Introduction to Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 38

Centric and pennate diatoms

1 2

3 4 5

Diatoms can be separated into two major group-
ings - centric and pennate diatoms, based primarily
on cell shape and frustule morphology (Fig. 1.12).
Centric diatoms typically have a discoid or cylindri-
calshape,witharadialsymmetrywhenseeninfaceor
'valve' view. Pennate diatoms have an elongate bilat-
eral symmetry, with longitudinal and transverse axes,
and often have the appearance of a 'feathery' (hence
'pennate') ornamentation when seen in surface view
(Fig. 1.13). Pennate diatoms have a range of shapes
(Barber and Haworth, 1981), with broad division
(Fig. 1.15) into isopolar (ends of valve similar in size
andshape),heteropolar(endsofvalvedifferinginsize
and shape), asymmetrical (outline dissimilar either
sideofthelongitudinalaxis)andsymmetrical(outline
of valve similar either side of the longitudinal axis).
The fundamental differences between centric and
pennate diatoms in relation to structure and symme-
try reflect a range of other distinguishing features
(Table 1.13) - including motility, number and size
of plastids, sexual reproduction and ecology. Centric
diatoms are mainly planktonic algae, and the occur-
rence of oogamy - with the production of a large
number of motile sperms, is regarded as a strategy for
increasing the efficiency of fertilisation in open-water
environments. Pennate diatoms are almost always
isogamous, with equal-sized non-flagellate gametes
(two per parent cell). In these algae, efficiency of
fertilisation is promoted by the pairing of gamete
parental cells prior to gamete formation, a possible
adaptation to more restricted benthic environments.
Fossil and molecular evidence suggests that centric
diatoms arose prior to pennate ones, implying that
in these algae isogamy has been phylogenetically
derived from oogamy (Edlund and Stoermer, 1997).
This is in contrast to the evolutionary sequence nor-
mally accepted for other algal groups - particularly
green and brown algae.

6 7 8 9 10 11

12 13 14

15 16 17

18 19 20

Figure 1.14 Diatom valve markings: (1) internal septa;
(2) transapical costae; (3) raphe in thickened ribs; (4) nor-
mal raphe; (5) shortened raphe in ribs; (6) parallel striae;
(7) radiate striae; (8) central area round; (9) central area
transverse; (10) central area small; (11) central area acute
angled; (12) punctae in radial rows interspersed with sub-
radial rows; (13) coarse areolae; (14) areolae grouped in
segments; (15) sections of valve alternately raised and
depressed giving shaded appearance; (16) central region
raisedordepressed;(17)pennatewithraisedordepressed
areas; (18) ocelli on surface; (19) rudimentary raphe, as
in Eunotia ; (20) raphe curved to point at centre, as in
Epithemia .

1.10.2 Morphological diversity

Unicellular and colonial diatoms

The morphological diversity of diatoms can be
considered in relation to two main aspects - the
distinction between centric/pennate diatoms and the
range of unicellular to colonial forms.

The genetic diversity of diatoms is not matched by
the complexity of their morphological associations -
which are limited to small chains and groups of cells.