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A flagellate amoeba; the connection of the flagellum to the moving nucleus and the corona of variable length bristles or setae measuring up to 5 µm suggests Mastigina setosa from the sabrobic bottom of the northernmost edge of freshwater spring-fed coastal pond at Ocean Dunes Apartments in the Atlantic Double Dunes Reserve. Imaged in Nomarski DIC on Olympus BH2S using SPlan 40x objective plus variable phone camera cropping on Samsung Galaxy S9+.

A dense and opaque cell which varies from 28 um up to 37 um in length with a hyaline projection (and pseudopodial base) around the edge-circling nucleus. The cell as a whole only sometimes seems to rotate with this movement, and sometimes apparently rotates independently. The flagellum seems to show a conical attachment to the circumnavigating nucleus, the most visible part is around 50 µm in length, but in review traces can be seen in some clips up to 100 µm from the cell – which seems long, given the cell diameter of about 38 µm. [But this is a floating and possibly compact form]. Spikes or bristles of various lengths visible up to about 5 µm form a corona on all parts of the cell (including at the “hyaline tide bump” of the moving nucleus and flagellum). Very slow and steady when moving, occasionally swimming or gliding forward with the flagellum, sometimes seeming to use it to connect with surfaces. There is a lot of slow cell rotation and/or nuclear movement and flagellar probing.

Mastigina is currently incertae sedis in the Archamoebae. Walker, Zadrobílková, and Čepička, accept the genus Mastigina but others consider this a junior synonym to Mastigamoeba as proposed by Lemmermann 1917. As Bruce Taylor wrote:"that placement is still respectable (though contended!), and both Ferry and Penard Labs have the species there. Most observations of the species on the net also seem to be assigned to that genus".

From Walker, Zadrobílková, and Čepička 2017: "Mastigina is a poorly known genus, with few sightings and no molecular data, currently classified as incertae sedis (Pánek et al. 2016) though likely to be a member of the mastigamoebids. It has many similarities to Mastigamoeba but has a limax body shape where pseudopodia emerge only at the anterior or posterior ends. Its identity has historically been confused with that of Tricholimax.

Mastigina Frenzel, 1897 Mastigina contains limax-shape amoeboid cells with a flagellum and no lateral pseudopodia and with a connection between the base of the flagellum and the rounded nucleus. Its microtubular ultrastructure has not been studied by electron microscopy.

Circumscription Archamoebae with a uniflagellated trophic stage, with limax amoeboid shape and no lateral pseudopodia, and in which the round nucleus is connected to the base of the flagellum, which has the distinctively languid flagellar beat typical of other pelobionts. One species, Mastigina setosa, has fountain-flow cytoplasmic movement (Goldschmidt 1907a). The outside of the cell is covered with closely packed spines in two species, Mastigina chlamys and M. setosa (Frenzel 1897; Goldschmidt 1907a; Skibbe and Zölffel 1991). Reported from anoxic or low-oxygen freshwater sediments.

Remarks Mastigina was not clearly distinguished from other pelobiont genera when it was introduced, in the original descriptions of M. chlamys and M. paramylon (Frenzel 1897). Goldschmidt (1907a) defined the genus on the basis of limax body shape with no lateral pseudopodia, round apical nucleus connected closely to the base of the flagellum (as opposed to elongated or drop-shaped, sometimes appearing removed from the base of the flagellum, in Mastigamoeba), characters that are adopted here. Goldschmidt (1907a) also regarded Tricholimax hylae as belonging to Mastigina, leading to later confusion about the characters displayed by Mastigina species: he used fountain-flow cytoplasmic movement as a defining feature of Mastigina, despite it being present only in M. setosa and T. hylae and despite the two species in the original description of Mastigina not displaying it. Ultrastructural studies of Tricholimax hylae have been carried out under the name Mastigina hylae (Brugerolle 1982, 1991), and summaries of the genus have subsequently relied largely on descriptions of T. hylae (Griffin 1988; Brugerolle and Patterson 2000). Perhaps on the basis that fountain-flow cytoplasm is also present in Mastigamoeba aspera (Chystjakova et al. 2012; Schulze 1875b), Lemmermann (1914) in turn regarded Mastigina as a junior synonym of Mastigamoeba, leading to further confusion about the distribution of fountain-flow cytoplasm and spines in pelobionts. Frolov (2011) redefined Mastigina as containing only taxa with spines on the surface, disregarding the criteria used by Frenzel (1897) and Goldschmidt (1907a). Although 50% of the species currently assigned to Mastigina do have spines, 50% do not, and there are also members of Mastigamoeba and Mastigella with spines. In the absence of clear ultrastructural and phylogenetic data on any of these species, we see no reason to reject Goldschmidt’s circumscription of the genus" (1).

1. Chapter 37- Archamoebae 37 Giselle Walker, Eliška Zadrobílková, and Ivan Čepička, pp 1353, 1386-8 In: John M. Archibald • Alastair G. B. Simpson Claudio H. Slamovits Editors Handbook of the Protists Second Edition. Springer International Publishing AG 2017 2nd Edition