Actinocyclus aquae-dulcis, Chiba & Osada & Tuji & Idei, 2025

Chiba, Takashi, Osada, Ryosuke, Tuji, Akihiro & Idei, Masahiko, 2025, Actinocyclus aquae-dulcis sp. nov. (Coscinodiscophyceae, Coscinodiscales, Hemidiscaeae), a new centric diatom from Lake Akan and northern coasts of Japan, Phytotaxa 687 (1), pp. 65-83 : 68-71

Actinocyclus aquae-dulcis sp. nov. ( Figs. 4–35 )

Designated holotype: — The individual illustrated in Figs. (7–9), on slide TNS-AL-58999s in TNS (Department of Botany, National Museum of Nature and Science).

Isotype material: — TNS-AL- 58999m

Synonym: Actinocyclus aquae-dulcis Idei in Idei et al. Plos one 7: e41890: 2. 2012. nom. nud.

Type locality: — Lake Akan (43°26’25.95”N, 144°05’07.05”E, Akan-cho, Kushiro-shi, Hokkaido, Japan) ( Figs. 1, 2 ).

Description:—Valves circular, 31–71 µm in diameter, with concentric undulations ( Figs. 4–16, 19–21 ). The center of these concentric undulations sometimes deviates slightly from the center of the frustule. Valve face with radially arranged vaguely fasciculate areolae (8–10/10 µm in a radial areola) ( Figs. 16–21 , Table 1, 2). Shallow mantle, 2–5 µm in height, with fine areolae (7–9/10 µm in an areolar row) ( Figs. 6 , 16, 19–21 , Table 1, 2). Areolae with an internal narrow passage, a loculus, and an external cribrum ( Figs. 22–24 ). Loculate areolae on the valve face ( Figs. 22–24 ). Valve wall without a bullulate structure. Rimoportulae born from valve face/mantle junction. Rimoportula slightly long, about 1–2 μm long. Rimoportula internally with a long, wide, and bent(horseshoe-shaped or slightly horseshoe-shaped), ambiguous labium and fan-shaped tube, externally with an elliptical opening on the mantle ( Figs. 7–15 , 17, 18 , 25–31 , Table 1). A single pseudonodulus at the valve face/mantle junction ( Figs. 8 , 10, 11 , 22 ).

The rimoportula of this species appears fan-shaped in LM and SEM observations when observed in valve view; in contrast, when observed in girdle view, it appears horseshoe-shaped under both LM and SEM. The microstructure observed in this study is consistent with that reported by Idei et al. (2012). The number of rimoportulae relative to valve diameter was almost the same in both samples (Lake Akan and Yoneshiro River mouth, Fig. 36 ); in other words, the number of rimoportulae decreases as the valve diameter decreases.

All Japanese Actinocyclus strains in this study and Idei et al. (2012) isolated from Lake Akan, Lake Ogawara, and the Yoneshiro River, fell within a single clade with high bootstrap values (100 for NJ and 100 for ML) ( Fig. 37 ). Thus the rbc L extraction results showed that the diatoms from Lake Akan and the Yoneshiro River mouth sample in this study were the same as those reported by Idei et al. (2012) ( Fig. 37 ). In addition, it was not only confirmed that A. aquae-dulcis and A. normanii (in Lake Kasumigaura) are different species, but also that both are completely different from A. octonarius and A. subtilis .

In the culture experiments, Actinocyclus aquae-dulcis from Lake Akan and from the Yoneshiro River mouth were observed to proliferate in salinities of 0%–25‰ ( Table 3).

Samples from Lake Akan (n = 20) ( Figs. 4–31 , Table 1)

Valve diameter: 31–71 μm

Areolae density: 7–9/10 μm

Striae density: 8–10/10 μm

Number of rimoportulae: 4–8, horseshoe-shaped, somewhat horseshoe-shaped

Habitat and sediment type: attached to sand

Salinity tolerance: 0%–25%

Samples from the Yoneshiro River mouth (n = 20) ( Figs. 32–35 )

Valve diameter 33–62 μm

Areolae density: 7–10/10 μm

Striae density: 8–10/10 μm

Number of rimoportulae: 4–8, horseshoe-shaped, somewhat horseshoe-shaped Habitat and sediment type: attached to sand

Salinity tolerance: 0%–25%

Figures

FIGURE 36. Relationship between valve diameter and number of rimoportulae in Actinocyclus aquae-dulcis from Lake Akan and the Yoneshiro River mouth. White circles show the Yoneshiro River mouth sample and black diamonds show the Lake Akan sample.

FIGURE 37. The phylogenetic tree of Actinocyclus aquae-dulcis and related strains determined using the Maximum Likelihood (ML) method with rbcL (GTR+G+I) gene. Accession numbers, taxonomic names, and strain names follow localities for Japanese strains, and NJ/ML bootstrap support values are represented on branches (only values above 70 are shown).

FIGURES 4–6. LM images of Actinocyclus aquae-dulcis (living cells) from Lake Akan.The white arrow indicates a chloroplast; the black arrow marks a nucleus. (4, 5) Valve views. (6) Girdle view. Scale bar = 10 μm.

FIGURES 7–9. LM images of Actinocyclus aquae-dulcis holotype from Lake Akan. Scale bar = 10 μm.

FIGURES 10–15. LM images of Actinocyclus aquae-dulcis from Lake Akan and variations in cell size. The white arrows each indicate a rimoportula; the black arrows each mark a pseudonodule. Scale bar = 10 μm.

FIGURES 16–24. SEM images of Actinocyclus aquae-dulcis from Lake Akan. (16) External valve view. (17, 18) Internal valve views. White arrows denote rimoportulae. (19–21) External girdle views. (22) Pseudonodule in external valve view; black arrows mark pseudonodules. (23) Loculate areolae in external valve view. (24) Cross section of the areolae. Scale bar = 10 μm in (16–21), and 1 μm in (22–24).

FIGURES 25–31. SEM images of Actinocyclus aquae-dulcis from Lake Akan, internal valve and the rimoportulae. (25–28) Complete rimoportulae. (27) Complete rimoportula and the split ring spacing (open band, on the left side). (29–31) Broken rimoportulae. White arrow denotes a rimoportula. Scale bar = 1 μm in (25–27) and 5 μm in (28–31).

FIGURES 32–35. LM images of Actinocyclus aquae-dulcis from the Yoneshiro River mouth. The white arrow indicates a rimoportula; the black arrow marks a pseudonodule. (32) Valve view of living cell and chloroplasts. (33) Girdle view of living cell and chloroplasts. Scale bar = 20 μm.

FIGURES 1–3. Study area. (1) Maps showing the study area and areas where Actinocyclus aquae-dulcis has been reported. The sampling locality of Actinocyclus normanii (Lake Kasumigaura) is also shown. (2) Sampling locality of A. aquae-dulcis in Lake Akan. (3) Sampling locality of A. aquae-dulcis in the Yoneshiro River mouth area.