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A1. Nonmotile algae without chloroplasts: Cyanobacteriophyta
B1. Vegetative cells usually solitary
C1. Thick walled resting cells:
akinetes
C2. Cells broadly oval or spherical; solitary unless dividing:
Synechocystis sallensis
C3. Cells occasionally appear to be solitary, but are usually in small 2-4 cell groups:
Chroococcus
B2. Vegetative cells united into colonies or agglomerations
C1. Cells arranged in a single layer to form flat colonies
D1. Cells form groups of four in rectangular colonies:
Merismopedia
D2. Cells tiny, spherical or oval; cells form single flat layer (not in regular groups of four cells):
Pannus
C2. Colonies comprised of cubical, four-cell subgroups
D1. Cells form cubical or irregular subgroups surrounded by bright red mucilage:
Gloeocapsopsis magma
D2. Cells in groups of 2 or 4; colonies occasionally cubical but more commonly amorphous; cells not surrounded by bright red mucilage:
Chroococcus/Limnococcus
C3. Colonies approximately spherical
D1. Cells form tangled filaments enclosed in spherical or nearly spherical mucilage:
Nostoc
D2. Cells appear mottled brown when viewed under the microscope
E1. Cells spherical or oval, distributed throughout colony (not in a single layer around hollow colony center):
Microcystis
E2. Spherical or hemispherical colony comprised of filaments radiating from common center:
Gloeotrichia echinulata
E3. Cells arrached in single layer on outer perimeter of a hollow colony:
Woronichinia naegeliana
D3. Cells various colors but not mottled brown when viewed under the microscope
E1. Cells form cubical or irregular subgroups surrounded by bright red mucilage:
Gloeocapsopsis magma
E2. Cells spherical or nearly spherical
F1. Cells spherical, in groups of 2 or 4 at end of thin, radiating mucilage strands:
Snowella
F2. Cells bright bluegreen, often in groups of 2 or 4, arranged on the outer perimeter of a hollow colony:
COLEOSPHAERIUM
F3. Colony surrounded by unstriated mucilage; individual cells without additional mucilage layers:
Aphanocapsa
F4. Cells, cell subgroups, and colony surrounded by striated mucilage; mucilage often pale red or pink:
Gloeocapsa
E3. Cells oval or elliptical
F1. Colony surrounded by unstriated mucilage; individual cells without additional mucilage layers:
Aphanothece
F2. Cells, cell subgroups, and colony surrounded by striated mucilage:
Gloeothece
E4. Cells wedge-shaped, forming heart-shaped pairs on thick, radiating mucilage strands:
Gomphosphaeria
C4. Cells arranged into irregular, amorphous colonies
D1. Cells appear brown and mottled under microscope
E1. Cells spherical or oval; distributed througout colony (not forming single layer around hollow colony center):
Microcystis
E2. Cells arranged in single layer on outer perimeter of a hollow colony:
Woronichinia naegeliana
E3. Spherical or hemispherical colony comprised of filaments radiating from common center:
Gloeotrichia
D2. Cells various colors but not mottled brown when viewed under the microscope
E1. Cells form irregular subgroups surrounded by bright red mucilage:
Gloeocapsopsis magma
E2. Cells spherical, in groups of 2 or 4 at end of thin, radiating mucilage strands:
Snowella
E3. Cells spherical or nearly spherical
F1. Cells tiny, spherical or oval; cells form single flat layer (not in regular groups of four cells):
Pannus
F2. Cells in groups of 2 or 4; young colonies occasionally cubical, but colony becomes amorphous as cell groups increase:
Chroococcus/Limnococcus
F3. Colony surrounded by unstriated mucilage; individual cells without additional mucilage layers:
Aphanocapsa
F4. Cells, cell subgroups, and colony surrounded by striated mucilage; mucilage often pale red or pink:
Gloeocapsa
cf.
sanguinea
E4. Cells oval, elliptical, or wedge-shaped
F1. Cells, cell subgroups, and colony surrounded by striated mucilage:
Gloeothece
F2. Colony surrounded by unstriated mucilage; individual cells without additional mucilage layers:
Anathece/Aphanothece
F3. Cells wedge-shaped, forming heart-shaped pairs on thick, radiating mucilage strands:
Gomphosphaeria
B3. Vegetative cells aligned end-to-end forming filaments or pseudofilaments
C1. Trichomes with heterocysts (heterocytes) at one or both ends (basal) or along the trichome (intercalary)
D1. Trichomes unbranched
E1. Cells form tangled, unbranched trichomes enclosed in firm, spherical or nearly spherical mucilage:
Nostoc
E2. Trichomes untapered, with basal heterocyst at one end; akinetes, when present, adjacent to basal heterocyts:
Cylindrospermum
E3. Trichome tapered, with basal heterocysts at one end; akinetes, when present, adjacent to basal heterocyst
G1. Tapered trichomes radiating from common center to form hemispherical or spherical colonies:
Gloeotrichia
G2. Trichomes mostly solitary, not radiating from common center; trichome surrounded by sheath:
Calothrix
E4. Trichomes with intercalary heterocysts (heterocysts scattered along trichome)
F1. Trichomes nearly straight, slightly tapered, clumped into flake-like colonies:
Aphanizomenon
F2. Trichomes with bead-like cells; trichomes may be straight, gently curved, spiralling, or tangled; scattered heterocysts usually abundant:
Anabaena
and
Dolichospermum
D2. Trichomes with false branching
E1. Trichomes tapered
F1. Filaments rarely solitary; usually forming dense mats:
Dichothrix
F2. Trichomes mostly solitary:
Calothrix
E2. Trichomes untapered; false branching mostly between vegetative cells (rarely with heterocyst at the point of branch initiation); trichomes enclosed in visible sheath:
Scytonema
E3. Trichomes not tapered; false branching, with one or more heterocysts at the base of each branch
F1. Trichomes gently curved, usually with single basal heterocyst; trichomes surrounded by thick, yellow or orange mucilage:
Tolypothrix elenkinii
F2. Trichomes straight or gently curved, with one or more heterocysts at the point of branch initiation; trichomes usually surrounded by thin sheath:
Tolypothrix
F3. Trichomes densely tangled, with one or more heterocytes at the point of branch iniation and occasionally along trichome (intercalary):
Hassallia
D3. Trichomes form true branches; main trichome usually multiseriate (more than one cell wide):
Stigonema
C2. Trichomes lack heterocysts at all stages of development
D1. Trichomes are spiralling or coiled and usually highly motile:
Arthrospira
and
Spirulina
D2. Multiple trichomes enclosed in a wide, common mucilage:
Microcoleus
D3. Cell width considerably shorter than length; cells often coin-like
F1. Trichomes short, usually with fewer than 20-30
G1. Trichomes short, with no visible sheath; cells with constricted cross-walls; apical cell conical:
Komvophoron
G2. Trichomes short, sometimes surrounded by thin mucilage layer; cells with constricted cross-walls; apical cell rounded and slightly inflated:
Trichodesmium
cf.
iwanoffianum
F2. Trichomes surrounded by a thin sheath (note that trichomes may creep out of their sheath)
Limnoraphis
and
Lyngbya
F3. Trichomes lack a sheath (check carefully - sheath may be inconspicuous):
Oscillatoria
D4. Cell width greater than or equal to length; cells cubical, cylindrical, or oval
F1. Trichomes narrow, with tapered, hooked apex:
Geitlerinema splendidum
F2. Cells oval, elliptical, or cylindrical, with constricted cross-walls; cell length ≥ width:
Pseudanabaena/Johanseninema
F3. Cells cubical or cylindrical; isodiametric (L=W), longer than wide or slightly shorter than width, but not coin-like
G1. Cells keritomized (net-like), appearing nearly empty:
Tychonema
G2. Trichomes very narrow, with large aerotopes (gas vesicles) near end-walls:
Limnothrix
cf.
redekei
G3. Simple, untapered trichomes with cells that are cylindrical, isodiametric (L=W), or slightly longer or shorter than width, but not coin-like; trichomes with or without a thin sheath:
Oscillatoriales/Phormidium
group
C3. Vegetative cells aligned end-to-end, but not joined along the endwall (pseudofilaments)
D1. Epiphytic or epilithic with elongated basal cell enclosed in an open ended sheath, often with one or more exocytes (reproductive cells):
Chamaesiphon
D2. Cells oval, elliptical, or cylindrical, with constricted cross-walls; cell length ≥ width:
Pseudanabaena/Johanseninema
A2. Nonmotile algae with chloroplasts
B1. Cells usually solitary
C1. Thick walled cells produced at end of growth cycle; usually found with decomposing vegetative cells of the same species:
Cysts, zygotes, and other resting cells
C2. Cells spherical, broadly oval, or lemon shaped in outline
D1. Cells spherical or nearly spherical; cell wall may be thick but cells are not surrounded by visible mucilage layer
E1. Cells roughly spherical in outline, with flexible wall and long, thin pseudopods; cells may exhibit amoeboid movement:
Chrysamoeba
E2. Spherical or nearly spherical, dull or bright green cells with dense (not star-shaped) chloroplast; solitary or in colonies; cells may or may not be surrounded by mucilage; colonies usually surrounded by diffuse mucilage:
Sphaerocystis
E3. Cells spherical with single long, sheathed hair; solitary or in irregular colonies or filaments:
Chaetosphaeridium
E4. Spherical cells lacking a visible mucilage layer and other distinctive features; cells stain dark brown in Lugol's solution indicating presence of starch:
nonmotile single cell green algae
(
Chlorococcum
,
Chlorella
, and others)
D2. Cells spherical or nearly spherical (broadly oval) in outline and surrounded by mucilage layer
E1. Spherical or oval, bright green cells with star-shaped chloroplast radiating from central pyrenoid; solitary or in small colonies surrounded by diffuse but (usually) visible mucilage:
Asterococcus
E2. Spherical or nearly spherical, bright green cells with dense (not star-shaped) chloroplast; solitary or in small colonies surrounded by mucilage layer:
Planktosphaeria
E3. Spherical or nearly spherical, dull or bright green cells with dense (not star-shaped) chloroplast; solitary or in colonies; cells may or may not be surrounded by mucilage; colonies usually surrounded by diffuse mucilage:
Sphaerocystis
D3. Cells elliptical or lemon-shaped; cell may or may not be surrounded by mucilage; also check spherical section for broadly oval cells
E1. Elliptical green cells surrounded by distinctly striated mucilage; solitary or in small groups surrounded by striated mucilage:
Gloeocystis
E2. Cells oval, elliptical, or lemon-shaped; occasionally solitary; usually in small colonies enclosed in expanded parent cell wall:
Oocystis
C3. Cells cylindrical, fusiform, or crescent-shaped in outline
D1. Cells broadly or narrowly fusiform; occasionally solitary but usually in small groups or short pseudofilaments; cells enclosed in lens-shaped or elliptical mucilage:
Elakatothrix
D2. Cells broadly fusiform or cylindrical; dense chloroplast with reddish end caps:
Spirotaenia
and
Tortitaenia
D3. Cells narrowly fusiform; solitary; lacking any visible mucilage layer; may form blooms:
Koliella
,
Monoraphidium
,
Schroederia
, and similar
D4. Cells broadly crescent-shaped with rounded or bluntly acute apices; occasionally solitary but usually loosely distributed in colony surrounded by indistinct mucilage (visible if stained):
Kirchneriella
C4. Cells angular in outline; three or four sided, flattened
D1. Cells very small; rectangular; flattened; usually with concave sides and short knobs or teeth at angles:
Tetraedron
D2. Cells very small, flattened, triangular in outline:
Goniochloris
B2. Nonmotile colonies with two or more cells
C1. Cells flattened, forming ribbon-like chains, short cylinders, or plate-like groups of cells
D1. Colonies joined on long axis to form flat ribbon-like colonies (singel row) or plate-like colonies (double rows)
E1. Cells fusiform or crescent-shaped; apex acutely pointed, without spines, but sometimes with rounded knobs; colonies twisted in somes species:
Tetradesmus
E2. Cells elliptical, cylindrical, or oval; apex rounded, mostly without spines, teeth, or knobs:
Scenedesmus
E3. Cells oval, elliptical, cylindrical, or fusiform; cells with spines, teeth, or granulations (not completely smooth):
Desmodesmus
D2. Cells form distinct groups of 4; colonies mostly rectangular in outline
E1. Apex of each wedge-shaped cell is deeply incised:
Stauridium
E2. Colonies comprised of 4-cell groups; cells green, oval, elliptical, or broadly wedge-shaped:
Crucigenia
and
Willea
E3. Colonies comprised of 4-cell groups, forming irregular, somewhat rectangular clumps; cells golden brown; usually epiphytic:
Plaeoplaca
D3. Cells angular and strongly flattened; inner and outer cells dissimilar:
Pediastrum
, and
Pseudopediastrum
C2. Colonies spherical, oval, or elliptical
D1. Cells spherical, oval, or lemon-shaped
E1. Cells form groups of 4, with multiple cell groups enclosed in colonial mucilage; cells may have non-motile pseudocilia but will not be attached to other cells by radiating strands:
Pseudosphaerocystis
E2. Cells spherical or nearly spherical; may be in 4-cell groups, but will not have pseudocilia
G1. Cells joined along flattened wall segment or short wall extensions to form hollow colony:
Coelastrum
G2. Cells form groups of 2-4 at the ends of radiating mucilage strands:
Mucidosphaerium
G3_ Spherical or oval, bright green cells with star-shaped chloroplast radiating from central pyrenoid; solitary or in small colonies surrounded by diffuse but (usually) visible mucilage:
Asterococcus
G4. Spherical or nearly spherical, bright green cells with dense (not star-shaped) chloroplast; solitary or in small groups surrounded by mucilage:
Planktosphaeria
G5. Spherical or nearly spherical, dull or bright green cells with dense (not star-shaped) chloroplast; solitary or in colonies; cells may or may not be surrounded by mucilage; colonies usually surrounded by diffuse mucilage:
Sphaerocystis
E3. Cells oval, broadly elliptical, or lemon-shaped
G1. Cells broadly oval; solitary or in small colonies enclosed in gelatinized parent cell wall:
Oonephris
G2. Cells oval, elliptical, or lemon-shaped; occasionally solitary; usually in small colonies enclosed in expanded parent cell wall:
Oocystis
D2. Cells cylindrical, fusiform, or narrowly elliptical
E1. Cells narrowly fusiform, aligned lengthwise into tangled linear or spherical colonies:
Ankistrodesmus
E2. Cells cylindrical or fusiform; aligned along long axis, forming bundles enclosed in diffuse mucilage; colonies irregular but mucilage is often spherical or lens-shaped:
Quadrigula
D3. Cells asymmetric, bluntly crescent-shaped, or sausage-shaped; colonies usually containing 4-8 cells surrounded by gelatinized parent cell wall:
Nephrocytium
C3. Colonies various shapes but not flat, spherical, oval, or elliptical
D1. Cells enclosed in rigid vase-like or cylindrical lorica:
Epipyxis
D2. Cells spherical or broadly oval
E1. Cells with one or more long, nonmotile pseudocilia
F1. Cells nearly spherical, with one flattened side with tufts of long pseudocilia; cells loosely arranged in an irregular (amorphous) colony; persistent fragments of old cell walls retained within colony:
Schizochlamys
F2. Cells form groups of 4, with multiple cell groups enclosed in colonial mucilage; cells may have non-motile pseudocilia but will not be attached to other cells by radiating strands:
Pseudosphaerocystis
E2. Pseudocilia absent; colonies lacking distinct mucilage layers; mucilage may be present, but may be difficult to see unless stained
F1. Cells oval, forming tetrahedral groups of 4 in irregular (amorphous) colonies; colony surrounded by nearly invisible mucilage:
Stichogloea
F2. Cells oval or spherical, golden brown, in loosely arranged, irregular (amorphous) colonies; colonies enclosed in mucilage that is rarely visible unless stained:
Gloeobotrys
F3. Spherical or nearly spherical, dull or bright green cells with dense (not star-shaped) chloroplast; solitary or in colonies; cells may or may not be surrounded by mucilage; colonies usually surrounded by diffuse mucilage:
Sphaerocystis
F4. Irregular colonies with numerous tiny, spherical, yellowish green cells; resembles
Chlorella
, but does not contain starch:
Nannochloropsis
E3. Pseudocilia absent; colonies surrounded by distinct mucilage layer or connected by mucilage strands; mucilage visible without staining
F1. Colonies consisting of clumps of cells embedded in dense mucilage; clumps connected by mucilage strands or residual parent cell walls:
Botryococcus
F2. Spherical or nearly spherical, dull or bright green cells with dense (not star-shaped) chloroplast; solitary or in colonies; cells may or may not be surrounded by mucilage; colonies usually surrounded by diffuse mucilage:
Sphaerocystis
F3. Elliptical green cells surrounded by distinctly striated mucilage; solitary or in small groups surrounded by striated mucilage:
Gloeocystis
D3. Cells club-shaped, fusiform, cylindrical, crescent-shaped, or spindle-shaped (not spherical or oval)
E1. Colony surrounded by mucilage (may require staining)
F1. Cells broadly or narrowly fusiform; occasionally solitary but usually in small groups or short pseudofilaments; cells enclosed in lens-shaped or elliptical mucilage:
Elakatothrix
F2. Cells cylindrical or fusiform; aligned along long axis, forming bundles enclosed in diffuse mucilage (not always visible); colonies irregular but mucilage often spherical or lens-shaped:
Quadrigula
E2. Cells and colonies lacking distinct mucilage; mucilage may be difficult to see, so some of these taxa are cross-listed
F1. Cells narrowly fusiform, aligned lengthwise, forming tangled spherical colonies:
Ankistrodesmus
F2. Cells long, cylindrical, joined at the ends to form nets comprised of 5-sided or 6-sided subunits:
Hydrodictyon
F3. Cells broadly crescent-shaped with rounded or bluntly acute apices; occasionally colitary but usually loosely distributed in colony; surrounded by indistinct mucilage (visible if stained):
Kirchneriella
E3. Cells broadly spindle-shaped; colonies formed from 4-cell groups joined by mucilage strands:
Diplochloris lunata
cf.
B3. Filamentous algae including pseudofilaments (see epiphyte links for epiphytic filaments)
C1. Cells form pseudofilaments where they are aligned end-to-end but not joined across the endwall
D1. Pseudofilaments short (≤10 cells); cells long, cylindrical, broadly rounded; pseudofilaments surrounded by very wide, inconspicuous mucilage layer:
Gloeotila
D2. Pseudofilaments usually longer than 10 cells; cells short, cylindrical, broadly rounded, often forming pairs; pseudofilaments surrounded by distinct mucilage layer:
Geminella
C2. Unbranched filamentous algae
D1. Cells cylindrical, mostly longer than wide, with ribbon-like, band-like, or star-shaped chloroplasts
E1. Cells with single flat chloroplast extending down the center of the cell (axial); chloroplast often twisted; very common, diverse genus:
Mougeotia
E2. Cells with one or more narrow, ribbon-like, spiralling chloroplasts; very common, diverse genus:
Spirogyra
E3. Cells with one narrow, ribbon-like chloroplast that forms a partial or complete ring circling the center of the cell; zoospores often form quickly in samples waiting to be examined:
Ulothrix
E4. Cells with two star-shaped chloroplasts located near the center of the cell (axial); very common, diverse genus:
Zygnema
D2. Chloroplasts not ribbon-like, band-like, or star-shaped
E1. Cell endwalls with H-shaped extensions, transverse rings, or thickening; chloroplasts not ribbon-like, band-like, or star-shaped
F1. Cells with H-shaped endwall extensions visible at end of filament; chloroplasts yellowish green (will not stain for starch in Lugol's):
Tribonema
F2. Cells cylindrical or oval, often paired; cell wall becoming increasingly thick and striated, especially at the crosswalls:
Binuclearia
F3. Cells cylindrical (longer than wide); some cells have ring-like cell division scars that resemble pipe threads; difficult to identify species unless mature oogonia present:
Oedogonium
E2. Cells without endwall rings, thickenings, or H-shaped extensions
G1. Filaments lacking crosswalls except below reproductive structures; mostly unbranched except for short reproductive filaments; chloroplasts numerous, tiny, disk-like, yellowish green; filaments often contain large, single or paired oogonia:
Vaucheria
G2. Cells oval, elliptical, or cylindrical; often paired; chloroplast plate-like or girdle-shaped (not stellate), usually with central pyrenoid; filaments surrounded by wide, diffuse mucilage:
Geminella
C3. Branched filamentous algae
D1. Cells in main branch of filament short, wide, barrel-shaped, with fringed, ring-like (parietal) chloroplast; cells in side branched much smaller, cylindrical or strongly tapered, with net-like (reticulate) chloroplasts:
Draparnaldia
D2. Filaments with approximately similar cells forming primary and secondary branches; filaments may have dissimilar terminal cells and slightly larger cells in the main branch
E1. Cells pinkish purple; vegetative cells in secondary branches often form long, hair-like setae; clumped spore-forming branches often present:
Audouinella
E2. Cells long, cylindrical, joined at the ends to form net comprised of 5-sided or 6-sided subunits:
Hydrodictyon
E3. Filaments with some cells extended into long, hair-like setae (variable feature)
F1. Cells spherical with single long, sheathed hair; solitary or in irregular colonies or filaments:
Chaetosphaeridium
F2. Cells irregular, broadly rounded, with long setae; filamentous form not always apparent; filaments technically epiphytic but often detached from substrate:
Coleochaete pulvinata
F3. Cells mostly cylindrical, forming regularly branching filaments
G1. Cells with very long hair-like setae with an expanded (bulbous) base; may have specialized holdfast at base of filament:
Bulbochaete
G2. Filaments form diffuse, irregularly shaped clumps; often has rhizoidal basal cells; terminal cells acutely pointed or extended into long, hair-like setae that are not expanded at the base:
Stigeoclonium
E4. Filaments lacking (or mostly lacking) cells that form long, hair-like setae
F1. Filaments with distinct crosswalls, unbranched or very sparsely branched; cells long, cylindrical; chloroplasts dense and reticulated (net-like); starch present (test using Lugol's):
Cladophora
F2. Filaments mostly attached to submerged vegetation, forming diffuse, irregularly shaped clumps; often has rhizoidal basal cells; terminal cells acutely pointed or extended into long, hair-like setae that are not expanded at the base:
Stigeoclonium
F3. Filaments branch into tree-like structures
G1. Cells smooth (not spiny); enclosed in vase-like lorica, forming tree-like colonies; often motile, so cross-listed in motile algae section:
Dinobryon
G2. Cells sausage-shaped, forming structure resembling palm tree; usually epiphytic, but occasionally dislodged into plankton:
Ophiocytium abuscula
B4. Cells living inside zooplankton or other aquatic animals (endozoic) or attached to aquatic plants (epiphytic)
C1. Chlorella-like cells living inside zooplankton and other aquatic animals (endozoic):
Zoochlorella group
C2. Some or all cells with one or more long hair-like extensions (setae)
D1. Cells spherical with single long, sheathed hair; solitary or in irregular colonies or filaments:
Chaetosphaeridium
D2. Cells rectangular or trapezoidal; joined into long filaments radiating from central point to form flat, spherical or oval colony:
Coleochaete scutata
D3. Cells within filament irregularly shaped, rounded; filament irregularly branched
E1. Cells irregularly cylindrical with one or more setae; cells usually joined to form irregular creeping epiphytic filaments:
Aphanochaete
E2. Cells irregular, broadly rounded, with long setae; filamentous form not always apparent; filaments often detached from substrate:
Coleochaete pulvinata
C3. Cells lack hair-like extensions(setae)
D1. Colonies comprised of 4-cell groups, forming irregular, somewhat rectangular clumps; cells golden brown; usually epiphytic:
Plaeoplaca
D2. Cells enclosed in rigid vase-like or cylindrical lorica:
Epipyxis
D3. Cells fusiform, oval, elliptical, or crescent-shaped; cells sessile or attached by long or short stipe:
Characium
,
Characiopsis
,
Pseudocharacium
, and similar
D4. Cells form tree-like structure; usually epiphytic, but occasionally dislodged into plankton:
Ophiocytium abuscula
A3. Motile algae with chloroplasts and flagella
B1. Solitary motile algae
C1. Cells with one visible flagellum
D1. Cells covered with scales (scaled chrysophytes); most species also have long spines:
Mallomonas
D2. Cell enclosed in a rigid spherical, oval, or elliptical lorica:
Trachemolomas
D3. Cells not covered with scaled or enclosed in a lorica; wall may be rigid or flexible
E1. Cells rigid and flattened, usually, but not always, with long posterior tail (caudus):
Phacus
E2. Cell wall flexible; cells with long, thin pseudopods; cells may exhibit amoeboid movement:
Chrysamoeba
E3. Cells elliptical, cylindrical, or teardrop-shaped, with worm-like euglenoid movement:
Euglena
and similar
E4. Cell wall flexible; cells spherical, golden brown, often with warty surface:
Ochromonas
C2. Cells with two equal length flagella
D1. Cells with bright red or partly red chloroplast surrounded by spherical mucilage:
Haematococcus lacustris
D2. Cells slipper-shaped, oval, teardrop-shaped; asymmetric; two nearly equal flagella inserted below apex (subapical):
Cryptomonads
D3. Spherical, oval, or elliptical cells with two equal flagella inserted at the apex (large, difficult group):
Chlamydomonas
and similar
C3. Cells with 2 dissimilar flagella
D1. Cells slipper-shaped, oval, teardrop-shaped; asymmetric; two nearly equal flagella inserted below apex (subapical):
Cryptomonads
D2. Cells with one transverse and one trailing flagellum (Dinoflagellates)
E1. Cells with flexible walls divided by cingulum into epicone and hypocone (difficult group):
athecate dinoflatellates
(
Amphidinium
, Gymnodinium,
Hemidinium
, and others)
E2. Cells with rigid, plate-like walls divided by cingulum into epitheca and hypotheca (difficult group):
thecate dinoflatellates
(
Ceratium
, Peridinium, and others)
C4. Cells heart-shaped and flattened, with 4 equal flagella:
Tetraselmis
B2. Motile colonies with two or more cells
C1. Cells with two equal flagella
D1. Spherical colonies with 16-32 compressed, wedge-shaped cells:
Pandorina
D2. Spherical colonies with 16-32 well-separated spherical cells:
Eudorina
C2. Cells with two dissimilar flagella
E1. Cells smooth (not spiny); enclosed in vase-like lorica, forming tree-like colonies:
Dinobryon
E2. Cells covered with tiny, ornate scales (no long spines):
Synura
A4. Cells divided into paired semi-cells that are approximate mirror images of each other:
Desmids
B1. Cells mostly solitary unless dividing
C1. Cells are rectangular or lens-shaped (length greater than width):
Genicularina
,
Gonatozygon
,
Penium
,
Roya
,
Spirotaenia
C2. Semi-cell with depressed or notched apex:
Euastrum
,
Cosmarium
(in part)
C3. Cells are crescent-shaped, with little resemblance to other desmids:
Closterium
C4. Cells angular in cross-section (mostly tri-radiate); some species with arm-like lateral extensions; very large, diverse genera:
Staurastrum
and
Staurodesmus
C5. Cells are spherical, oval, or elliptical in cross-section (view from apex) and lack long spines:
Cosmarium
B2. Cells linked end-to-end to form filaments
A5. Cells enclosed in glassy, ornamental frustules; many species exhibit gliding movement:
Diatoms
B1. Cells shaped like a disk or cylinder; circular in cross-section; raphe absent (mostly nonmotile):
Centric Diatoms
B2. Cells mostly elongated and bilaterally symmetric; raphe absent (mostly nonmotile):
Araphid Diatoms
B3. Valve view asymmetric and often curved; side (girdle) view bilaterally symmetric; raphe weakly developed (minimal gliding movement):
Eunotioid Diatoms
B4. Cells bilaterally symmetric in top (valve) and side (girdle) views; well developed raphe on both sides; very large, diverse group:
Symmetric Biraphid Diatoms
B5. Cells bilaterally symmetric in top (valve) view; well developed raphe on one side (other side lacks raphe):
Monoraphid Diatoms
B6. Cells bilaterally asymmetric in top (valve) or side (girdle) views, or in both views; well developed raphe on both sides; very large, diverse group:
Asymmetric Biraphid Diatoms
B7. Cells asymmetric in valve view, with a curved raphe:
Epithemioid Diatoms
B8. Cells bilaterally symmetric or asymmetric, but with distinctive raphe that is either twisted or off-set to one side of valve:
Nitzschioid Diatoms
B9. Cells bilaterally symmetrical in valve view, but often curved; raphe often located along margin:
Surirelloid Diatoms