A new record of the blackfin goby (Egglestonichthys melanoptera, Gobiidae) from China, with complete mitochondrial genomes of two Egglestonichthys species and their phylogenetic implications

This study documents the northernmost record of Egglestonichthys melanoptera in the South China Sea (Hainan Island, China), thereby extending its distribution in the Indo-Pacific region from the Indian Ocean, and confirms the sympatric occurrence of E. bombylios . The morphological characteristics of E. melanoptera are described in detail, and measurable data is presented. The complete mitochondrial genomes of both species have been presented for the first time (17,743 bp for E. bombylios , 17,350 bp for E. melanoptera ), revealing a conserved gobiid genomic architecture with a pronounced A+T bias (60.3–62.7%) and codon position-specific nucleotide preferences. A further objective of this study was to construct a phylogeny of 110 species of the family Gobiidae, extending previous studies. Phylogenetic analyses resolved E. melanoptera within Gobiidae, supported by osteological synapomorphies (single epural; pterygo-phore formula 3-22110), and placed Egglestonichthys within the Priolepis lineage. The validity of Benthophilinae remains contentious due to conflicting swim bladder and spine morphology, necessitating integrative reassessment. Comparative mitogenomic analyses reveal that the mitochondrial genome currently attributed to Oxyurichthys formosanus in GenBank (accession: KC237282) does not represent the genus Oxyurichthys but should be reassigned to Cryptocentrus yatsui . A revised diagnostic key has been synthesised, synthesising historical descriptions with novel morphological-molecular data, giving taxonomy for Egglestonichthys species. These findings contribute to our understanding of the evolutionary trajectories of gobiid fish, emphasising the synergy of mitogenomic and morphological approaches in resolving complex marine fish systematics.