Phylogenetic position, systematic status, and divergence time of the Procarididea (Crustacea: Decapoda)

Sequence comparison of the mitochondrial genomes of five caridean shrimps of the infraorder Caridea: phylogenetic implications and divergence time estimation

BACKGROUND

The Caridea, affiliated with Malacostraca, Decapoda, and Pleocyemata, constitute one of the most significant shrimp groups. They are widely distributed across diverse aquatic habitats worldwide, enriching their evolutionary history. In recent years, considerable attention has been focused on the classification and systematic evolution of Caridea, yet controversies still exist regarding the phylogenetic relationships among families.

METHODS

Here, the complete mitochondrial genome (mitogenome) sequences of five caridean species, namely Heterocarpus sibogae, Procletes levicarina, Macrobrachium sp., Latreutes anoplonyx, and Atya gabonensis, were determined using second-generation high-throughput sequencing technology. The basic structural characteristics, nucleotide composition, amino acid content, and codon usage bias of their mitogenomes were analyzed. Selection pressure values of protein-coding genes (PCGs) in species within the families Pandalidae, Palaemonidae, and Atyidae were also computed. Phylogenetic trees based on the nucleotide and amino acid sequences of 13 PCGs from 103 caridean species were constructed, and divergence times for various families within Caridea were estimated.

RESULTS

The mitogenome of these five caridean species vary in length from 15,782 to 16,420 base pairs, encoding a total of 37 or 38 genes, including 13 PCGs, 2 rRNA genes, and 22 or 23 tRNA genes. Specifically, L. anoplonyx encodes an additional tRNA gene, bringing its total gene count to 38. The base composition of the mitogenomes of these five species exhibited a higher proportion of adenine-thymine (AT) bases. Six start codons and four stop codons were identified across the five species. Analysis of amino acid content and codon usage revealed variations among the five species. Analysis of selective pressure in Pandalidae, Palaemonidae, and Atyidae showed that the Ka/Ks values of PCGs in all three families were less than 1, indicating that purifying selection is influencing on their evolution. Phylogenetic analysis revealed that each family within Caridea is monophyletic. The results of gene rearrangement and phylogenetic analysis demonstrated correlations between these two aspects. Divergence time estimation, supported by fossil records, indicated that the divergence of Caridea species occurred in the Triassic period of the Mesozoic era, with subsequent differentiation into two major lineages during the Jurassic period.

CONCLUSIONS

This study explored the fundamental characteristics and phylogenetic relationships of mitogenomes within the infraorder Caridea, providing valuable insights into their classification, interspecific evolutionary patterns, and the evolutionary status of various Caridea families. The findings provide essential references for identifying shrimp species and detecting significant gene rearrangements within the Caridea infraorder.

Across the benthic and pelagic realms: a species‐level phylogeny of Benthesicymidae (Crustacea:Decapoda)

Benthesicymidae is a monophyletic group of Decapoda adapted to a life on the sea-floor, in the near-bottom layer, in the bathy- and in the mesopelagic, within an impressive depth range from a few hundred metres (Gennadas) to several thousand metres (Benthesicymus). Higher taxa are known to conquer all main oceanic biotopes such as the benthic, benthopelagic, and pelagic and a wide depth range but few family-level groups have clades evolved within all these oceanic realms. Therefore, the global fauna of Benthesicymidae provides a rare opportunity for an insight into phylogenetic processes favouring colonisation of all principal oceanic biotopes. The first comprehensive phylogenetic study of Benthesicymidae (all 37 valid species) is based on six molecular markers and 105 morphological characters (including 72 female and male copulatory characters). Analyses resulted in trees with similar topology and the same set of robust clades. Molecular methods based on 167 sequences (84 new) provided better resolution of deeper nodes and generally higher support of the clades, while morphological methods allowed analyses of all valid species of the global fauna. Phylogenetic analyses support the monophyly and robustness of all currently known genera except Gennadas, which was split into Gennadas Bate, 1881, Amalopenaeus Smith, 1882, and Notogennema gen. nov. We also retrieved two major clades for which we erected two new subfamilies: Benthesicyminae subfam. nov. (presumably benthic, genera Altelatipes, Bathicaris, Benthesicymus, and Benthonectes) and Gennadinae subfam. nov. (presumably pelagic, genera Amalopenaeus, Bentheogennema, Benthoecetes, Boreogennema, Gennadas, Maorrancaris, and Notogennema gen. nov.). We revealed two groups of morphological characters, that are interlinked evolutionarily: (1) petasma and thelycum; (2) body, mouthparts, and pereopods. Morphological traits within benthic and pelagic clades are different, a model explaining the differences is proposed. Along with previous studies, our results confirm the idea that the elaboration of the copulatory structures is a key to successful colonisation of the pelagic realm. These results extend our knowledge about evolution in the largest habitual biotope of our planet and phylogenetic processes favouring colonisation of all principal oceanic biotopes.

Molecular phylogeny and species delimitation of the freshwater prawn Macrobrachium pilimanus species group, with descriptions of three new species from Thailand

Specific status and species boundaries of several freshwater prawns in the Macrobrachium pilimanus species group remain ambiguous, despite the taxonomic re-description of type materials and additional specimens collected to expand the boundaries of some species. In this study, the "pilimanus" species group of Macrobrachium sensu Johnson (1958) was studied using specimens collected from montane streams of Thailand. Molecular phylogenetic analyses based on sequences of three molecular markers (COI, 16S and 18S rRNA) were performed. The phylogenetic results agreed with morphological identifications, and indicated the presence of at least nine putative taxa. Of these, six morphospecies were recognised as M. malayanum, M. forcipatum, M. dienbienphuense, M. hirsutimanus, M. eriocheirum, and M. sirindhorn. Furthermore, three morphologically and genetically distinct linages were detected, and are described herein as M. naiyanetri Siriwut sp. nov. , M. palmopilosum Siriwut sp. nov. and M. puberimanus Siriwut sp. nov. The taxonomic comparison indicated wide morphological variation in several species and suggested additional diagnostic characters that are suitable for use in species diagnoses, such as the shape and orientation of fingers, the rostrum form, and the presence or absence of velvet pubescence hairs and tuberculated spinulation on each telopodite of the second pereiopods. The "pilimanus" species group was portrayed as non-monophyletic in both ML and BI analyses. The genetic structure of different geographical populations in Thailand was detected in some widespread species. The species delimitation based on the four delimitation methods (BIN, ABGD, PTP and GMYC) suggested high genetic diversity of the "pilimanus" species group and placed the candidate members much higher than in previous designations based on traditional morphology. This finding suggests that further investigation of morphological and genetic diversity of Southeast Asian freshwater prawns in the genus Macrobrachium is still required to provide a comprehensive species list to guide efforts in conservation and resource management.

A phylogenomic framework, evolutionary timeline and genomic resources for comparative studies of decapod crustaceans

Comprising over 15 000 living species, decapods (crabs, shrimp and lobsters) are the most instantly recognizable crustaceans, representing a considerable global food source. Although decapod systematics have received much study, limitations of morphological and Sanger sequence data have yet to produce a consensus for higher-level relationships. Here, we introduce a new anchored hybrid enrichment kit for decapod phylogenetics designed from genomic and transcriptomic sequences that we used to capture new high-throughput sequence data from 94 species, including 58 of 179 extant decapod families, and 11 of 12 major lineages. The enrichment kit yields 410 loci (greater than 86 000 bp) conserved across all lineages of Decapoda, more clade-specific molecular data than any prior study. Phylogenomic analyses recover a robust decapod tree of life strongly supporting the monophyly of all infraorders, and monophyly of each of the reptant, 'lobster' and 'crab' groups, with some results supporting pleocyemate monophyly. We show that crown decapods diverged in the Late Ordovician and most crown lineages diverged in the Triassic-Jurassic, highlighting a cryptic Palaeozoic history, and post-extinction diversification. New insights into decapod relationships provide a phylogenomic window into morphology and behaviour, and a basis to rapidly and cheaply expand sampling in this economically and ecologically significant invertebrate clade.

Mushroom body evolution demonstrates homology and divergence across Pancrustacea

Descriptions of crustacean brains have focused mainly on three highly derived lineages of malacostracans: the reptantian infraorders represented by spiny lobsters, lobsters, and crayfish. Those descriptions advocate the view that dome- or cap-like neuropils, referred to as 'hemiellipsoid bodies,' are the ground pattern organization of centers that are comparable to insect mushroom bodies in processing olfactory information. Here we challenge the doctrine that hemiellipsoid bodies are a derived trait of crustaceans, whereas mushroom bodies are a derived trait of hexapods. We demonstrate that mushroom bodies typify lineages that arose before Reptantia and exist in Reptantia thereby indicating that the mushroom body, not the hemiellipsoid body, provides the ground pattern for both crustaceans and hexapods. We show that evolved variations of the mushroom body ground pattern are, in some lineages, defined by extreme diminution or loss and, in others, by the incorporation of mushroom body circuits into lobeless centers. Such transformations are ascribed to modifications of the columnar organization of mushroom body lobes that, as shown in Drosophila and other hexapods, contain networks essential for learning and memory.

A phylogeny-based revision of the shrimp genera Altelatipes, Benthonectes and Benthesicymus (Crustacea: Decapoda: Benthesicymidae)

A phylogenetic study of deep-sea dendrobranchiate genera Altelatipes, Benthesicymus and Benthonectes based on four molecular markers and 91 morphological characters is presented. All currently recognized species of these genera, representatives of all other genera and species groups of Benthesicymidae, and three outgroups were included in the analyses. The molecular and morphological methods retrieved similar results, the molecular methods provided better resolution of deeper nodes and higher clade support. Both types of analyses showed paraphyly of Benthesicymus, which encompass five robust clades, four of which are diagnosed as new genera (type species in parentheses): Benthesicymus s.s. (B. crenatus), Bathicaris gen. nov. (Benthesicymus brasiliensis), Dalicaris gen. nov. (Benthesicymus altus), Trichocaris gen. nov. (Benthesicymus bartletti) and Maorrancaris gen. nov. (Benthesicymus investigatoris). Altelatipes was found to be monophyletic. The evolution of the major clades of Benthesicymidae is shown to be linked to trophic specialization, while further divergence at the genus level is mainly related to sexual evolution seen in the elaboration of the copulatory structures. We provide amended diagnoses of the previously recognized and new genera, key to species of each of these genera and include an updated key to genera of Benthesicymidae.

A hard-earned draw: phylogeny-based revision of the deep-sea shrimp Bentheogennema (Decapoda: Benthesicymidae) transfers two species to other genera and reveals two new species

The phylogenetic study of the deep-sea genus Bentheogennema is based on four molecular markers and 79 morphological characters. All four previously recognized species and two new species of Bentheogennema, representatives of all other genera and species groups of Benthesicymidae, and three outgroups were included in the analyses. We have examined and coded six major groups of morphological characters related to the carapace (three characters), the pleon and the telson (14), the mouthparts (nine), the armature of the pereopods (five), the thelycum (27) and the petasma (21). Results of morphological and molecular analyses were similar. Two species were transferred from Bentheogennema to other genera (for one of them a new genus was erected) and two new species of Bentheogennema were described. Three pelagic genera (Gennadas, Bentheogennema and a new genus) created a robust clade. The divergence of this clade is linked to ‘smoothening’ of the body (reduction of the branchiostegal spine on the carapace, reduction and loss of the dorsolateral spines and the end-piece on the telson) and elaboration of the copulatory structures. We provide amended diagnoses of these three pelagic genera and key to species of Bentheogennema.

Divergence history and hydrothermal vent adaptation of decapod crustaceans: A mitogenomic perspective

Decapod crustaceans, such as alvinocaridid shrimps, bythograeid crabs and galatheid squat lobsters are important fauna in the hydrothermal vents and have well adapted to hydrothermal vent environments. In this study, eighteen mitochondrial genomes (mitogenomes) of hydrothermal vent decapods were used to explore the evolutionary history and their adaptation to the hydrothermal vent habitats. BI and ML algorithms produced consistent phylogeny for Decapoda. The phylogenetic relationship revealed more evolved positions for all the hydrothermal vent groups, indicating they migrated from non-vent environments, instead of the remnants of ancient hydrothermal vent species, which support the extinction/repopulation hypothesis. The divergence time estimation on the Alvinocarididae, Bythograeidae and Galatheoidea nodes are located at 75.20, 56.44 and 47.41–50.43 Ma, respectively, which refers to the Late Cretaceous origin of alvinocaridid shrimps and the Early Tertiary origin of bythograeid crabs and galatheid squat lobsters. These origin stories are thought to associate with the global deep-water anoxic/dysoxic events. Total eleven positively selected sites were detected in the mitochondrial OXPHOS genes of three lineages of hydrothermal vent decapods, suggesting a link between hydrothermal vent adaption and OXPHOS molecular biology in decapods. This study adds to the understanding of the link between mitogenome evolution and ecological adaptation to hydrothermal vent habitats in decapods.

Comparative mitogenomics of the Decapoda reveals evolutionary heterogeneity in architecture and composition

The emergence of cost-effective and rapid sequencing approaches has resulted in an exponential rise in the number of mitogenomes on public databases in recent years, providing greater opportunity for undertaking large-scale comparative genomic and systematic research. Nonetheless, current datasets predominately come from small and disconnected studies on a limited number of related species, introducing sampling biases and impeding research of broad taxonomic relevance. This study contributes 21 crustacean mitogenomes from several under-represented decapod infraorders including Polychelida and Stenopodidea, which are used in combination with 225 mitogenomes available on NCBI to investigate decapod mitogenome diversity and phylogeny. An overview of mitochondrial gene orders (MGOs) reveals a high level of genomic variability within the Decapoda, with a large number of MGOs deviating from the ancestral arthropod ground pattern and unevenly distributed among infraorders. Despite the substantial morphological and ecological variation among decapods, there was limited evidence for correlations between gene rearrangement events and species ecology or lineage specific nucleotide substitution rates. Within a phylogenetic context, predicted scenarios of rearrangements show some MGOs to be informative synapomorphies for some taxonomic groups providing strong independent support for phylogenetic relationships. Additional comparisons for a range of mitogenomic features including nucleotide composition, strand asymmetry, unassigned regions and codon usage indicate several clade-specific trends that are of evolutionary and ecological interest.

Complete mitochondrial genome of the first deep-sea spongicolid shrimp Spongiocaris panglao (Decapoda: Stenopodidea): Novel gene arrangement and the phylogenetic position and origin of Stenopodidea

Stenopodidea Claus, 1872 (Crustacea: Decapoda) is one of the major groups of decapods crustaceans. Hitherto, only one complete mitochondrial genome (mitogenome) from the family Stenopodidae is available for the infraorder Stenopodidea. Here, we determined the complete mitogenome of Spongiocaris panglao de Grave and Saito, 2016 using Illumina sequencing, representing the first species from the family Spongicolidae. The 15,909 bp genome is a circular molecule and consists of 13 protein-coding genes (PCGs), 2 ribosomal RNA (rRNA) genes, 22 transfer RNA (tRNA) genes and one control region. Although the overall genome organization is typical for metazoans, the mitogenome of S. panglao shows some derived characters. A + T content of 77.42% in S. pamglao mitogenome is second-highest among the dacapods described to date. The trnR gene exhibit modified secondary structure with the TψC loop completely missing, which might be a putative autapomorphy of S. pamglao mitogenome. Compared with the shallow-water stenopodidean species S. hispidus, the control region of S. pamglao exhibits three characteristics: larger size, higher A + T content, and more tandem repeat sequences. The gene order exhibited difference from the ancestral mitogenome pattern of the Pancrustacea, with 5 tRNA genes rearrangement. The result from BI was agreed with most morphological characters and molecular evidences, revealing that Stenopodidea and Reptantia had the closest relationship, as the sister group of Caridea. Still, the alternative hypothesis supported from ML topology cannot be completely rejected based on the current data. Estimated times revealed that the two stenopodideans families Stenopodidae and Spongicolidae diverged from each other around 122 Mya. The divergence time of spongicolid shrimp is in good agreement with the origin of their hexactinellid hosts (78-144 Mya).

Freshwater transitions and symbioses shaped the evolution and extant diversity of caridean shrimps

Understanding the processes that shaped the strikingly irregular distribution of species richness across the Tree of Life is a major research agenda. Changes in ecology may go some way to explain the often strongly asymmetrical fates of sister clades, and we test this in the caridean shrimps. First appearing in the Lower Jurassic, there are now ~3500 species worldwide. Carideans experienced several independent transitions to freshwater from marine habitats, while many of the marine species have also evolved a symbiotic lifestyle. Here we use diversification rate analyses to test whether these ecological traits promote or inhibit diversity within a phylogenetic framework. We demonstrate that speciation rates are more than twice as high in freshwater clades, whilst symbiotic ecologies are associated with lower speciation rates. These lower rates amongst symbiotic species are of concern given that symbioses often occur in some of the most diverse, delicately balanced and threatened marine ecosystems.

Katie Davis et al. test the hypothesis that ecological traits are linked to diversification in caridean shrimps. They find that transitions from marine to freshwater habitats contributed to higher diversification rates, whereas symbiosis is associated with a slight decrease in diversification rates.

More limbs on the tree: mitogenome characterisation and systematic position of ‘living fossil’ species Neoglyphea inopinata and Laurentaeglyphea neocaledonica (Decapoda : Glypheidea : Glypheidae)

Glypheids first appeared in the Lower Triassic period and were believed to be extinct until specimens of Neoglyphea inopinata Forest & Saint Laurent and Laurentaeglyphea neocaledonica Richer de Forges were described in 1975 and 2006, respectively. The finding of extant species has meant that molecular data can now be used to complement morphological and fossil-based studies to investigate the relationships of Glypheidea within the Decapoda. However, despite several molecular studies, the placement of this infraorder within the decapod phylogenetic tree is not resolved. One limitation is that molecular resources available for glypheids have been limited to a few nuclear and mitochondrial gene fragments. Many of the more recent large-scale studies of decapod phylogeny have used information from complete mitogenomes, but have excluded the infraorder Glypheidea due to the unavailability of complete mitogenome sequences. Using next-generation sequencing, we successfully sequenced and assembled complete mitogenome sequences from museum specimens of N. inopinata and L. neocaledonica, the only two extant species of glypheids. With these sequences, we constructed the first decapod phylogenetic tree based on whole mitogenome sequences that includes Glypheidea as one of 10 decapod infraorders positioned within the suborder Pleocyemata. From this, the Glypheidea appears to be a relatively derived lineage related to the Polychelida and Astacidea. Also in our study, we conducted a survey on currently available decapod mitogenome resources available on National Center for Biotechnology Information (NCBI) and identified infraorders that would benefit from more strategic and expanded taxonomic sampling.

The role of isolation on contrasting phylogeographic patterns in two cave crustaceans

BACKGROUND

The underlying mechanisms and processes that prompt the colonisation of extreme environments, such as caves, constitute major research themes of evolutionary biology and biospeleology. The special adaptations required to survive in subterranean environments (low food availability, hypoxic waters, permanent darkness), and the geographical isolation of caves, nominate cave biodiversity as ideal subjects to answer long-standing questions concerning the interplay amongst adaptation, biogeography, and evolution. The present project aims to examine the phylogeographic patterns exhibited by two sympatric species of surface and cave-dwelling peracarid crustaceans (Asellus aquaticus and Niphargus hrabei), and in doing so elucidate the possible roles of isolation and exaptation in the colonisation and successful adaptation to the cave environment.

RESULTS

Specimens of both species were sampled from freshwater hypogean (cave) and epigean (surface) habitats in Hungary, and additional data from neighbouring countries were sourced from Genbank. Sequencing of mitochondrial and nuclear loci revealed, through haplotype network reconstruction (TCS) and phylogenetic inference, the genetic structure, phylogeographic patterns, and divergence-time estimates of A. aquaticus and N. hrabei surface and cave populations. Contrasting phylogeographic patterns were found between species, with A. aquaticus showing strong genetic differentiation between cave and surface populations and N. hrabei lacking any evidence of genetic structure mediated by the cave environment. Furthermore, N. hrabei populations show very low levels of genetic differentiation throughout their range, which suggests the possibility of recent expansion events over the last few thousand years.

CONCLUSIONS

Isolation by cave environment, rather than distance, is likely to drive the genetic structuring observed between immediately adjacent cave and surface populations of A. aquaticus, a predominantly surface species with only moderate exaptations to subterranean life. For N. hrabei, in which populations exhibit a fully 'cave-adapted' (troglomorphic) phenotype, the lack of genetic structure suggests that subterranean environments do not pose a dispersal barrier for this surface-cave species.

Genome Sequences of Marine Shrimp Exopalaemon carinicauda Holthuis Provide Insights into Genome Size Evolution of Caridea

Crustacea, particularly Decapoda, contains many economically important species, such as shrimps and crabs. Crustaceans exhibit enormous (nearly 500-fold) variability in genome size. However, limited genome resources are available for investigating these species. Exopalaemoncarinicauda Holthuis, an economical caridean shrimp, is a potential ideal experimental animal for research on crustaceans. In this study, we performed low-coverage sequencing and de novo assembly of the E. carinicauda genome. The assembly covers more than 95% of coding regions. E. carinicauda possesses a large complex genome (5.73 Gb), with size twice higher than those of many decapod shrimps. As such, comparative genomic analyses were implied to investigate factors affecting genome size evolution of decapods. However, clues associated with genome duplication were not identified, and few horizontally transferred sequences were detected. Ultimately, the burst of transposable elements, especially retrotransposons, was determined as the major factor influencing genome expansion. A total of 2 Gb repeats were identified, and RTE-BovB, Jockey, Gypsy, and DIRS were the four major retrotransposons that significantly expanded. Both recent (Jockey and Gypsy) and ancestral (DIRS) originated retrotransposons responsible for the genome evolution. The E. carinicauda genome also exhibited potential for the genomic and experimental research of shrimps.

Phylogeny of Stenopodidea (Crustacea : Decapoda) shrimps inferred from nuclear and mitochondrial genes reveals non-monophyly of the families Spongicolidae and Stenopididae and most of their composite genera

The infraorder Stenopodidea is a relatively small group of marine decapod crustaceans including the well known cleaner shrimps, but their higher taxonomy has been rather controversial. This study provides the most comprehensive molecular phylogenetic analyses of Stenopodidea using sequence data from two mitochondrial (16S and 12S rRNA) and two nuclear (histone H3 and sodium–potassium ATPase α-subunit (NaK)) genes. We included all 12 nominal genera from the three stenopodidean families in order to test the proposed evolutionary hypothesis and taxonomic scheme of the group. The inferred phylogeny did not support the familial ranking of Macromaxillocarididae and rejected the reciprocal monophyly of Spongicolidae and Stenopididae. The genera Stenopus, Richardina, Spongiocaris, Odontozona, Spongicola and Spongicoloides are showed to be poly- or paraphyletic, with monophyly of only the latter three genera strongly rejected in the analysis. The present results only strongly support the monophyly of Microprosthema and suggest that Paraspongiola should be synonymised with Spongicola. The three remaining genera, Engystenopus, Juxtastenopus and Globospongicola, may need to be expanded to include species from other genera if their statuses are maintained. All findings suggest that the morphological characters currently adopted to define genera are mostly invalid and substantial taxonomic revisions are required. As the intergeneric relationships were largely unresolved in the present attempt, the hypothesis of evolution of deep-sea sponge-associated taxa from shallow-water free-living species could not be verified here. The present molecular phylogeny, nevertheless, provides some support that stenopoididean shrimps colonised the deep sea in multiple circumstances.

Phylogenetics reveals the crustacean order Amphionidacea to be larval shrimps (Decapoda: Caridea)

We present evidence that the single representative of the crustacean order Amphionidacea is a decapod shrimp and not a distinct order. After reviewing available morphological evidence, it is concluded that Amphionides is a larval form, but with an as yet unknown parentage. Although the most likely adult form is in the family Pandalidae, the limited molecular data available cannot fully resolve its affinity. We therefore propose to treat Amphionides reynaudii as incertae sedis within Caridea, rather than a separate family. In view of the large scale, tropical and subtropical distribution of the taxon, the possibility is discussed that Amphionides is more likely to be a composite taxon at generic level, rather than larvae of a single shrimp species.

The emergence of lobsters: phylogenetic relationships, morphological evolution and divergence time comparisons of an ancient group (decapoda: achelata, astacidea, glypheidea, polychelida)

Lobsters are a ubiquitous and economically important group of decapod crustaceans that include the infraorders Polychelida, Glypheidea, Astacidea and Achelata. They include familiar forms such as the spiny, slipper, clawed lobsters and crayfish and unfamiliar forms such as the deep-sea and "living fossil" species. The high degree of morphological diversity among these infraorders has led to a dynamic classification and conflicting hypotheses of evolutionary relationships. In this study, we estimated phylogenetic relationships among the major groups of all lobster families and 94% of the genera using six genes (mitochondrial and nuclear) and 195 morphological characters across 173 species of lobsters for the most comprehensive sampling to date. Lobsters were recovered as a non-monophyletic assemblage in the combined (molecular + morphology) analysis. All families were monophyletic, with the exception of Cambaridae, and 7 of 79 genera were recovered as poly- or paraphyletic. A rich fossil history coupled with dense taxon coverage allowed us to estimate and compare divergence times and origins of major lineages using two drastically different approaches. Age priors were constructed and/or included based on fossil age information or fossil discovery, age, and extant species count data. Results from the two approaches were largely congruent across deep to shallow taxonomic divergences across major lineages. The origin of the first lobster-like decapod (Polychelida) was estimated in the Devonian (∼409-372 Ma) with all infraorders present in the Carboniferous (∼353-318 Ma). Fossil calibration subsampling studies examined the influence of sampling density (number of fossils) and placement (deep, middle, and shallow) on divergence time estimates. Results from our study suggest including at least 1 fossil per 10 operational taxonomic units (OTUs) in divergence dating analyses. [Dating; decapods; divergence; lobsters; molecular; morphology; phylogenetics.].

Molecular phylogeny of the superfamily Palaemonoidea (Crustacea : Decapoda : Caridea) based on mitochondrial and nuclear DNA reveals discrepancies with the current classification

Palaemonoidea is one of the most speciose superfamilies of Caridea. Since it was established, several classification schemes of Palaemonoidea have been proposed and modified. However, the current classification of Palaemonoidea is still in dispute. In this study, one mitochondrial gene (16S rRNA) and three nuclear genes (histone 3, 18S rRNA and 28S rRNA) were used to explore the phylogenetic relationships among the subgroups of the superfamily Palaemonoidea, including seven families with 25 affiliated genera. Based on the combined data with both maximum likelihood and Bayesian inference analyses, the results support the monophyly of Anchistioididae and Hymenoceridae. In contrast, Gnathophyllidae is suggested to be paraphyletic and Palaemonidae is shown to be a polyphyletic group. Our analyses reveal that the subfamily Palaemoninae could be approximately divided into three clades, and the branchiostegal groove is the probable morphological evidence of the environmental transition from sea to fresh water. Besides, for some of the Palaemonoidea families, their taxonomic status is obscure. A revision of Palaemonoidea and a re-evaluation of its constituent taxa appear to be necessary even though the systematic status of the subfamily Pontoniinae is still undetermined.

Evolution and phylogeny of the mud shrimps (Crustacea: Decapoda) revealed from complete mitochondrial genomes

BACKGROUND

The evolutionary history and relationships of the mud shrimps (Crustacea: Decapoda: Gebiidea and Axiidea) are contentious, with previous attempts revealing mixed results. The mud shrimps were once classified in the infraorder Thalassinidea. Recent molecular phylogenetic analyses, however, suggest separation of the group into two individual infraorders, Gebiidea and Axiidea. Mitochondrial (mt) genome sequence and structure can be especially powerful in resolving higher systematic relationships that may offer new insights into the phylogeny of the mud shrimps and the other decapod infraorders, and test the hypothesis of dividing the mud shrimps into two infraorders.

RESULTS

We present the complete mitochondrial genome sequences of five mud shrimps, Austinogebia edulis, Upogebia major, Thalassina kelanang (Gebiidea), Nihonotrypaea thermophilus and Neaxius glyptocercus (Axiidea). All five genomes encode a standard set of 13 protein-coding genes, two ribosomal RNA genes, 22 transfer RNA genes and a putative control region. Except for T. kelanang, mud shrimp mitochondrial genomes exhibited rearrangements and novel patterns compared to the pancrustacean ground pattern. Each of the two Gebiidea species (A. edulis and U. major) and two Axiidea species (N. glyptocercus and N. thermophiles) share unique gene order specific to their infraorders and analyses further suggest these two derived gene orders have evolved independently. Phylogenetic analyses based on the concatenated nucleotide and amino acid sequences of 13 protein-coding genes indicate the possible polyphyly of mud shrimps, supporting the division of the group into two infraorders. However, the infraordinal relationships among the Gebiidea and Axiidea, and other reptants are poorly resolved. The inclusion of mt genome from more taxa, in particular the reptant infraorders Polychelida and Glypheidea is required in further analysis.

CONCLUSIONS

Phylogenetic analyses on the mt genome sequences and the distinct gene orders provide further evidences for the divergence between the two mud shrimp infraorders, Gebiidea and Axiidea, corroborating previous molecular phylogeny and justifying their infraordinal status. Mitochondrial genome sequences appear to be promising markers for resolving phylogenetic issues concerning decapod crustaceans that warrant further investigations and our present study has also provided further information concerning the mt genome evolution of the Decapoda.

Phylogenetics links monster larva to deep-sea shrimp

Mid-water plankton collections commonly include bizarre and mysterious developmental stages that differ conspicuously from their adult counterparts in morphology and habitat. Unaware of the existence of planktonic larval stages, early zoologists often misidentified these unique morphologies as independent adult lineages. Many such mistakes have since been corrected by collecting larvae, raising them in the lab, and identifying the adult forms. However, challenges arise when the larva is remarkably rare in nature and relatively inaccessible due to its changing habitats over the course of ontogeny. The mid-water marine species Cerataspis monstrosa (Gray 1828) is an armored crustacean larva whose adult identity has remained a mystery for over 180 years. Our phylogenetic analyses, based in part on recent collections from the Gulf of Mexico, provide definitive evidence that the rare, yet broadly distributed larva, C. monstrosa, is an early developmental stage of the globally distributed deepwater aristeid shrimp, Plesiopenaeus armatus. Divergence estimates and phylogenetic relationships across five genes confirm the larva and adult are the same species. Our work demonstrates the diagnostic power of molecular systematics in instances where larval rearing seldom succeeds and morphology and habitat are not indicative of identity. Larval-adult linkages not only aid in our understanding of biodiversity, they provide insights into the life history, distribution, and ecology of an organism.

Complete mitochondrial DNA sequence of Stenopus hispidus (Crustacea: Decapoda: Stenopodidea) and a novel tRNA gene cluster

As a phylogenetically valuable decapoda, a complete mitochondrial genome from Stenopodidea has not been reported to date. Here, we determined the complete mitochondrial DNA sequence of Stenopus hispidus (Olivier, 1811). The 15,528 bp genome is a circular molecule and consists of 13 protein-coding genes (PCGs) and two ribosomal RNA (rRNA) genes plus the putative control region (CR). This finding is similar to other metazoan animals but with the exception of 23 transfer RNA (tRNA) genes, which contain an additional tRNA-Gln compared with other crustaceans. With respect to the pancrustacean ground pattern mitochondria gene order, 5 tRNAs appear to be rearranged (tRNAs-Leu (CUN), Arg, Glu, Gln, and Met), one of which has also undergone inversion (tRNA-Leu (CUN)). Phylogenetic analyses reveal Stenopodidea and Reptantia form a clade sister to Caridea, which agrees with Abele and Felgenhauer's (1986) hypothesis. This topology contrasts with previous results based on morphological and some molecular data.