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Starfish (phylum Echinodermata) are ecologically important and diverse members of marine ecosystems in all of the world's oceans, from the shallow water to the hadal zone. The deep sea is recognized as an extremely harsh environment on earth. In this study, we present the mitochondrial genome sequence of Mariana Trench starfish Freyastera benthophila, and this study is the first to explore in detail the mitochondrial genome of a deep-sea member of the order Brisingida. Similar to other starfish, it contained 13 protein-coding genes, two ribosomal RNA genes, and 22 transfer RNA genes (duplication of two tRNAs: trnL and trnS). Twenty-two of these genes are encoded on the positive strand, while the other 15 are encoded on the negative strand. The gene arrangement was identical to those of sequenced starfish. Phylogenetic analysis showed the deep-sea Brisingida as a sister taxon to the traditional members of the Asteriidae. Positive selection analysis indicated that five residues (8 N and 16 I in atp8, 47 D and 196 V in nad2, 599 N in nad5) were positively selected sites with high posterior probabilities. Compared these features with shallow sea starfish, we predict that variation specifically in atp8, nad2, and nad5 may play an important role in F. benthophila's adaptation to deep-sea environment.

Temperature and salinity are two of the most potent abiotic factors influencing marine mollusks. In this study, we investigated the individual and combined effects of temperature and salinity on the survival and growth of juvenile Pacific abalone, Haliotis discus hannai Ino, and also examined the DNA methylation alteration that may underpin the phenotypic variation of abalone exposed to different rearing conditions. The single-factor data showed that the suitable ranges of temperature and salinity were 16–28°C at a constant salinity of 32, and 24–40 at a constant temperature of 20°C, respectively. The two-factor data indicated that both survival and growth were significantly affected by temperature, salinity and their interaction. The optimal temperature-salinity combination for juveniles was 23–25°C and 30–36. To explore environment-induced DNA methylation alteration, the methylation-sensitive amplified polymorphism (MSAP) technique was used to analyze the genomic methylation profiles of abalone reared in optimal and adverse conditions. Neither temperature nor salinity induced evident changes in the global methylation level, but 67 and 63 differentially methylated loci were identified in temperature and salinity treatments, respectively. The between-group eigen analysis also showed that both temperature and salinity could induce epigenetic differentiation in H. discus hannai Ino. The results of our study provide optimal rearing conditions for juvenile H. discus hannai Ino, and represent the first step toward revealing the epigenetic regulatory mechanism of abalone in response to thermal and salt stresses.

Starfish (phylum Echinodermata) are ecologically important and diverse members of marine ecosystems in all of the world's oceans, from the shallow water to the hadal zone. The deep sea is recognized as an extremely harsh environment on earth. In this study, we present the mitochondrial genome sequence of Mariana Trench starfish Freyastera benthophila, and this study is the first to explore in detail the mitochondrial genome of a deep‐sea member of the order Brisingida. Similar to other starfish, it contained 13 protein‐coding genes, two ribosomal RNA genes, and 22 transfer RNA genes (duplication of two tRNAs: trnL and trnS). Twenty‐two of these genes are encoded on the positive strand, while the other 15 are encoded on the negative strand. The gene arrangement was identical to those of sequenced starfish. Phylogenetic analysis showed the deep‐sea Brisingida as a sister taxon to the traditional members of the Asteriidae. Positive selection analysis indicated that five residues (8 N and 16 I in atp8, 47 D and 196 V in nad2, 599 N in nad5) were positively selected sites with high posterior probabilities. Compared these features with shallow sea starfish, we predict that variation specifically in atp8, nad2, and nad5 may play an important role in F. benthophila's adaptation to deep‐sea environment.

Complete mitochondrial genomes play important roles in studying genome evolution, phylogenetic relationships, and species identification. Sea cucumbers (Holothuroidea) are ecologically important and diverse members, living from the shallow waters to the hadal trench. In this study, we present the mitochondrial genome sequence of the sea cucumber Benthodytes marianensis collected from the Mariana Trench. To our knowledge, this is the first reported mitochondrial genome from the genus Benthodytes. This complete mitochondrial genome is 17567 bp in length and consists of 13 protein-coding genes, two ribosomal RNA genes and 22 transfer RNA genes (duplication of two tRNAs: trnL and trnS). Most of these genes are coded on the positive strand except for one protein-coding gene (nad6) and five tRNA genes which are coded on the negative strand. Two putative control regions (CRs) have been found in the B. marianensis mitogenome. We compared the order of genes from the 10 available holothurian mitogenomes and found a novel gene arrangement in B. marianensis. Phylogenetic analysis revealed that B. marianensis clustered with Peniagone sp. YYH-2013, forming the deep-sea Elasipodida clade. Positive selection analysis showed that eleven residues (24 S, 45 S, 185 S, 201 G, 211 F and 313 N in nad2; 108 S, 114 S, 322 C, 400 T and 427 S in nad4) were positively selected sites with high posterior probabilities. We predict that nad2 and nad4 may be the important candidate genes for the further investigation of the adaptation of B. marianensis to the deep-sea environment.