Samples from coastal tropical waters of Central Sulawesi, Bangka Island and Bawean Island in Indonesia and from the Great Barrier Reef at Fitzroy Island in Queensland, Australia were analysed for species composition of diatom assemblages with a focus on Olifantiella. Whereas samples from Fitzroy Island littoral in Australia retrieved only one species of Olifantiella, in Poso Bay, Indonesia, we observed at least six species. All established taxa were documented with light (LM) and scanning electron microscope (SEM) and principal component analysis (PCA) analysis was used to compare the species, based on the basic valve parameters of length, width, length to width ratio and striae density. A new species of the genus Olifantiella, O. gondwanensis is described from Australia. In addition, we showed the distinct nature of O. pilosella var. rhizophorae permitting to species status. Particular attention is placed on girdle bands in this genus.

Bacillariophyta, buciniportula, Diadesmidaceae, marine coasts, new combination, new species, taxonomy

Olifantiella Riaux-Gobin and Compère is a relatively recently described diatom genus consisting mainly of small, marine naviculoid diatoms (Riaux-Gobin and Compère 2009). To date, 12 taxa have been described within the genus, 11 at the species rank (Riaux-Gobin and Compère 2009; Riaux-Gobin and Al-Handal 2012; Van de Vijver et al. 2016; Kaleli et al. 2018; Van de Vijver et al. 2018; Jung and Park 2019; Wetzel and Ector 2020) and one variety (Riaux-Gobin 2015). The major characters of the genus are marginal elevated ridge, parallel to slightly radiate striae, composed of single transapically elongated macroareola with finely perforated hymenate occlusions which are not interrupted under the elevated ridge, simple raphe slit, several girdle bands and presence of a trumpet-like (‘olifant’) internal tubular process called buciniportula after which the genus is named (Riaux-Gobin and Al-Handal 2012). The exact function of the buciniportula is still unclear, but probably may have a function in excretion of metabolites (Riaux-Gobin 2015). Due to the small size of the cells and the mainly very dense striation, the identification of individual Olifantiella taxa is currently only possible using SEM. In addition, due to only single molecular sequences being available, genetic sequencing does not allow identification of most known taxa.

The genus Olifantiella exhibits high morphological similarity to other genera that possess a central isolated process, such as Labellicula Van De Vijver and Lange-Bertalot, Luticola D.G. Mann and Luticolopsis Levkov, Metzeltin & Pavlov. Olifantiella can be discriminated from Labellicula, based on differences in isolated pores (Riaux-Gobin and Compère 2009). Furthermore, both genera differ in the presence of a longitudinal channel in Olifantiella, but which is absent in Labellicula (Majewska et al. 2017). Additionally, the genus Olifantiella shows a small, heavily silicified nodule, placed between the proximal raphe endings (similar to Neidium Pfitzer), but which is absent in Labellicula. Moreover, the genus Labellicula shows curved T-shaped grooves on distal raphe endings, while in Olifantiella, the distal raphe endings are simple or only curved on one side. In contrast to Olifantiella, the monotypic genus Luticolopsis shows strongly apically twisted valves and striae composed of two rows of areolae (Levkov et al. 2013). The genus Luticola differs from Olifantiella mainly by its larger size, punctate striae and less tubular internal opening of the isolated process (Levkov et al. 2013). Luticola is also different in its ecological preferences, as the genus contains mainly freshwater and terrestrial species with only few taxa known from marine habitats (Levkov et al. 2013; Rybak et al. 2021). Han et al. (2018), based on molecular data of a Chinese O. muscatinei culture (strain DB21-1), confirmed that OIifantiella and Luticola are not only similar, but also closely related genera. Unfortunately, molecular data for Labellicula and Luticolopsis are currently not available and, hence, the degree of their evolutionary relationship with Olifantiella and Luticola remains unclear.

The genus seems to occur in various climatic zones of the world ocean, including temperate waters of the North Atlantic, South Atlantic and North West Pacific (Van de Vijver et al. 2016; Kaleli et al. 2018; Jung and Park 2019). However, most species have been described from the tropical Indian and Pacific Oceans. The first Olifantiella species was formally described from coral sands of Mascarenes in the Western Indian Ocean (Riaux-Gobin 2015), but retrospectively, it was realised that the first documented Olifantiella originated from the Gulf of Eilat in the Red Sea and originally characterised as Navicula muscatinei Reimer and Lee = Olifantiella muscatinei (syn. O. pseudobiremis Riaux-Gobin) present as an endosymbiont from the foraminiferan Amphistegina lessonii (Reimer and Lee 1988). Interestingly, O. muscatinei has also been isolated from foraminiferans of the Great Barrier Reef within which Fitzroy Island, the type habitat of our newly-described Olifantiella species, is located. A few Olifantiella species have been described from the tropical South Pacific (Riaux-Gobin 2015).

During our recent studies of the coastal diatom floras of Australia and Indonesia, a large number of Olifantiella were observed. Amongst them, an unknown species was recorded necessitating its description as a new species – O. gondwanensis M.Rybak, A.Witkowski & C.Riaux-Gobin, sp. nov. Additionally, updated information about the distribution and morphology of established taxa are given.

Diatom samples were cleaned with 10% hydrochloric acid and washed thereafter with deionised water, followed by boiling with 30% hydrogen peroxide (H₂O₂) for a few hours and washed with deionised water. Cleaned diatom material was pipetted on to coverslips and dried and then mounted on glass slides using Naphrax mounting medium (Brunel Microscopes Ltd, Wiltshire, U.K.). Identification, counting and the measurements of diatom basic morphological features were performed under a Nikon ECLIPSE 80i light microscope (LM), equipped with a 100× Planapochromatic objective with differential interference contrast (DIC) for oil immersion (NA 1.4) and captured with a Nikon DS-Fi1c camera. For the observations in scanning electron microscope, the samples were applied to a polycarbonate membrane filter with a 3 μm mesh, attached to aluminium stubs and sputtered with 20 nm of gold using a turbo-pumped Quorum Q 150T ES coater. Diatoms were observed using a Hitachi SU 8010 SEM at University of Rzeszów, Poland. Diatom terminology follows Round et al. (1990) and Riaux-Gobin and Compère (2009).

The following samples were used in the present work:

• SZCZ 27565 – Bangka Island near the SE coast of Sumatra, 2°4'52.55"S, 105°8'14.65"E, 22 June 2022, sandy sediment.
• SZCZ 27652 – Bawean Island, North of Surabaya in the Central Sea of Java, 5°50′57.5′′S, 112°43′3.6′′E, 07 January 2021, the sample was taken from mangrove roots, temperature – 29 °C, salinity – 31‰.
• SZCZ 28814 – Indonesia, Sulawesi, Poso Regency, Tanjung Perak, 1°18′2.974′′S, 120°37′37.009′′E, 29 September 2022, the sample was collected during low tide as a scrape from exposed rock (depth ± 10–20 cm), temperature approximately above 30 °C, salinity – 28.5‰, pH – 7.76.
• SZCZ 28526 – Fitzroy Island, Australia, 16°55′38′′S, 149°59′24′′E, 15 August 2022, the sample was taken from green algae – Chlorodesmis sp. exposed on the Great Barrier Reef during the low tide.

To determine the similarity and/or dissimilarity of newly-described species (based on basic morphological features: length, width, length-to-width ratio and number of striae) with other members of the genus, Principal Component Analysis (PCA) was performed using Canoco 5.03 software (Ter Braak and Šmilauer 2012). Prior to analysis, diatom data were square-root transformed. Comparison was made, based on data from current observations and from the following publications: Riaux-Gobin and Compère (2009); Riaux-Gobin and Al-Handal (2012); Riaux-Gobin (2015); Kaleli et al. (2018) and Riaux-Gobin et al. (2019). Data for the following number of specimens were used for the analysis: O. gondwanensis sp. nov. (n = 42), O. gorandiana (n = 94), O. infirmitata (n = 7), O. mascarenica (n = 21), O. muscatinei (n = 15), O. cf. muscatinei (n = 25), O. onnuria (n = 7), O. paucistriata (n = 5), O. pilosella (n = 46), O. rhizophorae stat. nov. (n = 27), O. rodriguensis (n = 22), O. seblae (n = 33), O. societatis (n = 32) and O. visurgis (n = 26). In the case of Olifantiella onnuria, data obtained from photographs included in their original description were used (Jung and Park 2019).

Despite its small size and more than a dozen of species discriminated, the genus Olifantiella presents great morphological diversity. This manifests itself in the shape of the valves, the number and structure of the internal buciniportula openings, the structure of the areolae and in the details of the distal and proximal raphe endings.

Amongst known Olifantiella taxa, the newly-described O. gondwanensis sp. nov. is characterised by the lowest striae density (Fig. 7), which makes it the only species that can be easily identified using light microscopy. The greatest similarity to the newly-described species is shown by O. pilosella, O. rhizophorae, O. paucistriata, O. gorandiana and O. muscatinei (Reimer and J.J.Lee) Van de Vijver, Ector and C.E.Wetzel. Interestingly, O. muscatinei has also been isolated from foraminiferans of the Great Barrier Reef within which Fitzroy Island, the type habitat of our newly-described Olifantiella species, is located.

Figure 7. 

PCA ordination of all currently known Olifantiella taxa, based on their basic morphological features L length W width L/W length/width ratio S stria density nov O. gondwanensis sp. nov. rhi O. rhizophorae stat. nov. vis O. visurgis gor O. gorandiana mas O. mascarenica cfm O. cf. mascarenica inf O. infirmitata seb O. seblae pil O. pilosella rod O. rodriguensis soc O. societatis pau O. paucistriata mus O. muscatinei onn O. onnuria.

Olifantiella pilosella like the newly-described species, has areolae bearing long thin spines (pili), but in contrast to O. gondwanensis sp. nov., it has additional siliceous plates (flabella) in the areolae lumen (Riaux-Gobin and Al-Handal 2012; Riaux-Gobin 2015) which are absent in our newly-described species (Fig. 2Q–U). Additionally, both species can be separated, based on the external opening of the buciniportulae, which are positioned midway between the valve centre and valve margin in O. pilosella, but positioned in a small depression close to the proximal raphe endings in O. gondwanensis sp. nov. Internal views also show that O. pilosella possesses wider longitudinal channels when compared to the whole valve than O. gondwanensis sp. nov. (Fig. 2V–X vs. Fig. 5E, F). Another feature that allows the two species to be distinguished is the more elliptical shape of the vales in the newly-described species when compared to O. pilosella.

In addition, due to the elliptical shape of the valves, it is possible to distinguish O. gondwanensis sp. nov from the similar O. rhizophorae stat. nov. The latter species is also characterised by the simpler structure of areolae that show only short warts on the margin, instead of long and thin pili. Moreover, the external opening of buciniportula is shifted near the valve margin in O. rhizophorae stat. nov. (Riaux-Gobin 2015), but not positioned almost in the valve centre like in O. gondwanensis sp. nov. Additionally, O. rhizophorae stat. nov. possesses simpler raphe branches, which have both proximal and distal endings gently bent to the side opposite to the buciniportula (Riaux-Gobin 2015). This feature distinguishes this species from both O. gondwanensis sp. nov. and O. pilosella which have shallow depressions at both raphe endings (Riaux-Gobin and Al-Handal 2012; Riaux-Gobin 2015, Fig. 5B–D)

Olifantiella paucistriata is the least known member of the genus, with fewer than 10 individuals of this species documented so far (Riaux-Gobin and Al-Handal 2012). Despite that fact and despite the similar shape of the valve to O. gondwanensis sp. nov., both species can be easily discriminated. The main distinguishing characters are the striae density (higher in O. paucistriata, see Table 1) and presence of the pili in the areolae lumen in O. gondwanensis sp. nov. However, similar structures of the raphe, location of the external opening of the buciniportula near the valve centre and the presence of small warts in the areolae lumen in O. paucistriata indicate its close relationship to O. gondwanensis sp. nov. This requires further research supported by better documentation for the first of these species.

Olifantiella muscatinei (syn. O. pseudobiremis Riaux-Gobin) not only has a valve similar in shape to the newly-described species, but also overlapping size dimensions including length, width and striae density (see Table 1). Nevertheless, the distinction of both species is possible on the basis of morphological features of its valves and girdle bands. O. muscatinei is the only known member of the genus showing well-developed external perforated areola occlusions, both on the valve face and vale mantle (Riaux-Gobin and Al-Handal 2012; Jung and Park 2019). Additionally, the external opening of the buciniportula is shifted more towards the edge of the valve than with O. gondwanensis sp. nov. An additional difference between the two species is their girdle bands. They are hyaline in O. gondwanensis sp. nov. (Fig. 1R, S), while O. pseudobiremis has girdle bands with double rows of pores (Riaux-Gobin and Al-Handal 2012; Jung and Park 2019).

Of the five described morphotypes of O. gorandiana (Riaux-Gobin et al. 2019), the greatest similarity to O. gondwanensis sp. nov. is shown by the morphotypes ‘a’ and ‘b’. However, based on morphological characterss of the valves, both species are easily separated. Regardless of the morphotype, O. gorandiana has definitely denser striae (see Table 1), composed of simple areolae without any silica projections (Fig. 3A–D; Riaux-Gobin and Al-Handal 2012; Riaux-Gobin et al. 2019). In addition, the external opening of the buciniportula is strongly shifted to the edge of the valve face in O. gorandiana and not centrally positioned like in the case of O. gondwanensis sp. nov.

Olifantiella gorandiana is the only species in the genus within which several morphotypes have been distinguished, based on the shape of the valves. Based on the observation of a population developing epizoic on sea turtle (Chelonia mydas Linnaeus, 1758), five morphotypes have been distinguished (Riaux-Gobin et al. 2019). In the population documented in this work, all observed individuals represented the ‘b’ morphotype. However, the observed population showed a polymorphism in the construction of girdle bands. Most of them showed two rows of puncta (Fig. 3B–D), while some specimens also possessed plain girdle bands without any pores (Fig. 3C). The presence of two different types of girdle bands in a single species, even in single specimens, has not been observed so far in the genus Olifantiella. Most species within the genus have girdle bands with clearly visible two rows of pores, which were originally considered to be a characteristic feature of the genus (Riaux-Gobin and Compère 2009). However, later described species showed some plasticity in this character. For example, the here-described O. gondwanensis sp. nov. and O. rhizophorae stat. nov. have girdle bands without pores (Fig. 2R, S; Riaux-Gobin 2015). Additionally, O. pilosella also shows girdle bands which are not typically with two rows of areolae, but possess small and poorly visible pores (Fig. 4C). It seems that the morphology of the girdle bands is not a good criterion for defining members of this genus.

Studies focusing on Olifantiella have been conducted in Mascarenes (La Réunion, Rodrigues), Tahiti and Moorea Islands and coastal waters of the Mediterranean, Red, Baltic and North Seas. Until now, the occurrence of this genus has not been observed in the coastal waters of south-east Asia as well as in Australia, except the Great Barrier Reef area. Despite the great morphological diversity of representatives of the genus Olifantiella, only a few species are known, which may be due to their small size and, thus, difficulties in their observation and even detection in the studied materials. Future studies of coastal waters may result in descriptions of further members of this elusive genus. Additionally, further research (including molecular methods) is required to determine the relationships within the genus and with the other isolated pore-bearing members of family Diadesmidaceae, especially with the morphologically similar genus Labellicula.

The authors would like to acknowledge Aydin Kaleli from Istanbul University for sharing his data.