{"id":635785,"date":"2026-04-29T06:11:24","date_gmt":"2026-04-29T06:11:24","guid":{"rendered":"https:\/\/www.newsbeep.com\/ca\/635785\/"},"modified":"2026-04-29T06:11:24","modified_gmt":"2026-04-29T06:11:24","slug":"evolution-keeps-making-crabs-but-a-key-feature-has-only-evolved-once-sciencealert","status":"publish","type":"post","link":"https:\/\/www.newsbeep.com\/ca\/635785\/","title":{"rendered":"Evolution Keeps Making Crabs \u2013 But a Key Feature Has Only Evolved Once : ScienceAlert"},"content":{"rendered":"

As you may have heard via a barrage of internet memes, evolution can’t get enough<\/a> of crab-like forms. The body shape of crabs has arisen on many separate occasions in natural history, in a phenomenon known as ‘carcinization<\/a>‘.<\/p>\n

However, one feature sets true crabs (Brachyura<\/a>) apart from their lookalikes: their near-exclusive, characteristic sideways walk.<\/p>\n

“Sideways locomotion may have contributed significantly to the ecological success of true crabs,” says<\/a> behavioral ecologist Yuuki Kawabata from Nagasaki University in Japan.<\/p>\n

New research, conducted by Kawabata and a team from institutions across Japan, Taiwan, and the US, suggests the evolution of this crab walk was a singular event, which may have given the group an advantage that allowed them to rapidly diversify into different habitat niches.<\/p>\n

“This single event contrasts starkly with carcinization, which has occurred repeatedly across decapod species,” Kawabata explains<\/a>. “This highlights that while body shapes may converge multiple times, behavioral changes such as sideways walking can be rare.”<\/p>\n

“There are around 7,904 species of true crabs, far exceeding that of their sister group, Anomura<\/a> [hermit crabs], or their closest relatives, Astacidea<\/a> [clawed lobsters and crayfish]; they have colonized diverse habitats around the world, including terrestrial, freshwater, and deep-sea environments,” Kawabata points out<\/a>.<\/p>\n

Could it be that the sideways walk is the reason for this survival success story? Kawabata and team think so.<\/p>\n

Crabs have no shortage of predators: humans, seals, otters, seabirds, octopuses, sharks, and even other larger crabs all want a bite.<\/p>\n

The sideways walk is considered a defensive advantage<\/a> for crabs to quickly dodge these mostly forward-moving predators, and if you’ve ever walked along a rocky groin<\/a>, you’ll know how fast they can scuttle away.<\/p>\n

Contrary to the memes, evolution’s ‘goal’ is not really to produce more crabs. If it can even be said to have a goal, it would be to produce more diversity<\/a>, which improves the odds<\/a> of living things surviving whatever the world throws at them.<\/p>\n

“Such innovations can open new adaptive opportunities and yet remain constrained by phylogenetic history and ecological contexts,” Kawabata says<\/a>.<\/p>\n

“With direct behavioral observations and a phylogenetic framework, this work expands our understanding of how modes of travel in animals diversify and persist through evolutionary time.”<\/p>\n

It’s slightly unsettling, but some true crabs are actually an exception to the sideways rule.<\/p>\n

In lab experiments, the researchers watched the walks of 50 different species of true crab. They found that 35 qualified as predominantly side-walkers, while only 15 walked forwards most of the time.<\/p>\n

\"The(a) Experimental setup used to record each crab’s behavior. (b) Extraction of two-dimensional position coordinates from video frames. (Taniguchi et al., eLife, 2026)<\/p>\n

By combining this analysis with a recently-constructed ‘family tree’ of crab genetics, the researchers found the archetypal sideways walk occurred in a single common ancestor of all true crabs that followed.<\/p>\n

This innovation appears to have occurred about 200 million years ago, immediately after the Triassic-Jurassic extinction<\/a>, when movements in Earth’s crust resulted in an expansion of crabs’ favorite habitat: shallow marine environments.<\/p>\n

This also suggests that any true crabs who are now forward-movers arrived at that locomotion via the sideways route, rather than maintaining it through the millennia.<\/p>\n

As one peer-reviewer points out<\/a>, the experiment did only use one representative for each species, which is a limitation, since there’s no guarantee that any one individual walks ‘normally’ for its kind. Further research could replicate this experiment with greater numbers from each species.<\/p>\n

Peer reviewers<\/a> also note that further detail on the crab species’ anatomical differences could reveal whether body plan or locomotion style played more of a leading role in the evolution of true crabs.<\/p>\n

They also questioned the way crabs were categorized into forward- or sideways-walkers, based on a boundary set at 60 degrees from the crab’s bilateral axis.<\/p>\n

\"Subscribe<\/a><\/p>\n

“This boundary may be reasonable as a convention, but the paper does not explain why it is the right place to draw the line, and there is a plausible biological concern that a fixed angular cutoff does not mean the same thing across taxa,” one reviewer states<\/a>.<\/p>\n

Related: Crabs Aren’t The Only Things Evolution Keeps Making. An Expert Explains.<\/a><\/p>\n

Future research could address these issues and shed more light on evolution’s apparent obsession with crabs.<\/p>\n

“To disentangle the relative roles of innovation and environmental change, we need further analyses of trait-dependent diversification, fossil-informed timelines, and performance tests that link true crabs’ sideways movement to adaptive advantages,” Kawabata adds<\/a>.<\/p>\n

The study is available as a peer-reviewed preprint in eLife<\/a>.<\/p>\n","protected":false},"excerpt":{"rendered":"As you may have heard via a barrage of internet memes, evolution can’t get enough of crab-like forms.…\n","protected":false},"author":2,"featured_media":635786,"comment_status":"","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[7],"tags":[49,48,315,66],"class_list":["post-635785","post","type-post","status-publish","format-standard","has-post-thumbnail","category-science","tag-ca","tag-canada","tag-msft-content","tag-science"],"_links":{"self":[{"href":"https:\/\/www.newsbeep.com\/ca\/wp-json\/wp\/v2\/posts\/635785","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.newsbeep.com\/ca\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.newsbeep.com\/ca\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/ca\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/ca\/wp-json\/wp\/v2\/comments?post=635785"}],"version-history":[{"count":0,"href":"https:\/\/www.newsbeep.com\/ca\/wp-json\/wp\/v2\/posts\/635785\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/ca\/wp-json\/wp\/v2\/media\/635786"}],"wp:attachment":[{"href":"https:\/\/www.newsbeep.com\/ca\/wp-json\/wp\/v2\/media?parent=635785"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.newsbeep.com\/ca\/wp-json\/wp\/v2\/categories?post=635785"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.newsbeep.com\/ca\/wp-json\/wp\/v2\/tags?post=635785"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}