The researchers searched Google Scholar, Web of Science and PubMed using combinations of terms including “Arabian Gulf zooplankton,” “Persian Gulf zooplankton,” “Arabian Gulf copepod” and “Persian Gulf copepod.” They then examined the reference lists of eligible publications to identify additional studies. A publication entered the database only if it addressed some aspect of pelagic copepod biodiversity or ecology and provided collection locations in tables, maps or other sufficiently precise forms. Peer-reviewed articles, regional journals, technical reports, theses and dissertations were considered, whereas studies confined to physiological responses, parasitic copepods or meiobenthic copepods were excluded.

Taxonomic names were checked against the World Register of Marine Species, and synonyms, obsolete names and spelling variants were screened to avoid counting the same organism more than once. Each occurrence was assigned to the Western or Eastern Gulf according to its position relative to the Qatar Peninsula, which the authors selected because the basin’s circulation is organized into western and eastern gyres on either side of the peninsula. Sampling sites were plotted using GPS coordinates or maps supplied in the original studies, allowing researchers to identify concentrations of research and areas with limited coverage.

Among the 280 unique stations, 144 were in the Western Gulf and 136 in the Eastern Gulf. Sampling coverage was strongest in Kuwaiti and Saudi Arabian waters in the west and off the eastern side of Qatar in the east, while gaps were evident along the United Arab Emirates and the central coast of Iran. The inventory contained 25 families and 43 genera. Pontellidae contributed the most taxa, with 17, followed by Acartiidae with 12, Paracalanidae with 11, Coryceidae with 10 and Oithonidae with nine. Six families were represented by a single taxon each.

The Gulf-wide total of 107 taxa was lower than the 150 or more reported from the Red Sea and considerably below the 270 or more documented in the Arabian Sea, where nearly 200 species of Calanoida alone were recorded in one study. The review relates this comparatively modest richness to environmental and physical constraints, including a shallow basin, seasonal temperatures from 11 to 40 °C, and salinities that exceed 45 psu in some areas and reach above 65 psu locally. Species dependent on habitats below 100 metres or extensive vertical migration may lack suitable habitat, while the basin’s geologically young history limits the time available for colonization and diversification.

Extreme conditions were presented as one possible explanation for the regional pattern, alongside circulation, environmental gradients, distance from the Strait of Hormuz and differences in sampling effort. Surface water enters through the Strait of Hormuz, travels northwest along Iran, circles counterclockwise around the basin and exits as dense bottom water. This circulation is modified by seasonal shamal winds, tidal mixing, freshwater runoff, eddies and surface buoyancy fluxes. The review proposes that the two-gyre system may help maintain differences between eastern and western copepod communities, but the literature compilation did not directly demonstrate that circulation caused the observed distribution.

Salinity gradients may also shape where copepods occur. Water entering through the Strait of Hormuz is less saline, while evaporation removes an estimated 1–2 metres of water annually. The Shatt Al-Arab Estuary supplies more than 40% of the Gulf’s freshwater at its western end, keeping salinity comparatively lower there. Reduced freshwater inflow and the discharge of hypersaline brine from more than 200 desalination plants are weakening this gradient, while brine at 65 psu or higher is estimated to raise Gulf salinity by more than 2 psu by 2050. The review notes that salinity alone is unlikely to explain the regional separation because these effects interact with circulation and other factors.

Distance from the Strait of Hormuz introduces another possibility. Incoming surface water may carry copepods from the Gulf of Oman and Arabian Sea, and some eastern records could represent vagrants transported into the basin. However, the Western Gulf contained more region-specific taxa, including 47 Western-only records and both known Gulf endemics, Bestiolina arabica and Labidocera kuwaitiana. The review also examines potential human influences. The Gulf receives substantial shipping traffic, with an estimated 2.7 × 10⁶ cubic metres of ballast water discharged annually, and 30 copepod taxa have been found in ballast water in Iran. The authors state that outside introductions of copepods have not been confirmed, while treating future port surveys as a possible way to detect non-native plankton.

Although the historical database is broad, its temporal coverage and sampling methods are uneven. Two short-term studies conducted in 2010–2012 accounted for 44% of Eastern Gulf sampling stations, whereas long-term monitoring around Kuwait provides a stronger temporal record for the west. Most studies used bongo nets towed near the surface, reflecting the basin’s shallow depth, but net mesh sizes ranged from 55 to 300 micrometres and horizontal tow durations from 4 to 10 minutes. Coarser meshes can miss small copepods, so some apparent differences in recorded diversity may reflect gear selectivity as well as biological distribution. Long-term studies around Kuwait, extending back to the late 1970s, have also provided more repeated observations than are generally available for the Eastern Gulf.

The authors recommend standardizing future copepod surveys through horizontal surface tows with 120-micrometre mesh. Studies targeting microcopepods or ichthyoplankton could pair that net with a smaller or larger mesh, preserving their specific objectives while improving comparability with other Gulf surveys. They also recommend establishing a regional natural-history collection to retain voucher specimens for verification and consultation, and holding taxonomic workshops so specialists can reconcile identifications and train early-career researchers. These measures are important because copepods occupy herbivorous, omnivorous and carnivorous roles, have short life spans of weeks or months and can provide sensitive records of spatial and temporal changes in plankton communities.

Subject of Research: Historical diversity and regional distribution of pelagic copepods in the Arabian Gulf

Article Title: Historic diversity of pelagic copepods across the Arabian Gulf

Article References: Clardy, T. R., Al-Nuwairah, M., Das, P. B., Thomas, B. K., Hikmawan, T. I., Prihartato, P., Abdulqader, K. A., & Qurban, M. A. (2026). Historic diversity of pelagic copepods across the Arabian Gulf. Discover Oceans, 3(1), Article 69. https://doi.org/10.1007/s44289-026-00178-6

Image Credits: AI Generated

DOI: 10.1007/s44289-026-00178-6

Keywords: Arabian Gulf, pelagic copepods, biodiversity, zooplankton, marine ecology, biogeography, taxonomy, sampling gaps, desalination, ballast water, Historic, diversity