A Key to the More Frequently Occurring Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 210

2.5.2) so that the cell contents and external
mucilage can be seen together with any
markings on the silica frustules. Readers
are strongly recommended to consult the
more detailed literature mentioned in the
bibliography.

(b) No spines present at rim or margin of
valve............................. 134

NB. Some centric diatoms secrete chitin
fibres or threads that protrude from the
valve margin and may superficially look
like spines. These may function to bind
cells together in loose chains or act as a
deterrent to grazing animals.

128 (127) (a) Valve with clear (hyaline) central area
devoid of regular markings and 5-7 wide
hyaline rays extending to the cell margin
....................... Asteromphalus

131 (130) (a) Cells usually in chains joined together
by mucilage strands originating from the
valve surface centre or embedded in
mucilage. Valve face gently undulating,
with punctae in a radiating mesh-like pat-
tern..................... Thalassiosira

Asteromphalus is a widely distributed
marine and estuarine species common in
cool temperate waters. The valves have
a relatively flat surface and between the
hyaline rays, there may be fine or coarse
areolae (depending upon the species).
Cell diameter 25-100 μm. Numer-
ous discoid chloroplasts. Bacillariophyta.
Plate XXI.

Cells of Thalassiosira can be solitary
but are more frequently joined to form
loose chains or may even be embedded in
mucilage. Valve face circular, fairly flat to
gently undulating, and covered with quite
coarse punctae which may be radial or
arranged in arcs. Small spines may be vis-
ible at valve margin. Chloroplasts small
discs and numerous. Very common and
widely distributed in marine and estuar-
ine habitats. Bacillariophyta. Plates XVIII
and XIX.

(b) Valve without wide hyaline rays . 129

129 (128) (a) Markings on valve surface divided
into (usually) six segments that are
alternately raised or level giving an
alternating light and dark appearance
........................ Actinoptychus

Actinoptychus is a cosmopolitan marine,
coastal and estuarine species common at
most times of the year. Markings on the
valve are characteristic, with segments are
covered in coarse areolation. Chloroplasts
plate-like or irregular. Cells 20-80 μm
in diameter. Bacillariophyta. Plates XVIII
and XXI.

(b) Cells solitary, or if in loose chains not
embedded in mucilage or with mucilagi-
nous strands joining from valve face to
valveface......................... 132

132 (131) (a) Valves with small central area bearing
2-5 pores. Distinct striae radiate out from
this central area to the valve margin. Usu-
ally linked to form short chains by means
ofsmallspines.Valvefacesthenheldclose
together. ................... Orthoseira

(b) Valve markings not as above . . . . 130

130 (129) (a) Margin or rim of valve with spines
.................................. 131 Cells short cylinders with numerous dis-

coid chloroplasts. Found on damp rocks,
in wet moss or on stream banks, usually in
upland (sometimes alkaline) areas. Bacil-
lariophyta. Plate XXI.

NB. Coscinodiscus (138) may have spines
which may be difficult to see using a light
microscope.