Recently NHK news reported up the following:
カニの「横歩き」は約2億年前に始まったか 長崎大など研究
2026年4月29日4:06
サイエンス
カニが「横歩き」を始めたのは、およそ2億年前だった可能性が高いとする研究結果を長崎大学などのグループが発表しました。動物の移動様式がどのように進化してきたのかを理解することにつながる成果だとしています。
長崎大学などの研究グループは自然の環境に近づけた水槽の中で50種のカニの行動を10分間ずつ撮影して観察し、「横歩き」か「前歩き」かを調べました。
この観察データとカニが祖先からどのように枝分かれして進化してきたかを示す最新の「系統樹」を照らし合わせたところ、カニの「横歩き」は、およそ2億年前にもともと「前歩き」をしていた祖先が「横歩き」に進化し、その後、多くの種に受け継がれてきた可能性が高いことがわかったということです。
カニが「横歩き」を始めたとされるおよそ2億年前は大陸の分裂で浅い海が広がり、新しい環境でカニが多様化しやすい条件が整っていたと考えられ、左右どちらにも素早く動くことができ、移動する方向が予測されにくい「横歩き」の利点が生存に有利に働いたのではないかとしています。
長崎大学大学院総合生産科学研究科の河端雄毅 准教授は「『横歩き』はすごくヘンテコな行動で、動物界を見渡してもほかにこの歩き方をする動物はほとんどいない。この革命的な移動様式が生物の繁栄にどう関わってくるのかを理解することにつながる成果だと思う」と話していました。
Translation
Crabs'
"Side walking" Might Have Begun Around 200 million Years Ago:
Research by Nagasaki University and Others
April 29,
2026, 4:06 AM
Science
A research
group from Nagasaki University and other institutions had published findings
suggesting that crabs likely began "side walking" approximately 200
million years ago. This finding was expected to contribute to understanding how
animal locomotion patterns evolved.
The research group from Nagasaki University and other institutions filmed and observed the behavior of 50 species of crabs for 10 minutes each in a tank simulating their natural environment to determine whether they were "side walking" or "forward walking."
By comparing this observational data with the latest "phylogenetic trees" showing how crabs had branched out and evolved from their ancestors, the researchers found that it was highly likely that crabs' "side walking" evolved approximately 200 million years ago from ancestors that originally "forward walked," and that this behavior was subsequently passed down to many species.
It was
believed that around 200 million years ago, when crabs were thought to have
begun "sideways walking," the continents split, creating shallow seas
and providing ideal conditions for crab diversification in this new
environment. The ability to move quickly to either the left or right, and the
unpredictable nature of their movement through "sideways walking," might
have been advantageous for their survival.
Associate
Professor Yuki Kawabata (河端雄毅) of the Graduate School of Integrated Science and
Technology Nagasaki University commented, "'Sideways walking' is a very
unusual behavior; there are hardly any other animals in the animal kingdom that
walk this way. I believe this finding will help us understand how this
revolutionary mode of locomotion relates to the prosperity of organisms."
So, a research group in Japan suggests that
crabs likely began "side walking" approximately 200 million years
ago. Apparently, this finding can help us understand how animal locomotion
patterns have evolved over time.