Article Highlight | 16-Jun-2025

Linking multi‑gene and morphological data in the subclass scuticociliatia (protista, ciliophora) with establishment of the new family homalogastridae fam. Nov

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fig 1 — **image:**
**Fig. 1** Morphological schematic illustration of a representative of the subclass Scuticociliatia (A) and the phylogenetic position of the class Oligohymenophorea in the phylum Ciliophora according to Gao et al. (2016) (B).
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Credit: MLST

https://doi.org/10.1007/s42995-024-00264-8

Announcing a new publication for Marine Life Science & Technology journal. In this research article Professor Chundi Wang from the Marine College of Shandong University, Weihai, China considers some long-standing taxonomic uncertainties and evolutionary relationships within Scuticociliatia, a diverse group of ciliates.

Scuticociliates are a speciose and cosmopolitan group of ciliates that have been found in a wide range of aquatic and terrestrial habitats. They exhibit diverse lifestyles, existing as free-living forms, symbionts, or facultative parasites, often associated with invertebrates and vertebrates. Their presence can have significant ecological and economic implications, particularly in aquaculture, where they can cause diseases such as scuticociliatosis, leading to economic losses. The classification of Scuticociliatia has been unclear for long term due to the homogeneity of their morphology and the insufficiency of molecular data. With 21 novel sequences from 13 morphospecies, the researchers conducted phylogenetic analyses using multiple genes, including SSU rRNA, LSU rRNA, ITS1-5.8S-ITS2 region, and COI genes. A total of 308 SSU rRNA gene sequences, 64 LSU rRNA gene sequences, 90 ITS1-5.8S-ITS2 gene region sequences and 116 COI gene sequences, were used in the construction of phylogenetic trees.

The results of the phylogenetic analyses revealed that all scuticociliates cluster into two well-supported and one poorly supported group, representing three order-level taxa: Philasterida, Pleuronematida, and Loxocephalida. The order Loxocephalida was found to be polyphyletic, with only the family Conchophthiridae forming a monophyletic group. The order Pleuronematida was also polyphyletic, since the sequence of Pseudocyclidium longum located outside the Pleuronematida + Philasterida clade. In contrast, the order Philasterida formed a monophyletic group, although some of its families, such as Orchitophryidae and Philasteridae, were polyphyletic.

One of the major contributions of this study is the establishment of a new family, Homalogastridae fam. nov., within the order Philasterida. The genus Homalogastra, which was previously assigned to the family Uronematidae, was found to differ significantly from other members of Uronematidae in both morphology and molecular phylogeny. Based on these differences, the researchers proposed the new family Homalogastridae to accommodate the genus Homalogastra. This new family is characterized by free-living philasterids with a small, spindle-shaped cell, three reduced oral polykinetids, and a sub equatorially positioned cytostome.

In addition, the study also proposes several other taxonomic changes. Parauronema is formally transferred to the family Uronematidae, based on its phylogenetic relationship with Uronema. Potomacus, on the other hand, is treated as incertae sedis within the order Philasterida, and Parauronematidae is considered as a junior synonym of Uronematidae. The genus Madsenia and two species, Parauronema longum and Pseudocyclidium longum, are also considered incertae sedis, as they did not fit clearly into any existing family. The genus Protophyra was retained in the family Ancistridae, although its taxonomic position remains somewhat uncertain.

The study also analyzed the putative secondary structure of the internal transcribed spacer 2 (ITS2) of representative taxa from the three orders of Scuticociliatia. The results showed that the ITS2 secondary structures of these taxa are composed of a common loop and at least two unequal-length helices, with conserved motifs and bulges. These findings provide further insights into the evolutionary relationships within Scuticociliatia and may be useful for future taxonomic studies.

Overall, this study reconsidered the systematic relationships within the subclass Scuticociliatia according to both morphological and molecular information, which will serve as a resource for future studies on scuticociliates and related taxa. The proposed taxonomic changes and the establishment of the new family Homalogastridae clarified some long-term unresolved systematic relationships within Scuticociliatia and enhanced our knowledge of ciliate diversity.

Article reference: Liu, M., Jiang, L., Zhang, Z. et al. Linking multi-gene and morphological data in the subclass Scuticociliatia (Protista, Ciliophora) with establishment of the new family Homalogastridae fam. nov. Mar Life Sci Technol 7, 1–22 (2025). https://doi.org/10.1007/s42995-024-00264-8

Keywords: Ciliated protists, Homalogastridae fam. nov., Phylogeny, rRNA secondary structure, Scuticociliates

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