Ulva ohnoi M.Hiraoka & S.Shimada

Ng, Yu Fei & Huang, Danwei, 2024, Species diversity and phylogeny of the green macroalga Ulva (Ulvophyceae, Chlorophyta) in Singapore, Phytotaxa 662 (1), pp. 67-79 : 71

Ulva ohnoi M.Hiraoka & S.Shimada in Hiraoka et al. (2004: 20)

Material examined:— SINGAPORE. Changi: Changi Beach Carpark 1, 16 July 2022, NYF220701 , NYF220702 , NYF220703 , NYF220704 , NYF220705 , NYF220707 ( SING) ; Changi : Changi Point Ferry Terminal, 16 July 2022, N YF220708 , NYF220711 , NYF220712 , NYF220713 ( SING) ; Pulau Ubin : Chek Jawa, 17 July 2022, NYF220724 , NYF220726 , NYF220731 , NYF220732 ( SING) ; Pulau Ubin : Sensory Trail, 17 July 2022, NYF220735 ( SING) ; East Coast : East Coast Park Carpark G, 18 July 2022, NYF220738 , NYF220739 , NYF220740 , NYF220741 , NYF220743 , NYF220745 , NYF220746 , NYF220755 ( SING) ; Changi : Changi Beach Carpark 7, 12 August 2022, NYF220811 , NYF220812 , NYF220813 , NYF220814 , NYF220815 , NYF220816 , NYF220821 ( SING) ; Pulau Hantu , 14 August 2022, NYF220829 , NYF220834 , NYF220840 ( SING) ; Sentosa : Tanjong Rimau, 23 December 2022, NYF221209 ( SING) ; Changi : Changi Beach Carpark 6, 24 December 2022, NYF221211 ( SING) .

Description:— U. ohnoi comprises five morphotypes: reticulate, expanded, expanded with some reticulations, lobe-shaped, and filamentous thalli. The reticulate morphotype has reticulated ribbon-shaped thallus without perforations, 5–50 cm in length and 0.2–2.5 cm in width ( Figs. 3A, 3F ). The morphotype with expanded thalli shows foliose, irregularly expanded thallus with no reticulations, but sometimes with perforations and/or tears, 5–30 cm in length and 5–30 cm in width ( Fig. 3C ). The morphotype with expanded thalli possessing some reticulations shows foliose thalli expanding irregularly at the base along with some reticulations and perforations; sometimes with ribbonlike structures with reticulations towards the tips, 8–20 cm in length and 20–50 cm in width ( Fig. 3G ). The small lobe-shaped morphotype has foliose thallus forming small tufts, without reticulations or perforations, 0.5–1.5 cm in length and 1.2–3 cm in width ( Fig. 3H ). The filamentous morphotype has long, filamentous, non-tubular thallus with wrinkles and some reticulations, sometimes with secondary branches, 17–53.5 cm in length and 0.3–0.9 cm in width ( Figs. 3D–E ). Surface cells are irregular, polygonal, or rectangular, 7–19 μm in length, 7–12 μm in width (n = 20). Chloroplast with 1 to 2 pyrenoids ( Fig. 3I ). Various microscopic teeth present at margin ( Figs. 3J–M ), but lacking teeth in filamentous morphotype ( Fig. 3N ). The transverse width of the thallus is 58–59 μm (n = 5) and transverse cell shape is rectangular, 21–24 μm in length, 13–18 μm in width (n = 6), and contains parietal chloroplast with pyrenoids ( Fig. 3O ).

Distribution in Singapore:—Chek Jawa, Sensory Trail, East Coast Park Carpark G, Changi Point Ferry Terminal, Changi Beach Carpark 1, 6, and 7.

Habitat:—Mostly found in seagrass meadows and occasionally on sandy beaches. Morphotypes reticulate, expanded and expanded with some reticulates are either free-floating or attached to sandy substrate ( Fig. 3A ). Lobe-shaped thallus morphotype found attached to hard substrates like rocks or logs ( Fig. 3B ) while filamentous thallus morphotype attaches to sandy substrate ( Fig. 3D ).

Remarks:—New species record for Singapore. In the tuf A tree, Ulva ohnoi DNA sequences from Singapore formed a well-supported clade with five other species: Ulva pseudo-ohnoi , U. lactuca , U. spinulosa , U. reticulata and U. taeniata ( ML bootstrap / BI posterior probability: 97/1) ( Fig. 2 ). There was also strong support for the same clade in the ITS and rbc L trees (Supplementary Files 3, 4). Based on morphological data, we excluded U. taeniata , U. reticulata , and U. spinulosa from our sampled collections. Ulva taeniata thalli have wrinkles (Setchell & Gaedner 1920), U. spinulosa has either smooth or heavily denticulate margins, and U. reticulata has smooth microscopic margins ( Tran et al. 2023). These morphological characteristics are not found in the Ulva samples collected in this study.

Ulva ohnoi cell sizes measured at surface view and transverse section in our specimens were within the range reported for the U. ohnoi holotype ( Hiraoka et al. 2004). U. ohnoi intraspecific DNA sequence divergences observed in this study were 0.0–0.79% and 0.0–0.36% for tuf A and rbc L, respectively. When comparing our sequences with that of the U. ohnoi holotype ( Hiraoka et al. 2004), the genetic distances were 0.0–0.53% based on rbc L, supporting conspecificity. There appear to be no phylogenetic patterns associated with the five U. ohnoi thallus morphotypes, indicating that all non-tubular Ulva samples collected in this study are of the same species.

Three of five U. ohnoi morphotypes reported here have similar gross morphology to U. ohnoi reported in other studies ( Hiraoka et al. 2004, O’Kelly et al. 2010, Tran et al. 2023). Two new morphotypes have been uncovered, one with lobe-shaped thallus that attaches to hard objects and the other with filamentous thallus found on sandy substrate. However, the lobe-shaped thallus is smaller in length and width when compared to other morphotypes suggesting that these could be young U. ohnoi . There are gross and microscopic variations within the U. ohnoi species (Hiraoko et al. 2004, O’Kelly et al. 2010, Lee et al. 2019) that may be driven by various environmental factors. For example, pollution from wastewater discharge and surface runoffs could affect Ulva morphology as Zeroual et al. (2020) reported that U. intestinalis and U. rigida thickened their cell wall and had a reduction of thalli size in the presence of heavy metals. Such environmental variations could result in morphological variation among U. ohnoi specimens. Hiraoka et al. (2004) recommended distinguishing U. ohnoi from other Ulva species by examining the life history and morphology such as the number of perforations in the middle of the thallus. As U. ohnoi has not been recorded in Singapore until this study, the newly discovered morphotypes in this study expand upon the morphological range described for the species thus far ( Hiraoka et al. 2004). We note that the filamentous morphotype is represented by a single specimen (NYF221211), so its identity requires further validation. Ulva ohnoi has similar gross morphology to some Ulva species which were previously recorded in Singapore, including U. beytensis , U. reticulata , U. lactuca , U. australis , and U. rigida .

Figures

FIGURE 2. Phylogenetic tree of Ulva with Umbraulva kaloakulau as outgroup based on tufA DNA sequences. Bootstrap values ≥ 50 for maximum likelihood and posterior probability values ≥ 0.8 for Bayesian phylogenetic analyses are stated above branches. Samples sequenced from this study are coloured in green (U. ohnoi), blue (U. meridionalis), and red (U. tepida).

FIGURE 3. Morphology of Ulva ohnoi. A. In-situ seagrass meadow habitat of reticulate morphotype (NYF220703). B. In-situ rocky habitat of small lobe-shaped thallus morphotype (NYF220745). C. Expanded morphotype (NYF220732). D. In-situ sandy habitat of filamentous morphotype (NYF221211). E. Filamentous morphotype (NYF221211). F. Reticulate morphotype (NYF220712).G. Expanded with reticulations morphotype (NYF220708). H. Small lobe-shaped thallus morphotype (NYF220821). I. Surface cells (NYF220702). J- M. Thallus margins showing the variations of microscopic teeth (J: NYF220812, K: NYF220821, L: NYF220732, M: NYF221211). N. Thallus margin with no microscopic teeth (NYF221211). O. Cells in transverse section (NYF220713).