Xenobennettella coralliensis a new monoraphid diatom genus characterized by the alveolate sternum valve with cavum, observed from coral reef habitats

Abstract

During a survey of benthic diatoms from the coral reefs of the Indian Ocean (Scattered Islands) and Pacific Ocean (Tuamotu Archipelago), an interesting monoraphid diatom was observed and examined by light microscopy and various electron microscopy methods including Focus Ion Beam milling. Our thorough analysis revealed the similarity of this diatom to Bennettella R.W.Holmes, which we reference in the name: Xenobennettella Witkowski & Riaux-Gobin gen. nov., with Xenobennettella coralliensis Witkowski & Riaux-Gobin sp. nov. as the generitype. The type habitat for this new species is the sublittoral coral reef of Juan de Nova in the Mozambique Channel. The sternum valve of the new genus is characterized by an alveolate ultrastructure with the rim of the alveola opening along the valve margin, resembling the sternum valve of Bennettella. Internally, Xenobennettella differs from the latter by possessing a cavum (horseshoe-shaped chamber) on one side of the valve, in a central axial position. The raphe valve of Xenobennettella has small, marginal, apically elongate chambers, which are internally delineated by transapical ribs that are very similar to Bennettella. However, the raphe in the new genus is different from the latter, resembling some Cocconeis and Planothidium with internal raphe endings bent in the opposite direction, while resembling some Planothidium taxa externally by ending on the apical part of the mantle. This contrasts to Bennettella, which has a unique raphe system, with external raphe endings below the apices, a prominent axial structure and a transapically expanded central area. Likewise, the external surface of Bennettella is different from that of the new genus with a complex mantle structure and biseriate striae. In Xenobennettella, the valve mantle of the raphe valve is simple and perforated by areola. The transapical striae occur in the valve margin and the axial area is ornamented along its course with a single row of densely packed areola on both sides. The characteristics of the raphe valve and alveolate sternum valve place the new genus among the Achnanthidiaceae.

Keywords: Achnanthidiaceae; Cavum; Coral reef; Heterovalvy; Juan de Nova; Mozambique channel; New diatom; Nukutavake; Tuamotu Archipelago; Xenobennettella.

Figure 1. Location of the study areas with satellite images.

(A) Juan de Nova coral island, Western Indian Ocean. (B) Nukutavake atoll at Tuamotu archipelago, South Pacific. Yellow stars indicate location of samples used in this study. Figure source credit: Google. (C) Photograph of sampled microhabitat from Juan de Nova–corals taken by scuba diver. Photo source credit: Andrzej Witkowskiv.

Figure 2. Xenobennettella coralliensis Witkowski & Riaux-Gobin sp. nov. imaged in light microscope.

(A–N) The sternum valve—SV. (O–U) The raphe valve—RV. (A) Holotype specimen. (arrow in A) The presence of cavum.

Figure 3. Xenobennettella coralliensis Witkowski & Riaux-Gobin sp. nov. SV external surface.

(A–D) Lanceolate sternum and unilateral central area imaged in SEM. White arrows in A point the striae forming areolae positioned in quincunx between distinct virgae. (E) The external surface of broken SV with attached plain SVVC. (F) Openings of alveolae along the margin (black arrowheads).

Figure 4. Xenobennettella coralliensis Witkowski & Riaux-Gobin sp. nov. SV internal surface imaged in SEM.

(A) Internal valve surface with SVVC (white arrow) and alveolar openings (black arrows). (B) Valve interior devoid of SVVC with the position of cavum (white arrowhead) and the break in alveolae along the cavum entrance (black arrowhead). (C–D) Slightly elevated and plain sternum with a small depression around the contact with cavum (arrowhead in C).

Figure 5. Xenobennettella coralliensis Witkowski & Riaux-Gobin sp. nov. RV imaged in SEM.

(A–B) External and (C–D) internal surface of the RV. (black arrow in A) Transapical striae composed of bi- to triseriate areolae positioned along the valve margin. (black arrowheads in A) Raphe branches with slightly expanded proximal ends. (white arrowheads in A) Apical ends gently bent towards the valve margin. (black arrowheads in B) Valve middle with transapical striae marked only by solitary rows of areolae. (black arrows in B) Single row of areolae framing the raphe system. (black arrow in C–D) Internal view illustrating the chambered valve margin (black arrow on Figs. 5C–5D), (white arrow in C–D) Transapical striae with gradually decreasing areolae. (white arrowheads in C–D) Raphe branches with proximal raphe endings bent in opposite directions. (black arrowhead in C–D) Raphe terminating at apices in a small and indistinct helictoglossae.

Figure 6. Details of Xenobennettella coralliensis Witkowski & Riaux-Gobin sp. nov. valve ultrastructure.

(A–D) The internal view of the SV. (E–F) The internal view of the RV. (A) Close up of a break in alveolae along the cavum entrance (arrows point to alveolae). (B) Close up of the SVVC. (arrows in B) Triangular fimbriae between each alveolar opening on the. (C–D) SV external surface with (arrow in C) areola occlusions. (E) Interior of the RV with structures resembling chambers. (white arrow in E) Small and indistinct helictoglossa. (white arrowheads in E) areolae occlusions with slits. (F) Internal valve margin bent inwards. (arrow in F) Robust ribs between stria.

Figure 7. Xenobennettella coralliensis Witkowski & Riaux-Gobin sp. nov. Details of SV valve ultrastructure revealed with FIB nanocuts.

(A) General view of SV cut with FIB. (B) Close up of the cut illustrated in A. (arrowhead in B) Internal valve surface membrane. (arrow in B) Solid sternum. (C) Removed with FIB valve interior membrane of the same specimen. (arrow in C) Robust virgae. (arrowhead in C) Depressed sternum. (D) Close up of the cut through the cavum. (arrows in D) Lack of alveolar openings within cavum solid structure. (E) Close up of the cut through alveolae. (arrow in E) Robust virgae. (Black arrowhead in E) Alveolar openings. (White arrowhead in E) Biseriate areolae of the striae.