Xanthidae MacLeay, 1838 (Decapoda: Brachyura: Xanthoidea) systematics: A multi-gene approach with support from adult and zoeal morphology

Phylogenomic analysis of brachyuran crabs using transcriptome data reveals possible sources of conflicting phylogenetic relationships within the group

Despite extensive morphological and molecular studies, the phylogenetic interrelationships within the infraorder Brachyura and the phylogenetic positions of many taxa remain uncertain. Studies that used a limited number of molecular markers have often failed to provide sufficient resolution, and may be susceptible to stochastic errors and incomplete lineage sorting (ILS). Here we reconstructed the phylogenetic relationships within the Brachyura using transcriptome data of 56 brachyuran species, including 14 newly sequenced taxa. Five supermatrices were constructed in order to exclude different sources of systematic error. The results of the phylogenetic analyses indicate that Heterotremata is non-monophyletic, and that the two Old World primary freshwater crabs (Potamidae and Gecarcinucidae) and the Hymenosomatoidea form a clade that is sister to the Thoracotremata, and outside the Heterotremata. We also found that ILS is the main cause of the gene-tree discordance of these freshwater crabs. Divergence time estimations indicate that the Brachyura has an ancient origin, probably either in the Triassic or Jurassic, and that the majority of extant families and superfamilies first appeared during the Cretaceous, with a constant increase of diversity in Post-Cretaceous-Palaeogene times. The results support the hypothesis that the two Old World freshwater crab families included in this study (Potamidae and Gecarcinucidae) diverged from their marine ancestors around 120 Ma, in the Cretaceous. In addition, this work provides new insights that may aid in the reclassification of some of the more problematic brachyuran groups.

First records of some coral reef-associated brachyuran crabs from the Nicobar archipelago, India

The coral reefs off the Andaman and Nicobar archipelagos harbour some of the world's diverse reef-associated crustacean fauna, which have not been adequately surveyed. A recent faunal survey in the Andaman Sea on-board the Fishery Oceanographic Research Vessel Sagar Sampada (cruise number 388) revealed a rich diversity of coral reef-associated brachyuran crabs. The article reports four genera, Singhaplax Serène & Soh, 1976, Cavoportunus Nguyen & Ng, 2010, Cyrtocarcinus Ng & Chia, 1994 and Visayax Mendoza & Ng, 2008 occurring in Indian waters for the first time. Seven species namely Singhaplax ockelmanni (Serène, 1971), Urnalana angulata (Rathbun, 1911), Pseudolambrus confragosus (Calman, 1900), Cavoportunus dubius (Laurie, 1906), Cyrtocarcinus truncatus (Rathbun, 1906), Visayax osteodictyon Mendoza & Ng, 2008, and Xanthias joanneae Mendoza, 2013, are likewise reported for the first time from Indian waters.

A new species of the coral-symbiont crab genus Cymo de Haan, 1833 (Decapoda, Brachyura, Xanthidae) from Nansha Islands, the South China Sea

A new species of coral-symbiont crab, Cymo mazu sp. nov., is described from the Nansha Islands in the South China Sea. The new species is distinguished from its congeners by several unique morphological characteristics, including a smooth carapace armed with isolated spiny granules, chelipeds featuring large spines and granules, and a strongly concave endopodite of the first maxilliped. Molecular analysis using mitochondrial cytochrome oxidase I DNA barcodes provides further support for the identification of the new species. The relationships between the new species and its congeners were elucidated through a combination of morphological and molecular evidence. Diagnostic characteristics for differentiation among species of Cymo are discussed, and an updated key to the species of the genus is provided.

Allopatric mosaics in the Indo-West Pacific crab subfamily Chlorodiellinae reveal correlated patterns of sympatry, genetic divergence, and genitalic disparity

Molecular studies have revealed that many species once thought to be wide-ranging in the Indo-West Pacific contain allopatric mosaics of endemic lineages. These lineages provide compelling evidence that substantial time is needed to evolve isolating mechanisms sufficient to permit successful secondary sympatry, and that divergence is initiated in allopatry. In this context, questions arise regarding the nature, timing, and origin of isolating mechanisms that permit secondary sympatry. We present a phylogeny of the crab subfamily Chlorodiellinae which displays allopatric mosaics within species. These allopatric lineages typically do not have divergent male genitalia, while older sympatric lineages do. We tested the relationship between genetic distance (proxy for time), sympatry, and the divergence of male genitalic morphology. Our results suggest that male genitalic divergence is not involved in the initiation of speciation in chlorodielline crabs, having likely occurred only after isolation began in allopatry. However, morphological evolution of genitalia seemingly does play an important role in completing the process of speciation in these crabs.

A comprehensive molecular phylogeny of the brachyuran crab superfamily Xanthoidea provides novel insights into its systematics and evolutionary history

Maximum likelihood and Bayesian phylogenies for the brachyuran crab superfamily Xanthoidea were estimated based on three mitochondrial and four nuclear genes to infer phylogenetic relationships and inform taxonomy. Habitat data was then used in conjunction with several diversification rates analyses (BAMM, BiSSE, HiSSE, and FiSSE) to test evolutionary hypotheses regarding the diversification of xanthoid crabs. The phylogenies presented are the most comprehensive to date in terms of global diversity as they include all four constituent families (Xanthidae, Panopeidae, Pseudorhombilidae, and Linnaeoxanthidae) spanning all oceans in which xanthoid crabs occur. Six Xanthoidea families are recognised. Panopeidae and Xanthidae sensu stricto are the two largest family-level clades, which are reciprocally monophyletic. Pseudorhombilidae is nested within and is here treated as a subfamily of Panopeidae. Former subfamilies or tribes of Xanthidae sensu lato are basally positioned clades in Xanthoidea and are here assigned family-level ranks: Garthiellidae, Linnaeoxanthidae, Antrocarcinidae, and Nanocassiopidae. The genera Linnaeoxantho and Melybia were recovered in separate clades with Linnaeoxantho being sister to the family Antrocarcinidae, while Melybia was recovered within the family Panopeidae. The existing subfamily classification of Xanthidae and Panopeidae is drastically restructured with 20 xanthid and four panopeid subfamilies provisionally recognised. Diversification-time analyses inferred the origin of Xanthoidea and Garthiellidae in the Eocene, while the other families originated during the Oligocene. The majority of genus- and species-level diversification took place during the Miocene. Ancestral state reconstruction based on depth of occurrence (shallow vs. deep water) shows some ambiguity for the most recent common ancestor of Xanthoidea and Nanocassiopidae. The most recent common ancestors of Antrocarcinidae and Panopeidae were likely deep-water species, while those of Garthiellidae and Xanthidae were probably shallow-water species. Several shifts in net diversification rates were detected but they were not associated with depth-related habitat transitions.

A review of the common crab genus Macromedaeus Ward, 1942 (Brachyura, Xanthidae) from China Seas with description of a new species using integrative taxonomy methods

Macromedaeus is one of the most common xanthid genera in shallow waters of the Indo-West Pacific. In this study, we describe a new species, Macromedaeus hainanensis sp. nov., and report on two newly recorded species, M. quinquedentatus (Krauss, 1843) and M. orientalis (Takeda & Miyake, 1969) from Hainan Island, South China Sea. M. hainanensis is most related to M. distinguendus (De Haan, 1833-1850) and M. orientalis on the carapace shape and granular appearance, but can be distinguished by unique morphological characteristics especially its front, pereopods and male first gonopod. Taxonomic identities of the six Macromedaeus species recorded from China seas are discussed, and a phylogenetic analyzation is performed on Macromedaeus and related taxa based on three mitochondrial and two nuclear markers (12S, 16S, COI, H3, 18S). Integrated taxonomic evidence is used to support the taxonomic status of each species.

Annotated and updated checklist of marine crabs (Decapoda: Brachyura) of Mozambique supported by morphological and molecular data from shelf and slope species of the MOZAMBIQUE surveys

An updated checklist of Mozambican marine brachyuran crabs is generated based on an exhaustive revision of the existing literature, together with the additional records provided by the specimens collected throughout the three MOZAMBIQUE surveys carried out in Mozambican waters during three consecutive years (20072009) by the Instituto Espaol de Oceanografa, (Spanish Institute of Oceanography, IEO). A total of 269 species, grouped in 15 superfamilies, 26 families and 172 genera are reported in the checklist, and a detailed inventory is produced with the list and remarks about the brachyuran species collected. Thirty-nine crab species belonging to 19 families were identified based on morphological characteristics and/or genetic tools. DNA barcode sequences (16S rRNA and/or COI) were obtained for 37 species, including 16S and COI sequences that are new for 26 and 14 species, respectively. Colour photographs of fresh specimens illustrate the comments about most species, being the first time that the original colour pattern is described for some of them. New records in Mozambican waters are reported for the species Paromolopsis boasi, Mursia aspera, Carcinoplax ischurodous, Tanaoa pustulosus, Euclosiana exquisita, Oxypleurodon difficilis, Naxioides robillardi, Samadinia galathea, Cyrtomaia gaillardi, Paramaja gibba, Pleistacantha ori, Parathranites granosus, Parathranites orientalis, Ovalipes iridescens and Charybdis smithii, and second records for Moloha alcocki, Samadinia pulchra and Charybdis africana. In addition, Raninoides crosnieri, S. galathea and P. ori were collected for the first time after their descriptions. The female of Samadinia galathea is described for the first time, and a potential new species of Mursia is reported. Some records expand the known bathymetric range of certain species and/or their general distribution. New molecular and morphological data suggest the necessity of the revision of P. boasi, R. crosnieri, C. africana and the genera Platymaia and Carcinoplax. The variability and taxonomic validity of some morphological characters in brachyuran systematic is discussed.

DNA barcoding allows identification of undescribed crab megalopas from the open sea

Megalopas of 15 brachyuran crab species collected in the open sea plankton, and unknown until now, were identified using DNA barcodes (COI and 16S rRNA). Specimens belonging to the families Portunidae, Pseudorhombilidae and Xanthidae (Crustacea, Decapoda, Brachyura), and corresponding to the species Achelous floridanus, Arenaeus mexicanus, Callinectes amnicola, C. arcuatus, C. ornatus, C. toxones, Charybdis (Charybdis) hellerii, Portunus hastatus, Thalamita admete, Scopolius nuttingi, Etisus odhneri, Liomera cinctimanus, Neoliomera cerasinus, Pseudoliomera variolosa, and Williamstimpsonia stimpsoni, are described and illustrated, and compared with other congeneric species previously described. We also provide a new geographical record for N. cerasinus and the most remarkable features for each species.

The complete mitochondrial genome of spiny spooner Etisus dentatus (Herbst, 1785) using high-throughput sequencing

The whole mitochondrial genome of the spiny spooner Etisus dentatus collected from the South China Sea was determined for the first time using high-throughput sequencing. The circular mitogenome of E. dentatus is 15,884 bp, with 13 protein-coding genes (PCGs), 22 tRNA genes, two rRNA genes. The base composition is significantly biased (A, G, T, and C was 37.9%, 17.8%, 34.0%, and 10.3%, respectively) with A + T contents of 71.9%. Among 13 PCGs, all PCGs use a normal ATN as the start codon, and ten of them end with TAA or TAG except COX1, COX2 and CYTB gene using a single T as the stop codon. The phylogenetic tree showed that E. dentatus forms a cluster E. anaglyptus while Atergatis integerrimus form a sister group to this cluster.

The oldest freshwater crabs: claws on dinosaur bones

With approximately 1,500 extant species, freshwater crabs (Decapoda: Brachyura) are among the most diverse decapod crustaceans. Nevertheless, their fossil record is extremely limited: only Potamidae, Potamonautidae and Trichodactylidae are reported up to the Eocene of the Neotropics so far. This work documents unusually large decapod claws from the Upper Cretaceous (Campanian) continental deposits of Velaux and vicinity (southern France), in close association with large vertebrate remains. In addition to (1) the systematic assignment of these claws, the study addresses (2) the salinity trends in the deposit environment from its faunal assemblage and the elementary chemical patterns of fossils, and (3) the likely scenario for their auto/allochthony in the Velaux fluvial system. These claws belong to a new taxon, Dinocarcinus velauciensis n. gen. n. sp., referred to as Portunoidea sensu lato, a group of “true” crabs nowadays linked to marine systems. However, the faunal assemblage, the claw taphonomy and the carbonates Y/Ho signatures support their ancient freshwater/terrestrial ecology, making them the oldest reported continental brachyurans and extending the presence of crabs in freshwater environments by 40 Ma. Either as primary or as secondary freshwater crabs, the occurrence of these portunoids in Velaux is an evidence for the independent colonizations of continental environments by multiple brachyuran clades over time, as early as the Campanian.

A collection of crabs (Crustacea, Brachyura) from the southwestern coast of India, with a discussion of the systematic position of Nectopanope Wood-Mason in Wood-Mason & Alcock, 1891 (Euryplacidae)

A report on the brachyuran crabs collected from the southwestern coast of India by the Indian research vessel FORV Sagar Sampada is presented. The material consists of 13 species from three genera and five families, of which four are new records for India: Heteroplaxmaldivensis (Rathbun, 1902) (Euryplacidae), Cryptopodiacollifer Flipse, 1930 (Parthenopidae), Thalamitamacrodonta Borradaile, 1903 (Portunidae), and Paraxanthodescumatodes (MacGilchrist, 1905) (Xanthidae). The cruise also obtained the first known male of Nectopanoperhodobaphes Wood-Mason in Wood-Mason & Alcock, 1891 (type species of Nectopanope Wood-Mason in Wood-Mason & Alcock, 1891), and its characters show that it is in fact a member of the Euryplacidae Stimpson, 1871. The genus had previously been incorrectly classified in the Xanthidae MacLeay, 1838.

Analysis of complete mitochondrial genome of Etisus anaglyptus (Arthropoda, Decapoda, Xanthidae) with phylogenetic consideration

The complete mitochondrial genome is sequenced and analyzed from a xanthid crab Etisus anaglyptus, which is the first complete mitochondrial genome for the genus. The mitochondrial genome length of E. anaglyptus is 16,435 bp and it is composed of 13 protein coding genes, two ribosomal RNA genes, and 22 tRNA genes. The structure and gene orientation of the mitochondrial genome is identical with the other brachyuran records. Furthermore, phylogenetic relationships of the infraorder Brachyura evaluated by mitochondrial protein coding genes. The phylogenetic study showed that E. anaglyptus is positioned in the superfamily Xanthoidea and the closest species to E. anaglyptus is Leptodius sanguineus.

Improved heat tolerance in air drives the recurrent evolution of air-breathing

The transition to air-breathing by formerly aquatic species has occurred repeatedly and independently in fish, crabs and other animal phyla, but the proximate drivers of this key innovation remain a long-standing puzzle in evolutionary biology. Most studies attribute the onset of air-breathing to the repeated occurrence of aquatic hypoxia; however, this hypothesis leaves the current geographical distribution of the 300 genera of air-breathing crabs unexplained. Here, we show that their occurrence is mainly related to high environmental temperatures in the tropics. We also demonstrate in an amphibious crab that the reduced cost of oxygen supply in air extends aerobic performance to higher temperatures and thus widens the animal's thermal niche. These findings suggest that high water temperature as a driver consistently explains the numerous times air-breathing has evolved. The data also indicate a central role for oxygen- and capacity-limited thermal tolerance not only in shaping sensitivity to current climate change but also in underpinning the climate-dependent evolution of animals, in this case the evolution of air-breathing.

A new genus for Chlorodiella longimana (H. Milne Edwards) supported by morphology and molecular data, with a preliminary phylogeny of the Chlorodiellinae (Crustacea : Decapoda : Xanthidae)

Chlorodiella longimana is the only chlorodielline species presently known from the western Atlantic Ocean. Although C. longimana superficially resembles other species of the genus in general appearance of the carapace, morphological analyses revealed a suite of characters that separate it from all other known species of Chlorodiella – in particular, ambulatory legs having dactyli with a single tip and a basal antennal segment with a lateral flange that extends halfway into the orbital hiatus, excluding the antennal flagellum. In addition, a phylogenetic analysis of the Chlorodiellinae inferred from three mitochondrial markers (12S, 16S, COXI) and a nuclear marker (histone H3), confirms that C. longimana is genetically distinct from its congeners. A new genus, Ratha, is proposed to accommodate C. longimana. In addition, a phylogenetic analysis of six chlorodielline genera indicates that the subfamily is polyphyletic as presently defined.