Amazing stuff!
"True crabs—members of the infraorder Brachyura—are an evolutionary success story. These ten-limbed crustaceans can be found from the darkest depths of the ocean all the way up onto land, and with nearly 8000 described species, they dwarf the diversity of their closest kin, including hermit crabs, lobsters, and crayfish. But what gave them such an evolutionary edge? According to analyses recently published as a preprint, it could be that they gave up on going forward. ..."
From the abstract:
"The evolutionary change in the mode of locomotion is often a major evolutionary event, triggering diversification.
Sideways locomotion is a defining feature of true crabs (Brachyura) and may have contributed to their ecological success. Yet, the evolutionary origin of this unique behavior remains unknown.
Here we show that the prevalence of sideways locomotion in true crabs reflects a single evolutionary origin from a forward-moving ancestor.
Our behavioral analysis of 50 live crab species indicates that crab locomotion can be separated into two distinct types, sideways and forward locomotion, with no intermediates.
The phylogenetic comparative analysis revealed a single origin of sideways locomotion, with multiple independent reversions to forward locomotion in ecologically specialized groups.
The species richness data show that the lineage in which sideways locomotion originated is far more diverse than its nearest outgroups.
These results suggest that sideways locomotion acted as a key innovation, potentially promoting adaptive radiation by enabling true crabs to exploit a wide range of ecological niches. Such a rare but innovative behavioral trait provides a framework for understanding how locomotor modes shape evolutionary diversification in animals."
Evolution of sideways locomotion in crabs (preprint, open access)
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