Mohammad Abdul Baki

The present study investigated microplastic (MP) contamination in surface seawater, intertidal sediments, and commercially important fish and shellfish from Cox's Bazar and Saint Martin's Island, two of the most popular tourist destinations in Bangladesh. Seawater and sediment samples were collected from three locations per site, with each site having three replicates. Additionally, seven fish and four crustacean species were obtained from local markets and barbecue shops. MPs were morphologically identified under a stereomicroscope and chemically characterized using Fourier transform infrared spectroscopy (FTIR). The average MP concentrations in surface water were higher at Saint Martin's Island (35.1 ± 29.5 particles L ⁻¹ ) than at Cox's Bazar (10.45 ± 7.65 particles L ⁻¹ ). In sediments, concentrations averaged 0.211 ± 0.05 items g ⁻¹ at Cox's Bazar and 0.2055 ± 0.1 at St. Martin's Islands. MP abundance in fish and shellfish was tissue‐dependent and varied by location, generally following the order: gut > dorsal muscle > ventral muscle. Overall, mean MP concentrations in the corresponding tissues were higher in samples from Cox's Bazar than in those from St. Martin's Island. Fibers, predominantly black and red (0.1–0.5 mm), were the most dominant form of MPs across all samples. FTIR analysis identified polyethylene terephthalate (53.33%), polyethylene (26.67%), and polypropylene (20%) as major polymer types. The estimated daily MP intake via seafood consumption was highest in adults (74.99 items day ⁻¹ ) and children (49.99 items day ⁻¹ ), primarily through the ingestion of Awaous grammepomus . These findings revealed widespread MP contamination in the Bay of Bengal region, underscoring the potential ecological and human health risks associated with it.

Glycerol Dialkyl Glycerol Tetraethers (GDGTs) are microbial membrane lipids that can provide crucial information for identifying organic carbon sources and understanding paleoenvironments. Despite numerous studies reporting the presence of GDGTs in various terrestrial and marine environments, there is a paucity of reports concerning GDGTs in mangrove wetlands that are characterized by unique hydrological conditions and disproportionately high accumulation rates of blue carbon (i.e., carbon sequestered in coastal ecosystems, where tidal flooding and anaerobic sediments facilitate exceptional long-term carbon storage). This study investigates GDGTs in 81 sediment samples from 5 sediment cores collected from three Asian mangrove wetlands in Bangladesh, Hong Kong, and Guangxi Province, China. The Hong Kong mangrove sediments had the highest GDGT concentration (370.18 ± 58.00 ng·g⁻¹ dws), followed by Bangladesh mangrove sediments (136.70 ± 41.70 ng·g⁻¹ dws), while Guangxi mangrove sediments had the lowest (100.80 ± 28.71 ng·g⁻¹ dws). All samples demonstrated high BIT index values (>0.8), low IIIa/IIa index values (0.09–0.19) and the predominance of tetramethylated brGDGTs (70.38 ± 2.21%), indicating that terrestrial inputs are the primary source of organic carbon. Despite overall low methylation index (MI) values (0.15–0.35) and GDGT-0/Cren ratios, deeper sediment samples in the lower part of HK exhibited GDGT-0/Cren > 2, likely reflecting enhanced contributions of methanogenic archaea under distinct redox conditions compared to upper sediments. This in situ production may complicate the application of GDGT-based paleo-proxies, as indicated by the substantial deviations between CBT’-pH (MBT’_5ME-temperature) and measured pH (instrumental temperature). The dominant bacterial phyla in the mangrove sediments of Guangxi and Bangladesh were Proteobacteria, Actinobacteriota, Chloroflexi, Acidobacteriota, and Firmicutes (>70% relative abundance). However, correlations between microbial community compositions and brGDGT isomers are different among sampling sites. Our study emphasizes that site- and depth-specific microbial activity may significantly contribute to organic matter cycling and the in situ production of GDGTs in mangrove sediments. These factors should be taken into account for organic carbon sequestration and the validity of GDGT-based paleo-proxies in mangrove wetlands.

Anadromous migration toward riverine tributaries is often challenged by altered environmental cues, food scarcity, and energy demands, sometimes at the expense of life itself. Tenualosa ilisha (Clupeiformes: Dorosomatidae), the national fish of Bangladesh, an anadromous shad, offers a model for understanding the molecular mechanisms of migration. To this end, we present a chromosome-level genome of T. ilisha and compare its transcriptomic imprints from muscle and liver across environments to trace the physiological shifts driving the migration. We observed rapid expansion of gene families to facilitate efficient signaling and osmotic balance, as well as a substantial selection pressure in metabolism regulatory genes, potentially relevant to a highly anadromous fish. We detected 1298 and 252 differentially expressed transcripts between sea and freshwater in the liver and muscle of T. ilisha, respectively, reflecting habitat and organ-specific adaptations. Co-expression analysis led us to hypothesize that the strength required for breeding migration toward upstream rivers is fueled by muscle protein catabolism forming ubiquitin-proteasomal complexes. In the liver, we observed a group of genes promoting fatty acid (FA) synthesis significantly in the riverine habitat. Regulation of FADS2 and ELOVL2 in the river reasoned the natural abundance of LC-PUFAs with better energy utilization in T. ilisha. Moreover, active gluconeogenesis and reduced insulin signaling in the liver are possibly linked to glucose homeostasis, potentially induced by prolonged starvation during migration. These genomic resources will accelerate the future evolutionary and functional genomics studies of T. ilisha.

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.

Objectives: American shad (Alosa sapidissima) is an important migratory fish under Alosinae and has long been valued for its economic, nutritional and cultural attributes. Overfishing and barriers across the passage made it vulnerable to sustain. To protect this valuable species, aquaculture action plans have been taken though there are no published genetic resources prevailing yet. Here, we reported the first de novo assembled and annotated transcriptome of A. sapidissima using blood and brain tissues. Data description: We generated 160,481 and 129,040 non-redundant transcripts from brain and blood tissues. The entire work strategy involved RNA extraction, library preparation, sequencing, de novo assembly, filtering, annotation and validation. Both coding and non-coding transcripts were annotated against Swissprot and Pfam datasets. Nearly, 83% coding transcripts were functionally assigned. Protein clustering with clupeiform and non-clupeiform taxa revealed ~ 82% coding transcripts retained the orthologue relationship which improved confidence over annotation procedure. This study will serve as a useful resource in future for the research community to elucidate molecular mechanisms for several key traits like migration which is fascinating in clupeiform shads.

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.