When a familiar species is not what it seems

by Sulastri Arsad |

As an archipelagic country, Indonesia is rich in marine bioresources and biodiversity, including diatoms. Sulawesi, one of its largest islands, remains relatively underexplored for marine diatom diversity. To fill this gap, samples were collected from the Gulf of Tomini, Sulawesi. From these samples, several strains of diatoms were isolated, cultivated, and examined.

Among the examined material, one species collected from Poso Pesisir (Fig. 1) caught our attention. Under the microscope, it looked familiar, closely resembling Caloneis egena (Fig. 2A), a species widely regarded as cosmopolitan. At first, the identification seemed straightforward. However, a closer look and further analyses showed consistent differences in size, striae density and pattern. These observations raised an important question: were these cosmopolitan records truly C. egena, or another species?

Figure 1. Tanjung Perak Beach, Poso Pesisir, Sulawesi

Figure 2. (A) Caloneis egena from Campeche Bay, Mexico (Schmidt 1890, public domain). (B). Caloneis paraegena (scale bar: 10 µm) from Sulawesi, Indonesia (Arsad et al. 2026, CC BY 4.0)

To answer this, an integrative approach combining light microscopy (LM), scanning electron microscopy (SEM), and genomic analyses was used.

Despite its strong resemblance, the strain did not match the original concept of C. egena and was therefore described as Caloneis paraegena (Fig. 2B). Morphologically, the two species can be distinguished. Caloneis egena is longer (33 µm), whereas C. paraegena is smaller (13–27 µm) and differs in striae density (11 in 10 µm in C. egena vs. 26–39 in 10 µm in C. paraegena).

This finding suggests that many previous records of C. egena from the Pacific region may actually correspond to C. paraegena.

In addition, this work provides the first complete plastid and mitochondrial genomes for the genus Caloneis, providing new evolutionary insights. The maximum likelihood molecular phylogeny inferred from a 3-gene dataset (18S, rbcL, psbC) derived from the sequencing data shows that C. paraegena is closely related to C. fontinalis, C. silicula, and C. lewisii.

This study shows that even well-known “cosmopolitan” species may not be what they seem and emphasizes the importance of combining morphology and molecular data when studying diatoms.

For more details, see Arsad et al. (2026) published in Frontiers in Marine Science.

References:

1. Arsad S, Gastineau R, Bąk M, Rybak M, Górecka E, Lemieux C, Turmel M, Prasetiya FS, Lobban CS. 2026. Not Caloneis egena: Morphological description and complete plastid and mitochondrial genomes of Caloneis paraegena nov. from the Gulf of Tomini, Indonesia. Frontiers in Marine Science 13: 1749044. DOI: 10.3389/fmars.2026.1749044
2. Cleve, P. T. (1894). Synopsis of the naviculoid diatoms I. K. Sven. Vet. Akad. Handl. 26, 1–194 (5 plates)
3. Schmidt, A. (1890). Atlas der Diatomaceen-Kunde. Tafel 160. Reisland, Leipzig

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Arsad et al. (2026) (https://www.frontiersin.org/journals/marine-science/articles/10.3389/fmars.2026.1749044/full?utm_source=email&utm_medium=email&utm_campaign=imp_aral_en_dat_regiuser-ww)