Taxonomy, phylogeny, and geographical distribution of the little-known Helicoprorodon multinucleatum Dragesco, 1960 (Ciliophora, Haptorida) and key to species within the genus

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Abstract

A little-known haptorid ciliate, Helicoprorodon multinucleatum Dragesco, 1960, was found in a sandy beach at Qingdao, China. Its morphology was studied based on microscopic observations of live and protargol-stained specimens and morphometrics, and the phylogeny was analyzed using SSU rRNA gene sequences. Helicoprorodon multinucleatum is characterized by the combination of the following features: (i) a very narrowly worm-like body with a size of about 300–1500 μm × 30–60 μm in vivo, and two circles of horn-like protuberances around the head; (ii) 50–160 spherical macronuclear nodules scattered throughout the body; (iii) rod-shaped, 10–50 μm long extrusomes gathered into several bunches, which are randomly distributed beneath pellicle; and (iv) 42–88 somatic kineties, including four oralized kineties and two dorsal brush rows. Phylogenetic analyses reveal that both the family Helicoprorodontidae and the genus Helicoprorodon might be monophyletic. In addition, we provide an illustrated key to the species and the geographical distribution of the genus Helicoprorodon.

Introduction

Ciliates belonging to the order Haptorida Corliss, 1974 often feed on flagellates, other ciliates, and even small metazoans, and thus act as secondary consumers in aquatic environments (Lynn 2008). The taxonomy and systematics of these ciliates is quite confused. The order was first included in the subclass Gymnostomata Bütschli, 1889 and class Kinetofragminophora de Puytorac et al., 1974 (Corliss 1979). Later, it was transferred to class Litostomatea Small and Lynn, 1981. In the classification proposed by Lynn (2008), the Haptorida was assigned to the Haptoria, one of two subclasses in the Litostomatea, with two other orders, viz. Pleurostomatida Schewiakoff, 1896 and Cyclotrichiida Jankowski, 1980 (incertae sedis). During the last three decades, many haptorids have been described from various soil and freshwater biotopes (e.g. Bourland 2017; Dragesco 1960; Foissner et al. 2002; Kahl 1933; Vd’ačný 2007; Yıldız 2018); however, marine forms are relatively insufficiently investigated using modern methods except for a few researches on members of the genera Chaenea Quennnerstedt, 1867 and Phialina Bory de St. Vincent, 1824 (Al-Rasheid 1999; Dragesco 1960; Fan et al. 2015; Hu et al. 2019; Kwon et al. 2014; Lipscomb and Riordan 1991; Song et al. 2009; Wang et al. 2019).

Kahl (1933) described a giant, worm-shaped ciliate and classified it in Chaenea as C. gigas, though he later acknowledged that this species was very different from other members of the genus (Kahl 1935). Fauré-Fremiet (1950) noticed the unique oralized somatic kineties in C. gigas, and established the genus Helicoprorodon for it. Hereafter, five new species were added (Bock 1952; Dragesco 1954a, b, 1960; Raikov 1963). So far six species are included in the genus and are all psammophiles inhabiting marine sediments (Carey 1992), i.e. H. barbatus Dragesco, 1954, H. gigas (Kahl, 1933) Faure-Fremiet, 1950, H. maximus Dragesco, 1954, H. minutus Bock, 1952, H. multinucleatum Dragesco, 1960, and H. orientalis Raikov, 1963.

Most of the above-mentioned species were relatively incompletely described on limited occasions. Details on the ciliature are only available for Helicoprorodon gigas, and the SSU rRNA gene sequence is known only for H. maximus, which highlights the need for detailed re-descriptions with modern taxonomic methods (Al-Rasheid 1999).

In the present study, Helicoprorodon multinucleatum is redescribed as first recorded from Asia, and redefined using a multidisciplinary analytical approach, which combines morphological (i.e. live observation and silver staining) with morphometric and molecular analyses.

Section snippets

Sample collection and morphological methods

Samples were collected from the intertidal zone of the No. 2 Bathing Beach at Qingdao (36°03′25″N; 120°21′27″E), China on July 17, 2019, when the water temperature was 26 °C and salinity was 31‰ (Fig. 1). The sampling method was mainly according to Xu et al. (2011). Generally, a 15 cm-deep hole was dug in the sand into which seawater gradually seeped. The sample comprised a mixture of sand and a little seawater from the bottom of the hole. Ciliates were extracted using the seawater-ice method (

Results

Class Litostomatea Small and Lynn, 1981

Subclass Haptoria Corliss, 1974

Order Haptorida Corliss, 1974

Family Helicoprorodontidae Small and Lynn, 1985

Genus Helicoprorodon Fauré-Fremiet, 1950

Helicoprorodon multinucleatum Dragesco, 1960

Comparison with other populations

Helicoprorodon multinucleatum was first discovered at the French Atlantic coast (Dragesco 1960) and then reported from White Sea, Russia (Raikov 1962), Brazilian coast (Kattar 1970), Bermuda (Hartwig 1977), and Delaware, East coast of the USA (Lipscomb and Riordan 1991). The Qingdao isolate was identified as Helicoprorodonmultinucleatum because it agrees with the original description and redescriptions in the species diagnostic features, viz., the numerous, scattered macronuclear nodules,

Author contributions

XH conceived and guided the study. LJ conducted sampling and performed laboratory work. XH and LM identified the species. CW did the phylogenetic analyses and the results interpretation. LJ drafted the manuscript, and WZ, SL and XH made further revisions. All authors read and approved the final version of manuscript.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (project number: 41976086). We are grateful to the editor and anonymous reviewers for their constructive suggestions.

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