Morphology and taxonomy of Aulacoseira muzzanensis (Bacillariophyta)

. Morphological investigation of wild populations of Aulacoseira granulata shows signi-ficant variability of its quantitative morphological characteristics: valve diameter and height; number of rows of areolae and number of areolae in 10 µm on the valve mantle; and number of spines and their length on separation valves. Low values of mantle height to valve diameter ratio and large numbers of areolae in 10 µm, typical for A. muzzanensis , are also found in populations of A. granulata . The valve height/diameter ratio decreases as the valve diameter increases, and such relationships also occur in other taxa of this genus ( A. baicalensis , A. islandica , and A. subarctica ). No correlation is found between valve diameter and number of spines in separation valves. Our investigations confirm the correctness of referring A. muzzanensis to the synonymy of A. granulata , and based on original and published data, we suggest emendation of the diagnosis of A. granulata .

The independent taxonomic status of A. muzzanensis was recognized in other publications Algae…, 2009;. Notably,  believe that A. muzzanensis differs from A. granulata by a lower mantle height to valve diameter ratio (h/d) and a greater number of areolae in a row on the valve mantle. However, one more combination, A. granulata var. australiensis (Grunow) Moro was described based on the difference from the type variety in the valve diameter, position of areolae on the valve face and the number of spines on the separating spines .
There are other species similar to A. muzzanensis. One is A. gessneri (Hust.) Simonsen . Valves of A. muzzanensis have usually slightly spiral dextrorse rows of areolae on the valve mantle, which have never been observed in A. gessneri and clearly separate these species. A species similar to A. muzzanensis, A. brasiliensis Tremarin, Torgan et T. Ludwig , has been recently described. It differs from A. muzzanensis by the length of spines, position of areolae on the valve face and mantle, and the position and structure of rimoportulae. Even more recently, A. pseudomuzzanensis Olsyński et Żelana-Wieczorek was described by Olszyński et Żelazna-Wieczorek (2018). This species differs from A. muzzanensis by the number of areolae in 10 µm on the mantle, position of areolae on the valve face, and structure of rimoportula.
The aim of this study is to investigate the morphological variability in populations of A. granulata and specify the systematic position of A. muzzanensis.
Diatom frustules were treated from organic matter by cold burning (Balonov, 1975) with sulphuric acid and potassium dichromate. Cleaned specimens were dried on stubs, coated with gold using an EIKO-IB-3 sputter coater, and examined using a JSM-25S scanning electron microscope operating at 15 kV.

Results and discussion
In the population from the Cheboksar Reservoir, the range of variability of quantitative morphological characteristics, including mantle height/diameter (h/d) ratio, is similar in separation valves and valves in the filament (Table 1). These features correspond to the published data on A. granulata and A. muzzanensis, except for the separation valves by the number of areola rows in 10 µm and areolae in 10 µm, and fine-structure valves in the filament of A. muzzanensis by the number of areola rows in 10 µm and areolae in 10 µm (Table 2). Unfortunately, no information is available on the number of spines on separation valves.  reported 2-4 long spines for A. granulata. In valves from the Cheboksar Reservoir population, we recorded 1 to 7 spines, including long and more short ones (Fig. 1b). There were usually more longer spines, and we found no relationship between their number and the valve diameter ( Fig. 1c). In their quantitative morphological characteristics, the valves from the Ivankovo Reservoir and Sestroretsk Razliv (Table 1) also corresponded to published data (Table 2), except for maximum values of valve diameter in the population from the highly eutrophic Sestroretsk Razliv (37.1 µm).  . (b) A. granulata, relationship between valve diameter (axis X, µm) and long and more short spines (axis Y). (c) A. granulata, relationship between valve diameter (axis X, µm) and long spines (axis Y).   A certain relationship was observed in all A. granulata populations (Fig. 1a, 2a) with increasing valve diameter, the mantle height to valve diameter ratio decreases and the h/d ratio typical for A. muzzanensis (0.3-0.8), is found in large-diameter valves (Fig. 1a, 2a). Such correlation occurs in other representatives of the genus Aulacoseira (Fig. 2b, 3a, b). Skabichevsky (1960) also noted that A. granulata narrow cells have longer valves and its cells with a larger diameter are shorter. According to the diagnosis of  A. muzzanensis is characterized by shorter valves, a lower h/d ratio and a larger number of areolae in 10 µm than A. granulata (Table 2). According to the published data the first two characteristics correspond with A. granulata (Table 2), but the valve diameter and the height of its mantle significantly vary in a manner that may be due to seasonal, longterm or interpopulational variability (Genkal, 1990(Genkal, , 2007. In A. granulata the ratio of the mantle height to valve diameter is more than 0.8, and in A. muzzanensis it is 0.3-0.6 ( Table 2). Measurement and calculation of this characteristic in the other populations of A. granulata has shown similar results (Table 1). Measurements from published micrographs (Florin, 1970: Fig. 21-23) show that the mantle height to valve diameter ratio in A. granulata varies from 0.19 to 0.24. Krammer, Lange-Bertalot (1991: Taf. 20, Fig. 1) present a micrograph of A. muzzanensis with a ratio of mantle height to valve diameter of about 0.8. Noteworthy that measurements of A. muzzanensis from micrographs (Krammer, 1991: Fig. 3, 4, 6, 8, 10, 10a, 11, 11a, 12, 13, 16) have shown that in this species population from the type locality the h/d ratio varies quite a bit -from 0.3 to 1.06 -and this ratio depends on the valve diameter as in other representatives of the genus (Fig. 1a, 2, 3). In A. muzzanensis separation valves and those in filaments can have the same number areolae in 10 µm as in A. granulata, as well as much more (16-21) ( Table 2). However, according to the literature, such numbers of areolae were found in A. granulata too: Genkal (1992) and Genkal and Gorokhova (2008) reported 16 and Maystrova et al. (2007) reported 18. In illustrations of A. granulata (Florin, 1970: Fig. 21-23) the number of areolae in 10 µm ranges from14 to 20. Measurements from micrographs indicate that the number of areolae in 10 µm on separation valves of A. muzzanensis can differ even on one valve of the colony (14 and 18: Krammer, 1991: Fig. 11a), which indicates a wider range of variability of this characteristic coinciding with that of A. muzzanensis. Skabichevskiy (1960) also noted variations in the number of rows and areolae on the valve mantle and attributed it to cell age -old frustules have 11.2 rows of pores in 10 µm and 6.7 pores in a row and young ones have 13.2 rows and 13.3 pores.
In our opinion the results of our investigations and data from the literature confirm the correctness of referring Melosira granulata var. muzzanensis to the synonym of Aulacoseira granulata and make it possible to refine the diagnosis of the latter.
Aulacoseira granulata var. australiensis has a range of variability of quantitative features (valve diameter 18-31 µm, height 11-17 µm, 8-16 striae in 10 µm, 8-16 areolae in 10 µm) coinciding with those of A. granulata var. granulata (Table 2). According to the diagnosis, areolae on the valve face of A. granulata var. australiensis are located over the entire surface; however, they are absent in the type variety in the author's opinion . This statement is untrue, and areolae in A. granulata var. granulata are distributed over the entire surface (Krammer, Lange-Bertalot, 1991: Taf. 17, Fig. 7-10;Houk, 2003: Tab. XXV, Fig. 14, 15). One of the features that differentiates A. granulata var. australiensis from the type species is a smaller number of spines in the latter (2-3) compared to A. granulata var. australiensis (3-4). However, according to our data, their number may vary from 1 to 7 (Fig. 1b). In our opinion, in view of the above, A. granulata var. australiensis belongs to the group A. granulata var. granulata.