First confirmed record of adult Pacific squaretail, Tetragonurus pacificus (Teleostei: Perciformes: Stromateoidei) from the Indian Ocean, with an expanded description

Three adult individuals (129-133 mm SL) of the rare Pacific squaretail Tetragonurus pacificus Abe were collected during a pelagic trawl by the R/V Dr. Fridtjof Nansen off the coast of Sri Lanka in July 2018. The specimens represent the first confirmed adult records from Sri Lankan waters and the entire Indian Ocean, where previously only early juveniles (9-21 mm SL) were recorded from the region nearly seven decades ago. Taxonomic identification of the specimens was validated through an integrated approach combining morphological and molecular analyses, resulting in an expanded description of T. pacificus and a revision of diagnostic characters used to distinguish it from its congeners. Analysis of the COI gene revealed clear genetic divergence between T. pacificus and its two congeneric species, with pairwise distances of 8.6% from T. cuvieri Risso and 14.3% from T. atlanticus Lowe, offering new insights into the phylogenetic relationships within the genus Tetragonurus. A revised key to the species of Tetragonurus is also provided, informed by the newly available material of T. pacificus.