crabs. To collect data on such a diverse range of crabs, the cooperation of many people was indispensable."
"Director Hirai of the Susami Crustacean Aquarium, assisted us in collecting various crab samples. In addition, Taniguchi and Kohara stayed at National Kaohsiung University of Science and Technology, a partner university of Nagasaki University, for one year as double-degree students, where they acquired data on many crabs in Taiwan under Professor Huang, who specializes in crab taxonomy. During the process of analysis and manuscript preparation, we also received tremendous cooperation from Nobuaki Mizumoto at Auburn University in the United States and Fumio Takeshita at the Kitakyushu Museum of Natural History and Human History."
"Crabs walking sideways might seem obvious at first glance, but it is an extremely rare mode of locomotion across the entire animal kingdom. I am delighted to have been able to publish these results, which truly excite me as well regarding when and how such a locomotion mode evolved, together with the students and joint researchers."
Journal Information
Publication: eLife
Title: Evolution of sideways locomotion in crabs
DOI: 10.7554/eLife.110015.1
This article has been translated by JST with permission from The Science News Ltd. (https://sci-news.co.jp/). Unauthorized reproduction of the article and photographs is prohibited.