Seawater temperature values, as measured with Sea Bird™ SBE911plus and SBE19plus CTDs, were as high as 35.6 °C in August 2024 in the southwestern part of the study area and were uniformly above 35 °C throughout the water column for 200 km offshore, down to 35 m of water depth. Further offshore, at the sites from which planktonic foraminifers are reported here (Fig. 1E), and approaching the main basin of the Gulf, sea surface temperature was similar but dropped to 26 °C at 60 m depth.

In March 2025, the coastal and sea surface water had already started to warm from the winter minimum, and the lowest observed temperature was 21 °C at a depth of >20 m in the restricted basin to the south of the Great Pearl Bank (see Steuber et al., 2026, for further details).

Salinity of 44 to 45 psu throughout the water column was observed with CTD surveys in the basin south of the Great Pearl Bank in August 2024 and March 2025. Further to the east of the study area, salinity reached 42 psu in summer 2024 and 43 psu during March 2025 in coastal bottom waters at 20 m depth. In both surveys, salinity of surface waters dropped to 39 psu further offshore, i.e. at the sampling sites of the planktonic foraminifera (Fig. 1E), showing the influx of surface water with a lower salinity from the central basin of the Gulf, approaching the values of the surface inflow from the Arabian Sea of 37 psu (Reynolds, 1993; Steuber et al., 2026).

In the sediment samples provided from offshore Abu Dhabi, there are four localities, one from the first sampling campaign (sample F) and three sites from the second campaign (samples L12, L13, and L14), that contain planktonic foraminifera (Fig. 1E). In the duplicate samples that we picked, we found similar occurrences of planktonic foraminifera in both sample splits. The percentages of planktonic foraminifera for each sites range from 0.12 % to 5.22 % from the total picked foraminiferal specimens (relative to the benthic foraminifera or total assemblages). The collected materials were based on seafloor sediments; hence, the planktonic foraminifera specimens were already dead. The stained samples collected during the second campaign confirmed this assumption.

We found, in total, three species of true planktonic foraminifera (Plate 1), namely Globigerinoides tenellus Parker, 1958, Turborotalita cf. clarkei (Rögl & Bolli, 1973), and Neogallitellia vivans (Cushman, 1934), as well as one species, Tretomphalus bulloides (d'Orbigny, 1839), that, according to Banner et al. (1985), can be considered to be transitional between benthic and planktonic owing to the presence of a planktonic phase during its life cycle. Among all planktonic species, N. vivans is the most abundant species. The other records are just single occurrences, with a maximum of three counts per species in the studied samples, and not all samples contain similar occurrences of planktonic foraminifer (Tables 1 and 2).

Additionally, we found one possible planktonic shell of uncertain origin (incertae sedis), which only occurs in one sample (L14-2). This taxon is characterized by its spherical morphology, with multiple pores that are scattered over the shell or outer layer (referring to Versteegh et al., 2009). It resembles the spherical planktonic foraminifera Orbulina universa but is morphologically distinctive in terms of the absence of apertures and its smaller size. The morphologically closest form is Bachmayerella sp., a microfossil of uncertain origin initially described by Rögl and Franz (1979) from the Miocene of the Paratethys.

Most of the specimens were found in samples collected from the deeper stations on the UAE shelf (sample L14). Even though this species is well known for its small size (less than 120 µm) and possibly passes through the 125 µm sieve, some of the specimens that are retained on the sieve are quite large (Table 1, Fig. 2).

Order ROTALIIDA Lankester, 1885

Type species. Guembelitria vivans Cushman, 1934

Remarks. Neogallitellia Özdikmen, 2009 is a replacement name for Gallitellia Loeblich and Tappan, 1986, which was a junior homonym of the anthozoan Gallitellia Cuif, 1977.

Neogallitellia vivans (Cushman, 1934) Plate 1, figs. 1–3; Plate 2, figs. 1–3.

1934 Gümbelitria(?) vivans Cushman, p. 105, pl. 13, figs. 9–10.

Material. 79 specimens.

Description. Test tiny (averaging 130 µm based on the picked specimens retained on a 125 µm sieve), elongate, chambers globular and rapidly enlarging as added, arrangement triserial throughout, with chamber proliferation occasionally observed in the final stage, sutures deeply depressed, margins lobulate; wall calcareous, hyaline, consisting of a single continuous layer, microperforate, thin and translucent, surface smooth to grainy in appearance, with fine pores (<0.5 µm) randomly and sparsely scattered over the surface, often occurring in doublets or triplets, not elevated on pore mounds or in pore pits; aperture a simple rounded and umbilical arch at the base of the final chamber, without a lip.

Remarks. Cushman (1934) described the species from Challenger Station 192A off Little Ki Island, Indonesia, at 129 fathoms. Cushman's illustrated paratype shows strong and irregular chamber proliferation in the final growth stage. Specimens from the Arabian Gulf may become terminally quadriserial (Plate 1, fig. 1a–c), but we have not observed such irregular chamber proliferation in our material.

Our investigation of un-etched broken specimens (Plate 2, figs. 1–3, sample S1a–S3c) reveals that the test wall is very thin and consists of a single, contiguous calcitic layer lacking a visible primary organic sheet, with randomly (not regularly) spaced micropores, less than half a micron in diameter. This feature was previously displayed by Kroon and Nederbragt (1990) in Plate 1, fig. 9, but was not mentioned explicitly, and it was later observed by Kimoto et al. (2009, Fig. 3) from light microscope observation. Pores are sparse or absent in the proximity of the sutures (Plate 2, fig. 1b) and often occur as closely spaced pairs (doublets) or triplets in a triangular arrangement (Plate 2, figs. 1b, 2b, and 3b). This characteristic was also previously displayed in the SEM images of Loeblich and Tappan (1986, figs. 9–12), Kroon and Nederbragt (1990, plate 1, figs. 5 and 8), Pawlowski (1991, plate 1, fig. 4), Ujiié et al. (2008, fig. 2), Kimoto et al. (2009, fig. 4), and Schiebel and Hemleben, (2017, plate 2.32) but was not mentioned or emphasized in greater detail. The surface of the test is smooth, without pore mounds or pits.

According to Loeblich and Tappan (1986), the genus differs from Guembelitria Cushman and Chiloguembelitria Hofker (= Jenkinsina Haynes) in terms of its very thin, hyaline, smooth, and sparsely perforate wall; the occasional chamber proliferation; and the simple open aperture that has no bordering lip. It also differs from Guembelitria in terms of the wall lacking pore mounds. The mid-Cretaceous Guembelitriella also has chamber proliferation but differs in its finely pustulose surface, with pores between the pustules, and in the low arched aperture with a bordering lip.

Distribution. The specimens recovered from offshore the southeastern Arabian Gulf were found in sampled sediments at a water depth of 35–40 m, where it was the most abundant planktonic foraminiferal species. Neogallitellia vivans is regarded to be a shallow-dwelling epipelagic species – Kroon and Nederbragt (1990) collected living specimens from a water depth of 0–5 m off the southern tip of India, and Kimoto et al. (2009) recovered living specimens from plankton tows between 1 and 30 m water depth in the Tsushima Strait, at the entrance to the Japan Sea (see Fig. 1). Pawlowski (1991) reported a sediment sample with common N. vivans dated to 11 900 ka in core EN190-GGC1 from the Bermuda Rise. Kimoto et al. (2009) regarded the species to be adapted to warm, nutrient-rich coastal waters, noting that it was absent in the colder waters of the Japan Sea. The common occurrence of Neogallitellia offshore Abu Dhabi suggests that the species tolerates a wide range of salinities, including the hypersaline conditions of the southeastern Arabian Gulf.

From the extracted and later interpolated datasets, the general distribution pattern from both aspects is relatively clear (Fig. 2). For the planktonic foraminifera, the highest percentages (up to 50 %) are found in the eastern part of the Arabian Gulf, and their relative abundance decreases westward (close or moving to 0). On the other hand, the percentage of N. vivans tends to be higher in the inner part of the Gulf, particularly in the central part (including in our sites, where values exceed 50 %), and is reduced towards the northeast and northwest.

The presence of planktonic foraminifera in the inner Arabian Gulf is not well documented and established, with only sporadic reports over the last 50 years, with almost 3 decades of no reports (1998 to 2025). Hughes Clarke and Keij (1973) reported occurrences as far west as eastern Qatar, though most were concentrated near the Indian Ocean inlet. Lutze (1974) and Cherif et al. (1997) identified several species near the Strait of Hormuz. Cherif et al. (1997) classified the Abu Dhabi region as “Group 4 Southern (Arabian) Shallow Shelf”, noting that planktonic foraminifera were either absent or present in unspecified, negligible quantities. However, no studies have previously documented the occurrence of planktonic foraminifera along the southeastern coast, specifically from the United Arab Emirates and particularly from offshore Abu Dhabi. Therefore, this study presents the first report of their occurrence in this region, expanding current knowledge regarding their distribution within the modern-day Arabian Gulf.

From the distribution maps provided from the interpolated data, we infer that planktonic foraminifera in the Arabian Gulf originate from the Indian Ocean. As shown by Hughes Clarke and Keij (1973) and in the combined datasets of Lutze (1974) and our sites, the general pattern of planktonic foraminifera shows higher numbers closer to the inlet on the eastern side of the Arabian Gulf (Strait of Hormuz), with abundances decreasing westward (Fig. 2). This finding corroborates the additional report by Cherif et al. (1997) and aligns with general ideas about the presence of planktonic foraminifera in the Gulf of Oman and the northern Arabian Sea (Brock et al., 1992; Schiebel et al., 2004).

There are several mechanisms that might explain the presence of planktonic foraminifera in a semi-isolated, shallow marine basin like the Arabian Gulf, even as dead assemblages, despite their true nature as a plankton organism. One plausible explanation is that surface currents from the Indian Ocean might transport planktonic organisms, including foraminifera and calcareous nannoplankton, into the basin (Subba Rao and Al-Yamani, 1998; Vasou et al., 2020; Steuber et al., 2026). Alternatively, another explanation could be invasion from outside the basin through the introduction of ballast water, a known vector for invasive marine species in this region due to heavy commercial shipping (Al-Yamani et al., 2015). A third possibility is that planktonic foraminifera are simply part of a viable plankton community that had established itself in the deepest sectors of the Gulf at some stage during the Holocene, during the postglacial reflooding of the marine basin (Lambeck, 1996; Alsharhan and Kendall, 2003; Sheppard et al., 2010). Particularly in the case of the enigmatic genus Bachmayerella, this taxon was quite common in a semi-isolated setting within the Paratethys (Spezzaferri and Rögl, 2004), which was, in many ways, similar to the modern-day Arabian Gulf in terms of its paleoenvironment and paleoecological parameters (temperature and salinity context; see Schenk et al., 2018; Kranner et al., 2021; Peryt et al., 2024).

However, there is currently insufficient evidence to validate any of these hypotheses (apart from the last one, partially, which will be discussed below). Data on the spatio-temporal variation and historical record of foraminiferal assemblages in the Gulf's deeper sections are either unavailable or unexplored, with limited accessibility due to the proximity of offshore oil platforms (Hamza and Munawar, 2009). Furthermore, existing plankton studies have focused on other meiofaunal groups, such as copepods (Liu et al., 2022), or were conducted in the adjacent Arabian Sea rather than in the Arabian Gulf itself (e.g. Peeters et al., 1999; Schulz et al., 2002).

The presence and dominance of N. vivans in our samples support the hypothesis that this species functions as a stress-tolerant “disaster taxon” within the Arabian Gulf since the basin itself mostly cited as a naturally and anthropogenically stressed environment (Sheppard et al., 1992, 2010). In the fossil record, minute, triserial planktonic foraminifera have thrived during extreme environmental crises, such as the K–Pg boundary event (Keller, 2003; Keller and Pardo, 2004; Pardo and Keller, 2008). Additionally, this is also the case for the extant biserial form Streptochilus, which displays blooms even in oceanic settings.

This hypothesis is consistent with the geological history of the Arabian Gulf, a relatively young marine basin (Sheppard et al., 2010) formed by reflooding during the early-Holocene transgression (see Lambeck, 1996). Although modern hypersaline conditions differ from the oceanic conditions after the Cretaceous–Paleogene extinction event, the persistence of adaptive traits in N. vivans is highly plausible. The species appears to function as an opportunistic niche occupier in environments where typical planktonic foraminifera cannot survive (Darling et al., 2009; Schiebel and Hemleben, 2017). This interpretation is supported by Lutze (1974), who observed that, while standard planktonic assemblages comprise up to 50 % of the seafloor fauna near the Strait of Hormuz, N. vivans dominates the deepest region of the northern Arabian Gulf, which was the first to be submerged during the Holocene reflooding (Lambeck, 1996; Alsharhan and Kendall, 2003). Consequently, as the only extant triserial form in the modern ocean, N. vivans, appears to be uniquely adapted to flourish in eutrophic and hypersaline coastal environments that do not support other planktonic foraminiferal species. Future work needs to check for the presence of living specimens in plankton samples collected from the region.

With regard to the significance of this taxon's morphology, our un-etched SEM cross-sections consistently reveal a thin, single-layered wall structure. This structural simplicity aligns with the previous observations of Kimoto et al. (2009), who suggested a possible monolamellar wall structure for this species. In addition, we performed classical yet detailed microstructural observations on broken specimens, similarly to what Hemleben (1969) carried out for his observations of molamellar structure in Hastigerina pelagica. While microperforate taxa are classically defined as bilamellar (Schiebel and Hemleben, 2017), our specimens present a structural simplicity that resembles a monolamellar wall structure, aligning with the previous light-microscope observations of Kimoto et al. (2009). We report this single-layered appearance strictly as a morphological observation of our regional specimens rather than as an attempt to definitively revise the higher-level taxonomy of the clade. In the meantime, we point out that future ultrastructural studies utilizing chemical etching protocols would be required to definitively confirm the presence or absence of a primary organic sheet; these distinctive wall features and pore patterns likely reflect its evolutionary and ecological background.

Regarding the pore pattern, foraminiferal porosity is primarily functional for gas exchange, with pore size and density often serving as proxies for ambient oxygen levels (Glock et al., 2012). This is consistent with the ecology of N. vivans, which is frequently associated with upwelling regions and oxygen-minimum zones (OMZs), such as those in the Arabian Sea (Kroon and Nederbragt, 1990; Ghosh et al., 2008). Phylogenetically, Ujiié et al. (2008) indicated that N. vivans diverged from its benthic ancestor Stainforthia fusiformis during the late early Miocene (∼ 18.2 Ma). This species (S. fusiformis) is characterized by minute pores (Hofker, 1956; Williamson, 1858), and its initial chambers bear a strong resemblance to those of N. vivans. Stainforthia is well-known for its tolerance of dysoxic environments (Alve, 2003). We suggest that these retained traits are linked to the species' transition from a benthic to a planktonic lifestyle, similarly to the extant biserial forms (see Darling et al., 2009), though this aspect is beyond the scope of the current study, and further investigations are needed to observe its living behaviour, such as laboratory culture and/or micro-mesocosm experiments.