UC Davis

Much of Earth’s biodiversity currently exists on land and in the air, but all life began in the oceans. Nevertheless, sea-to-land transitions are rare and physiologically challenging; few marine groups have successfully invaded land and diversified there. Decapod crabs are a notable exception; at least 11 families have colonized terrestrial environments. Furthermore, these lineages occupy different transitional stages: some lineages live intertidally and spend short periods of time above water, while others live up to several kilometers inland and only seasonally return to the ocean to hatch eggs that then undergo normal planktonic development. No matter where they exist on this spectrum of terrestriality, they all have successfully overcome significant osmotic challenges and evolved solutions to address the vast physical differences between seawater and air. The evolutionary pathways to overcoming these obstacles are largely unexplored, and little is known about the genomic basis of these impressive phenotypic and behavioral changes. For my dissertation, I sought to characterize the physiological, behavioral, and genomic changes associated with the transition from marine to terrestrial habitats in gecarcinid crabs that exhibit varying grades of terrestrial adaptation. In my first chapter, I reclassified extant decapod land crab diversity by designing a novel framework that assigned crabs to specific “grades” based on the association between their habitats and the key traits they possess that permit them to survive in their particular habitat. In this framework, I also considered the fact that brachyuran and anomuran crabs colonized terrestrial habitats independently, and that both infraorders may have transitioned onto land either via marine environments or freshwater ones. This framework presents testable hypotheses for the sequence of key trait evolution in the land crabs, and is the first framework of its kind that classifies land crab diversity in a phylogenetic and evolutionary context. In my second chapter, we measured tissue-specific differential gene expression in two congeneric land crab sister species displaying different degrees of terrestrial adaptation, Tuerkayana celeste and T. magna, and a highly terrestrial confamilial species, Gecarcoidea natalis after placing the crabs into increasingly severe desiccation conditions. We found that while most of the differentially expressed genes were more likely to be conserved across all three species, genes from families expanded in one or more lineage or genes not shared across species also appear to play a critical role in how land crabs from different terrestrial grades adapt to the unique selective challenges that accompany a terrestrial life. In my third chapter, we sought to understand the genomic basis of red-blue color polymorphisms in Birgus latro, the coconut crab. Coconut crabs are the largest terrestrial arthropods on Earth and exist in two color morphs, the adaptive significance of which is currently unknown. We investigated whether sequence-level variation in the gene crustacyanin, which has been implicated in exoskeletal coloration in many other malacostracan crustaceans, played a similar role in this system using whole genome resequencing and transcriptomics. We found that while there were no significant differences in Fst between single nucleotide polymorphisms within crustacyanin genes and those randomly selected from across the genome, the presence of 40 paralogs of crustacyanin in the coconut crab genome might play some role in the mechanistic basis of coloration in this system. This body of work also produced novel genomic resources for the land crab study system (i.e., four transcriptomes and one ultra long-read genome) that open the door for future studies that seek to understand the genomic and transcriptomic basis of terrestrial adaptations in this, and potentially other, biological systems.