A Key to the More Frequently Occurring Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 229

(b) Costae absent. Cells either with one
C-shaped/plate-like or two elongate/plate-
likechloroplasts................... 173

174 (173) (a) Cells gently curved with a small
swelling at centre of concave surface.
Slightly capitate ends. Costae and septa
absent....................... Hannaea

172 (171) (a) Cells with arched dorsal surface and
concave ventral surface. Raphe character-
istically V-shaped at centre of concave sur-
face (see Plate XXV) ........ Epithemia

(see also key No. 163)

(b) Cells gently curved or bent at centre
in girdle view. Elliptical to oval or nearly
rectangular in valve view . .......... 175

Epithemia is a widely distributed epi-
phyte found in freshwaters and sometimes
in brackish conditions but rarely in more
acid waters. Some species found in habi-
tats where phosphorus is more readily
available. The cells are solitary with a
strongly dorsiventral shape, and show dis-
tinct transverse striae. The single chloro-
plast has distinctly lobed margins. Cells
(8-200 μm long, 4-35 μm wide) often
containcyanophyceanendosymbiontsthat
play a role in nitrogen fixation (DeYoe et
al., 1992). Bacillariophyta. Plate XXV.

175 (174) (a) Cells gently curved, no small cen-
tral swelling. Dorsal or convex margin
sometimes undulate. Ends of raphe visi-
ble towards poles in valve view . Eunotia

(see also key No. 161)

(b) Cells with raphe clearly visible on
one valve. Other valve with pseudoraphe.
Cells rectangular to elliptical in valve
view and bent (genuflexed) in girdle view
.......................... Achnanthes

(b) Raphe close to dorsal surface but not
V-shaped . ................. Rhopalodia

Cells of Achnanthes are heterovalvar, one
valve bearing a true raphe and the other
a pseudoraphe. Cell apices rounded or
variable. Cells (5-35 μm long, 3-10 μm
wide) contain a single plate-like chloro-
plast and are either single or form short
chains. A widespread and common alga
attached to a range of surfaces in flow-
ing waters. Many species in marine or
brackish waters. Recent revisions of the
old genus Achnanthes has resulted in a
number of new genera being separated out
including Achnanthidium , Eucocconeis ,
Planothidium , Kolbesia and Psammoth-
idium.Achnanthesminutissima , which is
verycommon,hasnowbeenreclassiiedas
Achnanthidium minutissimum . Bacillar-
iophyta. Plate XXV. Fig. 4.69.

Rhopalodia cells are solitary, either
attached or free-floating, in freshwater or
brackish waters. They are dorsiventral in
shape often having turned-down apices.
Cells (40-150 μm long, 7-12 μm wide)
have a single-lobed chloroplast and show
coarse transverse striae on the valve sur-
face. The raphe curves along the margin
of the cell in a keel but can be difficult to
see. Bacillariophyta. Plate XXIV.

173 (171) (a) Cells with a strongly curved appear-
ance but with margins/sides parallel.
Bluntly rounded ends, raphe hardly visi-
ble. Transverse ridges on the frustule sur-
face ending in short spines .... Semiorbis

(see also key No. 159)

176 (170) (a) Cells linked to form colonies or chains
.................................. 177 (b) No prominent transverse ridges or

short spines. Cells slightly but not strongly
curved............................ 174

(b) Cells mostly solitary . . ......... 191