izi_izi's Identifications

Phacus orbicularis Hübner 1886 from a sample fed with boiled wheat seed from the edge benthos of freshwater glacial kettle hole Kellis Pond. Imaged in Nomarski DIC on Olympus BH2S using SPlan 100 1.25 oil objective plus variable phone camera cropping on Samsung Galaxy S24.

The cells measure from 46 up to 52 um in length. The s[pepcies was firsts described by Hübner 1886 (1). We can clearly see the horizontal "struts" perpendicular to the longitudinal pellicular strips first described by Lefevre in 1931 and, after much confusion see (2) felt by Kozmala et al 2007 to be the distinguishing character for P. orbicularis (2).

"Species. Ph. orbicularis n. spec. My illustration Fig. 1. Body is almost circular, not painted at the front. The crooked torsion is very developed, the body appears flat. The mouth fold only leads to a slight upward bulging of the dorsal ligaments. The abdominal area on the right side (back view) is visible at the top, but it does not cause any abnormal curvature of the back area. Cuticle delicately striped. Chromatophores, cell nucleus, main vacuole according to the genus character. A large granule in the shape of a disc with a central opening, resting on the ventral surface, in front of or behind the cell nucleus. Terminal spine clearly visible, 1:6 of the body length, pointed left and upwards (back view). Flagellum of body length. Size = 0.07 mm. end spine. Br. = 0.045 mm. Occasionally, among other Euglenaceae, bird meadows" (1).

"Phacus orbicularis Hübner, Programm d. Realgymnasiums Stransund: 5, fig. 1, 1886. Emend. Zakrys´ et Kosmala. Emended diagnosis: Cells flat, 29–75 um long and 22–49 um wide, widely ovoid, ending with a more or less prominent and curved tail. Fine, numerous struts—perpendicular to the longitudinal axis—located between periplast strips" (2). "Periplast ornamentation was recognized as a main diagnostic character, distinguishing P. orbicularis from P. pleuronectes and P. hamelii. Phacus orbicularis has struts running perpendicular to the longitudinal axis of the strips, while P. pleuronectes and P. hamelii do not" (2). Support for this morphologic character being distinctive to P. orbicularis among congeners: " On the SSU rDNA tree, obtained by the Bayesian method, P. orbicularis, P. pleuronectes, and P. hamelii belong to three distinct clades" (2).

"Our studies also point out periplast ornamentation, described as ‘‘struts’’ by Leedale (1985), which are positioned perpendicular to the longitudinal axis of the strips. The struts were present in all P. orbicularis strains surveyed by us (Fig. 1, n and u)— all SSU sequences for the P. orbicularis strains clade together on the 18S rDNA tree (Fig. 2)—but struts were not observed in P. pleuronectes or P. hamelii (Fig. 1, o and t). Lefevre (1931) was the first to notice ‘‘perpendicular stripes’’ in P. orbicularis and take them into account in his drawings. Lefevre’s drawings were used later by Pochmann (1942, fig. 78, k and n), albeit without any commentary. This action suggests that both Lefevre and Pochmann, as well as others, did not consider struts to be a diagnostic feature distinguishing P. orbicularis from P. pleuronectes. This also might be the reason for not considering struts as diagnostic in spite of their appearing on drawings of several other species (P. rostafinski [Drezepolski _ 1921 ⁄ 1922, pl. 1, fig. 3]; P. platalea [Drezepolski _ 1925, pl. 3, fig. 110]; P. caudata var. minor and var. ovalis [Drezepolski 1925, pl. 3, figs. 107 and 111]; _ P. longicauda [Lefe´vre 1931, pl. 3, fig. 32]; Phacus triqueter [Leander and Farmer 2001, fig. 3a]). The presence of perpendicular struts on the iconotype of P. platalea Drezepolski 1925 (fig. 110), as well as its _ other features, such as the size and shape of the cell and the relatively long (12–15 lm) tail, prompted us to consider P. platalea as a synonym of P. orbicularis. In our view, the presence of struts is a good diagnostic feature for distinguishing P. orbicularis from P. pleuronectes since it is not susceptible to individual, developmental, and environmental variability. Moreover, struts are clearly visible under the light microscope, even in very small cells (in which case, a brief drying out of the material facilitates observations; Fig. 1, n and u)" (2)

1. Hübner, E.F.W. (1886). Euglenaceen-Flora von Stralsund. Programm des Realgymnasiums Stralsund 1886: 1-20, 1 pl..
2. MORPHOLOGICAL AND MOLECULAR EXAMINATION OF RELATIONSHIPS AND EPITYPE ESTABLISHMENT OF PHACUS PLEURONECTES, PHACUS ORBICULARIS, AND PHACUS HAMELII. Sylwia Kosmala, Magdalena Bereza, Rafał Milanowski, Jan Kwiatowski, and Bozena Zakrys. J. Phycol. 43, 1071–1082 (2007)