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Issue: Vol. 770 No. 1: 14 Aug. 2026
Type: Article
Published: 2026-08-14
DOI: 10.11646/phytotaxa.770.1.1
Page range: 1-19
Abstract views: 28
PDF downloaded: 1

On the non-homologous nature of the ‘cavum’ in Achnanthidiaceae (Bacillariophyta) as illustrated by Xenobennettella moai sp. nov., a marine benthic diatom from Easter Island (Rapa Nui, South Pacific)

CRIOBE-UAR 3278 CNRS-UPVD; 66860 Perpignan; France
Faculty of Science; Yamagata University; Yamagata 990-8560; Japan
CEFE; Univ Montpellier; CNRS; EPHE-PSL University; IRD; Montpellier; France
Natural History Museum; London SW7 5BD; United Kingdom
Koro Nui O Te Vaikava; Rapa Nui; Chile
Achnanthales cavum ultrastructural cladistics biogeography Algae

Abstract

A new marine benthic diatom belonging to the genus Xenobennettella Witkowski & Riaux-Gobin is described from the coastal waters of Easter Island, Rapa Nui, Polynesia, South Pacific. The newly proposed species, Xenobennettella moai sp. nov. exhibits a cavum on the sternum valve (SV), a morphological feature also observed in X. coralliensis, recently described from the Indian Ocean. Ultrastructural features observed in the scanning electron microscope clearly delineate both taxa, including differences in sternum valve (SV) stria patterns and the structure of the asymmetric SV fascia. A third taxon, Xenobennettella sp., recorded from Polynesia, but not fully characterized, also differs from the other two taxa. The alveolate (i.e., bilayered) status of the SV in X. moai sp. nov. is also considered a key feature, as well as the curvature and general design of the external raphe endings. In order to compare the morphological traits of ‘cavum-bearing’ Achnanthales, or any other distinctive asymmetric area devoid of areolae, we conducted a cladistic analysis based on ultrastructure elements. The reconstruction results, based on 21 morphological characters and 10 Achnanthales genera, often showing asymmetric areas, places Xenobennettella within these genera and delineate independent clades. Our results indicate that the cavum is a polyphyletic structure, likely non-homologous among the different concerned genera.

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How to Cite

Riaux-Gobin, C., Jordan, R.W., Delrieu-Trottin, E., Williams, D., Burns, L. & Valenzuela, T.S. (2026) On the non-homologous nature of the ‘cavum’ in Achnanthidiaceae (Bacillariophyta) as illustrated by Xenobennettella moai sp. nov., a marine benthic diatom from Easter Island (Rapa Nui, South Pacific). Phytotaxa 770 (1): 1–19. https://doi.org/10.11646/phytotaxa.770.1.1