The branchiostegal lung of Uca vocans (Decapoda: Ocypodidae): Unreported complexity revealed by corrosion casting and MicroCT techniques

Species-specific gill's microbiome of eight crab species with different breathing adaptations

Transitions to physically different environments, such as the water-to-land transition, proved to be the main drivers of relevant evolutionary events. Brachyuran crabs evolved remarkable morphological, behavioral, and physiological adaptations to terrestrial life. Terrestrial species evolved new respiratory structures devoted to replace or support the gills, a multifunctional organ devoted to gas exchanges, ion-regulation and nitrogen excretion. It was hypothesized that microorganisms associated with respiratory apparatus could have facilitated the processes of osmoregulation, respiration, and elimination of metabolites along this evolutionary transition. To test if crab species with different breathing adaptations may host similar microbial communities on their gills, we performed a comparative targeted-metagenomic analysis, selecting two marine and six terrestrial crabs belonging to different families and characterised by different breathing adaptations. We analysed anterior and posterior gills separately according to their different and specific roles. Regardless of their terrestrial or marine adaptations, microbial assemblages were strongly species-specific indicating a non-random association between the host and its microbiome. Significant differences were found in only two terrestrial species when considering posterior vs. anterior gills, without any association with species-specific respiratory adaptations. Our results suggest that all the selected species are strongly adapted to the ecological niche and specific micro-habitat they colonise.

Ecological, physiological and life-history traits correlate with genome sizes in decapod crustaceans

Crustaceans are characterized by some of the most variable genome sizes among animals. Significant relationships between genome size and specific eco-physiological and morphological features have been described in many crustacean taxa, such as Amphipoda, Ostracoda, Cladocera, and Copepoda. A consistent pattern of genome size variation is yet to be found, however, in one of the most specious order of crustaceans, the Decapoda. We investigated how genome size is related to selected eco-physiological and life-history traits in species belonging to this order. We analyzed the respiratory, excretory, and developmental adaptations of 179 decapod species inhabiting shallow marine, deep marine, freshwater, intertidal, and terrestrial habitats. Our results show that the genome size is significantly larger in decapods that have a direct development. Moreover, in Anomura, Astacidea, and Brachyura we found larger genome sizes in species (i) living in freshwater habitats, (ii) using gills as a strictly water-breathing organs, and (iii) presenting a direct development. While species that (i) live in shallow waters, and intertidal and terrestrial habitats, (ii) have some degrees of air-breathing adaptations, namely gills that can exchange oxygen with air if wet or branchiostegal lungs, and (iii) possess an indirect or extended development are characterized by significantly smaller genomes. Our analyses show that developmental complexity affects genome size in decapods, and that multiple eco-physiological and life-history traits correlate with genome size in Anomura, Astacidea, and Brachyura.

Comparative transcriptome analysis of the gills of Cardisoma armatum provides novel insights into the terrestrial adaptive related mechanism of air exposure stress

Cardisoma armatum is a typical member of the Gecarcinidae which show significant behavioral, morphological, physiological, and/or biochemical adaptations permitting extended activities on the land. The special gills (branchiostegal lung) of C. armatum play an important role in maintaining osmotic pressure balance and obtaining oxygen to adapt to the terrestrial environment. However, adaptive molecular mechanisms responding to air exposure in C. armatum are still poorly understood. In this study, transcriptomic analysis and histological analysis were conducted on the gills to test adaptive capabilities over 8 h between the aerial exposure (AE) and the water immersion (WI) group. Differentially expressed genes (DEGs) related to terrestrial adaptation were categorized into four broad categories: ion transport, acid-base balance, energy metabolism and immune response. This is the first research to reveal the molecular mechanism of terrestrial adaptation in C. armatum, and will provide new insight into the molecular genetic basis of terrestrial adaptation in crabs.

To the Land and Beyond: Crab Microbiomes as a Paradigm for the Evolution of Terrestrialization

The transition to terrestrial environments by formerly aquatic species has occurred repeatedly in many animal phyla and lead to the vast diversity of extant terrestrial species. The differences between aquatic and terrestrial habitats are enormous and involved remarkable morphological and physiological changes. Convergent evolution of various traits is evident among phylogenetically distant taxa, but almost no information is available about the role of symbiotic microbiota in such transition. Here, we suggest that intertidal and terrestrial brachyuran crabs are a perfect model to study the evolutionary pathways and the ecological role of animal-microbiome symbioses, since their transition to land is happening right now, through a number of independent lineages. The microorganisms colonizing the gut of intertidal and terrestrial crabs are expected to play a major role to conquer the land, by reducing water losses and permitting the utilization of novel food sources. Indeed, it has been shown that the microbiomes hosted in the digestive system of terrestrial isopods has been critical to digest plant items, but nothing is known about the microbiomes present in the gut of truly terrestrial crabs. Other important physiological regulations that could be facilitated by microbiomes are nitrogen excretion and osmoregulation in the new environment. We also advocate for advances in comparative and functional genomics to uncover physiological aspects of these ongoing evolutionary processes. We think that the multidisciplinary study of microorganisms associated with terrestrial crabs will shed a completely new light on the biological and physiological processes involved in the sea-land transition.

MicroCT imaging applied to description of a new species of Pagurus Fabricius, 1775 (Crustacea: Decapoda: Anomura: Paguridae), with selection of three-dimensional type data

A new species of hermit crab, Pagurus fraserorum n. sp. (family Paguridae) is described from rocky subtidal reefs off KwaZulu-Natal, South Africa, and illustrated using both conventional drawings and colour photographs, and via three-dimensional (3D) X-ray micro-computed tomography (μCT). Because of the limitation μCT has in detecting very fine and soft structures, a novel approach of manually drawing setation and spinulation onto the two-dimensional images of the 3D visualizations was used to illustrate the pereopods. In addition, an interactive figure and rotation movie clips in the supplement section complement the species description, and the 3D raw data of the 3D type data are downloadable from the Gigascience Database repository. The new species is the sixth species of Pagurus Fabricius, 1775 reported from South Africa and is closely allied to the Indo-Pacific P. boriaustraliensis Morgen, 1990 and P. pitagsaleei McLaughlin, 2002, from which it differs by its shorter ocular peduncles, by the armature of the carpus of the right cheliped, and also in colouration. This study presents the first description of a hermit crab in which a majority of taxonomic details are illustrated through 3D volume-rendered illustrations. In addition, colour photographs and COI molecular barcodes are provided, and the latter compared to COI sequences of specimens from Western Australia previously identified as P. boriaustraliensis and of specimens of P. pitagsaleei from Taiwan, as well as to three additional South African members of the genus. The South African taxon was confirmed to be genetically distinct from all species tested.