A small brown bivalve that has lived under stones along Kuwait’s intertidal shores for decades, catalogued under other names since 1984, turns out to be a species new to science. In a study published in Zoosystematics and Evolution, researchers describe Didimacar bajelaopsis, a noetiid clam from the northern Arabian Gulf, combining detailed shell and anatomical descriptions with mitochondrial DNA barcode evidence. The Kuwaiti name “bajela”, a small brown bean, gave the species its name, with the Latin suffix -opsis denoting resemblance.
The genus Didimacar belongs to the arcoid family Noetiidae, a group characterised by vertical ligament strips rather than the chevron arrangement found in most arcoids. Only two recent species were previously recognised in the genus: Didimacar tenebrica, described by Reeve in 1844 from Samar in the Philippines, and D. nigra, described by Lamy in 1907. Didimacar tenebrica has long been treated as a single, rather variable species with a vast recorded range stretching from the western Indian Ocean across the Indo-West Pacific, including Kuwait, Pakistan, India, Thailand, the Philippines, Queensland, Hong Kong, the East China Sea and Japan.
In Kuwait, Didimacar is a common bivalve living under stones on sandy or muddy substrates. It was first recorded there by Glayzer and colleagues in 1984 as Striarca tenebrica and most recently listed as D. tenebrica by Oliver and colleagues in 2023. An intertidal survey by Al-Kandari and colleagues recorded the clam from 12 localities across northern Kuwait, with an additional 10 sites added in 2024 to 2026. Given that earlier workers, including Oliver in 1990 and Huber in 2010, had questioned the identity of western Indian Ocean populations, the Kuwaiti material was included in a barcoding project investigating species found in Kuwait, a programme that has already uncovered several oyster and ark species unique to the country.
Specimens for barcoding were collected by Dr Manal Al-Kandari and her team over 2024 to 2026, and morphological study focused primarily on material from Shuwaikh and Al-Shumaimah. The researchers amplified the mitochondrial cytochrome c oxidase subunit I gene, the standard animal barcode region, using degenerate primers and Sanger sequencing. Newly generated sequences were queried against the NCBI nucleotide database with BLASTn, and all Didimacar sequences available in GenBank, updated to July 2026, were assembled into a single dataset alongside the Kuwaiti sequences.
The molecular results were unambiguous. Nine cox1 sequences from Kuwaiti specimens consistently returned Didimacar tenebrica from Korea, sequence OL877284, as the closest BLASTn match, but with only 82.5 percent sequence identity at 629 base pairs of query coverage. A maximum-likelihood tree inferred from a 632-base-pair alignment of 22 sequences, analysed in IQ-TREE under the TIM+F+I model selected by the Bayesian information criterion, recovered the sampled Didimacar sequences as a monophyletic clade with maximal ultrafast bootstrap support of 100, relative to three outgroup taxa. Within that clade, the nine Kuwaiti specimens formed one lineage, supported by an ultrafast bootstrap value of 74, clearly separated from the ten East Asian sequences deposited as D. tenebrica, which formed a clade with support of 100.
Genetic divergence between the two lineages, calculated from a trimmed 367-base-pair alignment in MEGA11, reached 24.3 percent under the Kimura 2-parameter model and 20.2 percent as uncorrected p-distance. The Kuwaiti clade therefore represents a deeply divergent lineage, and the authors formally described it as Didimacar bajelaopsis sp. nov., with a holotype from Al-Judailiat in Kuwait Bay collected in April 2025 and deposited in the National Museum of Wales.
The new species is a small noetiid, rarely exceeding 15 millimetres in length, tumid and distinctly umbonate in larger shells. Its ligament is entirely opisthodetic, a key feature distinguishing Didimacar from the similar genera Striarca, Arcopsis and Estellarca, all of which have amphidetic ligaments. The sculpture consists of dominantly radial riblets, roughly 70 at the margin of a 15-millimetre shell, increasing in number with growth as secondary and tertiary riblets are added in the interspaces. This pattern separates it from D. nigra, which is cancellate. The hinge plate is narrow with small taxodont teeth, up to eight anterior and up to 30 posterior. The prodissoconch, the larval shell, is domed and approximately 190 micrometres in diameter.
Anatomical dissection of specimens from Shuwaikh revealed large labial palps with up to 65 sorting ridges and a prominent smooth oral fold, single pigment spots or ocelli on the mantle edge, and gills of both demibranchs. The authors suggest that these large palps, which have been linked to survival in turbid conditions since work published in 1966, may explain why Didimacar flourishes in Kuwait’s turbid waters while the closely related Striarca symmetrica, present in the southern Arabian Gulf, is absent from Kuwait.
Morphology alone, however, could not have made the case. Comparisons with the syntypes of Arca tenebrica in the Natural History Museum, London, revealed only subtle differences: the Philippine shells grow slightly larger, up to 18.1 millimetres, show no allometric growth, are less umbonate, and have a stronger hinge with larger teeth and less differentiation between median and marginal riblets. But the variability of D. tenebrica across its enormous range means the two species cannot be reliably separated on shell characters. Compounding the problem, the GenBank sequences labelled D. tenebrica come from Chinese material, not from the Philippine type locality, and their vouchers were never compared with the type series, so it cannot be certain that those sequences represent the true D. tenebrica.
The study therefore urges caution for the wider region. Comparative shells from Pakistan and southern India show additional geographic variation, but without sequence data their taxonomic identity remains unresolved, and the authors recommend labelling these varieties, for now, under the aggregate designation D. tenebrica agg. Distinctive shells from Shek Ngau Chau in Hong Kong may even represent an additional undescribed taxon. The presence of two deeply divergent cox1 lineages signals that further diversity likely remains hidden across the Indo-West Pacific, and identifications of geographically distant populations lacking sequence data should be regarded as provisional until broader geographic sampling can test the emerging picture.
Subject of Research: Integrative taxonomy and DNA barcoding of a new noetiid bivalve species from Kuwait
Article Title: Integrative taxonomy reveals a new species of Didimacar ( Bivalvia , Noetiidae ) from Kuwait
Article References: Oliver, P. G., Al-Kandari, M., Garzia, M., & Salvi, D. (2026). Integrative taxonomy reveals a new species of Didimacar (Bivalvia, Noetiidae) from Kuwait. Zoosystematics and Evolution, 102(5), 1315–1328. https://doi.org/10.3897/zse.102.214669
Image Credits: AI Generated
Keywords: Didimacar bajelaopsis, Bivalvia, Noetiidae, DNA barcoding, cox1, Kuwait, Arabian Gulf, integrative taxonomy, marine bivalves, systematics, new species, phylogenetics
News Source: Drew Townsend. (October 29, 2026). DNA Barcodes Reveal a New Bean Clam Species Hidden in Kuwait’s Intertidal Waters. Scienmag.
