Cardiostomatella mononucleata (3), from the superficial intertidal benthic sands of marine estuary Acabonac Harbor. This Accabonac population measures 75-100 micrometers in length, which nicely matches the size bar key in the Fig. 5F illustration from Wang et al (1) but not the listed lengths from their table 3 where Dragesco's 1963 observations for C. mononucleata are quoted at 300 and 400 micrometers! Interestingly, Al-Rasheed reports regarding the type species C. vermiforme: "Size highly variable, 90–510 x 41–117 μm, usually 150–300 x 50–100 μm" (2) so perhaps C. mononcleata is similar in that regard. The Bonac version also has a single sausage-shaped macronucleus consistent with Cardiostomatella mononucleata as illustrated in Fig. 5F of Wang et al and Bill Bourland's opinion (3).

There are only four species in the genus. The genus Cardiostoma was erected by Kahl (1928) to accommodate the new species C. vermiforme, which contains some large, marine ciliates. Dragesco (1963) named another species Cardiostoma mononucleata but the generic name "Cardiostoma" was already taken for a species of mollusc and thus Corliss (1960), according to the naming rules of the ICZN (International Commission on Zoological Nomenclature) made a "replacement name", Cardiostomatella so all the ciliate species previously named "Cardiostoma" are thereafter named Cardiostomatella species. Later another two species were discovered: C. minuta and C. chesapeakensis (Dragesco, 1963; Dragesco, 1965; Small and Lynn, 1985).

Wang et al (1) provide a new diagnosis for the genus Cardiostomatella based on all data previously obtained: "cell elongated, with uniform somatic ciliation; both pre- and postoral sutures present; buccal cavity inconspicuous; three membranelles and one C-shaped paroral membrane arranged in Tetrahymena pattern; postoral kineties present". Apparently the protist enjoys a meal of diatoms which gives them extra energy to swim around rapidly and thus avoid my prying camera lens.
Imaged in Nomarski DIC using Olympus BH2S microscope under Olympus SPlan 40x and 20x objectives plus variable phone cropping on Samsung S9+.

Al-Rasheed writes about a curious proposal: "several authors reported the most common species,
C. vermiforme from various marine habitats, although, with many confusing morphological variations, which led one of the recent studies to suggest that all Cardiostomatellas should be combined into a single type
species; C. vermiforme (Fenchel et al. 1995) (2)!

Lineage: cellular organisms; Eukaryota; Sar; Alveolata; Ciliophora; Intramacronucleata; Oligohymenophorea; Scuticociliatia; Philasterida; Loxocephalidae

1. First Record and Redefinition of the Qingdao Population of Marine Ciliate Cardiostomatella vermiformis (Kahl, 1928) Corliss, 1960 (Protozoa, Ciliophora).
   WANG Yangang, HU Xiaozhong, LONG Hongan, and SONG Weibo. Journal of Ocean University of China (Oceanic and Coastal Sea Research) ISSN 1672-5182, October 30, 2007, Vol.6, No.4, pp.387-392

2. Redescription of the marine ciliate Cardiostomatella vermiforme (Kahl, 1928) Corliss, 1960. Khaled A. S. Al-Rasheid. Protistology 2 (1), 15–21 (2001) April, 2001
3. William Bourland, personal communication.