Karenia selliformis, A. I. Haywood, K. A. Steidinger et L. MacKenzie, 2004

4.1 Karenia selliformis

K. selliformis was the first species of the family Kareniaceae identified in Kuwait ’ s waters. Its appearance was linked to red tide and large fish kills in Kuwait Bay in September- October 1999 ( Heil et al. 2001; Husain and Faraj 2000). It was subsequently recorded again in 2001 but did not form a bloom and was reported at a concentration of 400 cells l −1 ( Glibert et al. 2002). This species was initially reported as Gymnodinium cf. mikimotoi ( Husain and Faraj 2000) and then as Gymnodinium sp. or G. selliforme ( Heil et al. 2001) , and Karenia sp. ( Glibert et al. 2002) ( Table 4). In the course of a taxonomic examination of bloom material by A. Haywood and K. Steidinger, the Kuwaiti taxon was believed to be conspecific with yet undescribed Gymnodinium sp. from New Zealand coastal waters, which possessed the laterally elongated nucleus in the hypocone ( Chang 1995; MacKenzie et al. 1995, 1996). Upon the description of the genus Karenia ( Daugbjerg et al. 2000) View in CoL , this taxon was recognized as fitting the newly erected genus and redescribed as K. selliformis ( Haywood et al. 2004) . The authors noted the conspecificity of material from Kuwait due to similar morphology ( Haywood et al. 2004; Steidinger et al. 2008). K. selliformis was recurrently found in Kuwait ’ s waters during the phytoplankton taxonomic survey in 2003 – 2007, when its morphology was illustrated by LM and SEM ( Al-Kandari et al. 2009, pl. 7) ( Table 4).

The observed morphological characters of K. selliformis cells from field samples and monoclonal culture (strain KW- D10-60) isolated from Kuwait ’ s waters are consistent with the original description of this species ( Haywood et al. 2004) and previous records from Kuwait ( Al-Yamani and Saburova 2019; Al-Kandari et al. 2009; Heil et al. 2001) in terms of their shape, apical groove length and path, and position of the nucleus ( Figure 3 View Figure 3 ; Table 3). The cells observed in this study were slightly larger than those described initially. The cells were 30 – 33 µm long, as compared to 26 – 32 µm ( MacKenzie et al. 1996) and 20 – 27 μm ( Haywood et al. 2004), but our measurements fall within the range previously reported from Kuwait (17 – 37 μm, Heil et al. 2001; 30 – 34 μm, Al-Yamani and Saburova 2019). The most notable characteristics distinguishing this species from the other Karenia View in CoL are the large, transversely elongated nucleus located posteriorly and the higher number of chloroplasts (Table 3).

Molecular phylogenetic analysis using partial LSU rDNA showed that the strain from Kuwait clearly belonged to the K. selliformis clade and was genetically distinct from other Karenia species (Figure 5). Kuwait ’ s strain was most closely related to isolates from New Zealand and was 99.86 % identical (1 bp difference) to the holotype sequence of K. selliformis ( U92250 View Materials ). This study provides the first molecular data on K. selliformis isolated from Kuwait ’ s waters and confirms the taxonomic identity of this dinoflagellate that has caused harmful blooms.

Recently, the genetic variability among the LSU rDNA and ITS sequences of K. selliformis strains originating from distant geographical areas worldwide was found to be high enough to form distinct nested subclades, suggesting the existence of two or three distinct phylotypes, depending on the molecular marker used ( Iwataki et al. 2022; Mardones et al. 2020; Orlova et al. 2022). Moreover, the delineated phylotypes were distinguished phenotypically and by their temperature tolerance ( Iwataki et al. 2022; Mardones et al. 2020; Orlova et al. 2022), pointing to the potential K. selliformis “ species complex ” ( Mardones et al. 2020).

Our morphological and molecular data support the hypothesis of phylotype separation within K. selliformis , differentiating phenotypically and by temperature preference. In the phylogeny inferred from partial LSU rDNA, the topology of the K. selliformis clade is consistent with that previously described ( Mardones et al. 2020). The K. selliformis strains were grouped into distinct nested subclades representing the different phylotypes (Figure 5). The basal subclade (phylotype I) comprised strains isolated from K. selliforms bloom along the coast of the Kamchatka Peninsula in 2020. The strain from subtropical Kuwait clustered with other warm-water strains originating from New Zealand, which were closely related to the Asian strains, belonging to phylotype II, and separated from another subclade that included the Chilean strains (phylotype III). The warm-water strain from Kuwait was represented by strongly dorso-ventrally flattened cells with a distinct antapical notch consistent with the described initially K. selliformis from New Zealand waters. In contrast, strains blooming in Chile (1999), Kamchatka, Russia (2020), and Hokkaido, Japan (2021), which belong to phylotype I, exhibited cells with abundant granular to strap-shaped chloroplasts and bloomed at low temperatures <20 °C ( Iwataki et al. 2022; Orlova et al. 2022; Uribe and RuÍz 2001). The strains isolated during the K. selliformis bloom in Chile in 2018 ( MN203220 View Materials and MN203221 View Materials ) were genetically distinct enough to form phylotype III. Morphologically , cells of this phylotype differed from the holotype strain by weak dorsal-ventral compression, shallower antapical excavation, and a smaller number of lateral pores in the hyposome ( Mardones et al. 2020).