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Background: Thoracotremata belong to the large group of "true" crabs (infraorder Brachyura), and they exhibit a wide range of physiological and morphological adaptations to living in terrestrial, freshwater and marine habitats. Moreover, the clade comprises various symbiotic taxa (Aphanodactylidae, Cryptochiridae, Pinnotheridae, some Varunidae) that are specialised in living with invertebrate hosts, but the evolutionary history of these symbiotic crabs is still partially unresolved., Methods: Here we assembled and characterised the complete mitochondrial genomes (hereafter mitogenomes) of three gall crab species (Cryptochiridae): Kroppcarcinus siderastreicola , Opecarcinus hypostegus and Troglocarcinus corallicola . A phylogenetic tree of the Thoracotremata was reconstructed using 13 protein-coding genes and two ribosomal RNA genes retrieved from three new gall crab mitogenomes and a further 72 available thoracotreme mitogenomes. Furthermore, we applied a comparative analysis to characterise mitochondrial gene order arrangement, and performed a selection analysis to test for selective pressure of the protein-coding genes in symbiotic Cryptochiridae, Pinnotheridae, and Varunidae ( Asthenognathus inaequipes and Tritodynamia horvathi )., Results: The results of the phylogenetic reconstruction confirm the monophyly of Cryptochiridae, which clustered separately from the Pinnotheridae. The latter clustered at the base of the tree with robust branch values. The symbiotic varunids A. inaequipes and T. horvathi clustered together in a clade with free-living Varunidae species, highlighting that symbiosis in the Thoracotremata evolved independently on multiple occasions. Different gene orders were detected in symbionts and free-living species when compared with the ancestral brachyuran gene order. Lastly, the selective pressure analysis detected two positively selected sites in the nad6 gene of Cryptochiridae, but the evidence for positive selection in Pinnotheridae and A. inaequipes and T. horvathi was weak. Adaptive evolution of mitochondrial protein-coding genes is perhaps related to the presumably higher energetic demands of a symbiotic lifestyle., Competing Interests: The authors declare there are no competing interests., (©2023 Xu et al.)

The endosymbiont-host relationship between a pea crab and its bivalve host has been studied previously using bivalve's multi-physiological parameters. The present study is first of its kind that uses hemocyte's immune functionality to evaluate the symbiotic relationship between bivalve Paphia malabarica and its symbiont crab, Pinnotheres vicajii. The sampling was carried out at two stations (differed in nutrient concentration, productivity, and bacterial abundance) located along the southwest coast of India. The results showed lower immunocompetence in the case of bivalves infested with pea crabs. The lowering of hemocyte population was the most prominent and perhaps the initial reaction in response to focal inflammation or wound inflicted by the pea crabs. A significant lowering of phagocytosis, lysozyme, and esterase activity was also observed, indicating the detrimental impact of crab infestation. Interestingly, no significant correlation was found between parasitism and condition index despite the lower immunocompetence, validating the effectiveness of using hematological parameters in comparison to other life-history traits. This indicates that the immune parameters being directly related to the organism's metabolic state can elucidate the early effect of stressors, therefore, proving to be a better proxy for understanding a host-pathogen relationship. Disparity found in bivalve's immunocompetence between the sampling sites could be linked to either variation in environmental parameters, the size of the symbiont or the infection level. Our preliminary results provide a direction towards examining immunodepression in parasitized bivalves by considering ex-situ controlled parasitism induction along with varying environmental conditions. In view of this, efforts must be directed towards minimizing pea crab's outbreaks, especially in cultivated shellfish farms. Future studies should elucidate molecular mechanisms involved in the immunocompetence of P. malabarica, hosting the parasitic pea crabs., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

Almost half of all known microsporidian taxa infect aquatic animals. Of these, many cause disease in arthropods. Hepatospora, a recently erected genus, infects epithelial cells of the hepatopancreas of wild and farmed decapod crustaceans. We isolated Hepatospora spp. from three different crustacean hosts, inhabiting different habitats and niches; marine edible crab (Cancer pagurus), estuarine and freshwater Chinese mitten crab (Eriocheir sinensis) and the marine mussel symbiont pea crab (Pinnotheres pisum). Isolates were initially compared using histology and electron microscopy revealing variation in size, polar filament arrangement and nuclear development. However, sequence analysis of the partial SSU rDNA gene could not distinguish between the isolates (~99% similarity). In an attempt to resolve the relationship between Hepatospora isolated from E. sinensis and C. pagurus, six additional gene sequences were mined from on-going unpublished genome projects (RNA polymerase, arginyl tRNA synthetase, prolyl tRNA synthetase, chitin synthase, beta tubulin and heat shock protein 70). Primers were designed based on the above gene sequences to analyse Hepatospora isolated from pea crab. Despite application of gene sequences to concatenated phylogenies, we were unable to discriminate Hepatospora isolates obtained from these hosts and concluded that they likely represent a single species or, at least subspecies thereof. In this instance, concatenated phylogenetic analysis supported the SSU-based phylogeny, and further, demonstrated that microsporidian taxonomies based upon morphology alone are unreliable, even at the level of the species. Our data, together with description of H. eriocheir in Asian crab farms, reveal a preponderance for microvariants of this parasite to infect the gut of a wide array of decapods crustacean hosts and the potential for Hepatospora to exist as a cline across wide geographies and habitats.