The crab body plan emerged multiple times throughout natural history due to a phenomenon termed “carcinization.” Nevertheless, one characteristic sets true crabs (Brachyura) apart from their look-alikes: their nearly unique, defining sideways gait.
“Lateral movement may have greatly contributed to the ecological success of true crabs,” states behavioral ecologist Yuki Kawabata from the University of Nagasaki in Japan.
A new study undertaken by Kawabata and a team of scientists from institutions across Japan, Taiwan, and the USA suggests that the evolution of this “crab walk” was a singular event, potentially granting the group an advantage that enabled its rapid diversification and occupation of various habitat niches. The research findings are presented as a peer-reviewed preprint article in the journal eLife.
“This single occurrence stands in stark contrast to carcinization, which has happened repeatedly among decapods,” Kawabata explains. “It highlights that while body forms can converge numerous times, behavioral shifts, such as sidestepping, might be rare occurrences.”
“With approximately 7,904 species, true crabs greatly outnumber their close relatives, the hermit crabs (Anomura), or their nearest kin, the crayfish (Astacidea); they have successfully colonized diverse environments worldwide, including terrestrial, freshwater, and deep-sea locales,” Kawabata notes.
Could it be that walking sideways was the very engine behind this survival success? Kawabata and his colleagues believe this to be the case.
Crabs face no shortage of predators: humans, seals, otters, seabirds, octopuses, sharks, and even other, larger crabs all vie for a meal.
Sideways locomotion is considered a defensive asset for crabs, allowing for swift evasion from these predators, which predominantly move in a forward direction.
Contrary to popular assumption, the “goal” of evolution is not truly to produce more crabs. If it can be said to have a goal at all, it is to increase biological variety, which bolsters the odds of living things persisting through any given set of circumstances.
“Such innovations can unlock new avenues for adaptation, yet remain constrained by phylogenetic history and ecological context,” says Kawabata. “Through direct behavioral observation and phylogenetic modeling, this work advances our comprehension of how animal gaits diversify and persist across evolutionary time.”
Slightly unsettlingly, some true crabs actually present an exception to the rule that they walk sideways.
In laboratory tests, researchers observed the gait of 50 distinct species of true crabs. They determined that 35 species primarily traverse sideways, while only 15 species favor moving forward most of the time.
By combining this analysis with a recently constructed “family tree” of crab genetics, the researchers found that the characteristic lateral gait originated in one common ancestor for all subsequent true crabs.
Apparently, this innovation appeared around 200 million years ago, immediately following the Triassic-Jurassic extinction event, when tectonic shifts caused the shallow marine areas favored by crabs to expand.
This also implies that all true crabs currently moving forward arrived at that mode of transport by shifting away from the sideways walk, rather than retaining it over millennia.
As one reviewer points out, the experiment utilized only a single specimen per species, which poses a limitation since there’s no guarantee any single individual walks “normally” for its species. Further studies could repeat this experiment with a larger sample size for each species.
Experts also comment that a more in-depth look at the anatomical differences between crab species could reveal whether body structure or locomotion played a more decisive role in the evolution of true crabs.
They also questioned the method of classifying crabs as forward- or sideways-crawling based on a threshold set at 60 degrees relative to the crab’s bilateral axis.
“This boundary might be reasonable as a convention, but the paper doesn’t elaborate on why that specific line should be drawn there, and there is a valid biological concern that a fixed angular threshold doesn’t signify the same thing across different taxa,” states one reviewer.
Future research could address these points and shed more light on evolution’s apparent preoccupation with crabs.
“To disentangle the relative roles of innovation and environmental change, we need further analyses of trait-dependent diversification, fossil-based chronologies, and performance tests that link true crab sidestepping to adaptive advantages,” adds Kawabata.
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