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A1. Nonmotile algae without chloroplasts:
Cyanobacteriophyta
B1. Vegetative cells usually solitary
C1. Cells attached to aquatic plants (epiphytic)
D1. Solitary elongated basal cell enclosed in open sheath, often with one or more exocytes (reproductive cells):
Chamaesiphon
D2. Solitary short basal cell enclosed in an open ended sheath, usually with many exocytes (reproductive cells):
Stichosiphon
D3. Solitary elongated basal cell (not in sheath), usually with one or more exocytes (reproductive cells):
Cyanophanon
D4. Solitary short, club-shaped basal cell with only one exocyte (reproductive cell):
Geitleribactron
C2. Cells solitary unless dividing; not epiphytic:
Cyanothece
C3. Cells solitary or in small colonies; colonies usually formed from 2-4 cell subgroups:
Chroococcus
C4. Thick walled resting cells:
akinetes
B2. Vegetative cells united into colonies or agglomerations
C1. Cells arranged in a single layer to form flat colonies
D1. Cells form groups of 4x in rectangular colonies:
Merismopedia
D2. Cells tiny (<4 µm), spherical or oval; blue-gray, in single flat layer; colonies spherical, irregular, or net-like (clathrate):
Pannus
C2. Colonies comprised of cubical, 4-cell subgroups
D1. Cells embedded in bright red mucilage:
Gloeocapsopsis
D2. Small spherical cells in dense, cubical colonies:
Eucapsis
D3. Cells forming cubical, spherical, or irregular subgroups within larger colonies:
Microcystis
(in part)
D4. Cells in groups of 2 or 4; colonies occasionally cubical but usually amorphous:
Chroococcus
C3. Colonies spherical, oval, or amorphous (irregular) in shape
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 attached at ends of thick, radiating mucilage strands (rarely visible), forming single layer on outer perimeter of hollow colony:
Woronichinia naegeliana
E2. Cells not attached to radiating mucilage strands; strands may be present but invisible under normal magnification
F1. Cells spherical or oval and distributed throughout colony (not forming single layer around hollow center):
Microcystis
F2. Cells form a single layer on outer perimeter of hollow colony. Cells are technically attached at ends of thick, radiating mucilage strands, but these strands are rarely visible:
Woronichinia naegeliana
F2. Spherical or hemispherical colony comprised of filaments radiating from common center:
Gloeotrichia
D3. Cells various colors but not mottled brown when viewed under the microscope
E1. Cells attached to radiating mucilage strands
F1. Cells spherical, in groups of 2 or 4 at end of thin, radiating mucilage strands:
Snowella
F2. Cells wedge-shaped, forming heart-shaped pairs on thick, radiating mucilage strands:
Gomphosphaeria
E2. Cells not attached to radiating mucilage strands
F1. Cells spherical or hemispherical
G1. Cell width ≥3 µm; bright blue-green; arranged in single layer around hollow colony center:
Coelosphaerium
and
Coelomoron
G2. Cells in groups of 2 or 4; small groups of cells may appear cubical, but colonies are usually amouphous:
Chroococcus/Limnococcus
G3. Colony surrounded by diffuse, unstriated mucilage (difficult to see!); individual cells without additional mucilage layers:
Aphanocapsa
F2. Cells oval, elliptical, cylindrical, or fusiform
G1. Cells, cell subgroups, and colony surrounded by striated mucilage:
Gloeothece
G2. Colonies with unstriate mucilage that may be difficult to see without staining
H1. Colony surrounded by unstriated mucilage; individual cells without additional mucilage layers:
Anathece/Aphanothece
H2. Cells narrowly elliptical, cylindrical, or fusiform; colonies spherical to elliptical;
Rhabdoderma
and
Rhabdogloea
G3. Clumps of thick walled resting cells: akinetes, especially from
Dolichospermum
B3. Vegetative cells aligned end-to-end forming filaments or pseudofilaments
C1. Trichomes with 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 with heterocytes at one or both ends
F1. At least some of the trichomes have heterocytes on both ends
G1. Heterocytes spherical or oval; cells slightly longer than wide; trichomes twisted or coiled:
Anabaenopsis
G2. Heterocytes spherical or oval, located on one or both ends or scattered along the trichome; cells shorter than width; trichomes straight or gently curved; usually in brackish water:
Nodularia
G3. Heterocytes conical, on one or both ends of trichome; trichomes straight or coiled; usually in warm, eutrophic lakes:
Raphidiopsis
G4. Trichomes endophytic in
Azolla
(Mosquitofern); heterocytes at one or both ends or scattered along trichome:
Trichormus
F2. Trichomes untapered, usually with basal heterocyte at one end; akinetes, when present, adjacent to basal heterocytes:
Cylindrospermum
F3. Trichome tapered, with basal heterocytes at one end; akinetes, when present, adjacent to heterocyte
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
E3. Trichomes with heterocytes scattered along the trichome (intercalary)
F1. Trichomes nearly straight, slightly tapered, usually with empty cells at the narrow end. Trichomes stick together to form large, flake-like colonies:
Aphanizomenon
F2. Trichomes with bead-like or cylindrical cells; trichomes straight, gently curved, spiraling, or tangled:
Anabaena
and
Dolichospermum
F3. Heterocytes spherical or oval, located on one or both ends or scattered along the trichome; cells shorter than width; trichomes straight or gently curved; usually in brackish water:
Nodularia
F4. Trichomes endophytic in
Azolla
(Mosquitofern); heterocysts at one or both ends or scattered along trichome:
Trichormus
D2. Trichomes with false-branching
E1. Trichomes tapered, at least near apex (opposite basal heterocyte)
F1. Filaments rarely solitary; trichome enclosed in visible sheath; usually forming dense mats:
Dichothrix
F2. Trichomes mostly solitary, not radiating from common center; trichome surrounded by sheath:
Calothrix
F3. Trichomes short, densely false-branched, untapered or abruptly tapered at apex, enclosed in firm, dark sheath; heterocytes basal or intercalary:
Hassallia
E2. Trichomes untapered, surrounded by thick sheath; false-branching mostly between vegetative cells, but with an occasional basal heterocyte:
Scytonema
E3. Trichomes untapered; heterocyte at base of false-branch
F1. Trichomes untapered; false-branching, usually with more than one heterocyte at the point of branch initiation; individual trichomes surrounded by thin sheath:
Tolypothrix
F2. Trichomes straight or gently curved; cells shorter than long, with constricted cross-walls; one or more trichomes completely enclosed in thick, yellow or orange sheath:
Tolypothrix elenkinii
F3. Trichomes short, densely false-branched, untapered or abruptly tapered at apex, enclosed in firm, dark sheath; heterocytes basal or intercalary:
Hassallia
F4. Trichomes untapered, densely false-branching, with mostly basal heterocytes; trichomes partially surrounded by thick, striated sheath (may extend beyond sheath):
Coleodesmium
D3. Trichomes form true branches
E1. Cells spherical, oval, or barrel-shaped; primary trichome usually multiseriate (more than one cell wide), surrounded by firm, very dark mucilage:
Stigonema
(in part)
E2. Cells spherical, oval, or compressed rectangular (L<W); trichomes mostly monoseriate (one cell wide), surrounded by firm, yellowish mucilage:
Stigonema
(in part)
E3. Cells and heterocytes cylindrical; primary and secondary branches uniseriate and similar in appearance:
Hapalosiphon
E4. Cells barrel-shaped in the primary branch and cylindrical in secondary branches; filaments mostly uniseriate (one cell wide) but may be multiseriate in the creeping primary branch:
Fischerella
C2. Trichomes lack heterocytes at all stages of development
D1. Trichomes are attached to aquatic plants (epiphytic), aquatic animals (epizoic), or found in mucilage surrounding algae, aquatic plants, or zooplankton (endogloeic)
E1. Trichomes mostly short (<10 cells), embedded in rotifer or algae mucilage (endogloeic):
Pseudanabaena
E2. Cells form very short filaments or pseudofilaments enclosed in an open ended sheath, often with one or more exocytes (reproductive cells)
F1. Epiphytic (or epilithic), with elongated basal cell and one or more exocytes (reproductive cells):
Chamaesiphon
F2. Epiphytic (or epilithic), with short basal cell and numerous exocytes (spore-like reproductive cells):
Stichosiphon
E3. Cells form very short filaments or pseudofilaments, but are not enclosed in an open-ended sheath
F1. Solitary elongated basal cell, usually with one or more exocytes (spore-like reproductive cells); sheath absent or reduced to small basal cup; older basal cells occasionally Y-shaped:
Cyanophanon
F2. Solitary short, club-shaped basal cell (sheath absent) with only one exocyte (spore-like reproductive cell):
Geitleribactron
D2. Trichomes are not epiphytic, epizoic, or endogloeic. This group contains many taxa that are difficult to identify correctly
E1. Trichomes are spiraling or coiled and usually highly motile:
Arthrospira
and
Spirulina
E2. Multiple trichomes enclosed in a wide, common mucilage; live specimens actively motile:
Microcoleus
group (in part)
E3. Trichomes with fewer than 10 cells (usually 3); no visible sheath; cells compressed (W>L) with constricted cross-walls:
Borzia
E4. Cell width considerably shorter than length; cells often coin-like
F1. Trichomes lack a sheath; cells relatively wide (>8 um). Check carefully - sheath may be inconspicuous:
Oscillatoria
F2. Trichomes surrounded by a thin sheath (note that trichomes may creep out of their sheath)
Limnoraphis
and
Lyngbya
E5. Cells cubical, cylindrical, or oval; isodiametric (L=W) or slightly shorter or longer than wide (not coin-like)
F1. Trichomes narrow (<5 µm wide), with tapered, hooked apex; cells longer than wide, with granular cross-walls:
Geitlerinema
F2. Cells with deeply constricted cross-walls or unattachead to adjacent cells, forming pseudofilaments
G1. Cells tiny (width <2 µm), rod-shaped, pale blue-gray; forming pseudofilaments inside spherical or amorphous colony:
Cyanodictyon
,
Lemmermanniella
, and similar
G2. Cells with deeply constricted end-walls or separated slightly, forming pseudofilaments:
Johanseninema
and
Pseudanabaena
F3. Cells cubical or cylindrical; isodiametric (L=W) or slightly shorter or longer than width (not coin-like)
G1. cells cylindrical, nearly isodiametric (L=W), keritomized (net-like), appearing nearly empty:
Tychonema
G2. Tapered or untapered trichomes with narrow, cylindrical cells (W≤6 µm); cells with numerous aerotypes resulting in a sparkly appearance:
Limnothrix
G3. Untapered trichomes with relatively wide, cylindrical cells (W=8-10 µm; L≤8 µm); trichomes usually without sheath; cells with numerous aerotopes, resulting in a sparkly appearance:
Planktothrix
G4. Trichomes tapered or untapered; cells cylindrical, isodiametric (L=W), or slightly longer or shorter than width (not coin-like); trichomes with or without a thin sheath:
Oscillatoriales/Phormidium
group
A2. Motile algae with chloroplasts and flagella
B1. Motile single-cell algae
C1. Cells with one visible flagellum
D1. Cells covered with scales (scaled chrysophytes); most species also have long spines:
Mallomonas
D2. Cells enclosed in a rigid lorica
E1. Lorica spherical, oval, or elliptical; reddish brown or dark brown; surface smooth, granular, spiny, warty, or ridges:
Trachelomonas
E2. Lorica vase-shaped with tapered neck and pointed caudal tail; transparent or yellowish brown; surface smooth, wrinkled, or granular (not spiny):
Strombomonas
E3. Cells tiny; lorica bowl-shaped, transparent, with spiralling ridges:
Kephyrion
and
Pseudokephyrion
D3. Cells not covered with scales or enclosed in a lorica; may be rigid or flexible
E1. Cells resemble
Euglena
but are usually epizoic on freshwater microcrustaceans:
Colacium
E2. Cell wall flexible; cells often exhibit worm-like or amoeboid movement in addition to flagellar movement
F1. Cells elliptical, cylindrical, or teardrop-shaped; cells usually with worm-like euglenoid movement:
Euglenoids
(
Euglena
and
Lepocinclis
, in part)
F3. Cells spherical, golden brown, often with warty surface:
Chromulina
F4. Cells with flexible walls and long, thin pseudopods; cells may exhibit amoeboid movement:
Chrysamoeba
and
Rhizochrysis
E3. Cell wall rigid
F1. Cells oval with deep central groove (coffee-bean shape):
Cryptoglena
F2. Cells rigid and flattened, usually with long posterior tail (caudus):
Euglenoids
(
Lepocinclis
and
Phacus
, in part)
F3. Cells oval, broadly elliptical, or lemon-shaped; not strongly flattened or elongated:
Euglenoids
(
Lepocinclis
and
Phacus
, in part)
F4. Cells cylindrical or spindle-shaped; not strongly flattened; wall more or less rigid:
Euglenoids
(
Lepocinclis
and
Monomorphina
, in part)
C2. Cells with two equal length flagella
D1. Cells enclosed in firm, transparant, spherical mucilage
E1. Cells with bright red or partly red protoplast surrounded by spherical mucilage:
Haematococcus
E2. Cells with green protoplasts (not red) surrounded by spherical or oval mucilage:
Vitreochlamys
D2. Cells enclosed in rigid lorica
E1. Cells enclosed in a rough, reddish brown lorica; lorica separates along transverse (equatorial) groove:
Hemitoma
E2. Cells circular in face view, lens-shaped in side view; enclosed in transparent or yellowish, granular lorica; lorica separated along longitudinal groove:
Phacotus
D3. Cells not enclosed in lorica or surrounded by spherical or oval muicilage
E1. Narrowly elliptical or spindle-shaped cells; flagella often directed backwards; cells often nonmotile and embedded in mucilage or other algae:
Chlorogonium
E2. Cells slipper-shaped, oval, teardrop-shaped; asymmetric; two nearly equal flagella inserted below apex (subapical):
Cryptomonads
E3. Spherical, oval, or elliptical cells with two equal flagella inserted at the apex (large, difficult group):
Chlamydomonas
and similar
C3. Cells with two dissimilar flagella
D1. Cells with one forward and one trailing flagellum
E1. Cells teardrop-shaped or oval; cell wall contains many scattered trichocysts:
Gonyostomum
E2. Cells oval or spherical; cell wall contains knobby mucilage granules (no trichocysts):
Vacuolaria
D2. Cells with one transverse and one trailing flagellum (Dinoflagellates)
E1. Cells with flexible walls divided by a transverse groove (cingulum) into an epicone and hypocone:
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
,
Parvodinium
, Peridinium, and others)
D3. Cells with two forward-directed or subapical flagella
E1. Cells tiny; lorica bowl-shaped, transparent, with spiralling ridges:
Kephyrion
and
Pseudokephyrion
E2. Cells teardrop-shaped, golden brown, with very short, subapical flagella:
Chlorochromonas danica
E3. Cells slipper-shaped, oval, teardrop-shaped; asymmetric; two nearly equal flagella inserted below apex (subapical):
Cryptomonads
E4. Cells oval or spherical; cell wall contains knobby mucilage granules (no trichocysts):
Vacuolaria
C4. Cells with more than two flagella
D1. Cells spherical or oval with what appears to be three flagella (two flagella and a haptonema):
Chrysochromulina/Prymnesium
D2. Cells spherical, oval, or elliptical, with four equal flagella:
Carteria
D3. Cells heart-shaped and flattened, with four equal flagella:
Tetraselmis
B2. Motile colonies with 4 or more cells
C1. Cells with two equal flagella in colonies with 4 or more cells
D1. Flat colonies with 4-16 spherical or oval cells:
Gonium
and
Tetrabaena
D2. Spherical, oval, or elliptical colonies (not flat)
E1. Spherical colonies with 16-32 compressed, wedge-shaped cells:
Pandorina
E2. Spherical colonies with 16-32 spherical or oval cells (not wedge-shaped):
Eudorina
E3. Large colonies with more than 250 tiny cells:
Volvox
E4. Large colonies with 32-128 cells; large and small cells on opposite hemispheres:
Pleodorina
C2. Cells with two dissimilar flagella
D1. Cells smooth (not spiny); enclosed in vase-like lorica, forming tree-like colonies:
Dinobryon
D2. Cells covered with small scales (scaled chrysophytes); may also have short or long spines
E1. Cells covered with tiny scales and stout spines:
Chrysosphaerella
E2. Cells covered with tiny, ornate scales (no long spines):
Synura
D3. Cells not covered with scales or spines; may be enclosed in grainy mucilage or lorica
E1. Cells smooth (not spiny); solitary or in colonies of oval or teardrop-shaped cells joined by long, fragile mucilage threads:
Uroglena
and
Uroglenopsis
E2. Cells smooth (not spiny); colonies spherical, surrounded by mucilage containing tiny scales excreted by cells:
Neotessella
A3. Nonmotile algae with chloroplasts but without flagella
B1. Cells usually epiphytic, epizoic, or endozoic
C1. Cells, colonies, filaments or pseudofilaments attached to aquatic plants (epiphytic)
D1. Colonies parasitic on tree pollen; cells pyramidal, with short apical spine, often lacking chlorophyll; cells joined by short extensions to form open, spherical colony:
Ducellieria
D2. Protoplast (living portion of cell) enclosed in rigid lorica
E1. Lorica attached to plant host by very short, thick stipe, or appearing to be sessile; lorica with narrow opening:
Derepyxis
E2. Lorica attached by long stipe:
Stylochrysalis
E3. Urn-shaped lorica attached to mucilage surrounding host plant by stipe that is slightly longer than the lorica:
Epipyxis aureus
E4. Lorica sessile on host plant
F1. Lorica attached by thin band that encircles plant substrate:
Chyrsopyxis
F2. Lorica sessile; cells flask-shaped, usually orange or brown:
Lagynion
F3. Lorica long, cylindrical, covered with scales, usually sessile:
Epipyxis
D3. Protoplasts (living portion of cell) not enclosed in rigid lorica; cell wall may be rigid or flexible
E1. Cells with long hair-like setae or pseudocilia; solitary or in irregular colonies or filaments; highly variable appearance
F1. Cells spherical or hemispherical
G1. Cells spherical, with long, flexible pseudocilia; loosely enclosed in a club-shaped colonial mucilage:
Apiocystis
G2. Cells hemispherical (flattened against substrate), with long, branched hairs; solitary or in irregular colonies or filaments:
Dicranochaete
G3. Cells spherical with single long, sheathed hair; solitary or in irregular colonies or filaments:
Chaetosphaeridium
F2. Cells rectangular or irregular in shape
G1. Cells rectangular, trapezoidal, or irregular; joined into long filaments:
Coleochaete
G2. Cells irregularly cylindrical with one or more long hair-like setae; cells joined to form irregular creeping epiphytic filaments:
Aphanochaete
E2. Cells lacking or mostly lacking long hair-like setae
F1. Cells spherical, forming loosely arranged pseudofilaments at ends of upright, highly branched mucilage tubes:
Mischococcus
F2. Cells narrowly cylindrical and highly branched; branches initiate below cross-walls:
Microthamnion
F3. Cells on stipes (not sessile)
G1. Cells broadly crescent-shaped, golden brown, with red pigment spot; often epiphytic:
Cystodinium
G2. Cells golden yellow, club-shaped or cylindrical, solitary or in tree-like colonies with progeny cells forming at end of elongated basal parent cell:
Ophiocytium
G3. Cells oval or teardrop-shaped; attached by one or more long thin stipes:
Stipitochrysis
G4. Cells mostly small, green or yellowgreen, on short, thick stipes
H1. Cells fusiform, oval, elliptical, or crescent-shaped; cells sessile or attached by long or short stipe:
Characium
,
Characiopsis
, and similar
H2. Cells oval, solitary, with large pyrenoid; attached to susbtrate by short stipe; usually epiphytic on
Woronichinia
:
Stylosphaeridium
F4. Cells lacking or mostly lacking long hair-like setae
G1. Cells rectangular, trapezoidal, or irregular; joined into long filaments:
Coleochaete
G2. Cells oval or elliptical and bright blue-green; solitary or in small colonies enclosed in old parent cell wall; often epiphytic, but also found in plankton samples:
Glaucocystis
G3. Cells golden brown, in rectangular groups of 4; forming irregular colonies or short filaments:
Phaeoplaca
G4. Cells club-shaped or elliptical; filaments highly branched, usually epiphytic:
Phaeothamnion
C2. Cells, colonies, filaments or pseudofilaments attached or associated with or attached to aquatic animals (epizoic)
D1. Cells crescent-shaped (gently curved) with stout apical and posterior spines; usually epizoic on
Daphnia
:
Korshikoviella
D2. Cells resemble
Euglena
but are usually epizoic on freshwater microcrustaceans, rotifers, and protozoa:
Colacium
C3. Chlorella-like cells living inside zooplankton and other aquatic animals (endozoic):
Zoochlorella group
B2. Nonmotile solitary and colonial algae; cells not (or usually not) epiphytic, epizoic, or endozoic
C1. Cells spherical, oval, or elliptical
D1. Protoplast (live portion of cell) enclosed in rigid lorica
E1. Protoplast (living portion of cell) inside scaly, cylindrical, lorica; usually sessile:
Epipyxis
E2. Protoplast (living portion of cell) inside oval lorica with long apical and posterior spines:
Bitrichia
D2. Cells oval, heart-shaped, or wedge-shaped with apical extensions; cells joined at colony center by short mucilage stalks:
Sorastrum
D3. Cells or colonies with visible mucilage layers or strands
E1. Cells almost always solitary or paired except during reproduction
F1. Cells solitary or paired, spherical, golden brown, surrounded by thick, striated mucilage:
Gloeodinium
F2. Cells solitary, spherical, golden brown, with red eyespot; mucilage layer very thin or absent:
Hypnodinium
F3. Spherical or broadly oval cells with star-shaped chloroplast; solitary or in small colonies surrounded by diffuse mucilage:
Asterococcus
F4. Spherical or nearly spherical green cells with dense (not star-shaped) chloroplast; solitary or in small colonies surrounded by narrow, unstriated mucilage:
Planktosphaeria
E2. Colonies flat; cells form two rows, each with 4 cells joined on long edge; mucilage very diffuse:
Comasiella
E3. Colonies spherical or oval
F1. Cells broadly oval to spherical; solitary or in small colonies enclosed in gelatinized parent cell wall:
Oonephris
F2. Cells solitary or in small colonies up to 8 cells
G1. Cells yellowish green with red pigment droplet; solitary or in very small colonies surrounded by diffuse mucilage:
Chlorobotrys
G2. Cells golden brown, fine rhizopodia; colony with numerous symbiotic bacteria:
Chrysostephanosphaera
G3. Cells bright or dull green; starch present (test using Lugol's)
H1. Cells oval, elliptical, or lemon-shaped; solitary or in small colonies enclosed in expanded parent cell wall:
Oocystis
and
Neglectella
H2. Cells in groups of 4 (or 8) enclosed in diffuse mucilage; long pseudocilia extending beyond the colonial mucilage:
Pseudosphaerocystis
H3. Spherical or broadly oval cells with star-shaped chloroplast; solitary or in small colonies surrounded by diffuse mucilage:
Asterococcus
H4. Spherical or nearly spherical green cells with dense (not star-shaped) chloroplast; solitary or in colonies with 4-32 cells surrounded by narrow, unstriated mucilage:
Planktosphaeria
H5. Cells broadly elliptical to spherical, surrounded by very thick mucilage that is often striated; images:
Gloeocystis/Chlamydocapsa
group
F3. Colonies with more than 8 cells
G1. Cells in groups of 4 at the end of radiating mucilage strands:
Mucidosphaerium
and cf.
Brachionococcus
G2. Cells spherical with fine rhizopodia; colonies usually with no more than 8 cells:
Chrysostephanosphaera
G3. Cells oval, elliptical, or lemon-shaped; solitary or in small colonies (usually with no more than 8 cells); enclosed in expanded parent cell wall:
Oocystis
, in part
G4. Cells broadly elliptical to spherical, surrounded by very thick mucilage that is often striated; images:
Gloeocystis/Chlamydocapsa
group
G5. Cells with dense but not star-shaped chloroplast; solitary or in small to large colonies; colonies often contain large parent cells and clumbs of small progeny cells:
Sphaerocystis
E4. Colonies with irregular (amorphous) shape
F1. Cells loosely enclosed in a club-shaped colonial mucilage; usually epiphytic:
Apiocystis
F2. Colonies consisting of clumps of cells connected by mucilage strands or residual parent cell walls
G1. Cells enclosed by firm mucilage forming large clumps connected by mucilage strands; colonies orange, green, or both:
Botryococcus
G2. Colonies with grape-like clusters of spherical cells connected to other clusters by thin mucilage strands:
Botryosphaerella
G3. Cells in 4x groups connected by mucilaginous strands created from parent cell wall:
Westella
F3. Amorphous colonies comprised of 4x cell subgroups
G1. Cells spherical, in groups of 4, with long, flexible pseudocilia:
Tetraspora
G2. Cells broadly elliptical to spherical, surrounded by very thick mucilage that is often striated; images:
Gloeocystis/Chlamydocapsa
group
G3. Cells oval, forming tetrahedral groups of 4 in irregular (amorphous) colonies; colony surrounded by diffuse mucilage not visible without staining:
Stichogloea
F4. Amorphous colonies; cells not in obvious 4x subgroups
G1. Cells golden brown; starch absent (test with Lugol's)
H1. Cells arranged in highly branched pseudofilaments; specimens have strong, unpleasant odor:
Hydrurus
H2. Cells oval or spherical, golden brown, in loosely arranged, irregular (amorphous) colonies:
Gloeobotrys
G2. Cells bright or dull green; starch present (test with Lugol's)
H1. Cells with one flattened side containing tufts of pseudocilia; colony contains fragments of old cell walls; mucilage nearly invisible:
Schizochlamys
H2. Cells broadly elliptical to spherical, surrounded by very thick mucilage that is often striated; images:
Gloeocystis/Chlamydocapsa
group
H3. Cells with dense but not star-shaped chloroplast; solitary or in small to large colonies; colonies often contain large parent cells and clumbs of small progeny cells:
Sphaerocystis
D4. Cells or colonies lacking visible mucilage layers or strands, or with mucilage that is not visible without staining
E1. Cells usually solitary and lacking visible mucilage layers
F1. Cells oval or elliptical and bright blue-green; solitary or in small colonies enclosed in old parent cell wall; often epiphytic, but also found in plankton samples:
Glaucocystis
F2. Cells usually golden brown; starch not present in most taxa - use Lugol's to confirm
G1. Cells solitary, spherical, golden brown, with red eyespot; mucilage layer absent or very thin:
Hypnodinium
G2. Cells solitary but often in large groups; cells relatively small, with tiny oil droplets:
Chrysosphaera
G3. Cells with flexible walls and long, thin pseudopods; cells may exhibit amoeboid movement:
Chrysamoeba
and
Rhizochrysis
F3. Cells usually green; starch present - test using Lugol's
G1. Spherical or broadly oval cells with star-shaped chloroplast; solitary or in small colonies; colonies are usually surrounded by diffuse mucilage:
Asterococcus
G2. Cells very large; many small chloroplasts attached to internal strands of mucilage radiating from center of cell:
Eremosphaera
G3. Cells have 4 or more long spines or flexible hairs
H1. Cells with many long, thin, slightly flexible spines; chloroplast with large central pyrenoid with starch ring:
Golenkinia
H2_ Cells oval or elliptical with long setae or spines; solitary or in small colonies enclosed in old parent cell wall:
Lagerheimia
G4. Small, spherical cells lacking visible mucilage and containing starch:
Chlorella
group
E2. Cells rarely solitary; colonies spherical or oval
F1. Cells lacking long spines or hairs
G1. Cells oval or elliptical and bright blue-green; solitary or in small colonies enclosed in old parent cell wall; often epiphytic, but also found in plankton samples:
Glaucocystis
G2. Cells yellowish green with red pigment droplet; solitary or in very small colonies surrounded by diffuse:
Chlorobotrys
G3. Cells joined along flattened wall segment or short wall extensions ro form hollow colony:
Coelastrum
and
Hariotina
G4. Cells with dense but not star-shaped chloroplast; solitary or in small to large colonies; colonies often contain large parent cells and clumbs of small progeny cells:
Sphaerocystis
F2. Cells with long spines or hairs
G1. Cells spherical with long, sharp spines; colonies usually with one or more 4-cell tetrahedral groups:
Micractinium
G2. Cells oval or elliptical with long setae or spines; solitary or in small colonies enclosed in old parent cell wall:
Lagerheimia
E3. Cells rarely solitary; colonies flattened
F1. Cells joined along their long axis to form ribbon-like colonies
G1. Cells elliptical, cylindrical, or oval; apex rounded, usually without spines, teeth, or knobs:
Scenedesmus
and
Comasiella
G2. Cells elliptical_cylindrical_or_oval; apex rounded, with spines, teeth, or knobs:
Desmodesmus
and
Scenedesmus
, in part
F2. Wedge-shaped golden brown cells in rectangular 4x groups:
Phaeoplaca
F3. Wedge-shaped cells in rectangular groups 4x groups; colonies usually comprised of numerous 4x subgroups:
Lemmermannia
F4. Colonies comprised of 4-cell groups; cells green, oval or elliptical:
Willea
E4. Cells rarely solitary; colonies with irregular (amorphous) shape
F1. Cells with one flattened side containing tufts of pseudocilia; colony contains fragments of old cell walls; mucilage nearly invisible:
Schizochlamys
F2. Cells with dense but not star-shaped chloroplast; solitary or in small to large colonies; colonies often contain large parent cells and clumbs of small progeny cells:
Sphaerocystis
F3. Cells golden brown or yellowish-green; starch absent (test with Lugol's)
G1. Cells spherical, golden brown, in loosely arranged, irregular (amorphous) colonies:
Gloeobotrys
G2. Cells oval, forming tetrahedral groups of 4 in irregular (amorphous) colonies; colony surrounded by diffuse mucilage not visible without staining:
Stichogloea
G3_ Irregular colonies containing many small, spherical, yellowish green cells:
Nannochloroposis
C2. Cells not spherical, oval, or broadly elliptical
D1. Cells are usually solitary unless reproducing
E1. Cells solitary, smooth walled, with no obvious spines or teeth
F1. Cells golden yellow, club-shaped or cylindrical, solitary or in tree-like colonies with progeny cells forming at end of elongated basal parent cell:
Ophiocytium
F2. Cells broadly cylindrical or spindle-shaped:
Spirotaenia
and
Tortitaenia
F3. Cells narrowly fusiform; solitary, with or without spines; lacking any visible mucilage layer; may form blooms:
Monoraphidium
group
E2. Cells solitary, with distinct teeth or spines
F1. Cells broadly crescent-shaped with short apical and posterior spines; cells golden or yellowish brown
G1. Cells broadly crescent-shaped and often asymmetric, with short, blunt apical and posterior spines:
Keratococcus
G2. Cells broadly crescent-shaped, golden brown, with red pigment spot; often epiphytic:
Cystodinium
F2. Cells with intact (not split) apical and posterior spines
G1. Cells usually solitary; narrowly cylindrical or fusiform, with long apical and posterior spines:
Centritractus
G2. Cells crescent-shaped (gently curved) with stout apical and posterior spines; usually epizoic on
Daphnia
:
Korshikoviella
G3. Cells narrowly fusiform; solitary, with or without spines; lacking any visible mucilage layer; may form blooms:
Monoraphidium
group
G4. Cells narrowly crescent-shaped, with intact apical spine and leaf-like posterior spine:
Lanceola
F3. Cells club-shaped to narrowly crescent-shaped, with apical spine and split or divided posterior spines:
Ankyra
D2. Taxa usually form colonies of 2 or more cells
E1. Colonies with dissimilar cells in 4-cell subgroups joined by mucilage strands:
Dimorphococcus
E2. Cells club-shaped or cylindrical
F1. Cells long, cylindrical, joined at ends to form nets with pentagonal or hexagona subunits:
Hydrodictyon
F2. Cells golden yellow, club-shaped or cylindrical, solitary or in tree-like colonies with progeny cells forming at end of elongated basal parent cell:
Ophiocytium
F3. Cells club-shaped or somewhat fusiform with long apical bristles and a short, split (bifurcate) posterior spine; usually in 2-cell colonieis:
Paradoxia
F4. Cells club-shaped, arranged at ends of radiating mucilage to form small colonies:
Dichomotococcus
E3. Colonies with spindle-shaped or fusiform cells; cells may be slightly curved, but not distinctly crescent-shaped
F1. Cells fusiform or spindle-shaped; colonies usually with 8 cells radiating from center for form tetrahedral shape:
Actinastrum
F2. Cells broadly or narrowly fusiform; occasionally solitary but usually in lens-shaped colonies enclosed in lens-shaped mucilage:
Elakatothrix
and
Fusola
F3. Cells cylindrical or fusiform, aligned along long axis forming bundles enclosed in diffuse mucilage:
Quadrigula
F4. Cells narrowly fusiform, aligned lengthwise into tangled, linear, or spherical colonies:
Ankistrodesmus
, in part
F5. Groups of (mostly) 4 fusiform or crescent-shaped cells joined along the long axis:
Tetradesmus
,
Pectinodesmus
E4. Colonies with distinctly crescent-shaped (lunate) cells; apex rounded or sharply acute
F1. Cells broadly crescent-shaped in 8-cell colonies enclosed in gelatinized parent cell wall:
Nephrocytium
F2. Cells range from narrowly crescent-shaped to narrowly fusiform in tightly clumped colony:
Ankistrodesmus spiralis
F2. Groups of (mostly) 4 fusiform or crescent-shaped cells joined along the long axis:
Tetradesmus
,
Pectinodesmus
F3. Cells strongly crescent-shaped, acutely pointed; joined along outer (dorsal) wall:
Messastrum
and
Selenastrum
F4. Cells bluntly or acutely pointed; not joined along outer (dorsal) wall, forming loosely clumps sourrounded by diffuse mucilage:
Kirchneriella
C3. Cells strongly angular, sometimes asymmetric or irregular in shape
D1. Cells 3-4 sided; if 4-sided, not flattened
E1. Cells 3-sided (triradiate) with long, straight spines at each angle:
Treubaria
E2. Cells triradiate with narrow arms; no angular spines, but wall distinctly rough; large, central nucleus surrounded by lipid droplets:
Pseudogoniochloris tripus
E3. Cells tetrahedral but mostly appearing tringular in outline; lacking spines or teeth on angles, but with faintly sculptured wall:
Goniochloris
and
Tetradriella
, in part
E5. Cells tetrahedral but usually appear tringular in outline; stout spines on angles; cell wall sculpted:
Tetradriella
cf. spinigera
E5. Cells essentially tetrahedral but strongly twisted:
Tetraplektron
D2. Cells flattened with 4-5 sides
E1. Cells golden-brown, 4-sided with 1-2 stout spines at each angle:
Tetradinium
E2. Cells angular, strongly flattened, with deep apical notch:
Stauridium
E3. Cells 4-5 sided, flattened, with central pyrenoid; 5-sided taxa may have spines on each angle:
Tetraedron
, in part
D3. Cells with irregular shapes or with more than one shape within a colony (dimorphic)
E1. Cells angular and strongly flattened; inner and outer cells dissimilar:
Lacunastrum
,
Monactinus
,
Pediastrum
,
Pseudopediastrum
, and
Stauridium
(in part)
E2. Colonies consisting of heart-shaped or crescent-shaped cells and broadly elliptical cells joined by mucilage strands:
Dimorphococcus
E3. Cells oval, heart-shaped, or wedge-shaped with apical extensions; cells joined at colony center by short mucilage stalks:
Sorastrum
E4. Cells highly branched, bright yellowish green, starch absent:
Isthmochloron
and
Pseudostaurastrum
B3. Nonmotile algae that form filaments or pseudofilaments
C1. Unbranched filamentous algae
D1. Cells very large; filaments coarse, unbranched, very sparsely branched; one genus lacks cellular crosswalls
E1. Filaments lack cross-walls except below reproductive structures; filament often contains large, single or paired oogonia:
Vaucheria
E2. Filaments with distinct cross-walls, unbranched or sparsely branched; cells long, cylindrical; chloroplasts dense and reticulated (net-like):
Cladophora
E3. Filaments with distinct cross-walls, unbranched or sparsely branched; cells long, cylindrical; chloroplast reticulated (net-like); dark akineted often formed at ends and within filament:
Pithophora
D2. Cells spherical, lens-shaped, oval, or elliptical
F1. Cells spherical, forming pseudofilaments of widely spaced cells enclosed in mucilage tube:
Palmodictyon
F2. Cells oval or elliptical, forming pseudofilaments, often with paired cells; filaments/pseudofilaments surrounded by wide, diffuse mucilage:
Geminella
, in part
F3_ Cells lens-shaped, with transverse ring; closely spaced in uniseriate filaments:
Radiofilum
F4. Cells compressed oval or slightly lens-shaped; closely spaced in uniseriate or multiseriate filaments:
Flintia
F5. Oval cells surrounded by striated mucilage that becomes inflated and increasingly striated with age:
Cylindrocapsa
D3. Cells cylindrical or narrowly elliptical
E1. Cells cylindrical, mostly longer than wide; some cells with ring-like cell division scars that resemble pipe threads:
Oedogonium
E2. Cells cylindrical or oval, often paired; cell wall becoming increasinly thick and stratified, especially at crosswalls:
Binuclearia
E3. Cells with H-shaped endwall extensions
F1. Endwall extensions robust, visible at ends of filaments and sometimes along length of filament; chloroplast bright green, reticulated (net-like):
Microspora
F2. Cells with very short endwall extensions; chloroplast green, saddle-like, extending about half way around cell:
Klebsormidium
F3. Cell extensions long, needle-like, fragile; chloroplasts small, numerous, disk-shape:
Tribonema
F4. Endwall extensions short, robust; chloroplasts disk-shaped; filaments very easily fragmented:
Bumilleria
E4. Cell wall without growth rings or H-shaped extensions
F1. Cells long and cylindridal; filaments large, coarse, unbranched or very sparsely branched
G1. Filaments with distinct crosswalls, unbranched or sparsely branched; cells long, cylindrical; chloroplasts dense and reticulated (net-like):
Cladophora
G2. Filaments with distinct crosswalls, unbranched or sparsely branched; cells long, cylindrical; chloroplast reticulated (net-like); dark akinetes often formed at ends and within filament:
Pithophora
F2. Cells with spiraling, stellate, ribbon-shaped, ring-like, or saddle-shaped chloroplasts; mucilage absent or, if present, not wide and diffuse
G1. Cells with two star-shaped chloroplasts located near the center of the cell (axial); very common, diverse genus:
Zygnema
G2. Cells with single flat chloroplast extending down the center of the cell (axial); chloroplast often twisted; very common, diverse genus:
Mougeotia
G3. Cells with one or more narrow, ribbon-like chloroplasts spiralling around the inside of the cell (parietal); very common, diverse group:
Spirogyra
and
Sirogonium
G4. Chloroplast forms a complete or nearly complete ring or saddle circling the center of the cell (parietal)
H1. Chloroplast forms a narrow, complete or nearly complete ring circling the center of the cell (parietal):
Ulothrix
H2. Chloroplasts broadly saddle-like, parietal, extending at least half way around cell; very short endwall extensions may be visible at ends of filaments:
Klebsormidium
H3. Cells long and very narrow (≤10 μm); chloroplasts broadly saddle-like, parietal, extending more than half way around cell; apical cell slightly inflated:
Uronema
F3. Filaments surrounded by wide, diffuse mucilage that may need staining or phase contrast to be visible
G1. Cells form short, easily fragmented pseudofilaments; mucilage layer wide, unstratifed, inconspicuous:
Gloeotila
G2. Cells variable depending on species; filaments surrounded by wide, diffuse, unstriated mucilage:
Geminella
G3. Filaments surrounded by striated mucilage that becomes inflated and increasingly striated with age:
Cylindrocapsa
C2. Branched filamentous algae
D1. Cells very large; filaments coarse, unbranched or very sparsely branched; one genus lacks cellular crosswalls
E1. Filaments lack crosswalls except below reproductive structures; filament often contains large, single or paired oogonia:
Vaucheria
E2. Filaments with distinct crosswalls, unbranched or sparsely branched; cells long, cylindrical; chloroplasts dense and reticulated (net-like):
Cladophora
E3. Filaments with distinct crosswalls, unbranched or sparsely branched; cells long, cylindrical; chloroplast reticulated (net-like); dark akinetes often formed at ends and within filament:
Pithophora
D2. Filaments with similar cells forming primary and secondary branches; filaments may have dissimilar terminal cells and slightly larger main branch cells
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 ends to form nets with pentagonal or hexagona subunits:
Hydrodictyon
E3. Cells golden yellow, club-shaped or cylindrical, solitary or in tree-like colonies with progeny cells forming at end of elongated basal parent cell:
Ophiocytium
E4. Cells at end of primary or secondary branches extended into hair-like setae
F1. Cells rectangular, trapezoidal, or irregular; joined into long filaments; may have long setae, but this feature may not be visible:
Coleochaete
F2. Cells with long, hair-like setae that are expanded at the base:
Bulbochaete
F3. Filaments mostly attached to submerged surfaces, forming dense hemispherical or shrubby clumps; terminal cells on secondary branched usually extended into hair-like setae:
Chaetophora
and
Chaetophoropsis
F4. Filaments usually attached to submerged surfaces, forming diffuse, irregularly shaped clumps; often with rhizoidal basal cells; terminal cells acutely pointed and sometimes extended into setae:
Stigeoclonium
E5. Filaments lacking (or mostly lacking) cells that form hair-like setae
F1. Filaments coarse, sparsely branched, often forming dense mats on submerged or damp surfaces
G1. Cells bright reddish-orange; filaments sparsely branched:
Trentepohlia
G2. Filaments mostly unbranched, but appear to branch when forming reproductive structures:
Vaucheria
G3. Filaments with distinct cross-walls, unbranched or sparsely branched; cells long, cylindrical; chloroplasts dense and reticulated (net-like):
Cladophora
G4. Filaments with distinct cross-walls, unbranched or sparsely branched; cells long, cylindrical; chloroplast reticulated (net-like); dark akineted often formed at ends and within filament:
Pithophora
F2. Cells golden brown, lacking starch; filaments mostly irregular or highly branched
G1. Cells arranged in highly branched pseudofilaments; specimens have strong, unpleasant odor:
Hydrurus
G2. Cells spherical, forming loosely arranged pseudofilaments at ends of upright, highly branched mucilage tubes:
Mischococcus
G3. Cells club-shaped or elliptical; filaments highly branched, usually epiphytic:
Phaeothamnion
F3. Cells bright or dull green; starch present (test with Lugol's)
G1. Cells spherical, oval, or lens-shaped and bright or dull green
H1_ Cells spherical, dull green; forming unbranched or very sparsely branched pseudofilaments enclosed in mucilage tube:
Palmodictyon
H2. Cells dull green, lens-shaped, with transverse ring; forming mostly unbranched filaments; occasionally appears branched:
Radiofilum
H3. Cells bright green, compressed oval or lens-shaped; forming uniseriate or multiseriate filaments surrounded by mucilage:
Flintia
G2. Cells cylindrical or quadrate and usually bright or dull green
H1. Cells rectangular, trapezoidal, or irregular; joined into long filaments; with or without setae:
Coleochaete
H2. Filaments mostly attached to submerged surfaces, forming dense hemispherical or shrubby clumps; terminal cells on secondary branched usually (not always) extended into hair-like setae:
Chaetophora
and
Chaetophoropsis
H3. Filaments usually attached to submerged surfaces, forming diffuse, irregularly shaped clumps; often with rhizoidal basal cells; terminal cells acutely pointed and sometimes extended into setae:
Stigeoclonium
H4. Cells narrowly cylindrical and highly branched; branches initiate below cross-walls:
Microthamnion
D3. Filaments with distinctly larger primary branches and smaller secondary branches
E1. Main branches with long, cylindrical cells; secondary branches with tear-drop, oval, or elliptical cells forming whorls:
Batrachospermum
and
Paludicola
E1. Cells arranged in highly branched pseudofilaments; specimens have strong, unpleasant odor:
Hydrurus
E2. Filaments highly differentiated; cells in main branch wide, barrel-shaped, with fringed, ring-like parietal chloroplast; cells in side branched small, cylindrical or tapered, with net-like (reticulate) chloroplasts:
Draparnaldia
A4. Cells with paired, symmetric semi-cells:
Desmids
B1. Cells rarely solitary; usually forming clumps or filaments
C1_ Cells oval or elliptical, enclosed in dense mucilage forming small colonies (occasionally solitary). Cells with one plate-like chloroplast:
Mesotaenium
C2. Semi-cells smooth-walled, oval or broadly elliptical; cells linked by mucilage threads to form branched colonies:
Cosmocladium
C3. Cells linked end-to-end to form filaments
D1. Cells with shallow sinus or unconstricted; cell length ≤ width
E1. Cells angular in cross-section, with angular projections or wall extensions:
Desmidium
E2. Cells short cylindrical; circular or oval in cross-section:
Hyalotheca
D2. Cells with shallow sinus or unconstricted; cell length > width
E1. Cells barrel-shaped, distinctly expanded at center:
Bambusina
E2. Semi-cells spherical, cells peanut-shaped with widely open sinus:
Cosmarium moniliforme
E3. Cells cylindrical; sinus absent or very shallow and indistinct
F1. Cells cylindrical, very long (L≫W), with no apparent sinus; solitary or in short filaments:
Gonatozygon
F2. Cells approximately cylindrical, but either slightly inflated or constricted at center:
Groenbladia
D3. Cells with moderately deep sinus
E1. Semi-cells oval or broadly elliptical; cells joined along smooth apical surface:
Spondylosium
E2. Semi-cells elliptical or kidney-shaped; cells joined by short, rod-like apical extensions:
Sphaerozosma
E3. Semi-cells oval or elliptical; cells joined by inconspicuous apical granules (look carefully):
Teilingia
B2. Cells mostly solitary unless dividing
C1. Cells lack an obvious sinus; semi-cells usually separated by a simple suture that may not be visible
D1. Cells crescent-shaped, strongly or weakly curved, usually with tapered apices; cell wall lacking spines or teeth, but often striated; apex with vacuole containing one or more granules:
Closterium
D2. Cells spherical, oval, or broadly elliptical, with no obvious sinus
E1. Cells oval or broadly elliptical in outline; semi-cells separated by thin suture; cell walls ridged, striated, or covered with rod-like projections:
Penium
(in part)
E2. Cells oval or broadly elliptical, with no obvious sinus, but with separate chloroplasts in each semi-cell:
Actinotaenium
and
Cosmarium
(in part)
D3. Cells elliptical, spindle-shaped, or cylindrical
E1. Cells with a spiraling chloroplasts; narrowly cylindrical or spindle-shaped:
Spirotaenia
E2. Cells with toothed whorls or horn-like lateral extensions; narrowly or broadly cylindrical
F1. Cells very long, cylindrical, with toothed whorls extending the full length of the cell:
Triploceras
F2. Cells cylindrical in outlilne, with 3 pairs of horn-like extensions on each semi-cell:
Micrasterias muricata
E3. Outer edge of chloroplast fringed or notched; cells broadly elliptical:
Cylindrocystis
E4. Cells broadly spindle-shaped; wider at the center than the apex
F1. Cells broadly spindle-shaped; outer edge of chloroplast deeply notched or incised:
Netrium
F2. Cells spindle-shaped, solitary; chloroplast with median notch containing the nucleus:
Tortitaenia
F3. Cells solitary, spindle-shaped or elliptical; both ends of the chloroplast have a large spherical vacuole containing a single granule
Planotaenium
E5. Cells cylindrical, with parallel lateral margins
F1. Cells cylindrical, very long (L≫W), with flattened apex and no apparent sinus:
Gonatozygon
F2_ Cells cylindrical, long (L≫W), with a rounded apex and no apparent sinus; chloroplast with median notch:
Roya
F3. Cells short, cylindrical with flattened apex; no obvious sinus:
Penium cylindrus
C2. Cells with relatively distinct sinus; sinus may be open or closed, deep or shallow
D1. Cells with toothed whorls or horn-like lateral extensions along entire lateral margin; cells narrowly or broadly cylindrical, with an indistinct sinus
E1. Cells very long, cylindrical, with toothed whorls extending the full length of the cell:
Triploceras
E2. Cells cylindrical in outline, with 3 pairs of horn-like extensions on each semi-cell:
Micrasterias muricata
D2. Cells circular, oval, or broadly elliptical in cross-section
E1. Cells broadly spindle-shaped or elliptical, with apical incision:
Tetmemorus
E2. Cells cylindrical in outline
F1. Cells cylindrical in outline; lateral margin undulating at least once above sinus, but lacking lateral teeth or spines:
Pleurotaenium
and
Haplotaenium
F2. Cells very long, cylindrical, with toothed whorls extending the full length of the cell:
Triploceras
E3. Semi-cells with smooth walls and 2 long spines or stout teeth at the angles on each semi-cell
Staurodesmus
, in part
E4. Cells oval or rectangular in outline, circular or oval in cross-section; most species with more than two long, stout spines on the semi-cell angles:
Xanthidium
E5. Cells circular or oval in cross-section; highly variable in outline (circular, oval, elliptical, trapezoidal, pyramidal, or rectangular):
Cosmarium
and
Actinotaenium
(in part)
D3. Cells moderately to strongly flattened in cross-section; cells circular, oval, or broadly elliptical in outline
E1. Cells oval, rectangular, or trapezoidal in outline; strongly or moderately flattened; apex (polar lobe) usually with median notch or incidion:
Euastrum
E2. Cells spherical or oval in outline, strongly flattened in cross-section; cell walls smooth, granular, or covered with short spines, but lacking long spines:
Micrasterias
E3. Cells oval or rectangular in outline, moderately to strongly flattened in cross-section; most species with 2 long spines or stout teeth on the semi-cell angles:
Staurodesmus
, in part
E4. Cells oval or rectangular in outline, moderately to strongly flattened in cross-section; most species with more than 2 long, stout spines on the semi-cell angles:
Xanthidium
, in part
D4. Cells biradiate, triradiate, or polyradiate (more than 3 angles) in cross-section, usually distinctly angular
E1. Semi-cells mostly triradiate in cross-section (or multiples of 3x); cell wall often covered with granules, teeth, or spines; angles may be extended to form long arms:
Staurastrum
E2. Semi-cells triradiate in cross-section, with short, blunt knobs on angles; wall smooth:
Xanthidium tumidum
E3. Semi-cells with smooth walls and 2 stout spines or teeth at the angles on each semi-cell; biradiate or triradiate:
Staurodesmus
A5. Cells enclosed in glassy, ornamental frustules:
Diatoms