*Geissleria tahitiensis* sp. nov. (*Cymbellaceae*, *Bacillariophyceae*), a common diatom in Society Islands rivers

Olivier Monnier, *French Biodiversity Agency, Vincennes, France* (correspondence: olivier.monnier@ofb.gouv.fr)

Gilles Gassiole, *MicPhyc, Salazie, France*

Frédéric Rimet, *INRAE, USMB, CARRTEL, Thonon-les-Bains, France*

After investigations on freshwater diatoms of Tahiti and Moorea islands, Coste & Ricard (1990) characterised a taxon new to science, *Navicula ignota* var. *tahitiensis* Coste & Ricard, 1990, referring it to the section *Annulatae* Hustedt, 1937 of *Navicula* Bory, 1822. This new variety of *N. ignota* Krasske, 1932 was distinguished based on a greater length and a non-undulated shape. The section *Annulatae* was based on the presence at each side of the distal ends of raphe branches of few rows of enlarged slit-like areolae with small teeth on the inner side of the valve. This feature is called an annulus and its function may be excretion of mucus. Mainly on the basis of this distinctive morphological characteristic, the section *Annulatae* was erected as a new genus, *Geissleria* Lange-Bertalot & Metzeltin, 1996, and *N. ignota* var. *tahitiensis* was transferred to *Geissleria* as *G. schoenfeldii* var. *tahitiensis* (Coste & Ricard) Lange-Bertalot 1996, in accordance with apically taller areolae and the absence of clearly separated areolae on the mantle, from the valve face (Lange-Bertalot & Metzeltin 1996).

However, as noted by Coste & Ricard (1990), and confirmed by our investigations with scanning electronic microscopy on material collected in 2018, there is a clear difference from *G. schoenfeldii* and some other related taxa such as *G. ignota* (Krasske) Lange-Bertalot & Metzeltin 1996 and *G. cummerowii* (Kalbe) Lange-Bertalot 2001 (Lange-Bertalot & Metzeltin 1996, Lange-Bertalot 2001): the presence of a pore at one side of the central nodule (Table 1, Figs 22-25). Differences between *G. schoenfeldii* var. *tahitiensis* and other known closely related *Geissleria* species, listed in AlgaeBase (Guiry & Guiry 2022), are compiled in Table 1. Species with a different, somewhat indiscernible, annulus system (e.g. *G. bourbonensis* Le Cohu, Ten-Hage & Coste, *G. creolorum* Le Cohu, *G. mafatensis* Le Cohu, Ten-Hage & Coste, *G. mascarenicensis* Le Cohu, Ten-Hage & Coste, Le Cohu & al. 2009), or without central pore, except for previously considered related taxa (see above), have been excluded from the comparison. We propose therefore to treat *G. schoenfeldii* var. *tahitiensis* as a species.

Additionally, the name *Navicula ignota* var. *tahitiensis* Coste & Ricard, published after 1 January 1990 is invalid, as no type was designated (Guiry 2023; Turland & al. 2017, Art. 40.6).

Consequently, the name *Geissleria schoenfeldii* var. *tahitiensis* (Coste & Ricard) Lange-Bertalot, is also invalid. We propose to create the following new species.

*Geissleria tahitiensis* Gassiole & O.Monnier sp. nov. (Figs 2-25)

Designation (in synonymy): “*Navicula ignota* var. *tahitiensis* Coste & Ricard”, In Ricard M. (ed.) *Ouvrage dédié à la Mémoire du Professeur Henry Germain (1903-1989)*, p. 44, figs 2/12, 13; 6/12-14, 1990.

Holotypus: Slide PC0677383 in Museum National d'Histoire Naturelle, Paris (PC) (Fig. 5 represents the holotype)

Registration: http://phycobank.org/103829

Etymology: the epithet “tahitiensis” refers to the island of Tahiti, where is the type locality.
Type locality: Stones in a lentic stretch of the lower Vaiha River (14 m.a.s.l.), Tahiti, French Polynesia (Fig. 1), 26 August 2018, leg. Laëtitia Bisarah.

<table><caption>**Table 1.** Morphometry of *Geissleria* species considered historically as close relatives of “*Navicula ignota* var. *tahitiensis* Coste & Ricard” and of other species with similar features. After Novais & al. (2013), Lange-Bertalot (2001), Kulikovskiy & al. (2012) and Coste & Ricard 1990.</caption><thead><tr><th>Taxon</th><th>Valve shape; apices shape</th><th>Length (μm)</th><th>Width (μm)</th><th>Striae (/10 μm)</th><th>Central pore</th><th>Annulus rows/areolae</th></tr></thead><tbody><tr><td><i>G. cummerowii</i> (Kalbe)<br>Lange-Bertalot 2001</td><td>linear-elliptical to elliptical; very slightly or not protracted</td><td>8–17</td><td>4–6.5</td><td>14–16</td><td>no</td><td>2/?</td></tr><tr><td><i>G. hinziae</i> Novais & Ector<br>2013</td><td>elliptical to linear-elliptical; slightly protracted</td><td>11,3–15.7</td><td>4.8–5.7</td><td>15–18</td><td>yes</td><td>2/1–2</td></tr><tr><td><i>G. ignota</i> (Krasske) Lange-Bertalot & Metzeltin 1996</td><td>linear with slightly triundulate margins; protracted, slightly or not capitate</td><td>17–25</td><td>4.5–5</td><td>12–14</td><td>no</td><td>2/2–3</td></tr><tr><td><i>G. lusitanica</i> Novais & Ector<br>2013</td><td>linear-elliptical to elliptical; slightly protracted to protracted</td><td>11.3–17</td><td>5–5.9</td><td>14–15</td><td>yes (2)</td><td>2-3/2–3</td></tr><tr><td><i>G. matrioschka</i> Kulikovskiy, Metzeltin & Lange-Bertalot<br>2012</td><td>rhombical-elliptic; obtusely rounded, almost truncated</td><td>11.6–12.7</td><td>4.5–4.7</td><td>15–17</td><td>yes</td><td>2/?</td></tr><tr><td><i>G. ovisimilis</i> Kulikovskiy, Metzeltin & Lange-Bertalot<br>2012</td><td>linear-elliptical to elliptical; broadly rounded</td><td>10.7–18.7</td><td>4.7–6</td><td>10–11</td><td>yes</td><td>3/3</td></tr><tr><td><i>G. paludosa</i> (Hustedt) Lange-Bertalot & Metzeltin 1996 (morphotype I)</td><td>linear to linear-elliptic slightly protracted, rounded</td><td>14–28</td><td>5–7</td><td>13–18</td><td>yes, usually</td><td>2-4/2–4</td></tr><tr><td><i>G. schoenfeldii</i> (Hustedt)<br>Lange-Bertalot & Metzeltin 1996*</td><td>elliptic-lanceolate to elliptical; broad round</td><td>11.3–18.3</td><td>6.3–6.7</td><td>13–14</td><td>no</td><td>2/1–3</td></tr><tr><td>“<i>N. ignota</i> var. <i>tahitiensis</i> Coste & Ricard 1990”</td><td>linear-elliptical to elliptical; not to slightly protracted, rounded</td><td>16.5–20</td><td>5–5.8</td><td>12–13</td><td>yes</td><td>2-3/3–6</td></tr></tbody></table>

Description (on the basis of LM and SEM observations):

Length: 9.2–22.9 µm, width: 4.5–6.3 µm, striae: 11–16 in 10 µm. Frustule in girdle view narrow, rectangular. Valve shape elliptical in small individuals to linear-elliptical in big ones, with broad rounded apices, not to slightly protracted. Valve plan flat. Mantle shallow. Valve plan/mantle transition rounded, fairly marked. Striae extending on the valve plan to the valve plan/mantle transition by two to three areolae, with a more or less clear interruption at the valve plan/mantle transition. At the level of the central area, more or less perpendicular to the apical axis, one to three more or less shorter striae, linear to weakly arcuate. Following striae slightly radiating and arcuate. Striae appearing as a weak furrow externally, deeper internally. Areolae openings apically elongated, very thin. Sometimes, one to three randomly oriented and shaped areolae at the apices of the valve, after the annulus. Annulus composed of two to three rows of three to five -exceptionally seven in larger individuals- more elongate and larger slit-like areolae. Annulus rows prolonged by clearly separated normal striae of a few areolae, extending only on the mantle, except for larger individuals. 70 to 80 areolae in 10 µm. Virgae larger than striae. Axial area (sternum) narrow, linear-lanceolate, with laterally expanded central and distal nodules. Inside of the valve, raphe sternum slightly prominent. Central area expanded laterally,

<footnote>*Morphometry based on the type material examination of *Navicula schoenfeldii* Hustedt 1930 by Novais & al. 2013.</footnote>
rectangular, bordered by one to three shorter striae. One central pore, at the secondary side of the central nodule, with a round to slightly elongated opening externally. Raphe branches coaxial, filiform, straight to weakly arcuate. Helictoglossae slightly prominent inside. Central raphe endings weakly expanded and somewhat deflected towards the primary side externally, straight and slit-shaped internally. Terminal fissures hooked externally, sometimes forked, turned towards the primary side at the valve plan/mantle transition.

Distribution: *G. tahitiensis* is presently known only from Tahiti and Moorea islands in French Polynesia.

This note is a contribution to the project "Diatoms Flora of Mayotte Rivers and other Indo-Pacific Islands", supported by the French Biodiversity Agency, MicPhyc and INRAE (Institut national de recherche pour l'agriculture, l'alimentation et l'environnement), and to the project "Diatoms of French Polynesia rivers", supported by the French National Inventory of the Natural Heritage. The Regional Council of Brittany, the General Council of Finistère, the urban community of Concarneau Cornouaille Agglomération and the European Regional Development Fund (ERDF) are acknowledged for the funding of the Sigma 300 FE-SEM of the Concarneau Marine Biology Station, used for this study. We choose to maintain the genus *Geissleria* for this taxon, despite more recent nomenclatural proposals, after discussions with Bart van de Vijver and Horst Lange-Bertalot during the 2022 ADLaF congress at Clermont-Ferrand. Samples were collected by Laëtitia Bisarah (Tahiti) and Magalie Lagant (Moorea). Our thanks to Michael Guiry and Wolf-Henning Kusber for their helpful comments.

Coste, M. & Ricard, M. (1990). Diatomées continentales des îles de Tahiti et de Moorea (Polynésie Française). *In* Ricard M. (ed.) *Ouvrage dédié à la Mémoire du Professeur Henry Germain (1903-1989)*. pp. 33-62. Königstein, Koeltz.

Guiry, M.D. & Guiry, G.M. (2022). AlgaeBase. World-wide electronic publication, National University of Ireland, Galway. http://www.algaebase.org; searched on 23 December 2022.

Guiry, M.D. in Guiry, M.D. & Guiry, G.M. (22 February 2023). *AlgaeBase*. World-wide electronic publication, National University of Ireland, Galway. https://www.algaebase.org; searched on 22 June 2023

Kulikovskiy, M., Lange-Bertalot, H., Metzeltin, D. & Witkowski, A. (2012) Lake Baikal: hotspot of endemic diatoms. *In* *Iconographia Diatomologica* 23: 1-608.

Lange-Bertalot, H. (2001). *Navicula* sensu stricto, 10 genera separated from *Navicula* sensu lato, *Frustulia*. *Diatoms of Europe* 2: 1-526.

Lange-Bertalot, H. & Metzeltin, D. (1996). Indicators of Oligotrophy. 800 taxa representative of three ecologically distinct lake types. Carbonate buffered - Oligodystrophic - Weakly buffered soft water. *Iconographia Diatomologica* 2: 1-390.

Le Cohu, R., Ten-Hage, L. & Coste, M. (2009). *Geissleria bourbonensis*, *G. mascarenicensis*, *G. creolorum* et *G. mafatensis*, quatre espèces nouvelles de diatomées d'eau douce de l'île de La Réunion (France). *Diatom Research* 24: 307-326.

Novais, M.H., Wetzel, C.E., Van de Vijver, B., Morais, M.M., Hoffmann, L. & Ector, L. (2013). New species and combinations in the genus *Geissleria* (Bacillariophyceae). *Cryptogamie, Algologie* 34: 117-148.

Turland, N.J., Wiersema, J.H., Barrie, F.R., Greuter, W., Hawksworth, D.L., Herendeen, P.S., Knapp, S., Kusber, W.-H., Li, D.-Z., Marhold, K., May, T.W., McNeill, J., Monro, A.M., Prado, J., Price, M.J. & Smith, G.F., editors (2018). International Code of Nomenclature for algae, fungi, and plants (Shenzhen Code) adopted by the Nineteenth International Botanical Congress Shenzhen, China, July 2017. *Regnum Vegetabile*, Vol. 159. pp. [i]-xxxviii, 1-253. Glashütten: Koeltz Botanical Books.
<figure><img src="image_1.png" /><figcaption>Figs 1-21. *Geissleria tahitiensis*. Fig. 1. The type locality in the lower Vaiha river, Tahiti, French Polynesia, 2018. Photograph by Laëtitia Bisarah. Figs 2-21. LM micrographs of the type population. Fig. 5 represents the holotype. Scale bars = 10 µm.</figcaption></figure>
<figure><img src="image_2.png" /><figcaption>Figs 22-25. *Geissleria tahitiensis* SEM micrographs. Figs 22, 23, 25. External valve views. Fig. 24. Internal valve view. Fig. 22. Vaitepiha River, Tahiti, 2018. Fig. 23. Vaiare River, Moorea, 2018. Figs 24, 25. Afareaitu River, Moorea, 2018. Scale bars = 1 µm.</figcaption></figure>