The Tetraconata concept: hexapod-crustacean relationships and the phylogeny of Crustacea

A growing body of evidence indicates that Crustacea and Hexapoda are sister groups, rather than Hexapoda and Myriapoda. Some recent molecular data even suggest that Mandibulata is not monophyletic, with Myriapoda and Chelicerata instead being sister groups. Here, arguments for homology of the mandible throughout mandibulate arthropods and for a monophyletic Mandibulata will be presented, as well as arguments supporting the taxon Tetraconata (i.e. Crustacea + Hexapoda). The latter include molecular data (nuclear and mitochondrial ribosomal RNAs and protein coding genes), and morphological characters such as ommatidial structure, the presence of neuroblasts and a very similar axonogenesis of pioneer neurons. However, crustaceans are insufficiently sampled for the molecular data, and studies of neurogenesis are lacking for many crustacean taxa. Remipedia, Cephalocarida and Maxillopoda are particularly problematic. This is important for the entire problem, because monophyly of the Crustacea has not yet been proven beyond doubt and several molecular analyses suggest a paraphyletic Crustacea. Here, arguments for the monophyly of the Crustacea are reviewed and two alternatives for the relationships between the five higher taxa Remipedia, Cephalocarida, Maxillopoda, Branchiopoda and Malacostraca are discussed: the Entomostraca concept sensu Walossek with Malacostraca as sister group to Cephalocarida, Maxillopoda and Branchiopoda, and the Thoracopoda concept sensu Hessler with Cephalocarida, Branchiopoda and Malacostraca forming a monophylum.     

Nearly complete mitochondrial genome of Polyascus gregaria and the phylogenetic relationships among maxillopodans

We determined for the first time the nearly complete mitochondrial genome sequence of the entozoic Polyascus gregaria, a representative of Rhizocephala, Cirripedia. The nearly complete mitogenome was 15, 465 bp in length, consisting of 11 protein-coding genes, two rRNA genes, 22 tRNA genes and one major incomplete noncoding region. In total there are 73 overlapping nucleotides and 17 spacers between genes. All genes sequenced in P. gregaria mtDNA (including RNAs) were encoded on the same strand of the DNA, and the gene arrangement differed from that of other metazoan animals. The mitochondrial genome rearrangements included translocation of at least 8 genes and even inversion of the coding polarity of at least 2 genes. Comparative analysis of the gene orders with other maxillopodan mtDNAs showed that the unique characteristics of the thoracican cirripeds lineage were not observed in this representative of rhizocephalan. Phylogenetic analyses supported a close affinity of Rhizocephala to Thoracica. By adding the mitochondrial genomes from 4 copepods, the reciprocally monophyletic cirripeds and copepods clustered as sister groups, refusing the close relationship between Cirripedia and Remipedia. However, the monophyly of Maxillopoda was not supported in this study.     

Reflections on the Phylogenetic Position of the Cephalocarida

This essay re-evaluates the phylogenetic position of the Cephalocarida in the light of the recently discovered Orsten (Upper Cambrian) crustaceans, the living Remipedia, and new interpretation of the Paleozoic ‘trilobitomorphs’. The Orsten crustaceans reinforce the belief that cephalocarid external morphology is primitive, except to show that the early crustacean trunk limb was almost surely biramous. The epipod appears to be a synapomorphy of the Cephalocarida, Branchiopoda and Malacostraca, thus justifying the existence of the Thoracopoda, which is coequal to the Maxillopoda and Remipedia. External morphology of the Remipedia is too specialized to support the hypothesis that it approximates that of the urcrustacean. Crustaceans and chelicerates are still best regarded as subgroups of the Schizoramia.     

Pancrustacean phylogeny: hexapods are terrestrial crustaceans and maxillopods are not monophyletic

Recent molecular analyses indicate that crustaceans and hexapods form a clade (Pancrustacea or Tetraconata), but relationships among its constituent lineages, including monophyly of crustaceans, are controversial. Our phylogenetic analysis of three protein-coding nuclear genes from 62 arthropods and lobopods (Onychophora and Tardigrada) demonstrates that Hexapoda is most closely related to the crustaceans Branchiopoda (fairy shrimp, water fleas, etc.) and Cephalocarida + Remipedia, thereby making hexapods terrestrial crustaceans and the traditionally defined Crustacea paraphyletic. Additional findings are that Malacostraca (crabs, isopods, etc.) unites with Cirripedia (barnacles, etc.) and they, in turn, with Copepoda, making the traditional crustacean class Maxillopoda paraphyletic. Ostracoda (seed shrimp)--either all or a subgroup--is associated with Branchiura (fish lice) and likely to be basal to all other pancrustaceans. A Bayesian statistical (non-clock) estimate of divergence times suggests a Precambrian origin for Pancrustacea (600 Myr ago or more), which precedes the first unambiguous arthropod fossils by over 60 Myr.     

Phylogenetic position of the Pentastomida and (pan)crustacean relationships

Pentastomids are a small group of vermiform animals with unique morphology and parasitic lifestyle. They are generally recognized as being related to the Arthropoda; however, the nature of this relationship is controversial. We have determined the complete sequence of the mitochondrial DNA (mtDNA) of the pentastomid Armillifer armillatus and complete or nearly complete mtDNA sequences from representatives of four previously unsampled groups of Crustacea: Remipedia (Speleonectes tulumensis), Cephalocarida (Hutchinsoniella macracantha), Cirripedia (Pollicipes polymerus) and Branchiura (Argulus americanus). Analyses of the mtDNA gene arrangements and sequences determined in this study indicate unambiguously that pentastomids are a group of modified crustaceans probably related to branchiurans. In addition, gene arrangement comparisons strongly support an unforeseen assemblage of pentastomids with maxillopod and cephalocarid crustaceans, to the exclusion of remipedes, branchiopods, malacostracans and hexapods.     

Setal morphology and cirral setation of thoracican barnacle cirri: adaptations and implications for thoracican evolution

Thoracic cirripedes are sessile crustaceans that use six pairs of thoracic appendages (the cirri) to catch and handle food. We used scanning electron microscopy to examine the cirri, which include one to three pairs of maxillipeds in six species of thoracican barnacles, in search of correlations between cirral setation and feeding mode. The species studied comprise both pedunculate and sessile forms and represent a wide range of marine habitats as well as morphologies, viz., Ibla cumingi , Octolasmis warwickii , Capitulum mitella , Pollicipes polymerus , Tetraclita japonica japonica and Megabalanus volcano . Of the pedunculates, I. cumingi has the least complex setation pattern consisting of only serrulate types. This is consistent with its very simplified feeding mode and an apparent inability to discriminate between food items. Octolasmis warwickii is slightly more modified, while both P. polymerus and C. mitella have a more diversified setation. The balanomorphan species exhibit by far the most complex cirral setation. This is consistent with the several types of suspension feeding seen in these species, their ability to identify and sort captured food items and even to perform microfiltration in the mantle cavity using the setae on their three pairs of maxillipeds. Our results indicate that in thoracican barnacles, adaptations in feeding behaviour are associated with changes in the setation pattern of the cirri. In addition, the setal types and their distribution on the cirri are potential new characters in future morphology-based analyses of the phylogeny of the Cirripedia Thoracica.     

Origin of the Ostracoda and their maxillopodan and hexapodan affinities

There are Cambrian fossils attributed to the Ostracoda but the extant subclasses Podocopa and Myodocopa do not appear until the Ordovician. At this time the morphologically similar, free-living ancestors of the now sedentary Thecostraca (Ascothoracida, Acrothoracica and Cirripedia) may have still been extant, and from an ecological point of view it seems likely that, by and large, ostracods replaced them. However, living ostracods have an abbreviated, direct development, and some key aspects of their morphology, such as the nature of the maxillary segment and abdomen, are conjectural. Thus the affinities between these and related taxa remain uncertain; e.g., while some contemporary carcinologists place Ostracoda as a taxon coordinate with the Branchiopoda, Remipedia, Cephalocarida, Maxillopoda, Malacostraca, others tentatively or unequivocally ally them with the Maxillopoda (generally Mystacocarida, Copepoda, Tantulocarida and Thecostraca, and sometimes Branchiura and Pentastomida). Others, largely involved with fossils, have stretched the definition of the Maxillopoda even further, to the point where it seems even less likely a monophyletic taxon. Until recently cladistic analyses utilizing genetic (largely 18S rDNA) as well traditional morphological characteristics have given confusing results regarding the affinities between these taxa, and an important one suggested the Ostracoda might even be diphyletic. Furthermore, a very recent genetic study utilizing protein encoding genes places a podocopine ostracod among the most primitive of the extant crustaceans (Branchiopoda, Cephalocarida Remipedia and Mystacocarida), and then generally at the base of a lineage leading to the Malacostraca, a lineage giving rise to copepods and cirripeds along the way. This indicates these so-called maxillopodan taxa evolved independently from a malacostracan-like ancestor, and if so they are convergent. And finally, from genetic studies it is not only becoming well documented the Crustacea rather than Myriapoda gave rise to the Hexapoda, but it appears the Hexapoda stem from among the lower rather than the higher crustaceans, possibly even from the Ostracoda. Whether there were terrestrial ostracods at the time hexapods appeared in the Lower Ordovician is unknown, but the modest diversity of terrestrial ostracods today are podocopines which also first appeared in the Lower Ordovician. Thus, if current interpretations of living ostracodan and fossil hexapodan body plans are largely correct, it can be hypothesized the Ostracoda are close to the ancestor of the Hexapoda.     

Phylogeny and evolution of life history strategies of the parasitic barnacles (Crustacea, Cirripedia, Rhizocephala)

The barnacles (Crustacea, Cirripedia) consist of three well-defined orders: the conventional filter-feeding barnacles (Thoracica), the burrowing barnacles (Acrothoracica), and the parasitic barnacles (Rhizocephala). Thoracica and Acrothoracica feed by catching food particles from the surrounding seawater using their thoracic appendages while members of Rhizocephala are exclusively parasitic. The parasite consists of a sac-shaped, external reproductive organ situated on the abdomen of its crustacean host and a nutrient-absorbing root system embedded into the heamolymph of the host. In order to resolve the phylogenetic relationship of the order Rhizocephala and elucidate the evolution of the different life history strategies found within the Rhizocephala, we have performed the first comprehensive phylogenetic analysis of the group. Our results indicate that Rhizocephala is monophyletic with a filter-feeding barnacle-like ancestor. The host-infective stage, the kentrogon larva, inserted in the lifecycle of the rhizocephalan suborder, Kentrogonida, is shown to be ancestral and most likely a homologue of the juvenile stage of a conventional thoracican barnacle. The mode of host inoculation found in the suborder Akentrogonida, where the last pelagic larval stage directly injects the parasitic material into the heamolymph of the host is derived, and has evolved only once within the Rhizocephala. Lastly, our results show that the ancestral host for extant rhizocephalans appears to be the anomuran crustaceans (Anomura), which includes hermit crabs and squat lobsters.     

The complete mitochondrial genome of the black mud crab, Scylla serrata (Crustacea: Brachyura: Portunidae) and its phylogenetic position among (pan)crustaceans

The black mud crab, Scylla serrata (Forsk?l 1775), is the most economically important edible crab in South-East Asia. In the present study, the complete mitochondrial genome of black mud crab, S.?serrata, was determined with the sequential polymerase chain reaction and primer walking sequencing. The complete mitochondrial genome was 15,721?bp in length with an A+T content of 69.2?% and contained 37 mitochondrial genes (13 protein coding genes (PCGs), 2 ribosomal RNA genes and 22 transfer RNA genes) and a control region (CR). The analysis of the CR sequence shows that it contains a multitude of repetitive fragments which can fold into hairpin-like or secondary structures and conserved elements as in other arthropods. The gene order of S. serrata mainly retains as the pancrustacean ground pattern, except for a single translocation of trnH. The gene arrangement of S. serrata appears to be a typical feature of portunid crabs. Phylogenetic analyses with concatenated amino acid sequences of 12 PCGs establishes that S. serrata in a well-supported monophyletic Portunidae and is consistent with previous morphological classification. Moreover, the phylogenomic results strongly support monophyletic Pancrustacea (Hexapoda plus “Crustaceans”). Within Pancrustacea, this study identifies Malacostraca?+?Entomostraca and Branchiopoda as the sister group to Hexapoda, which confirms that “Crustacea” is not monophyletic. Cirripedia?+?Remipedia appear to be a basal lineage of Pancrustacea. The present study also provides considerable data for the application of both population and phylogenetic studies of other crab species.     

On the phylogenetic position of insects in the Pancrustacea clade

The current views on the phylogeny of arthropods are at odds with the traditional system, which recognizes four independent arthropod classes: Chelicerata, Crustacea, Myriapoda, and Insecta. There is compelling evidence that insects comprise a monophyletic lineage with Crustacea within a larger clade named Pancrustacea, or Tetraconata. However, which crustacean group is the closest living relative of insects is still an open question. In recent phylogenetic trees constructed on the basis of large gene sequence data insects are placed together with primitive crustaceans, the Branchiopoda. This topology is often suspected to be a result of the long branch attraction artifact. We analyzed concatenated data on 77 ribosomal proteins, elongation factor 1A (EF1A), initiation factor 5A (eIF5A), and several other nuclear and mitochondrial proteins. Analyses of nuclear genes confirm the monophyly of Hexapoda, the clade uniting entognath and ectognath insects. The hypothesis of the monophyly of Hexapoda and Branchiopoda is supported in the majority of analyses. The Maxillopoda, another clade of Entomostraca, occupies a sister position to the Hexapoda + Branchiopoda group. Higher crustaceans, the Malacostraca, in most analyses appear a more basal lineage within the Pancrustacea. We report molecular synapomorphies in low homoplastic regions, which support the clade Hexapoda + Branchiopoda + Maxillopoda and the monophyletic Malacostraca including Phyllocarida. Thus, the common origin of Hexapoda and Branchiopoda and their position within Entomostraca are suggested to represent bona fide phylogenetic relationships rather than computational artifacts.