Introduction to Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 27

Epitheca

Cingulum

Transverse
flagellum

Hypotheca

Longitudinal flagellum

Sulcus

(b)

(a)

Figure 1.8 Dinoflagellate symme-
try and plates. Top: Diagrammatic
view of a typical dinoflagellate
cell showing flagellar insertion and
grooves within the plate system.
(a) Ventral view; (b) Dorsal view.
Bottom: Pattern of numbered thecal
plates characteristic of the dinoflag-
ellate Peridinium . (c) Ventral view;
(d) Dorsal view. See also Fig. 4.57.
Wehr & Sheath, 2003. Reproduced
with permission from Elsevier.

3 ′

3 ′

2 ′

4 ′

3a

2a

1 ′′

7 3 ′′

′′ 1 ′

4 ′′ 5 ′′

2 ′′′

5 ′′′

1 ′′′

3 ′′′

4 ′′′

1 ′′′′

1 ′′′′

2 ′′′′

(c)

(d)

that are typically condensed throughout the entire
cell cycle. These have an unusual structure,
with genetically active (transcriptional) DNA
on the outside and genetically inactive (struc-
tural) DNA forming a central core (Sigee,
1984).

by scanning electron microscopy (Fig. 4.57). The
number, shape and arrangement of thecal plates are
taxonomically diagnostic (compare Figs. 1.8 and
1.9). A typical arrangement of plates can be seen
in Peridinium (Fig. 1.8), with a division into two
major groupings, forming an apical epitheca (or
epicone) and a posterior hypotheca (or hypocone).
Heavily armoured species tend to be more angu-
lar in outline, and plates may be extended in some
species into elaborate projections. The presence of
such projections or horns in Ceratium, for exam-
ple (Fig. 1.9), increases the surface area to volume
ratio of the cell and possibly reduces its sinking
rate by increasing frictional resistance. Projections
may also help in reducing predation. In contrast to
armoured dinoflagellates, some species have very
thin plates or they may be absent altogether (naked
dinoflagellates).

Cell wall material lies beneath the cell mem-
brane, in contrast to many other algae - where
the cell wall, scales or extracellular matrix occurs
on the outside of the plasmalemma. The cell
wall in dinoflagellates is composed of cellulose
(within subsurface membrane-bound vesicles),
forming discrete discs (thecal plates) which give
the dinoflagellate a distinctive armoured appear-
ance. The presence of thecal plates is difficult to
see in light microscope images of living cells, but
is clear when chemically fixed cells are viewed