The deep-sea chemosynthetic environment is one of the most extreme environments on the Earth, with low oxygen, high hydrostatic pressure and high levels of toxic substances. Species of the family Vesicomyidae are among the dominant chemosymbiotic bivalves found in this harsh habitat. Mitochondria play a vital role in oxygen usage and energy metabolism; thus, they may be under selection during the adaptive evolution of deep-sea vesicomyids. In this study, the mitochondrial genome (mitogenome) of the vesicomyid bivalve Calyptogena marissinica was sequenced with Illumina sequencing. The mitogenome of C. marissinica is 17,374 bp in length and contains 13 protein-coding genes, 2 ribosomal RNA genes (rrnS and rrnL) and 22 transfer RNA genes. All of these genes are encoded on the heavy strand. Some special elements, such as tandem repeat sequences, “G(A)_nT” motifs and AT-rich sequences, were observed in the control region of the C. marissinica mitogenome, which is involved in the regulation of replication and transcription of the mitogenome and may be helpful in adjusting the mitochondrial energy metabolism of organisms to adapt to the deep-sea chemosynthetic environment. The gene arrangement of protein-coding genes was identical to that of other sequenced vesicomyids. Phylogenetic analyses clustered C. marissinica with previously reported vesicomyid bivalves with high support values. Positive selection analysis revealed evidence of adaptive change in the mitogenome of Vesicomyidae. Ten potentially important adaptive residues were identified, which were located in cox1, cox3, cob, nad2, nad4 and nad5. Overall, this study sheds light on the mitogenomic adaptation of vesicomyid bivalves that inhabit the deep-sea chemosynthetic environment.

The deep‐sea environment is characterized by darkness, hypoxia, and high hydrostatic pressure. Mitochondria play a vital role in energy metabolism; thus, they may endure the selection process during the adaptive evolution of deep‐sea organisms. In the present study, the mitogenome of Phymorhynchus sp. from the Haima methane seep was completely assembled and characterized. This mitogenome is 16,681 bp in length and contains 13 protein‐coding genes, 2 rRNAs, and 22 tRNAs. The gene order and orientation were identical to those of most sequenced conoidean gastropods. Some special elements, such as tandem repeat sequences and AT‐rich sequences, which are involved in the regulation of the replication and transcription of the mitogenome, were observed in the control region. Phylogenetic analysis revealed that Conoidea is divided into two separate clades with high nodal support. Positive selection analysis revealed evidence of adaptive changes in the mitogenomes of deep‐sea conoidean gastropods. Eight residues located in atp6, cox1, cytb, nad1, nad4, and nad5 were determined to have undergone positive selection. This study explores the adaptive evolution of deep‐sea conoidean gastropods and provides valuable clues at the mitochondrial level regarding the exceptional adaptive ability of organisms in deep‐sea environments.

The deep sea is one of the most extreme environments on earth, with low oxygen, high hydrostatic pressure and high levels of toxins. Species of the family Vesicomyidae are among the dominant chemosymbiotic bivalves found in this harsh habitat. Mitochondria play a vital role in oxygen usage and energy metabolism; thus, they may be under selection during the adaptive evolution of deep-sea vesicomyids. In this study, the mitochondrial genome (mitogenome) of the vesicomyid bivalve Calyptogena marissinica was sequenced with Illumina sequencing. The mitogenome of C. marissinica is 17,374 bp in length and contains 13 protein-coding genes, 2 ribosomal RNA genes ( rrnS and rrnL ) and 22 transfer RNA genes. All of these genes are encoded on the heavy strand. Some special elements, such as tandem repeat sequences, “G(A) _n T” motifs and AT-rich sequences, were observed in the control region of the C. marissinica mitogenome, which is involved in the regulation of replication and transcription of the mitogenome and may be helpful in adjusting the mitochondrial energy metabolism of organisms to adapt to the deep-sea environment. The gene arrangement of protein-coding genes was identical to that of other sequenced vesicomyids. Phylogenetic analyses clustered C. marissinica with previously reported vesicomyid bivalves with high support values. Positive selection analysis revealed evidence of adaptive change in the mitogenome of Vesicomyidae. Ten potentially important adaptive residues were identified, which were located in cox1, cox3, cob, nad2, nad4 and nad5 . Overall, this study sheds light on the mitogenomic adaptation of vesicomyid bivalves that inhabit the deep-sea environment.

Petrolisthes lamarckii (Leach, 1821) and P. haswelli Miers, 1884 are a pair of sister species of porcelain crabs, both of which are common in the intertidal zone of southern China, typically found under rocks and in the crevices of coral reefs. However, the distribution, genetic relationship and diversity of the two species in China have not been rigorously studied. Meanwhile, P. lamarckii is considered as a complex of cryptic species due to their diverse morphological features. In this study, we identified 127 specimens of the P. lamarckii–P. haswelli complex (LH complex) and recognised a new species through morphological and molecular analysis. Furthermore, we constructed a time-calibrated phylogeny of the LH complex using three mitochondrial and two nuclear genes from all three species, finding that the divergence of the LH complex can be traced back to the Miocene epoch, and that the genetic diversity increased during the Mid-Pleistocene transition period. Glacial refugia formed during the Pleistocene climatic oscillations has been regarded as one of the contributing factors to the diversification of marine organisms in the north-western Pacific. Petrolisthes haswelli demonstrates a wide distribution along the southern coast of China, while other lineages display more restricted distributions. The research on the demographic history and gene flow of P. haswelli revealed that the Chinese coastal populations experienced an expansion event approximately 12.5 thousand years ago (Kya) and the asymmetrical gene flows were observed between the two sides of the Taiwan Strait and Qiongzhou Strait, respectively, which is likely influenced by the restriction of ocean currents.

The genus Neocaridina, originating from East Asia and representing a small-size landlocked shrimp group of the family Atyidae, is an important group of ornamental shrimps and plays significant ecological roles in their natural habitats. Owing to the considerable variability of the taxonomic characters it employed, Neocaridina is constantly under revision, and the validation of several species is currently questionable. In the present study, several Neocaridina shrimps were collected from the Baiyangdian drainage area. Through morphological examination, they exhibited delicately diagnostical differences in the dactyli of the third pereiopod and the endopod of the first and second pleopod and were classified into morph A, morph B and morph C. According to the literature description, morph A and morph C were identified as N. denticulata denticulata and N. denticulata sinensis, respectively. Among them, morph B presents an intermediate state between morph A and morph C. Subsequently, we determined the mitogenomes of morph A, morph B and morph C. Based on the morphological characteristics, genetic variation and phylogenetic tree, we contend that N. davidi, N. d. denticulata, N. d. sinensis and N. heteropoda should belong to the same species, and we propose retaining the name N. denticulata. The reconstructed mitogenomic phylogeny indicated that the monophyly of several genera within Atyidae has been challenged, suggesting that the established classification of Atyidae requires substantial taxonomic revision at all taxonomic levels. Furthermore, the tree’s topologies supported Atyidae at a deeper base within Caridea. More comprehensive taxon sampling is still needed to resolve the explicit internal relationships among Caridea.