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

Area of Science:

• * Evolutionary Biology
• * Crustacean Ecology
• * Terrestrial Adaptation

Background:

• * Macroscopic terrestrial life evolved from a few ancestral lineages, with independent transitions occurring multiple times.
• * Decapod crabs exhibit at least ten independent transitions from marine to terrestrial environments, showcasing diverse habitats and varying degrees of terrestriality.
• * Previous classifications of land crabs were limited by reliance on single traits and failed to distinguish infraorders or colonization routes (freshwater vs. marine), omitting critical phylogenetic and ecological constraints.

Purpose of the Study:

• * To reclassify land crab terrestriality by introducing four transition pathways reflecting deep phylogenetic relationships and route-specific adaptations.
• * To describe adaptive traits that evolved in response to six key terrestrial selective pressures.
• * To propose six grades of terrestriality based on observable trait-environment associations and suggest future research directions.

Main Methods:

• * Phylogenetic analysis to delineate distinct transition pathways for decapod crabs.
• * Comparative analysis of adaptive traits in response to terrestrial challenges.
• * Development of a novel grading system for terrestriality based on trait-environment associations.

Main Results:

• * Identification of four distinct transition pathways for terrestrial colonization in decapod crabs, accounting for infraordinal relationships and colonization routes.
• * Description of six primary terrestrial selective challenges and the corresponding adaptive traits that evolved.
• * Proposal of a six-grade system for classifying terrestriality, linking observable traits to specific environments.

Conclusions:

• * A new framework for understanding land crab evolution and adaptation is presented, integrating phylogenetic and ecological factors.
• * The proposed classification provides a foundation for future research into trait evolution and convergent evolution in terrestrial crustaceans.
• * Future studies utilizing phylogenomic and transcriptomic tools are recommended to test evolutionary hypotheses and explore convergence mechanisms.