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 brachyuran 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.

Making the grade: Physiological adaptations to terrestrial environments in decapod crabs.

All extant macroscopic terrestrial diversity has evolved from the ancestors of a small group of successful terrestrial colonizers, but in a few lineages this transition has independently occurred multiple times in spite of the significant functional challenges it presents. Decapod crabs have transitioned from marine to terrestrial environments at least ten times, occupy diverse habitats, and display varying degrees of terrestriality. Previous attempts to categorize land crab diversity relied on single traits, did not explicitly distinguish between brachyuran and anomuran lineages, and did not separate lineages that colonized land via freshwater or marine environments. As a result, critical phylogenetic and ecological constraints were missing from these earlier classifications. In this paper, I reclassify terrestriality in the land crabs by designating four transition pathways that reflect deep phylogenetic relationships between the two decapod crab infraorders and the route-specific nature of this transition. I then describe the adaptive traits that evolved in response to six primary terrestrial selective challenges. I conclude by proposing six grades of terrestriality in this system that describe observable trait-by-environment associations, and propose studies that can test the hypothetical sequence of trait evolution and the nature of convergence in the land crabs using phylogenomic and transcriptomic tools.