Observation of a coral-dwelling gall crab (Cryptochiridae) in a dead coral host highlights their vulnerability to reef degradation

Coral-associated fauna contributes greatly to coral reef biodiversity. Many species are obligately associated with their hosts on which they depend for food and/or refuge from predators. Their close relationship with their hosts makes them vulnerable to coral mortality. Here I report a coral-dwelling gall crab (Cryptochiridae) inhabiting a partially dead Echinopora Lamarck, 1816 coral, at Magoodhoo Island, Faafu Atoll, Maldives. Cryptochirids are thought to feed off the mucus provided by their coral host, although some questions about their feeding biology remain. This observation highlights that these crabs remain associated with a dead host, even if it can no longer provide nutrients. The strong host association makes gall crabs vulnerable to widespread habitat degradation

<i>Lithoscaptus aquarius</i> sp. nov. (Decapoda: Cryptochiridae) described from a <i>Catalaphyllia jardinei</i> (Scleractinia) out of the aquarium trade

A new species of gall crab collected from elegance coral, Catalaphyllia jardinei, is described in this paper. The male holotype was collected from a reef tank in Germany in 2016, and it is described here using integrative taxonomy. This species, named Lithoscaptus aquarius sp. nov., is the thirteenth assigned to the genus. It is morphologically and phylogenetically closest to Lithoscaptus semperi, a cryptochirid associated with Trachyphyllia geoffroyi. Like L. semperi, it has a large, broad W-shaped depression on the anterior half of the carapace, but the carapace surface of L. aquarius sp. nov. is smooth overall, lacking spines or tubercles. This new species is so named because it was found in a reef tank after searching in vain for material during fieldwork campaigns over the course of several years

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Environmen

Phylomitogenomics elucidates the evolution of symbiosis in Thoracotremata (Decapoda: Cryptochiridae, Pinnotheridae, Varunidae)

BackgroundThoracotremata 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.MethodsHere 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).ResultsThe 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.<br/

Conservation of Coral-Associated Fauna

Coral reefs are some of the most biodiverse ecosystems harboring thousands of species, many of them symbionts that play important roles in the survival of their hosts. These associated taxa, mostly invertebrates, remain largely unstudied and the conservation status of the majority of these species is not assessed. With coral reefs under severe global and local threats, effective conservation measures based on a whole ecosystem approach are needed. The IUCN Red List is the most up to date inventory of species’ conservation status, yet does not include symbiont fauna of important host organisms. Here we suggest including associated taxa in future endangerment assessments, especially those living in obligate symbiosis

DNA barcoding, dwelling morphology, and fecundity of the gall-forming shrimp <i>Paratypton siebenrocki</i> Balss, 1914 (Caridea: Palaemonidae)

Tropical coral reefs offer a wide variety of habitats to countless invertebrate species. Sessile host organisms especially are inhabited by small taxa, of which decapod crustaceans form one of the most diverse communities. Symbiotic palaemonid shrimp species associ-ate with marine invertebrate hosts from multiple phyla, including cnidarians such as stony corals (Scleractinia). The intriguing gall- forming shrimp Paratypton siebenrocki, a symbiont of Acropora corals in the Indo-Pacific, was collected in the Saudi Arabian Red Sea, Kenya, and the Maldives. Based on morphology P. siebenrocki has been considered to be most closely related to the genera Anapontonia and Metapontonia; however, no clear clustering with either palaemonid genus was observed in a phylogenetic recon-struction based on 16S and COI mtDNA. Here we photo-document the dwellings of P. siebenrocki in Acropora spp. for the first time, and furthermore we report on the reproductive output of this species. The number of eggs ranged from 345 to 909 (n = 6), and embryo volume differed strongly between early- and late-stage embryos. The carapace length ranged from 2.58 to 4.55 mm for the females and 1.51 to 2.5 mm for the males (n = 5). The number and size of the embryos, combined with their specialised, secluded lifestyle, sug-gest that P. siebenrocki allocates highe

Genomic survey sequencing and complete mitochondrial genome of the elkhorn coral crab Domecia acanthophora (Desbonne in Desbonne &amp; Schramm, 1867) (Decapoda: Brachyura: Domeciidae)

The elkhorn coral crab Domecia acanthophora inhabits shallow-water coral reefs in the Western Atlantic. The species has a wide distribution and, although primarily associated with endangered Acropora corals, has been recorded from a myriad of hosts. Here we conducted the first genomic survey and complete mitochondrial assemblage and characterisation of any species of Domeciidae, as well as the first species within Trapezioidea. The estimated size of the nuclear genome ranged from 0.64 Gbp to 1.76 Gbp, revealing a small genome. Repetitive elements of the genome were estimated here at 66.4% and 74%, respectively, with the majority of the repetitive elements consisting of LINE, LTR, and satellite DNA. The assembled A-T rich mitochondrial genome consisted of 15,568 bp in length, with 13 protein-coding genes (PCGs), 22 transfer RNA (tRNA) genes and two ribosomal RNA (rRNA) genes. A 619 bp long non-coding region was identified as the supposed D-loop/control region, containing eight microsatellites. The 22 tRNA genes, ranging from 65 to 71 bp in length, displayed a typical “cloverleaf” secondary structure, with the exception of tRNA-Ser1 which lacked part of the DHU arm and tRNA-Asp displayed a deletion of the TΨC loop but not the arm. Two transposition events of two tRNA genes were also found when comparing the gene order of D. acanthophora to that of the brachyuran basic gene order, which had not been reported before. Despite belonging to a widely distributed, well-known superfamily of coral-associated crabs, the Trapezioidea, very little was known about this species from a genetics perspective, which is remedied here by providing a new genomic resource for D. acanthophora