A Key to the More Frequently Occurring Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 215

Cells of Cyclotella are disc-shaped with
circular-shaped valves having a slightly
undulate surface. Valve margins with-
out spines but in some species small
tubules are present that could be mis-
taken as spines. The central area is irreg-
ularly punctate and distinct from the
concentric outer area, which is regu-
larly striate. Cells frequently solitary but
may be attached in chains by mucilagi-
nous threads. There are several discoid
chloroplasts usually arranged around the
cell margin. Widespread in lakes, rivers,
marine and brackish water habitats. Has
been reported to produce geranium and
fishy odours and tastes in drinking water
(Palmer, 1962). Two of the more common
species in cool temperate waters are C.
menegheniana with cells 10-30 μmin
diameter and C. kuetzingiana with cells
10-40 μm in diameter. The former has 40-
50 rows of radiating striae and the latter
up to 90 radiating rows. Can be present as
large numbers in reservoirs and lakes and
may cause problems of filter blocking in
water treatment works. SEM has revealed
other features on the valve surface not
easily seen using light microscopy, result-
ing in some workers dividing the genus
into specific sub-groups. Bacillariophyta,
Plate XIX. Fig. 4.62.

15 μm

Figure 4.61 Hyalodiscus. Diatom with peripheral
clear area on valve surface. Prepared slide.

concentric area around the centre
.......................... Hyalodiscus

Hyalodiscus cells are either sessile, in
short chains or as individuals in the marine
plankton. Valves circular and strongly
convex with the centres slightly flattened.
Because of the markedly convex nature of
the valve face, only a small area can be
in focus under a light microscope at one
time, hence the apparent lack of mark-
ings. In fact the whole valve surface is
covered with areolae arranged in radial
rows. Cells (12-115 μm diameter) have
numerous rod-shaped chloroplasts. Bacil-
lariophyta. Plate XIX. Fig. 4.61.

(b) Valve markings not in two distinct
zones of punctae and striae ......... 138

138 (137) (a) Valve surface flat to slightly convex,
circular or slightly elliptical in outline.
Areolae in straight parallel lines (tangen-
tial to the radius) of decreasing length giv-
ing the surface a segmented cross-hatched
appearance.. ............. Actinocyclus

(b) Valve surface with obvious markings
over whole or a large part of the area. 137

Valves of Actinocyclus are circular to
slightly subcircular. There is a small cen-
tral hyaline area with irregular punctae.
The valve face is marked with areolae in
regular parallel rows of decreasing length

137 (136) (a) Valve surface with two distinct areas
of markings. The middle area is punc-
tate and the outer striate or ribbed
............................ Cyclotella