Convergent adaptation of true crabs (Decapoda: Brachyura) to a gradient of terrestrial environments

For much of terrestrial biodiversity, the evolutionary pathways of
adaptation from marine ancestors are poorly understood, and have usually
been viewed as a binary trait. True crabs, the decapod crustacean
infraorder Brachyura, comprise over 7,600 species representing a striking
diversity of morphology and ecology, including repeated adaptation to
non-marine habitats. Here, we reconstruct the evolutionary history of
Brachyura using new and published sequences of 10 genes for 344 tips
spanning 88 of 109 families. Using 36 newly vetted fossil calibrations, we
infer that brachyurans most likely diverged in the Triassic, with
family-level splits in the late Cretaceous and early Paleogene. By
contrast, the root age is underestimated with automated sampling of 328
fossil occurrences explicitly incorporated into the tree prior, suggesting
such models are a poor fit under heterogeneous fossil preservation. We
apply recently defined trait-by-environment associations to classify a
gradient of transitions from marine to terrestrial lifestyles. We estimate
that crabs left the marine environment at least seven and up to 17 times
convergently, and returned to the sea from non-marine environments at
least twice. Although the most highly terrestrial- and many
freshwater-adapted crabs are concentrated in Thoracotremata, Bayesian
threshold models of ancestral state reconstruction fail to identify shifts
to higher terrestrial grades due to the degree of underlying change
required. Lineages throughout our tree inhabit intertidal and marginal
marine environments, corroborating the inference that the early stages of
terrestrial adaptation have a lower threshold to evolve. Our framework and
extensive new fossil and natural history datasets will enable future
comparisons of non-marine adaptation at the morphological and molecular
level. Crabs provide an important window into the early processes of
adaptation to novel environments, and different degrees of evolutionary
constraint that might help predict these pathways. --