Molecular phylogeny of the benthic shallow-water octopuses (Cephalopoda: Octopodinae)

Population genetic structure of Uroteuthis edulis in the East China Sea based on the COI gene

BACKGROUND

Uroteuthis edulis is a significant economic cephalopod resource utilized by mainland China, Taiwan, and Japan. Understanding the population genetic structure of U.edulis is essential to evaluate the changes of its resource abundance. In this study, we used the COI gene as a marker to evaluate the population structure of U.edulis.

METHODS AND RESULTS

Molecular marker analysis indicated that the sampled individuals were categorized into three populations (ES1, ES2, and ES3). All populations exhibited high haplotype diversity indices (Hd) and low nucleotide diversity indices (Pi), with the ES2 population showing the lowest Hd and Pi values. Analysis of molecular variance (AMOVA) revealed that genetic variation among populations accounted for 89.06% of the total variation. The phylogenetic tree illustrated distinct lineage clustering, with each population forming a separate clade. Combined with the biological information, it was observed that the majority of female individuals were larger in size and exhibited high sexual maturity. Male individuals displayed notable differences across populations: ES1 individuals were primarily small to medium-sized with low sexual maturity, ES2 individuals were mainly medium-sized with a relatively even distribution of sexual maturity, and ES3 individuals were predominantly large-sized with high sexual maturity.

CONCLUSIONS

The findings indicate substantial genetic differentiation and ecological variation among U.edulis.

POPULATIONS

Strengthen monitoring of population resources and genetic diversity, long-term tracking of changes, and the timely formulation of scientifically sound conservation and management measures to ensure the sustainable development in future.

Genetic confirmation of Octopus insularis (Leite and Haimovici, 2008) in South Florida, United States using physical features and de novo genome assembly

The distribution of octopuses within the Octopus vulgaris species complex remains inadequately understood. Species determination can be complex and involves characterizing a specimen's physical features and comparing its genetic makeup to other populations. In this study, we present the first genetic confirmation of Octopus insularis (Leite and Haimovici, 2008) inhabiting the coastal waters of the Florida Keys, United States. We employed visual observations to identify species-specific body patterns of three wild-caught octopuses and used de novo genome assembly to confirm their species. All three specimens exhibited a red/white reticulated pattern on their ventral arm surface. Two specimens displayed body pattern components of deimatic display (white eye encircled by a light ring, with darkening around the eye). All visual observations were consistent with distinguishing features of O. insularis. We then compared mitochondrial subunits COI, COIII, and 16S in these specimens across all available annotated octopod sequences, including Sepia apama (Hotaling et al., 2021) as a control outgroup taxon. For species exhibiting intraspecific genomic variation, we included multiple sequences from geographically distinct populations. Laboratory specimens consistently clustered into a single taxonomic node with O. insularis. These findings confirm O. insularis presence in South Florida and suggest a more extensive northern distribution than previously assumed. Whole genome Illumina sequencing of multiple specimens enabled taxonomic identification with well-established DNA barcodes while also generating the first de novo full assembly of O. insularis. Furthermore, constructing and comparing phylogenetic trees for multiple conserved genes is essential for confirming the presence and delineation of cryptic species in the Caribbean.

SNP data reveals the complex and diverse evolutionary history of the blue-ringed octopus genus (Octopodidae: Hapalochlaena) in the Asia-Pacific

The blue-ringed octopus species complex (Hapalochlaena spp.), known to occur from Southern Australia to Japan, currently contains four formally described species (Hapalochlaena maculosa, Hapalochlaena fasciata, Hapalochlaena lunulata and Hapalochlaena nierstraszi). These species are distinguished based on morphological characters (iridescent blue rings and/or lines) along with reproductive strategies. However, the observation of greater morphological diversity than previously captured by the current taxonomic framework indicates that a revision is required. To examine species boundaries within the genus we used mitochondrial (12S rRNA, 16S rRNA, cytochrome c oxidase subunit 1 [COI], cytochrome c oxidase subunit 3 [COIII] and cytochrome b [Cytb]) and genome-wide SNP data (DaRT seq) from specimens collected across its geographic range including variations in depth from 3m to >100m. This investigation indicates substantially greater species diversity present within the genus Hapalochlaena than is currently described. We identified 10,346 SNPs across all locations, which when analysed support a minimum of 11 distinct clades. Bayesian phylogenetic analysis of the mitochondrial COI gene on a more limited sample set dates the diversification of the genus to ∼30mya and corroborates eight of the lineages indicated by the SNP analyses. Furthermore, we demonstrate that the diagnostic lined patterning of H. fasciata found in North Pacific waters and NSW, Australia is polyphyletic and therefore likely the result of convergent evolution. Several "deep water" (> 100m) lineages were also identified in this study with genetic convergence likely to be driven by external selective pressures. Examination of morphological traits, currently being undertaken in a parallel morphological study, is required to describe additional species within the complex.

Morphological description and mitochondrial DNA-based phylogenetic placement of a new species of Callistoctopus Taki, 1964 (Cephalopoda, Octopodidae) from the southeast waters of China

In this study, we described a new species of octopus and named it Callistoctopusxiaohongxu sp. nov. based on nine specimens captured in the waters of southeast China. Callistoctopusxiaohongxu sp. nov. is a small to moderate-sized octopus. The most characteristic and defining morphological features are the reddish-orange to reddish-brown skin, gills with 8 or 9 lamellae per demibranch, \∧/-shaped funnel organ, and small suckers. Fragments obtained from the mitochondrial cytochrome c oxidase subunit I (COI) gene of nine specimens were 593 bp in length, and the genetic distance among the specimens of C.xiaohongxu sp. nov. and the other 16 octopods ranged from 11.13 to 21.09%. Topologies resulting from ML and BI analyses of the COI gene showed a highly supported monophyletic clade (bootstrap value [BS] = 94%, posterior probability [PP] = 100%) containing all the specimens identified as C.xiaohongxu sp. nov.

Comparative brain structure and visual processing in octopus from different habitats

Octopods are masters of camouflage and solve complex tasks, and their cognitive ability is said to approach that of some small mammals. Despite intense interest and some research progress, much of our knowledge of octopus neuroanatomy and its links to behavior and ecology comes from one coastal species, the European common octopus, Octopus vulgaris. Octopod species are found in habitats including complex coral reefs and the relatively featureless mid-water. There they encounter different selection pressures, may be nocturnal or diurnal, and are mostly solitary or partially social. How these different ecologies and behavioral differences influence the octopus central nervous system (CNS) remains largely unknown. Here we present a phylogenetically informed comparison between diurnal and nocturnal coastal and a deep-sea species using brain imaging techniques. This study shows that characteristic neuroanatomical changes are linked to their habits and habitats. Enlargement and division of the optic lobe as well as structural foldings and complexity in the underlying CNS are linked to behavioral adaptation (diurnal versus nocturnal; social versus solitary) and ecological niche (reef versus deep sea), but phylogeny may play a part also. The difference between solitary and social life is mirrored within the brain including the formation of multiple compartments (gyri) in the vertical lobe, which is likened to the vertebrate cortex. These findings continue the case for convergence between cephalopod and vertebrate brain structure and function. Notably, within the current push toward comparisons of cognitive abilities, often with unashamed anthropomorphism at their root, these findings provide a firm grounding from which to work.

Octopus djinda (Cephalopoda: Octopodidae): a new member of the Octopus vulgaris group from southwest Australia

A new Octopus Cuvier, 1797 species, Octopus djinda Amor, 2021 (previously treated as O. cf. tetricus and O. aff. tetricus), is described from the shallow waters off southwest Australia. This species was classified as conspecific with O. tetricus Gould, 1852 from Australias east coast and New Zealand but is shown here to be morphologically and genetically distinct. This description is based on 25 individuals across three localities in southwest Australia, encompassing most of its distribution. Greater and non-overlapping sucker counts on the males hectocotylised arm delimit east and west coast forms. DNA barcoding using cytochrome c oxidase subunit I also successfully differentiates between these taxa; 13 polymorphisms along a 349 bp partial fragment (3.7% sequence divergence). A close relative of the O. vulgaris Cuvier, 1797 species-group, O. djinda, sp. nov. supports a highly productive fishery and is currently one of two octopod fisheries worldwide to have received sustainable certification from the Marine Stewardship Council. The taxonomic description presented here provides formal recognition of the taxonomic status of southwest Australias common octopus, O. djinda, sp. nov. and facilitates appropriate fisheries catch reporting and management.

Adaptive venom evolution and toxicity in octopods is driven by extensive novel gene formation, expansion, and loss

BACKGROUND

Cephalopods represent a rich system for investigating the genetic basis underlying organismal novelties. This diverse group of specialized predators has evolved many adaptations including proteinaceous venom. Of particular interest is the blue-ringed octopus genus (Hapalochlaena), which are the only octopods known to store large quantities of the potent neurotoxin, tetrodotoxin, within their tissues and venom gland.

FINDINGS

To reveal genomic correlates of organismal novelties, we conducted a comparative study of 3 octopod genomes, including the Southern blue-ringed octopus (Hapalochlaena maculosa). We present the genome of this species and reveal highly dynamic evolutionary patterns at both non-coding and coding organizational levels. Gene family expansions previously reported in Octopus bimaculoides (e.g., zinc finger and cadherins, both associated with neural functions), as well as formation of novel gene families, dominate the genomic landscape in all octopods. Examination of tissue-specific genes in the posterior salivary gland revealed that expression was dominated by serine proteases in non-tetrodotoxin-bearing octopods, while this family was a minor component in H. maculosa. Moreover, voltage-gated sodium channels in H. maculosa contain a resistance mutation found in pufferfish and garter snakes, which is exclusive to the genus. Analysis of the posterior salivary gland microbiome revealed a diverse array of bacterial species, including genera that can produce tetrodotoxin, suggestive of a possible production source.

CONCLUSIONS

We present the first tetrodotoxin-bearing octopod genome H. maculosa, which displays lineage-specific adaptations to tetrodotoxin acquisition. This genome, along with other recently published cephalopod genomes, represents a valuable resource from which future work could advance our understanding of the evolution of genomic novelty in this family.

Population genetics and comparative mitogenomic analyses reveal cryptic diversity of Amphioctopus neglectus (Cephalopoda: Octopodidae)

This study presented 96 cox1 and 76 cox3 genes of Amphioctopus neglectus populations. Three distinct lineages were formed from phylogenetic trees and networks constructed using haplotypes. Mitogenomes of A. neglectus-a and A. neglectus-b as the representatives of two lineages separated from population genetics were sequenced to compare with A. neglectus at the genome-level. Amphioctopus neglectus-a showed significant differences with A. neglectus, mainly reflected in gene length, intergenic regions and the secondary structure of tandem repeat motifs. Notably, two sequence deletions in mitogenomes of the two representative species were detected in different positions of major non-coding regions, which were the most distinct differences with A. neglectus. Pairwise genetic distances and the phylogenetic analysis supported the relationship of (A. neglectus-a + (A. neglectus + A. neglectus-b)). This study suggested that A. neglectus-a should be considered as a potential cryptic species of this complex, while A. neglectus-b needed further verification to be defined.

Redescription of Amphioctopus ovulum (Sasaki, 1917) (Cephalopoda : Octopodidae) and comparative morphological analyses among three species of violet-ringed octopods

Amphioctopus ovulum (Sasaki, 1917) is a small to moderate-sized octopus, which can be identified by the iridescent violet ring present in the dark ocellus on the web between the bases of arms II and III. Comprehensive taxonomic review is required to fully characterise this species because the syntypes are missing and the description is insufficiently complete for modern octopod taxonomy. In this study, the species A. ovulum is redescribed with morphological and morphometric characters of 18 specimens collected from the coastal waters of China. The distribution of A. ovulum extends from the Gulf of Thailand, through Cambodia, Vietnam, Philippines, through the South China Sea and the East China Sea to Japan. The swollen terminal organ diverticulum and long spermatophores make it possible to distinguish A. ovulum clearly from A. rex and A. neglectus, species with similar morphological characters of violet rings. Moreover, three species of violet-ringed octopods were clearly differentiated by sequences of the partial mitochondrial genes COI and COIII. Three monophyletic clades resolved in phylogenetic trees. Amphioctopus rex and A. neglectus clustered into a sister taxon, and clustered with the remaining Amphioctopus species.

Taxonomic assessment of species of the genus Octopus from the northeastern Pacific via morphological, molecular and morphometric analyses

Species of the genus Octopus from the northeastern Pacific are ecologically and economically important; however, their taxonomy is confusing and has not been comprehensively assessed. In this study, we performed a taxonomic evaluation of these species considering the morphological characteristics of the original descriptions, a molecular analysis of partial COI-gene sequences, and a traditional morphometry analysis of nine body measurements. Several interesting findings were obtained with our results: for instance, we updated the diagnoses of some species by including characters such as the number of lamellae per demibranch and the presence of chromatophores in the visceral sac; we deposited partial COI-gene sequences of species that had not been incorporated into the GenBank repository; and according to the morphometric analysis, we confirmed that the lengths of arms I–IV are relevant to discriminate the species under study. The taxa evaluated were morphologically, molecularly and morphometrically well-delimited; however, features such as funnel organ shape and arm length proportions in regard to dorsal mantle length are either not included in the diagnosis of the genus Octopus or overlap with other genera. Hence, this information, combined with the results obtained from the molecular analysis, supports the generic re-assignation of two of the species evaluated.

Surveying cephalopod diversity of the Amazon reef system using samples from red snapper stomachs and description of a new genus and species of octopus

The cephalopod fauna of the southwestern Atlantic is especially poorly-known because sampling is mostly limited to commercial net-fishing operations that are relatively inefficient at obtaining cephalopods associated with complex benthic substrates. Cephalopods have been identified in the diets of many large marine species but, as few hard structures survive digestion in most cases, the identification of ingested specimens to species level is often impossible. Samples can be identified by molecular techniques like barcoding and for cephalopods, mitochondrial 16S and COI genes have proven to be useful diagnostic markers for this purpose. The Amazon River estuary and continental shelf are known to encompass a range of different substrates with recent mapping highlighting the existence of an extensive reef system, a type of habitat known to support cephalopod diversity. The present study identified samples of the cephalopod fauna of this region obtained from the stomachs of red snappers, Lutjanus purpureus, a large, commercially-important fish harvested by fisheries using traps and hook-and-line gear that are capable of sampling habitats inaccessible to nets. A total of 98 samples were identified using molecular tools, revealing the presence of three squid species and eight MOTUs within the Octopodidae, representing five major clades. These include four known genera, Macrotritopus, Octopus, Scaeurgus and Amphioctopus, and one basal group distinct from all known octopodid genera described here as Lepidoctopus joaquini Haimovici and Sales, new genus and species. Molecular analysis of large predatory fish stomach contents was found to be an incredibly effective extended sampling method for biodiversity surveys where direct sampling is very difficult.

An integrative taxonomic approach reveals Octopus insularis as the dominant species in the Veracruz Reef System (southwestern Gulf of Mexico)

The common octopus of the Veracruz Reef System (VRS, southwestern Gulf of Mexico) has historically been considered as Octopus vulgaris, and yet, to date, no study including both morphological and genetic data has tested that assumption. To assess this matter, 52 octopuses were sampled in different reefs within the VRS to determine the taxonomic identity of this commercially valuable species using an integrative taxonomic approach through both morphological and genetic analyses. Morphological and genetic data confirmed that the common octopus of the VRS is not O. vulgaris and determined that it is, in fact, the recently described O. insularis. Morphological measurements, counts, indices, and other characteristics such as specific colour patterns, closely matched what had been reported for O. insularis in Brazil. In addition, sequences from cytochrome oxidase I (COI) and 16S ribosomal RNA (r16S) mitochondrial genes confirmed that the common octopus from the VRS is in the same highly supported clade as O. insularis from Brazil. Genetic distances of both mitochondrial genes as well as of cytochrome oxidase subunit III (COIII) and novel nuclear rhodopsin sequences for the species, also confirmed this finding (0-0.8%). We discuss our findings in the light of the recent reports of octopus species misidentifications involving the members of the 'O. vulgaris species complex' and underscore the need for more morphological studies regarding this group to properly address the management of these commercially valuable and similar taxa.

The genome of common long-arm octopus Octopus minor

Background

The common long-arm octopus (Octopus minor) is found in mudflats of subtidal zones and faces numerous environmental challenges. The ability to adapt its morphology and behavioral repertoire to diverse environmental conditions makes the species a promising model for understanding genomic adaptation and evolution in cephalopods.

Findings

The final genome assembly of O. minor is 5.09 Gb, with a contig N50 size of 197 kb and longest size of 3.027 Mb, from a total of 419 Gb raw reads generated using the Pacific Biosciences RS II platform. We identified 30,010 genes; 44.43% of the genome is composed of repeat elements. The genome-wide phylogenetic tree indicated the divergence time between O. minor and Octopus bimaculoides was estimated to be 43 million years ago based on single-copy orthologous genes. In total, 178 gene families are expanded in O. minor in the 14 bilaterian species.

Conclusions

We found that the O. minor genome was larger than that of closely related O. bimaculoides, and this difference could be explained by enlarged introns and recently diversified transposable elements. The high-quality O. minor genome assembly provides a valuable resource for understanding octopus genome evolution and the molecular basis of adaptations to mudflats.

Population genetic signatures of a climate change driven marine range extension

Shifts in species distribution, or 'range shifts', are one of the most commonly documented responses to ocean warming, with important consequences for the function and structure of ecosystems, and for socio-economic activities. Understanding the genetic signatures of range shifts can help build our knowledge of the capacity of species to establish and persist in colonised areas. Here, seven microsatellite loci were used to examine the population connectivity, genetic structure and diversity of Octopus tetricus, which has extended its distribution several hundred kilometres polewards associated with the southwards extension of the warm East Australian Current along south-eastern Australia. The historical distribution and the range extension zones had significant genetic differences but levels of genetic diversity were comparable. The population in the range extension zone was sub-structured, contained relatively high levels of self-recruitment and was sourced by migrants from along the entire geographic distribution. Genetic bottlenecks and changes in population size were detected throughout the range extension axis. Persistent gene flow from throughout the historical zone and moderate genetic diversity may buffer the genetic bottlenecks and favour the range extension of O. tetricus. These characteristics may aid adaptation, establishment, and long-term persistence of the population in the range extension zone.

Studying the genetic basis of speciation in high gene flow marine invertebrates

A growing number of genes responsible for reproductive incompatibilities between species (barrier loci) exhibit the signals of positive selection. However, the possibility that genes experiencing positive selection diverge early in speciation and commonly cause reproductive incompatibilities has not been systematically investigated on a genome-wide scale. Here, I outline a research program for studying the genetic basis of speciation in broadcast spawning marine invertebrates that uses a priori genome-wide information on a large, unbiased sample of genes tested for positive selection. A targeted sequence capture approach is proposed that scores single-nucleotide polymorphisms (SNPs) in widely separated species populations at an early stage of allopatric divergence. The targeted capture of both coding and non-coding sequences enables SNPs to be characterized at known locations across the genome and at genes with known selective or neutral histories. The neutral coding and non-coding SNPs provide robust background distributions for identifying FST-outliers within genes that can, in principle, identify specific mutations experiencing diversifying selection. If natural hybridization occurs between species, the neutral coding and non-coding SNPs can provide a neutral admixture model for genomic clines analyses aimed at finding genes exhibiting strong blocks to introgression. Strongylocentrotid sea urchins are used as a model system to outline the approach but it can be used for any group that has a complete reference genome available.

Combined Transcriptomic and Proteomic Analysis of the Posterior Salivary Gland from the Southern Blue-Ringed Octopus and the Southern Sand Octopus

This study provides comprehensive proteomic profiles from the venom producing posterior salivary glands of octopus (superorder Octopodiformes) species. A combined transcriptomic and proteomic approach was used to identify 1703 proteins from the posterior salivary gland of the southern blue-ringed octopus, Hapalochlaena maculosa and 1300 proteins from the posterior salivary gland of the southern sand octopus, Octopus kaurna. The two proteomes were broadly similar; clustering of proteins into orthogroups revealed 937 that were shared between species. Serine proteases were particularly diverse and abundant in both species. Other abundant proteins included a large number of secreted proteins, many of which had no known conserved domains, or homology to proteins with known function. On the basis of homology to known venom proteins, 23 putative toxins were identified in H. maculosa and 24 in O. kaurna. These toxins span nine protein families: CAP (cysteine rich secretory proteins, antigen 5, parthenogenesis related), chitinase, carboxylesterase, DNase, hyaluronidase, metalloprotease, phospholipase, serine protease and tachykinin. Serine proteases were responsible for 70.9% and 86.3% of putative toxin expression in H. maculosa and O. kaurna, respectively, as determined using intensity based absolute quantification (iBAQ) measurements. Phylogenetic analysis of the putative toxin serine proteases revealed a similar suite of diverse proteins present in both species. Posterior salivary gland composition of H. maculosa and O. kaurna differ in several key aspects. While O. kaurna expressed the proteinaceous neurotoxin, tachykinin, this was absent from H. maculosa, perhaps reflecting the acquisition of a potent nonproteinaceous neurotoxin, tetrodotoxin (TTX) produced by bacteria in the salivary glands of that species. The dispersal factor, hyaluronidase was particularly abundant in H. maculosa. Chitinase was abundant in both species and is believed to facilitate envenomation in chitinous prey such as crustaceans. Cephalopods represent a largely unexplored source of novel proteins distinct from all other venomous taxa and are of interest for further inquiry, as novel proteinaceous toxins derived from venoms may contribute to pharmaceutical design.

Application of DNA Barcoding for Controlling of the Species from Octopus Genus

The DNA barcoding proposes the use of a particular sequence from a single genomic region as the base for an identifying system capable to determine all animal species. This methodology comprises the analysis of a 655 base-pair region from the mithocondrial cytochrome C oxidase gene (COI). Its application in the species identification of fishery products has been very promising. However, in the last years some doubts about its usage have emerged. In this work, we make use of the DNA barcoding for the identification of some of the octopus species with higher commercial interest (Octopus membranaceus, Octopus vulgaris, Octopus aegina, Octopus cyanea) focusing the attention on the reliability and completeness of the available information on the databases. The study looked over 51 individuals apparently belonging to the Octopus genus. For the identification of O.aegina, O.cyanea, O.vulgaris species no particular problems were found. On the other hand, most of the samples of O.membranaceus, though they clearly presented the morphological characteristics of the species, were not identified with the biomolecular analyses.

Body size, growth and life span: implications for the polewards range shift of Octopus tetricus in south-eastern Australia

Understanding the response of any species to climate change can be challenging. However, in short-lived species the faster turnover of generations may facilitate the examination of responses associated with longer-term environmental change. Octopus tetricus, a commercially important species, has undergone a recent polewards range shift in the coastal waters of south-eastern Australia, thought to be associated with the southerly extension of the warm East Australian Current. At the cooler temperatures of a polewards distribution limit, growth of a species could be slower, potentially leading to a bigger body size and resulting in a slower population turnover, affecting population viability at the extreme of the distribution. Growth rates, body size, and life span of O. tetricus were examined at the leading edge of a polewards range shift in Tasmanian waters (40°S and 147°E) throughout 2011. Octopus tetricus had a relatively small body size and short lifespan of approximately 11 months that, despite cooler temperatures, would allow a high rate of population turnover and may facilitate the population increase necessary for successful establishment in the new extended area of the range. Temperature, food availability and gender appear to influence growth rate. Individuals that hatched during cooler and more productive conditions, but grew during warming conditions, exhibited faster growth rates and reached smaller body sizes than individuals that hatched into warmer waters but grew during cooling conditions. This study suggests that fast growth, small body size and associated rapid population turnover may facilitate the range shift of O. tetricus into Tasmanian waters.

Allopatric speciation within a cryptic species complex of Australasian octopuses

Despite extensive revisions over recent decades, the taxonomy of benthic octopuses (Family Octopodidae) remains in a considerable flux. Among groups of unresolved status is a species complex of morphologically similar shallow-water octopods from subtropical Australasia, including: Allopatric populations of Octopus tetricus on the eastern and western coasts of Australia, of which the Western Australian form is speculated to be a distinct or sub-species; and Octopus gibbsi from New Zealand, a proposed synonym of Australian forms. This study employed a combination of molecular and morphological techniques to resolve the taxonomic status of the ‘tetricus complex’. Phylogenetic analyses (based on five mitochondrial genes: 12S rRNA, 16S rRNA, COI, COIII and Cytb) and Generalised Mixed Yule Coalescent (GMYC) analysis (based on COI, COIII and Cytb) distinguished eastern and Western Australian O. tetricus as distinct species, while O. gibbsi was found to be synonymous with the east Australian form (BS = >97, PP = 1; GMYC p = 0.01). Discrete morphological differences in mature male octopuses (based on sixteen morphological traits) provided further evidence of cryptic speciation between east (including New Zealand) and west coast populations; although females proved less useful in morphological distinction among members of the tetricus complex. In addition, phylogenetic analyses suggested populations of octopuses currently treated under the name Octopus vulgaris are paraphyletic; providing evidence of cryptic speciation among global populations of O. vulgaris, the most commercially valuable octopus species worldwide.

The complete mitochondrial genomes of two octopods Cistopus chinensis and Cistopus taiwanicus: revealing the phylogenetic position of the genus Cistopus within the order Octopoda

In the present study, we determined the complete mitochondrial DNA (mtDNA) sequences of two species of Cistopus, namely C. chinensis and C. taiwanicus, and conducted a comparative mt genome analysis across the class Cephalopoda. The mtDNA length of C. chinensis and C. taiwanicus are 15706 and 15793 nucleotides with an AT content of 76.21% and 76.5%, respectively. The sequence identity of mtDNA between C. chinensis and C. taiwanicus was 88%, suggesting a close relationship. Compared with C. taiwanicus and other octopods, C. chinensis encoded two additional tRNA genes, showing a novel gene arrangement. In addition, an unusual 23 poly (A) signal structure is found in the ATP8 coding region of C. chinensis. The entire genome and each protein coding gene of the two Cistopus species displayed notable levels of AT and GC skews. Based on sliding window analysis among Octopodiformes, ND1 and DN5 were considered to be more reliable molecular beacons. Phylogenetic analyses based on the 13 protein-coding genes revealed that C. chinensis and C. taiwanicus form a monophyletic group with high statistical support, consistent with previous studies based on morphological characteristics. Our results also indicated that the phylogenetic position of the genus Cistopus is closer to Octopus than to Amphioctopus and Callistoctopus. The complete mtDNA sequence of C. chinensis and C. taiwanicus represent the first whole mt genomes in the genus Cistopus. These novel mtDNA data will be important in refining the phylogenetic relationships within Octopodiformes and enriching the resource of markers for systematic, population genetic and evolutionary biological studies of Cephalopoda.

Determination of the complete mitochondrial DNA sequence of Octopus minor

In this study, we have determined the complete nucleotide sequence of the mitochondrial genome of Octopus minor. It is 15,974 nucleotide pairs and encodes 13 proteins, two ribosomal RNAs and 22 tRNAs of the mitochondrion's own protein synthesizing system. Seven of thirteen proteins are encoded by the H-strand, while the other six proteins, as well as the two ribosomal RNAs are encoded by the L-strand. The nucleotide composition of the proteins showed a nucleotide bias against G encoded by the H-strand, while they showed a nucleotide bias against A and C encoded by the L-strand. Two of the 13 protein coding genes of O. minor began with the unorthodox translation initiation codon ATA and all others use the standard ATG. In addition, six of thirteen mt proteins of O. minor have unambiguous termination codons. There are four cases where tRNA genes appear to overlap. The long noncoding region (LNCR) of O. minor was 930 nucleotides and no repeated sequences were found in this LNCR. The gene arrangements of O. minor showed remarkable similarity to that of O. ocellatus and O. vulgaris. Phylogenetic analysis demonstrated that O. minor appears as sister taxan to the monophyletic group combined by O. ocellatus and O. vulgaris, suggesting a relative distant genetic relationship between O. minor and the other two octopus species.