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Major Groups

Note: keys designed only for the species represented herein
  1   Plants with vein-like vascular systems, consisting of true roots, stems and leaves MAGNOLIOPHYTA (SEAGRASSES, not included)  
  1   Thallus without vascular systems 2  
  2   Thallus often blue-green, gray-green to blue-gray; cell content appear granular, without distinct plastids (pigmented organelles) CYANOBACTERIA (Blue Green Algae)  
  2   Thallus not blue-green, gray-green or blue-gray (occasionally iridescent when living); cells with distinct plastids or organelles 3  
  3   Thallus red, violet, pink, white-pink to bluish-green; red pigments extract in boiling water RHODOPHYTA (Red Algae)  
  3   Thallus generally pale to dark green or olive to dark brown; pigments do not extract in boiling water. 4  
  4   Thallus predominately pale to dark green; starch (stains black with iodine) present in cells CHLOROPHYTA (Green Algae)  
  4   Thallus olive to dark brown; starch absent PHAEOPHYCEAE (Brown Algae)  

Rhodophyta Red Algae

Note: keys designed only for the species represented herein
  1   Thallus fine filaments or parts finely filamentous FILAMENTOUS, 8  
  1   Thallus not finely filamentous 2  
  2(1)   Thallus as thin delicate mesh or sheet-like membranes DELICATE MEMBRANES, 18  
  2(1)   Thallus not thin, delicate membranes 3  
  3(2)   Thallus mucilaginous, gelatinous or gooey SOFT AND GELATINOUS, 20  
  3(2)   Thallus corticated, coarsely branched or crust-like 4  
  4(3)   Thallus wiry, tough, often attached by prostrate runners [medulla formed by cells of similar size; some genera with internal, thick-walled, slender filaments (rhizines) intermixed with internal cells WIRY, 27  
  4(3)   Plants not wiry 5  
  5(4)   Thallus [firm, tough] with polysiphonous core (central and pericentral cells of similar length, obvious in younger branches), often, but not always, with terminal tufts of hair-like filaments on branches or branchlets, cylindrical, compressed FIRM, 32  
  5(4)   Thallus not as above 6  
  6(5)   Central medullary cells largest, grading smaller toward surface LEATHERY, 35  
  6(5)    Thallus calcified 7  
  7(6)   Thallus calcified with flexible non-calcified joints JOINTED CALCAREOUS, 36  
  7(6)   Thallus crustose (crust-like, thickened erect hard branches or with knob-like projections) CALCIFIED CRUST, KNOBBED or BRANCHED, 40  


  8(1)   Entire thallus or apices of thallus as fine filaments three cells in diameter, with each new segment offset 60º Asparagopsis taxiformis (Falkenbergia Stage)  
  8(1)   Thallus lacks filaments three cells in diameter 9  
  9(8)   Main axes and/or branches solely polysiphonous (central and pericentral cells of similar length, obvious in younger branches), tough; basal rhizoidal cortication rare 10  
  9(8)   Thallus not as above 12  
  10(9)   Not solely intertidal; some sort of apical filaments at least occasionally present 11  
  10(9)   Restricted to upper intertidal habitats; apical filaments (trichoblasts) lacking Bostrychia  
    a Branchlets unbranched; older areas may become corticated by rhizoidal filaments Bostrychia calliptera  
    a Branchlets forked or alternately branched; older areas not corticated b  
    b Branches 75–100 µm diam.; outermost branchlets entirely uniseriate Bostrychia moritziana  
    b Branches 150–200 µm diam.; outermost branchlets not entirely uniseriate Bostrychia radicans  
  11(10)   Branchlets persistent, consistently on every other segment Heterosiphonia crispella var. laxa  
  11(10)   Branchlets (apical filaments) soon deciduous, not consistently on every other segment Polysiphonia / Neosiphonia  
    a Axis of 4 pericentral cells b  
    a Axis of 5 pericentral cells c  
    b Thallus short, mat-like, 1–3(–5) mm high Polysiphonia cf. pseudovillum  
    b Thallus forming upright bushy tufts, to 30 mm high Neosiphonia tongatensis  
    c Tetrasporangia in short spiral series; anchored by horizontal rhizome (runner) Polysiphonia bifurcata  
    c Tetrasporangia in straight series; anchored by disc-like holdfast or basal cluster of rhizoids Polysiphonia homoia  
  12(9)   Thallus uniseriate (in single series or row of cells), finely filamentous, cortication lacking or only basal 13  
  12(9)   Uniseriate axes corticated at joints (axes faintly banded) or corticated in older portions or entirely corticated by one or two layers of small thin-walled cells 16  
  13(12)   Filaments unbranched or irregular sparsely branched 14  
  13(12)   Filaments with distinct branching pattern 15  
  14(13)   Attached by conspicuous stolons (runners) Lejolisia colombiana  
  14(13)   Attached by single basal cell Stylonema alsidii  
  15(13)   Main axes 120–200 µm diam.; branching consistently opposite alternate from every axis cell distally Pleonosporium mexicanum  
  15(13)   Main axes 80–110 µm diam.; branching radially alternate Callithamnion marshallense  
  16(12)    Cortication not as below 17  
  16(12)   Cortication complete in main axes with bands of short wider cells alternating with long thinner cells; cortication in branchlets limited to joints Spyridia  
    a Branching and branchlets in one plane (appearing flat, distichous) Spyridia cf. clavata  
    a Branching and branchlets irregularly radial Spyridia filamentosa  
  17(16)    Corticating cells at joints only Gayliella flaccida  
  17(16)   Corticating cells as rhizoid-like filaments originating from lowermost cell of lateral branchlets Wrangelia bicuspidata  

Delicate Membranes

  18(2)    Thallus tufts or clumps of mesh-like blades Haloplegma mexicanum  
  18(2)   Thallus other than network of fusing branchlets 19  
  19(18)   Tetrasporangial sori adjacent and parallel to midrib Hypoglossum attenuatum  
  19(18)   Tetrasporangial sori scattered on blade Grinnellia lanceolata  

Soft and Gelatinous

  20(3)   Thallus calcified, chalk-white below (stiff), pink-white above (soft) Titanophycus validus  
  20(3)   Thallus other than above 21  
  21(20)   Thallus of slender filaments within mucilaginous gel; texture soft, fleshy, mucilaginous or gelatinous 24  
  21(20)    Thallus having thin but firm cortex surrounding mucilaginous gel-filled interior; fine, sparse, internal filaments when present usually transverse 22  
  22(21)   Thallus radially or irregularly branched, generally opaque pink; texture mucilaginous (gooey) Acrosymphyton sp.  
  22(21)    Thallus thick, lobed, generally dark translucent red; texture gelatinous 23  
  23(22)   Blades irregularly lobed, in more than one plane; medullary filaments 2–3 µm diam. Predaea laciniosa  
  23(22)   Blades thickly flattened, in one plane; medullary filaments 4–14 µm diam. Platoma fanii  
  24(21)   Thallus cortex two cells thick, delicate; subsurface cells small, tear-shaped; longitudinal medullary filaments in loose central bundle of 6–50 strands [vaguely apparent when thallus held to light] Scinaia minima  
  24(21)   Thallus cortex 1–7 cells thick; cortical cells decreasing in size toward surface; medullary filaments scattered, tangled 25  
  25(24)   Blades or branches slick delicate 26  
  25(24)   Blades or branches soft to turgid, not delicate Sebdenia  
    a Blades cylindrical; branching strictly dichotomous Sebdenia flabellata  
    a Blades flat oval; blades occasionally split b  
    b Margins slightly thickened; medullary filaments 14–40 µm diam. Sebdenia actinophysa  
    b Margins not thickened; medullary filaments 4–8 µm diam. Sebdenia rubra  
  26(25)   Thallus with internal filaments both perpendicular and parallel cortex; carpogonia and auxiliary cells borne on specialized filamentous structures Kallymenia latiloba  
  26(25)   Thallus with majority of internal filaments perpendicular to cortex; carpogonia and auxiliary cells not borne on specialized filamentous structures Halymenia  
    a Medullary filaments 6–8 µm diam. Halymenia refugiensis  
    a Medullary filaments greater than 10 µm diam. b  
    b Medullary filaments 10–14 µm diam. Halymenia abyssicola  
    b Medullary filaments 14–24(–40) µm diam. Halymenia porphyroides  

Wiry (often coarsely branched)

  27(4)    Growth initiated by one apical cell at tip of branch 28  
  27(4)   Growth initiated by two apical cells or series of cells at tip of branch 30  
  28(27)   Rhizines absent Gelidiella trinitatensis  
  28(27)   Rhizines present (small dark staining, thick-walled filaments intermixed with medullary cells) 29  
  29(28)   Branching absent or rare; tetrasporangia 16–18 µm diam.; cystocarps with one chamber Pterocladiella caerulescens  
  29(28)   Branching common; tetrasporangia 25–40 µm diam.; cystocarps with two chambers each Gelidium  
    a Branching abundant Gelidium pusillum  
    a Branching sparse Gelidium pusillum var. pacificum  
  30(27)    Medulla filaments less than 10 µm diam.; cortical cells radially (anticlinally) arranged Ahnfeltiopsis pygmaea  
  30(27)   Medulla cells greater than 10 µm diam.; cortical cells not anticlinal 31  
  31(30)   Central medullary cells generally largest; tetrasporangia zonately divided Wurdemannia miniata  
  31(30)   Central medullary cells often smaller than outer medullary cells; tetrasporangia tetrahedrally divided Gelidiopsis  
    a Erect axes cylindrical Gelidiopsis variabilis  
    a Erect axes slightly compressed to flattened b  
    b Erect axes slightly compressed, 230–310 µm diam. Gelidiopsis intricata  
    b Erect axes compressed to flattened, 500–1,100 µm diam. Gelidiopsis planicaulis  

Firm, Crisp

  32(5)   Apices covered with deciduous (soon dropping off), nonpigmented filaments (trichoblasts) 33  
  32(5)   Apices not covered by non-pigmented filaments Hypnea  
    Branches 1–3 mm diam. Hypnea pannosa  
    a Branches less than 1 mm diam. b  
    b Branchlets lightly upcurved; tetrasporangial sori girdling middle part of lateral branchlet Hypnea spinella  
    b Branchlets generally straight; tetrasporangial sori as pad near tip of lateral branchlet. Hypnea nidulans  
  33(32)   Branches cylindrical; branchlets with uniformly abundant spur-like spines Acanthophora spicifera  
  33(32)   Branches compressed or flattened; branchlets without sharp spur-like spines. 34  
  34(33)    Ultimate branches generally constricted at base; apices either with terminal pit or pointed Chondria platyclada  
  34(33)   Ultimate branches generally not constricted at base; apical cells always sunken in terminal pit Osmundea  
    a Branching consistently opposite or slightly alternate, in one plane Osmundea splendens  
    a Branching irregularly opposite to alternate, not in one plane Osmundea sinicola  

Flat, Leathery

  35(6)   Blades strap-shaped, 2–10 mm wide; branching dichotomous; apices bluntly rounded Gracilaria brevis  
  35(6)   Main axes 1–2 mm wide; branching alternate in sets forming zigzag pattern; apices bluntly pointed Plocamium  
    a Thallus 4–30 cm high; branchlets straight or outwardly curved Plocamium cartilagineum subsp. pacificum  
    a Thallus 4–5 cm high; branchlets straight or inwardly curved Plocamium violaceum  

Jointed Calcareous

  36(7)   Segments obscure, moderately to lightly calcified, tough, often (but not always) with surface hair-like filaments 37  
  36(7)    Segments distinct, heavily calcified, hard, brittle. 39  
  37(36)   Distal branches smooth, without hair-like filaments 38  
  37(36)   Branches uniformly covered with hair-like filaments Galaxaura  
    a Thallus less than 5 cm high, soft; branching irregularly alternate Galaxaura filamentosa  
    a Thallus greater than 5 cm high, stiff; branching irregularly dichotomous Galaxaura rugosa  
  38(37)   Blades thick but flat Dichotomaria marginata  
  38(37)   Branches cylindrical Tricleocarpa fragilis  
  39(36)   Thallus segments more equal in length; conceptacles terminal or in axis of developing dichotomy Jania capillacea  
  39(36)    Thallus segments varied in length; conceptacles scattered on surface Amphiroa  
    a Mature branches cylindrical or slightly flattened, less than 2 mm diam. b  
    a Mature branches flattened, greater than 2 mm diam. f  
    Segments 1–2 mm diam. d  
    b Segments less than 1 mm diam. c  
    c Lateral fusions with adjacent segments common Amphiroa franciscana  
    c Lateral fusions absent Amphiroa valonioides  
    d Thallus more recumbent; conceptacles 400–500 µm diam. Amphiroa zonata  
    d Thallus generally erect; conceptacles 200–300 µm diam. e  
    e Joints well above dichotomies, often concealed by heavy calcification Amphiroa magdalenensis  
    e Joints at or just above dichotomies, generally not concealed by heavy calcification Amphiroa subcylindrica  
    Segments mainly strap-shaped g  
    Segments irregularly shaped h  
    g Segments short 2.5–4 mm long Amphiroa brevianceps  
    Segments longer 3–7 mm long Amphiroa dimorpha  
    h Terminal branchlets often finger-like Amphiroa foliacea  
    h Terminal branchlets not finger-like i  
    i Thallus to 1 cm long Amphiroa crosslandii  
    i Thallus to 4 cm long Amphiroa misakiensis  
  40(7)   Roof of conceptacles with one pore 41  
  40(7)   Roof of conceptacles with multiple pores Lithothamnion  
    a Forming highly branched heads; branches < 1.5 mm diam. Lithothamnion australe  
    a Crust or rhodoliths generally with short thicks knobs Lithothamnion indicum  
  41(40)   Not dark to medium red-brown, not bright white when broken 42  
  41(40)   Dark to medium red-brown, bright white when broken or chipped Sporolithon howei  
  42(41)   Trichocytes (unusually large cells) present 43  
  42(41)   Trichocytes not present Lithophyllum  
    a Extensive crust, generally on rock; surface with short, shiny glazed, deep rose red knobs, tips lighter Lithophyllum imitans  
    a Not as above b  
    b Generally forming knobby crusts, occasionally as rhodoliths c  
    b Generally forming branched rhodoliths (spherical free-living spheres) or attached heads d  
    c Knobs slender to stout, to 1 mm diam., to 8 mm high Lithophyllum brachiatum  
    c Knobs stout, to 3 mm diam., to 5 mm high Lithophyllum alternans  
    d Branching mostly dichotomous, generally not fusing with adjacent branches Lithophyllum divaricatum  
    d Branching irregular, generally fusing with adjacent branches Lithophyllum kotschyanum  
  43(42)   Surface bright pale lavender to deep purple Hydrolithon reinboldii  
  43(42)   Surface pink-gray 44  
  44(43)   Trichocytes large, 18-22 µm diam.; conceptacles large, to 500 µm diam. Fosliella fertilis  
  44(43)   Trichocytes smaller, 9-19 µm diam.; conceptacles smaller, 175-315 µm diam. Porolithon  
    a Margins lightly colored, occasionally having many anastomosing raised ridges Porolithon sonorense  
    a Margins not lightly colored, generally a flat crust Porolithon onkodes  

Phaeophyceae Brown Algae

Note: keys designed only for the species represented herein
  1   Thallus exclusively filamentous, generally in tufts or turfs FILAMENTOUS, 4  
  1   Thallus not filamentous 2  
  2(1)   Thallus hollow, saccate or coarsely branched (cylindrical branches and branchlets) SACCATE OR COARSELY RANCHED, 6  
  2(1)   Thallus flat, sheet-like 3  
  3(2)   Thallus crust-like or cluster of fan-shaped blades CRUSTOSE OR FAN-SHAPED BLADES, 7  
  3(2)    Thallus of strap-shaped blades STRAP-LIKE BLADES, 8  


  4(1)    Filaments multiseriate (more than one row of cells) Sphacelaria rigidula  
  4(1)   Filaments uniseriate (one row of cells) 5  
  5(4)   Thallus to 1.5 mm high; filaments 10–18 µm diam. Kuetzingiella elachistaeformis  
  5(4)   Thallus to 5 cm high; filaments 20–34 µm diam. Feldmannia indica  

Saccate or Coarsely Branched

  6(2)   Thallus coarsely branched, not saccate Chnoospora pannosa  
  6(2)   Thallus saccate, hollow Colpomenia  
    a. Thallus sack-like, often lobed b  
    a Thallus with short stubby cylindrical branches c  
    b Thallus medium to dark shiny brown Colpomenia peregrina  
    b Thallus light dull golden-brown Colpomenia sinuosa  
    c Thallus less than 1.5 cm high; surface membrane 50–100 µm diam. Colpomenia mollis  
    c Thallus to 3 cm high; surface membrane 400–500 µm diam. Colpomenia ramosa  

Crust-like or Fan-shaped Blades

  7(3)    Apical margin flat, not rolled, often crust-like. Lobophora cf. papenfussii  
  7(3)   Apical margin rolled Padina  
    a Thallus small; less than 4 cm high Padina crispata  
    a Thallus greater than 4 cm high b  
    Thallus thin; 3, occasionally 4 cells layers thick Padina fraseri  
    b Thallus thicker; generally greater than 4 layers thick c  
    Thallus to 40 cm high, upright, with distinct stalk Padina durvillaei  
    c Thallus to 10 cm high, low-lying, with matted base Padina concrescens  

Strap-like Blades

  8(3)   Blades without distinct midrib 9  
  8(3)   Blades with distinct midrib Dictyopteris  
    a Blades to 4 mm wide, often creeping Dictyopteris repens  
    a Blades to 20 mm wide, upright Dictyopteris cf. plagiogramma  
  9   Branches indistinctly forked, greater than 1 cm wide Spatoglossum ecuadoreanum  
  9   Branches distinctly forked, generally less than 1 cm wide Dictyota  
    a Thallus basal stalk upright; parallel to substrate toward apices Dictyota stolonifera  
    a Thallus not as above b  
    b Thallus delicate, creeping c  
    b Thallus upright d  
    c Thallus dark yellow-brown, adhering to hard substrate, overlapping, often becoming attached to one another Dictyota concrescens  
    c Thallus light brown to brilliant iridescent blue, creeping over various substrates, not attaching to one another Dictyota humifusa  
    d Thallus distinctly horizontally striped; blades to 15 mm wide; medullary cells to 130 µm thick Dictyota major  
    d Thallus without stripes; blades less than 8 mm wide; medullary cells less than 60 µm thick e  
    e Surface cells to 10 µm thick, 20–30 µm wide Dictyota flabellata  
    e Surface cells 10–15 µm thick, 40–50 µm wide Dictyota binghamiae  

Chlorophyta Green Algae

note: keys designed only for the species represented herein
  1   Plants filamentous throughout FILAMENTOUS, 4  
  1   Plants not filamentous 2  
  2(1)   Plants thin or gelatinous membranes or crusts (mesh-like, tube-like, parasol-shaped or sheet-like) MEMBRANES AND CRUST, 7  
  2(1)   Plants other than membrane-like 3  
  3   Plants not calcified COURSELY BRANCHED, NON-CALCAROUS, 10  
  3   Plants calcified [often brittle, white or whitish green in color; bubbles form when exposed to vinegar or hydrochloric acid (1% solution)] CALCAREOUS, 11  


  4   Thallus constricted at base of branches or branchlets; coenocytic 5  
  4   Thallus not constricted at base of branching; not coenocytic 6  
  5   Siphons repeatedly dichotomous or trichotomously branched Chlorodesmis caespitosa  
  5   Siphons feather-like, oppositely branched Bryopsis  
    a Lateral branchlets 30–65 µm diam. Bryopsis salvadoreana  
    a Lateral branchlets 75–150 µm diam. Bryopsis pennata  
  6   Filaments unbranched; 95–140 µm diam. Rhizoclonium rhizophilum  
  6   Filaments unilaterally branched, 375–560 µm diam. Cladophora herpestica  

Membranes and crusts

  7   Thallus other than mesh-like network 8
  7   Thallus formed by branchlets fusing to form mesh-like network Phyllodictyon  
    a Thallus sparse; mesh-like network obvious with many attaching hapteroid cells Phyllodictyon anastomosans  
    a Thallus crowded pad-like cushion; mesh-like network not obvious with few attaching hapteroid cells Phyllodictyon robustum  
  8   Thallus small, solitary, parasol-shaped (delicate stipe with terminal whorl of rays), 1–3(–5) mm high Parvocaulis parvulus  
  8   Thallus not as above 9  
  9   Thallus as small solid 2–3 cell layers thick disc-like epiphytic crust Ulvella lens  
  9   Thallus tube-like, often in clusters Ulva paradoxa  

Coursely Branched, Non-Calcareous

  10   Surface stolons lacking or obscure; siphons without trabeculae [tough, spongy, branched or as prostrate, mats; composed of complex siphons with cortex of tightly packed utricles (swollen siphon apices)] Codium  
    a Branches 3–8 mm diam.; utricle apices rough, rugose Codium foveolatum  
    a Branches 1.5–3 mm diam.; utricle apices smooth Codium isabelae  
  10   Stoloniferous, with obvious surface or shallow subsurface runners; siphons with microscopic internal transverse supports or trabeculae [highly variable genus with many forms] Caulerpa  
    a Fronds feather-like Caulerpa sertularioides  
    a Fronds other than feather-like b  
    b Fronds flat with serrulate or toothed margins Caulerpa serrulate  
    b Fronds without toothed margins c  
    c Branchlets spherical to slightly mushroom-shaped; margins smooth Caulerpa macrophysa  
    c Branchlets flattened discs; margins often slightly lobed or scalloped Caulerpa nummularia  


  11   Thallus not or very slightly calcareous, without heavily calcified segments 2  
  11   Thallus calcareous, calcified segments connected by uncalcified joints Halimeda  
    a Segments to 2 cm wide, 1.3 cm long, 300—600 µm thick; surface utricle walls fusing for 5–10 µm; subsurface utricles not exceptionally large (40–120 µm long) Halimeda cf. tuna  
    a Segments to 4 cm wide, 3 cm long, 0.7–1.4 mm thick; surface utricles walls not fusing; subsurface utricles exceptionally large (100–350 µm long) Halimeda cf. discoidea  

Cyanobacteria Blue Green Algae (Cyanophyta)

key designed only for the species represented herein

  1   Heterocysts (specialized cells, different in size, with denser pigmentation and thicker cell wall than vegetative cells) absent 2  
  1   Heterocysts present Nostoc caeruleum  
  2   Trichomes (in Cyanobacteria defined as chain of cells without the enveloping sheath); not tightly coiled 3  
  2   Trichomes coiled Spirulina  
    a Trichomes in tight spiral Spirulina versicolor  
    a Trichomes in loose spiral or consistently wavy Spirulina breviarticulata  
  3   Trichomes with sheath present 4  
  3   Trichomes without visible sheath or gelatinous matrix Oscillatoria  
    a Trichomes greater than 15 µm diam. b  
    a Trichomes less than 15 µm diam. c  
    b Trichomes loosely twisted Oscillatoria bonnemaisonii  
    b Trichomes straight or curved, not twisted in figure “8’s” Oscillatoria margaritifera  
    c Trichomes 8–10 µm diam. Oscillatoria subsalsa  
    c Trichomes generally less than 8 µm diam. d  
    d Apical cells very sharply pointed Oscillatoria acuminata  
    d Apical cells taper to blunt point e  
    e Cells mostly longer than wide Oscillatoria formosa  
    e Cells mostly wider than long Oscillatoria okenii  
  4   Multiple trichomes obvious within sheaths Hydrocoleus holdenii  
  4   Generally single trichome within sheath 5  
  5   Trichomes with false branching (forming erect wick-like bundles) Symploca hydnoides  
  5   Trichomes without false branching 6  
  6   Sheaths distinct, not fusing, not coalescing Lyngbya majuscula  
  6   Sheaths eventually fusing, coalescing, blending together Phormidium  
    a Cells less than 4 µm long Phormidium subfuscum  
    a Cells greater than 4 µm long b  
    b End cells rounded, without thickening or cap Phormidium retzii  
    b End cells cone-shaped, seldom rounded or flat, with distinct terminal thickening c  
    c Thallus forming mucilaginous soft tufts Phormidium corium var. capitatum  
    c Thallus forming mucilaginous finger-like masses Phormidium laysanense