Aim: Sillaginidae is commercially exploited fish species across the Indo‐West Pacific (IWP), and changes in its abundance are of major conservation concern. Cryptic species in the marine realm emphasizes the necessity to revise our current perception of marine biodiversity, and understanding cryptic diversity is a prerequisite for sustainable development of ocean fisheries. Therefore, our goal was to provide an in‐depth understanding of cryptic diversity and evolutionary diversification of the family Sillaginidae across the IWP to assess the currently uncertain global status of the species.Location: IWP shores spanning from Japan and Australia to South Africa.Methods: Samples from 23 valid and two undefined Sillaginidae species were obtained across their habitats in IWP. Phylogenetic analyses and molecular dating of both mitochondrial (12S, 16S, Cyt b, and COI) and nucleic (RAG2) loci, combined with morphometrics of the swim bladder, were used to evaluate their cryptic species diversity and phylogeography.Results: A total of 31 operational taxonomic units with two highly diverse cryptic complexes in Sillago ingenuua and Sillago sihama were identified across IWP, including a shallow dichotomous branch in the S. ingenuua complex and eight highly divergent clades from the S. sihama complex. The robust model‐based multigene phylogeny for 24 Sillaginidae species was well supported in accordance with the morphology evolution of swim bladder. The origin of the Sillaginidae was estimated during the Middle Eocene based on the relaxed molecular clock analysis calibrated by fossil and paleogeographic event.Main conclusions: Our results indicated that more cryptic species could be present in the family Sillaginidae and a thorough reappraisal of the current S. sihama taxonomy is warranted. The swim bladder evolution in Sillaginidae fishes was from simplicity to complex corresponding with phylogeny. Given the complex cryptic diversity of Sillaginidae species, we suggested treating them as different management units in utilization of offshore fisheries resources.

The present study contributes to the taxonomy of the family Sillaginidae, with comments on the distribution of its species in the Indian Ocean and an emphasis on the taxonomy and distribution of Sillago sihama. Thirty described and putative species with Indian Ocean distribution are listed, and a distribution range for each species is provided based on published data and results from the present study. A comprehensive phylogenetic analysis of the barcoding portion of the mitochondrial COI gene is provided together with three approaches for molecular species delimitation, which includes 44 to 47 genetic lineages (depending on the species delimitation approach used) in the family Sillaginidae, 33 of them applying to described species and also 8 putative species, formerly misidentified as S. sihama. Inclusion of specimens from South Africa, Iran, Pakistan, India, Bangladesh and the southern Red Sea (type locality) reveals one genetic lineage representing the true Sillago sihama. Distribution of the species is confined to the Red Sea and the Indian Ocean, and other records under the name S. sihama are based on misidentifications. Several undescribed species identified as S. sihama are distributed in the Indo-West Pacific region and closely resemble S. sihama, but are not identical with this species and can be identified as members of different evolutionary lineages. Two species, S. sihama and S. soringa, reported from Bangladesh, represent the easternmost record of both species. These two species are described in detail, including swimbladder morphology. The study also shows that specimens from India identified as Sillago ingenuua McKay, 1985 are nested within a lineage previously referred to as S. ingenuua A, but are different from the lineage S. ingenuua B, representing a confirmed record of the clade S. ingenuua in the northern Indian Ocean. Comments on misidentifications of S. sihama from the Indian Ocean and western Pacific are provided. Furthermore, we propose that Sillago erythraea should be resurrected from its synonymy with S. sihama. As Sillago suezensis is identical with the former species, it becomes a junior synonym of S. erythraea.

Due to difficulty in recognition, many true species have been covered under the synonyms of wide-spread species. To justify the identification of a widely distributed species, Sillago sihama from the Bay of Bengal, Bangladesh, an integrated approach including morphology and DNA barcoding was used. Two unrecognized species of Sillago, i.e., Sillagomuktijoddhai sp. nov. and S. mengjialensis sp. nov., were identified from the coastal area of Bangladesh. S. muktijoddhai sp. nov. has marked differences in the body color, anal fin color, number of gill rakers, snout length, and swimbladder. S. mengjialensis sp. nov. has notable differences in the anal fin color, snout length, and swimbladder and is distinguished from S. muktijoddhai sp. nov. by the body color and swimbladder. The morphological characters of 14 documented Sillago species with two posterior extensions of the swimbladder were referenced and distinguished to accredit the two new species. Genetic analyses of partial mitochondrial cytochrome oxidase subunit I and 16S ribosomal RNA also supported the validity of the new species. This study has increased the number of recognized species of Sillago in the world and confirmed the prevailing misidentification of these two new species in Bangladesh as so-called S. sihama. Moreover, the study confirmed the misidentification of S. mengjialensis sp. nov. in Indonesia as S. sihama and the identification of unknown Sillago sp.1 in India.

This study investigates the reproductive biology, feeding habits, and growth pattern of Sillago sihama ( N = 119) and Sillago mengjialensis ( N = 18) collected from Cox’s Bazar, Bay of Bengal, between August 2022 and October 2023. The total length and body weight of S. sihama ranged from 72.19 to 173.50 mm and from 2.49 to 41.32 g, respectively, while they ranged from 78.32 to 159.81 mm and from 3.08 to 27.19 g, in S. mengjialensis , respectively. The sex ratio was male‐dominated in both species. Peak gonadosomatic index (GSI) values were recorded in March for S. sihama (females: 0.653 ± 0.121; males: 0.351 ± 0.022) and in November for S. mengjialensis (females: 3.600 ± 0.371; males: 1.068 ± 0.115), indicating distinct and separate spawning seasons. In both species, males matured at smaller size, compared to females, with Lm ₅₀ estimated at 93.73 and 98.76 mm for male and female S. sihama , and 86.00 and 128.14 mm for male and female S. mengjialensis , respectively. Histological examination identified immature and mature males and immature and maturing females in S. sihama , whereas mature males and fully ripe females were observed in S. mengjialensis . Fecundity in S. mengjialensis ranged from 8548 to 16,716 eggs per ovary. Feeding analysis revealed omnivory in S. sihama (RLG: 1.12) and carnivory in S. mengjialensis (RLG: 0.75). Length–weight relationships indicated positive allometric growth ( b = 3.453) in S. sihama and isometric growth ( b = 3.08) in S. mengjialensis . These findings provide important baseline data for future management, conservation, and stock assessment of these species.

The complete mitochondrial genome of Sillaginopsis panijus has been determined for the first time using Sanger Dideoxy DNA sequencing. The mitogenome is a circular molecule of 16,529 bp in length. It contains 37 mitochondrial genes (13 protein-coding genes, two ribosomal RNA, and 22 transfer RNA) and a control region as other bony fishes. In the phylogenetic analysis using 12H-strand protein-coding genes, monotypic S. panijus is situated separately from the genus Sillago. The present phylogeny supports its taxonomic position according to morphology and will be helpful for evolutionary analysis.