Chaetoceros heterovalvatus Proschkina-Lavrenko (1953: 46)

Aké-Castillo, José Antolín, 2022, Morphology and taxonomy of Chaetoceros heterovalvatus, a rare marine diatom, Phytotaxa 544 (1), pp. 100-102 : 100-101

Chaetoceros heterovalvatus Proschkina-Lavrenko (1953: 46) Figs 1 –6

Type: — UKRAINE: Black Sea , northwest region in the river mouth of Dnepr and Bug (Holotype Fig. 1 !, (10 illustrations) in Proschkina-Lavrenko 1953, p. 47)

LM observations: The cells are solitary, cylindrical with 4 long setae ( Fig. 1 ). The frustule is heterovalvate, with a convex valve with the setae rising from the valve margin and directed to the opposite valve with an increasing angle to the pervalvar axis ( Fig. 2 ); the other valve is flat with the setae rising from the margin of the valve and diverging. The apical axis measures 3.2–9.5 µm, and the pervalvar axis 8–12 µm. It contains several small chloroplasts.

SEM observations: Only valves were observed. A valve is vaulted with a smooth mantle with the setae rising between the valve surface and the mantle, curved downwards ( Fig. 3 ). The other valve is slightly convex whose transition to a wide mantle is abrupt ( Fig. 4 ). At the center has a rimoportula composed by a short tube ( Fig. 4 ). The setae rise at the edge of the valve surface and mantle diverging perpendicularly to the valve surface. No other ornamentation was observed. The setae are cylindrical at the base and quadrangular (four sided) most of the length with conspicuous spines ( Figs 3, 4 ).

Distribution: —Black Sea ( Proschkina-Lavrenko 1953, Belenkova 2000, Cărăuş 2002, Terenko 2005), Elton Lake ( Russia) ( Burkova 2015), Mediterranean Sea ( Margalef 1963, 1969,1994), Gulf of Mexico ( Aké-Castillo & Vázquez 2008)

Comments: Range of measurements of apical axis is greater than the original description (5-8.75 µm) and range of pervalvar axis is smaller (12.5-20 µm). Direction of seta of the convex valve is at less angle comparing to the drawings of the type.

Ecological remarks: Found in the dry season (April) at salinities between 10 to 32 and temperatures of 29 to 32°C with the higher cell density (8000 cell/L) at salinity of 17 and temperature of 31.9 °C. ( Aké-Castillo & Vázquez 2008)

In this study, the general structure of the frustule is shown. Heterovalvate frustule, possession of four setae and direction, and possession of many chloroplasts fits mostly with the overall diagnosis of C. heterovalvatus . Variations were observed in range of sizes and in the direction of setae of the convex valve, which are not “perpendicular intensae”. Morphological data of the species is only available in the original description (Proschkina-Laverenko 1953) so, no information about variability exists. Range in size overlapped so the differences may be responses to the environmental condition or life stage of the cells. The seta of the convex valve, although not totally perpendicular to the pervalvar axis, it raises from the valve margin of the valve and curve downwards as the diagnosis indicates. Similar morphological species is C. throndsenii var. trisetosus Zingone , but this heterovalvate species has only three setae ( Marino et al. 1991).

Considering the morphological characters of the frustule and number of chloroplasts of C. heterovalvatus , this species belongs to the section Subconica. Sar et al. (2002) wrote the section Simplicia “consists of solitary (or in pairs as cell division takes place), small, delicate (weakly silicified) species, with 1 or 2 chloroplasts and very thin setae”. De Luca et al. (2019) described the section Simplicia similarly as follows “cells small and fragile, generally single or two or three together. In case of chain formation, there is no differentiation of terminal setae.” The section Minima comprises species with one seta at a valve and one or two setae at the other ( De Luca et al. 2019). So, considering the number of chloroplasts, the only option for C. heterovalvatus is to be allocated in section Subconica as Semina & Sergeeva (1982) and Strelnikova (2006) did. I agree De Luca et al. (2016) not consider the subgenus Hyalochaete in the taxonomy of Chaetoceros , as they demonstrated it is paraphyletic.

Chaetoceros heterovalvatus is a rare marine diatom with preferences to low salinity. This species belongs to the section Subconica. The possession of one rimoportula at least at the hypovalve confirms the occurrence of this character in most of the solitary forms of Chaetoceros . The single record of the occurrence of the species in America may indicate that the species has been overlooked and it has a wide-world distribution.

Figures

FIGURES 1-4. Chaetoceros heterovalvatus. LM fresh preparation, complete cell (1), LM fresh preparation, vaulted valve with curved setae (2), SEM vaulted valve with curved setae (3), SEM valve with divergent setae and rimoportula (arrow) (4), Scale bar 10 µm (1,2), 2 µm (3,4).