Quick Guide to Common Diatom Genera in Freshwaters - NIWA
Quick Guide to Common Diatom Genera in Freshwaters - NIWA
Quick Guide to Common Diatom Genera in Freshwaters - NIWA
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<strong>Quick</strong> <strong>Guide</strong> <strong>to</strong> <strong>Common</strong> Dia<strong>to</strong>m <strong>Genera</strong> <strong>in</strong> <strong>Freshwaters</strong><br />
By Cathy Kilroy<br />
Centric<br />
dia<strong>to</strong>ms<br />
YES<br />
Valves circular,<br />
raphe or axial<br />
area never present<br />
NO<br />
Pennate<br />
dia<strong>to</strong>ms<br />
Cells typically<br />
<strong>in</strong> filaments<br />
NO<br />
Araphid<br />
dia<strong>to</strong>ms<br />
YES<br />
Valves elongated,<br />
elliptical or swollen<br />
bilaterally. No<br />
raphe on either<br />
valve<br />
NO<br />
YES<br />
Very noticeable<br />
patterns <strong>in</strong> girdle<br />
and valve views<br />
Different pattern <strong>in</strong><br />
the middle and<br />
edge of valve<br />
Septa present<br />
(<strong>in</strong>ternal projections<br />
from the girdle band)<br />
NO<br />
Raphe present<br />
on at least one<br />
valve<br />
CONTINUED <strong>in</strong> Part B<br />
YES<br />
Costae present<br />
(<strong>in</strong>ternal<br />
projections on<br />
the valve face<br />
NO<br />
YES<br />
YES<br />
YES<br />
NO<br />
Valves<br />
asymmetrical<br />
transversely<br />
(lengthwise)<br />
NO<br />
NO<br />
Similar or cont<strong>in</strong>uous pattern<br />
from edge of valve <strong>to</strong> center<br />
Valves<br />
asymmetrical<br />
transversely<br />
YES<br />
YES<br />
NO<br />
Cells often form<br />
cha<strong>in</strong>s (most<br />
noticeable <strong>in</strong> live<br />
samples)<br />
NO<br />
YES<br />
Valves l<strong>in</strong>ear<br />
or oval<br />
YES<br />
NO<br />
YES<br />
Striae cont<strong>in</strong>uous<br />
across valve face (or<br />
axial area v. narrow)<br />
NO<br />
YES<br />
Aulacoseira<br />
Melosira<br />
Cyclotella<br />
Cyclostephanos<br />
Tabellaria<br />
Meridion<br />
Dia<strong>to</strong>ma<br />
Asterionella<br />
Fragilariforma<br />
(part)<br />
Fragilaria<br />
Staurosirella<br />
Centrally<br />
expanded<br />
Striae very thick<br />
NO<br />
Fragilariforma<br />
(part)<br />
Synedra<br />
Part A
Part B<br />
CONTINUED YES<br />
Monoraphid<br />
dia<strong>to</strong>ms<br />
Raphe present<br />
on one valve<br />
only<br />
NO<br />
YES<br />
Biraphid<br />
dia<strong>to</strong>ms<br />
Round-oval with<br />
different patterns on<br />
the two valves<br />
NO<br />
Raphes visible as<br />
long or short l<strong>in</strong>es<br />
(slits) on the<br />
valve surface of<br />
both valves<br />
NO<br />
YES<br />
CONTINUED <strong>in</strong> part C<br />
Usually a thick rim<br />
on the raphe valve<br />
An empty<br />
“depression” <strong>to</strong><br />
one side on the<br />
rapheless valve<br />
NO<br />
Raphes short on both<br />
valves, <strong>to</strong> one side<br />
(sometimes barely<br />
visible <strong>in</strong> valve view)<br />
Raphes<br />
visible <strong>in</strong><br />
girdle view<br />
YES<br />
NO<br />
YES<br />
Narrow axial<br />
area on the<br />
raphless valve<br />
often <strong>to</strong> one side<br />
NO<br />
YES<br />
Valve heteropolar (the<br />
two ends markedly<br />
different widths)<br />
YES<br />
Raphes markedly different<br />
lengths on the two valves.<br />
Curved <strong>in</strong> girdle view.<br />
Septa at valve poles.<br />
NO<br />
CONTINUED <strong>in</strong> part D<br />
NO<br />
Small narrow cells,<br />
striae denser<br />
<strong>to</strong>wards the poles<br />
than at the centre<br />
NO<br />
YES<br />
YES<br />
YES<br />
Striae evenly<br />
spaced<br />
NO<br />
Cocconeis<br />
<strong>Genera</strong> related <strong>to</strong><br />
Achnanthidium<br />
Planothidium<br />
Achnanthes<br />
Achnanthidium<br />
Rossithidium<br />
Act<strong>in</strong>ella<br />
Eunotia<br />
Valve shape and size<br />
very variable<br />
Raphes<br />
barely<br />
visible<br />
Rhoicosphenia
Part C<br />
Biraphid dia<strong>to</strong>ms<br />
CONTINUED<br />
Raphe slits not<br />
obvious<br />
(though a<br />
thick raphe<br />
canal may be<br />
visible)<br />
NO<br />
Raphe marg<strong>in</strong>al, or<br />
nearly marg<strong>in</strong>al, <strong>in</strong> a<br />
canal enclosed by<br />
struts (fibulae) – dark<br />
dots <strong>in</strong> LM<br />
YES<br />
Costae present<br />
(<strong>in</strong>ternal projections<br />
from the valve face,<br />
appear<strong>in</strong>g as dark<br />
l<strong>in</strong>es)<br />
YES<br />
Gap <strong>in</strong> costae<br />
at the valve<br />
centre. Raphe<br />
marg<strong>in</strong>al<br />
NO<br />
Raphes on<br />
opposite sites<br />
NO<br />
Valves narrow,<br />
l<strong>in</strong>ear, may be<br />
sigmoid<br />
YES<br />
NO<br />
Raphe canal visble<br />
on valve face,<br />
sometimes only at<br />
the centre<br />
YES<br />
YES<br />
NO<br />
Focus<strong>in</strong>g through<br />
whole frustules will<br />
show the raphe along<br />
both sides<br />
Valves wide, more robust,<br />
variable sizes<br />
Stenopterobia<br />
Surirella<br />
Epithemia<br />
Rhopalodia<br />
NB. Frustules appear<br />
bilaterally symmetrical<br />
because both valves are<br />
visible at the same time<br />
Nitzschia<br />
Hantzschia
Part D<br />
Biraphid dia<strong>to</strong>ms<br />
CONTINUED<br />
Raphes long<br />
on both<br />
valves, ends<br />
usually close<br />
<strong>to</strong>gether at<br />
the centre of<br />
the valve<br />
Valves<br />
clearly<br />
sigmoid<br />
NO<br />
S-shaped<br />
Valves<br />
asymmetrical<br />
bilaterally<br />
NO<br />
YES<br />
YES<br />
For whole frustules, <strong>in</strong><br />
valve view, both<br />
valves are visible ly<strong>in</strong>g<br />
side by side<br />
YES<br />
YES<br />
NO<br />
Cells bilaterally<br />
symmetrical (or<br />
nearly so), but<br />
asymmetrical<br />
lengthwise<br />
CONTINUED <strong>in</strong> part E<br />
Striae very<br />
short ventrally<br />
Valves usually<br />
clearly halfmoon<br />
shaped,<br />
often<br />
NO<br />
A discont<strong>in</strong>uity<br />
visible <strong>in</strong> the<br />
striae, parallel <strong>to</strong><br />
the valve edge<br />
NO<br />
YES<br />
YES<br />
NO<br />
Valves more or less<br />
elliptical, with a clear<br />
swollen area on one<br />
side<br />
YES<br />
Outer ends of<br />
raphe po<strong>in</strong>t<br />
downwards<br />
YES<br />
NO<br />
NO<br />
YES<br />
Valves very large,<br />
usually >100 mm<br />
long, triundulate<br />
NO<br />
YES<br />
Central raphe<br />
ends bend<br />
downwards<br />
Valves almost<br />
bilaterally<br />
symmetrical<br />
Valve shape and<br />
size very variable<br />
Gyrosigma<br />
Amphora<br />
Eunophora<br />
NOTE: closely<br />
related <strong>to</strong> Eunotia<br />
Encyonema<br />
Cymbella<br />
Reimeria<br />
Encyonopsis<br />
Gomphoneis<br />
Didymosphenia<br />
Gomphonema
Part E<br />
Biraphid dia<strong>to</strong>ms YES<br />
CONTINUED<br />
Valves<br />
bilaterally<br />
symmetrical<br />
Septa present<br />
(<strong>in</strong>ternal<br />
projections from<br />
the girdle<br />
NO<br />
Raphe<br />
with<strong>in</strong> a<br />
thickened<br />
band<br />
Septum across whole<br />
valve, 2 or 3 large<br />
holes centrally placed<br />
NO<br />
More biraphid<br />
naviculoid<br />
dia<strong>to</strong>ms<br />
YES<br />
NO<br />
Outer ends of raphe<br />
usually <strong>in</strong> an<br />
arrowhead shape.<br />
Striae very f<strong>in</strong>e<br />
YES<br />
Ornamented<br />
valve edges.<br />
Central raphe<br />
end<strong>in</strong>gs often<br />
curve <strong>in</strong> opposite<br />
directions<br />
Striae very<br />
thick (many<br />
NO t<strong>in</strong>y puncta,<br />
not visible)<br />
NO<br />
NO<br />
NO<br />
YES<br />
Septa on each side of the valve,<br />
form<strong>in</strong>g chambers (locules)<br />
YES<br />
A band of clear<br />
silica across the<br />
centre of the<br />
valve<br />
NO<br />
YES<br />
YES<br />
Areolae usually<br />
prom<strong>in</strong>ent <strong>in</strong> LM<br />
Striae form long,<br />
irregular l<strong>in</strong>es on the<br />
valve face<br />
NO<br />
YES<br />
YES<br />
Thickened<br />
clear areas at<br />
each pole<br />
Dia<strong>to</strong>mella<br />
Mas<strong>to</strong>gloia<br />
Frustulia<br />
Diploneis<br />
Neidium<br />
P<strong>in</strong>nularia<br />
Stauroneis<br />
Brachysira<br />
Sellaphora
Part F<br />
Biraphid dia<strong>to</strong>ms<br />
CONTINUED<br />
Raphes long,<br />
usually almost<br />
meet<strong>in</strong>g at the<br />
centre of the<br />
valve<br />
YES<br />
Valves often<br />
relatively large<br />
(>30 mm) typical<br />
naviculoid shape<br />
Valves very small,<br />
narrow, striae barely<br />
visible. Raphe<br />
straight, outer ends<br />
hooked on valve face<br />
Striae very regular,<br />
areolae often visible.<br />
In live material, a pair<br />
of lateral chloroplasts<br />
NO<br />
Frustules often<br />
form cha<strong>in</strong>s.<br />
Rounded ends.<br />
Short parallel<br />
striae<br />
NO<br />
YES Navicula<br />
Choroplast an xshaped<br />
plate Striae f<strong>in</strong>e, but usually<br />
visible, with wider<br />
spac<strong>in</strong>g at the valve<br />
NO centre. Raphe curves <strong>to</strong><br />
same side on valve face<br />
YES<br />
YES<br />
YES<br />
YES<br />
Valves often almost<br />
round, radial striae,<br />
outer raphe ends<br />
YES<br />
curved <strong>in</strong> opposite<br />
directions<br />
Central area oblong,<br />
NO one stigma (hole) <strong>to</strong><br />
one side, large areolae<br />
NO<br />
Prom<strong>in</strong>ent<br />
areolae at<br />
outer ends<br />
NO<br />
YES<br />
YES<br />
YES<br />
Very small<br />
forms with<br />
prom<strong>in</strong>ent<br />
striae<br />
Placoneis<br />
Adlafia<br />
NO Kobayasiella<br />
NO<br />
Many other small<br />
naviculoid genera have<br />
been described. Often SEM<br />
is required <strong>to</strong> verify their<br />
dist<strong>in</strong>guish<strong>in</strong>g features.<br />
NO<br />
Diadesmis<br />
Cav<strong>in</strong>ula<br />
Luticola<br />
Geissleria<br />
Chamaep<strong>in</strong>nularia<br />
Genus<br />
features<br />
hard <strong>to</strong> see<br />
<strong>in</strong> LM. ID<br />
<strong>to</strong> species<br />
will place<br />
<strong>in</strong> genus<br />
SEM required <strong>to</strong> see<br />
characteristic areolae <strong>in</strong><br />
some species
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