Craspedostauros E.J. Cox is a diatom genus comprising 17 taxa reported from various regions of the world. While many species of Craspedostauros are epibiontic, the taxa have variable ecological preferences. In this study we formally describe Craspedostauros nazmii sp. nov., an epilithic species discovered along the Turkish Black Sea Coast, based on light and scanning electron microscopy. Craspedostauros nazmii sp. nov. is characterized by valves that are lanceolate to narrowly lanceolate, slightly constricted near the apices with uniseriate, parallel throughout the whole valve, transapical striae and and the presence of an apical silica flap. The areolae are distributed over the valve face and the mantle. The differences and similarities between C. nazmii sp. nov. and established species of Craspedostauros are discussed. Based on shape and morphometrics, the most similar species is Craspedostauros capensis, but it is easily distinguished from C. nazmii sp. nov. by its lack of an apical silica flap.

Apical silica flap, Black Sea, Craspedostauros, epilithic marine diatom

The genus Craspedostauros E.J.Cox, 1999 was established to accommodate several marine species previously included in Stauroneis Ehrenberg (Lange-Bertalot and Genkal 1999; Cox 1999). As the latter genus comprised mostly freshwater species, a separation was proposed by different authors to accomodate the marine species with regards to their autecology and morphological differences. Species strictly conforming with Stauroneis Ehrenberg are characterized by naviculoid valves with two girdle appressed chloroplasts and possess a distinct and large central area called stauros (Round et al. 1990). The valves in Stauroneis are flat and the girdle is composed of a few plain copulae that is always rather narrow (Krammer and Lange-Bertalot 1986; Round et al. 1990; Cox 1999).

When establishing the genus Craspedostauros, Cox (1999) referred to cytological (shape and number of chloroplasts) and ultrastrutural characteristics (cribrate occlusions of areolae), also leading her to point out an affinity with Mastogloia Thwaites ex W. Smith (1856). With regards to these affinities, Craspedostauros has been included in Mastogloiaceae with a recommendation to also move Achnanthes Bory sensu stricto (Cox 1999; Cox and Williams 2006) there. The molecular phylogeny published by Ashworth et al. (2017) and inferred from a 3 genes dataset (18S, rbcL and psbC genes) tends to associate Craspedostauros to Achnanthes and Staurotropis, but clearly distinguishes them from Mastogloia spp., strongly questionning the monophyly of the Mastogloiales. This group of genera was later referred to as CAS genera, for Craspedostauros-Achnanthes-Staurotropis by Mann et al. (2021), whose phylogenetic works surprisingly tended to associate the CAS genera with Bacillariaceae. However, the lack of support at the nodes or strong morphological evidences led the authors to consider this result unlikely, probably resulting from an artefact of genes/species sampling and they emphasized the need for deeper phylogenomic investigations in order to elucidate the position of the CAS genera.

As a genus, Craspedostauros is relatively small, with 15 taxonomically accepted species listed on AlgaeBase, to which could be added two recently described taxa from Antarctica (Trentin et al. 2022). Thus, there remains a potential for the description of new species in unexplored habitats such as tropical coasts and biofilms on seaweeds in particular (Risjani et al. 2021; Witkowski unpublished observations).

The Turkish Republic is surrounded by four different seas, namely the Eastern Mediterranean Sea, the Aegean Sea, the Marmara Sea and the Black Sea. Craspedostauros decipiens was found in the Sea of Marmara (Witkowski et al. 2000; Akçaalan and Kaleli 2021) but this is so far the only species of Craspedostauros discovered in Turkish Black Sea waters. The Black Sea is a semi-closed sea located in southeastern Europe. It is considered an isolated sea since the Dardanelles and Bosphorus straits limit water exchange with the Mediterranean Sea. This semi-closure influences both sea water characteristics (typically salinity) as well as the dispersal potential of aquatic species inhabiting both sides of the straits (Nevrova et al. 2013). Studying the diatoms of the Black Sea may be of additional interest for science in that they may represent flora of an ancient marine basin isolated due to limited water exchange over a fairly long geological time period (Witkowski et al. 2010).

Investigations of the diatom flora of the Black Sea go as far back as the works of Mereschkowsky (1902), whose research resulted in the description of some globally distributed diatom genera and species (e.g, Catenula Mereschkowsky, Licmosphenia and Stauronella). Research on the Black Sea diatom assemblages was continued by Proshkina-Lavrenko (1955, 1963) with several new species and varieties described (e.g., Amphora inconspicua and Nitzschia rupestris). Later on, Guslyakov et al. (1992) produced an atlas of benthic diatoms for the Northwestern Black Sea extensively illustrated with electron microscope images. More recently, Witkowski et al. (2010) revised Navicula pontica (Mereschkowsky) A.Witkowski, M.Kulikovskiy, E.Nevrova and Lange-Bertalot 2010 and Navicula parapontica, A.Witkowski, M.Kulikovskiy, E.Nevrova and Lange-Bertalot 2010, whereas Witkowski et al. (2014) described Navicula petrovii Nevrova, Witkowski, Kociolek and Lange-Bertalot. Nevrova et al. (2013) studied Lyrella and described five novel species including Lyrella abruptapontica, L. karayevae and L. pontieuxinii. The Black Sea is inhabited by what seems to be an endemic taxon of the blue-pigment producing diatom, Haslea karadagensis Davidovich, Gastineau and Mouget (Davidovich et al. 2012a; Gastineau et al. 2012a, 2012b). An increasing number of studies on the biodiversity and species richness of the Black Sea diatoms have been published in recent years (Nevrova 2022, 2023; Nevrova and Petrov 2019a, 2019b; Zidarova et al. 2022b). Strains of diatoms from the Black Sea have also been used to investigate either their patterns of auxosporulating (Podunay et al. 2021; Davidovich et al. 2012b, 2017a, 2017b, 2019; Kaczmarska et al. 2018), physiology (Bedoshvili et al. 2021; Davidovich et al. 2016, 2018; Podunay et al. 2016) or genomic peculiarities (Gastineau et al. 2021).

From the list above, it is obvious that it is mostly the northern part of the Black Sea whose diatom communities have been investigated and in contrast, the Southern Turkish Coasts of the Black Sea only received attention very recently (Baytut 2013). The latter authors investigated the discharge zone of the Kizilirmak River into the Black Sea and among the diatom species list many new records for Turkey were published. Similarly, Kaleli et al. (2017) studied Akliman city in the Sinop area and provided a species list also with new records for Turkish waters.

In this article, we contribute to the expanding list of novel taxa by describing Craspedostauros nazmii sp. nov., a new epilithic species from the Turkish Coasts of the Black Sea. The results are based on light and Scanning Electron Microscopy. This is the first and, for now, only species of Craspedostauros observed along the Turkish Black Sea coast.

The sample was collected in July 2017 from epilithic substrata in Kastamonu Doganyurt, Southern Black Sea (42°0'29.24"N, 33°27'34.19"E) (Fig. 1). A single epilithic sample was collected using a toothbrush from the surfaces of submerged stones of this sampling station. Environmental parameters were measured using a Lange Hach HQ40d. No live observations of the samples were conducted as the sample was processed directly and boiled with H₂O₂ and 10% HCl to remove organic matter and calcium carbonate respectively. After washing the diatoms with distilled water several times, permanent slides were mounted with Naphrax synthetic resin. Light Microscope (LM) observations were conducted on an OLYMPUS BX51 Light Microscope with OLYMPUS EP50 camera at Kütahya Dumlupınar University. Scanning Electron Microscope (SEM) observations were made using a FEI Versa 3D at İstanbul University with secondary electron and backscatter excitation, 10 kV and a working distance 10 mm. For this purpose, samples were placed on polycarbonate membrane filters with a 5 μm mesh. The membranes were left to dry and then attached to aluminum stubs with double-sided carbon tape, and sputter coated with ca. 20 nm gold using a Turbo–Pumped Quorum Q 150OT ES coater.

Figure 1. 

Map of the sampling location A shows the Kastamonu Province (red rectangle) on the shores of the Black Sea. The red dot on B indicates the exact place where the sampling was conducted in Doğanyurt, north of Kastamonu. Figures obtained from Google Maps, Creative Commons CCO Licence, GNU Free Document Licence.

The taxa belonging in Craspedostauros originate from various geographic regions of the world with Craspedostauros britannicus E.J.Cox 1999 known from the East and West coast of Great Britain, C. neoconstrictus E.J.Cox 1999 from the English Channel coasts, C. australis E.J.Cox 1999 from the south coast of Australia, C. indubitabilis (Lange-Bertalot and S.I.Genkal) E.J.Cox 1999 from Europe, North America and the Subantarctic Islands, C. alyoubii J.Sabir and Ashworth 2016 and C. paradoxus Ashworth and Lobban 2016 in the Red Sea and the West coast of Guam respectively, C. amphoroides (Grunow ex A.W.F.Schmidt) E.J.Cox 1999 in the Philippines, C. decipiens (Hustedt) E.J.Cox 1999 in the English Channel and North Sea coasts, C. capensis E.J.Cox 1999 along the West coast of South Africa and C. laevissimus (West and G.S.West) Sabbe 2003 in Maritime Antarctic saline lakes (Cox 1999; Sabbe et al. 2003; Rivera et al. 2011; Ashworth et al. 2017). Recently, C. alatus Majewska & Ashworth, 2018, C. danayanus Majewska & Ashworth, 2021, C. legouvelloanus Majewska & Bosak, 2021, and C. macewanii Majewska & Ashworth, 2021 were described as epibionts on sea turtles by Majewska et al. (2018, 2021). It is important to note that most of the recent discoveries originated from the Southern Hemisphere (e.g. Majewska et al. 2021; Zidarova et al. 2022a; Trentin et al. 2022).

Based on comparative morphology (Table 1), C. alatus in Majewska et al. (2018), C. britannicus E.J. Cox (1999), C. capensis E.J. Cox (1999), C. indubitabilis (Lange-Bertalot and S.I.Genkal) E.J.Cox in Rivera et al. (2011) and C. macewanii Majewska and Ashworth in Majewska et al. (2021) are similar taxa. Among them, C. capensis is the most similar taxon to C. nazmii sp. nov., with similar stria density and valve width. The valve outline and cribrate areolae are also similar (linear to linear-lanceolate), however the apices are not as strongly constricted as in C. capensis. When compared with other taxa, the stria density is higher in C. britannicus (~24 in 10 µm), C. indubitabilis (25–27 in 10 µm) and C. alatus (26–28 in 10 µm) than in C. nazmii (20–21 in 10 µm). Regarding valve outline, C. nazmii sp. nov. resembles the other listed taxa except C. indubitabilis. Craspedostauros indubitabilis has a markedly elliptic outline with wider apices (Lange-Bertalot and Genkal 1999; Rivera et al. 2011: fig. 1B, C), and a larger valve width (6–7 µm). Moreover, there is no apical wing-like silica flaps (Rivera et al. 2011: fig. 1E, F). Craspedostauros britannicus and C. alatus also have shorter valves than C. nazmii (14.0–60.0, 20.0–37.0 and 29.6–41.8 µm respectively), have no apical wing-like silica flaps (Cox 1999: fig. 26) and the shape of the internal raphe endings are helictoglossae in C. britannicus but rectevelatum in C. alatus. The valve margin is straight in C. nazmii compared to C. alatus and C. britannicus. The average number of cribrum pores is higher in C. nazmii (6–17 in 10 µm) than in the other taxa (5–13 in C. capensis, 5(+) in C. britannicus and 3–11 in C. alatus). Unfortunately, despite numerous attempts, it was impossible to find in our samples any girdle bands that could be used for taxonomy in a similar way that was introduced by Cox (1999).

We are grateful to Mrs Genowefa Daniszewska-Kowalczyk and Mrs Agnieszka Kierzek (Palaeoceanology Unit, University of Szczecin, Szczecin, Poland) for laboratory assistance. We are also grateful to the editor and reviewer for their helpful comments.