 Consent to participate (Not applicable) Consent for publication (Not applicable)  Availability of data and materialThe whole genome sequence data obtained in this study was deposited at DDBJ/EMBL/GenBank under BioProject Accession: PRJDB11924.  Code availability (Not applicable) Author Contributions Chunqi Jiang conceived, designed and performed the experiments, analyzed the data, visualized the data, drafted and reviewed the manuscript. Mami Tanaka and Sayo Nishikawa performed the experiments, reviewed the manuscript.

A Gram-staining-negative, aerobic bacterium, designated strain PT3T was isolated from laboratory-reared larvae of the Japanese sea cucumber Apostichopus japonicus. Phylogenetic analysis based on the 16S rRNA gene nucleotide sequences revealed that PT3T was closely related to Amphritea ceti RA1T (= KCTC 42154T = NBRC 110551T) and Amphritea spongicola MEBiC05461T (= KCCM 42943T = JCM 16668T) both with 98.3% sequence similarity, however, average nucleotide identity (ANI) and in silico DNA-DNA hybridization (in silico DDH) values among these three strains were below 95% and 70%, respectively, confirming the novelty of PT3T. Furthermore, the average amino acid identity (AAI) values of PT3T against other Amphritea species were on the reported genus delineation boundary (64–67%). Multilocus sequence analysis using four protein-coding genes (recA, mreB, rpoA, and topA) further demonstrated that PT3T, Amphritea ceti and Amphritea spongicola formed a monophyletic clade clearly separate from other members of the genus Amphritea. Three strains (PT3T, A. ceti　KCTC 42154T and A. spongicola JCM 16668T) also showed higher similarities in their core genomes compared to those of the other Amphritea spp. Based on the genome-based taxonomic approach, Aliamphritea gen. nov. was proposed together with the reclassification of the genus Amphritea and Aliamphritea ceti comb. nov. (type strain RA1T = KCTC 42154T = NBRC 110551T), Aliamphritea spongicola comb. nov. (type strain MEBiC05461T = KCCM 42943T = JCM 16668T), and Aliamphritea hakodatensis sp. nov. (type strain PT3T = JCM 34607T = KCTC 82591T) were suggested.

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The genus Thalassotalea is ubiquitous in marine environments, and up to 20 species have been described so far. A Gram-staining-negative, aerobic bacterium, designated strain PTE2T was isolated from laboratory-reared larvae of the Japanese sea cucumber Apostichopus japonicus. Phylogenetic analysis based on the 16S rRNA gene nucleotide sequences revealed that PTE2T was closely related to Thalassotalea sediminis N211T (= KCTC 42588T = MCCC 1H00116T) with 97.9% sequence similarity. ANI and in silico DDH values against Thalassotalea species were 68.5–77.0% and 19.7–24.6%, respectively, indicating the novelty of PTE2T. Based on genome-based taxonomic approaches, strain PTE2T (= JCM 34608T = KCTC 82592T) is proposed as a new species, Thalassotalea hakodatensis sp. nov.