DISCLAIMER: These keys and species identities are provided for informational purposes only. We can take no responsibility for any consequences of misinterpreted information presented here. Always consult your local algae specialist if you have questions.


Key to Common Genera of Hawaiian Rhodophyta

    The 64 genera listed in this key represent about 50% of the red algal genera in the Hawaiian Islands. Some key lines end in a specific name which indicates that there is only one species in the genus or that a certain species is different from the remainder in that genus and can be isolated by itself. The number in parentheses that follows genus names is the number of species recognized for that genus at this time in Hawai’i. There are several genera that are followed by "(1 +?)" in the key; this notation indicates that at least one species is recognized for that genus in Hawaii. It is unclear at this time, how many more might be in our flora. Isabella Abbott, January 1997.

1 Plants gelatinous (slippery; hard to pick up) 2
1 Plants not gelatinous (may be soft, but not slippery) 9
2 Plants calcified (whitened or greyish) under mucus-like material 3
2 Plants not calcified 5
3 Plants with three or more orders of branching 4
3 Plants blade-like with fringed or dissected margins Titanophora pikeanum
4 Calcification mostly around axes and ultimate branches; when fresh, mucus 1-2 mm in diam; early Spring annual Liagora orientalis (see under plants not gelatinous for other species of Liagora)
4 Calcification continuous around axes; when fresh, mucus 2-8 mm in diam; late Spring annual Trichogloea (2)
5 Plants 8-28 cm long, branching orders 1-3, long, slender, round and tapering; shallow subtidal Helminthocladia (2)
5 Plants mucosoid paddles, branched clustered blades, or repeatedly divided axes, subtidal 6
6 Plants with stem-like stipe bearing blades that are confluent or paddle shaped clusters; cortical filaments extend beyond gelatinous material Gibsmithia (2)
6 Plants lacking stipes 7
7 Plants bladelike, with soft, pointed or blunt projections from a gelatinous surface; auxiliary cell conspicuous before fertilization because of numerous nutritive cells Predaea (2)
7 Plants not bladelike but with repeatedly divided axes 8
8 Central axes usually several times wider than laterals, spermatangia in terminal clusters; carposporophyte terminating a special fertile branchlet Acrosymphyton taylorii
8 Central axes and laterals nearly the same diameter; spermatangia in lateral clusters; carposporophyte in intercalary position on special fertile branchlet Dudresnaya (2)
9 Calcification not associated with gelatinous material; plants stony, some rigid, crustose, others with "sandy" or mealy surfaces (all producing bubbles in weak acid) 10
9 Plants neither gelatinous nor calcified 29
10 Plants mostly flattened, crustose, crust tightly adherent; if having erect portion, this usually composed of protuberances, not regular branches 11
10 Plants erect, branching pattern subdichotomous, or pinnate, with or without adherent crustose base 19
11 Crust difficult to remove from old coral, stones, pebble, other algae; usually lavender to purple, pinkish to white where exposed to sunlight; tetrasporangia zonately divided 12
11 Crust less difficult to remove from dead coral or rock owing to free margins; dark maroon, gold to orange; calcification usually limited to ventral surface; whole plant relatively easy to remove with weak acid; tetrasporangia cruciately divided Peyssonnelia (3)
12 Tetrasporangial plants producing cruciate tetrasporangia housed singly in compartments that may be isolated or grouped into soral arrangements; vegetative filaments lined both by lateral cell fusions and secondary pit-connections Sporolithon ptychoides
12 Tetrasporangial plants producing zonate tetrasporangia housed in groups within conceptacles; vegetative filaments linked either by lateral cell fusions or by secondary pit-connections (but not both) or not linked 13
13 Vegetative plant body endophytic within species of Jania or other geniculate coralline algae; only conceptacles protrude above host tissue Choreonema thuretii
13 Vegetative plant body conspicuous, growing on rock, algae, molluscs or other substances, not endophytic 14
14 Tetrasporangial/bisporangial plants having conceptacles with a number of pores in the roof (i.e., roofs multiporate) 15
14 Tetrasporangial/bisporangial plants having conceptacles with a single pore in the roof (i.e., roofs uniporate) 16
15 Surface layer of vegetative cells, when seen in cross-section, with rounded or flattened outer walls that are not prolonged into points at the upper corners Lithothamnion (1 +?)
15 Surface layer of vegetative cells when seen in cross-section with rounded or flattened outer walls that are not prolonged into points at the upper corners Mesophyllum (1 +?)
16 Cells of adjacent vegetative filaments linked by lateral cell fusions but not secondary pit- connections 17
16 Cells of adjacent vegetative filaments linked by secondary pit-connections but not by lateral cell fusions 18
17 Pores of tetrasporangial/bisporangial conceptacles having a conspicuous circle of elongate cells like gate posts around the pore canal Hydrolithon (4) (including Porolithon gardineri and P. onkodes)
17 Pores of tetrasporangial/bisporangial conceptacles lacking a conspicuous circle of elongate cells like gate posts around the pore canal; pore canal lined by cells whose tips more or less protrude into the pore canal Neogoniolithon (1 +?) (including N.brassicas-florida and its synonym N. fosliei)
18 Plants with flat branches occurring in storied layers, with markings resembling scrolling; internally each branch made up of a layer of vertically elongate cells 5- 6 times longer than wide and a 'discontinuous' surface layer of very small cells, with one small cell above each elongate cell Lithophyllum prototypum (includes Titanoderma tessellatum as a synonym)
18 Plants varying in growth-form but not made up of flat branches occurring in storied layers; internally with or without a basal layer of vertically elongate cells Lithophyllum (1 +?).(includes L. ganeopsis, L. insipidum, L. kotschyanum, L. pallescens, L. punctatum)
19 Erect, articulated (=jointed), with or without a crustose or disklike base; calcification embedded in cell walls, thus difficult to remove; reproductive structures in conceptacles 20
19 Erect, not articulated, without a crustose or disklike base; calcification around filaments or tissues, thus easily removed with weak acid; reproductive structures not in conceptacles 24
20 More than 4 cm tall, fronds clustered together, relatively coarse (segments more than 1 mm wide), epilithic (growing on rocks) 21
20 Plants less than 4 cm tall, tufted, some principally epiphytic (growing on plants), others with species epiphytic or epilithic 23
21 Branching dichotomous, conceptacles deeply embedded and inconspicuous Arthrocardia sp.
21 Branching pinnate, conceptacles protruding and conspicuous 22
22 Usually only 1 or 2 laterals from axial segments; tetrasporangial conceptacles containing +/- 30 sporangia Corallina elongata
22 Two laterals plus 1-3 adventitious branches from each axial segment; tetrasporangial conceptacles containing ca 10 sporangia Haliptilon subulatum
23 Plants frequently growing in mats on rocks; conceptacles on intergenicula, making surfaces lumpy Amphiroa (4)
23 Plants usually epiphytic on coarse algae, growing in tufts; branching dichotomous; conceptacles terminating axial intergenicula (segments) Jania (4)
24 Plants with smooth calcified surfaces 25
24 Plants with roughened, hairy, or chalky surfaces 26
25 Plants orange, yellow or dark maroon, branches frequently furrowed longitudinally; ends of branches pointed Stenopeltis (3)
25 Plants tan to coral colored, losing color rapidly, turning greenish grey, surface often appearing obscurely ringed, ends of branches cylindrical, inner tissue often protruding Tricleocarpa (2)
26 Under low magnification, surfaces of plants often chalky or mealy; mostly irregularly dichotomous, but occasionally pinnately divided 27
26 Under low magnification, surface appears like a fine pinkish net with a dusting of flour; branching variable with branches within a range of 1.5 to 25 mm in diam Ganonema farinosum
27 After decalcification, a colorless medulla observed from which an external layer of cortical cells is produced radially; surfaces frequently with colored, matted "hairs" 28
27 After decalcification, a colorless medulla observed bearing pigmented branched filaments up to 20 cells long, surfaces without matted hairs Liagora (12) (see also under gelatinous plants)
28 Cortical layer composed of inner colorless irregular cells and outer pigmented cells, the latter usually bearing pigmented "epidermal" cells or "hairs" Galaxaura (6)
28 Cortical layer containing rounded to pillow-like cells throughout; stiff, pigmented "hairs" penetrating calcification in rings or bands Actinotrichia fragilis
29 Plants opaque, multiaxial, branches and axes smooth and elongate or divided into ovals like branched chains; in cross section cortical layers separated from medullary filaments, cortical cells forming closely packed cell layers; subtidal only Scinaia (2)
29 Plants not opaque, firm in texture, often cartilaginous when dried; uniaxial or multiaxial; cortical layers grading imperceptibly into medulla 30
30 Plants terete (axes and branches cylindrical) or slightly flattened (compressed) with single apical cell often seen in surface view of young axes and branches (uniaxial) 31
30 Plants terete, compressed, or flattened, with more than one apical cell (multiaxial); axes best seen in a cross-section about 1 mm behind apices 36
31 Plants consisting of single conspicuous axial filament with or without branches constructed in similar way (filamentous) 43
31 Plants with single axial filament not conspicuous; internal branches from axes cohering to form a pseudoparenchymatous cortex not obviously filamentous 32
32 Branching pattern irregular or radial 33
32 Branching pattern pinnate (opposite or alternate) in one plane 34
33 Branching pattern irregular; branches often spine-like, long and short branchlets (spines) along axes, ends of branches pointed, or hooklike Hypnea (6)
33 Branching pattern radial; branches not spine-like but soft and fluffy; erect axes arising from creeping rhizomes Asparagopsis taxiformis
34 Plants bright scarlet with several orders of branching, the branchlets feathery 35
34 Plants usually maroon to dark red, pyramidal in shape, with no more than three orders of branching, the branchlets strap-like Gelidium / Pterocladiella (6+4)
35 Last order of branching on one side of branch, the branchlets arranged like teeth of a comb Plocamium sandvicense
35 Last order of branching on both sides of branch, these frequently claw-like; terminal branches bent over on themselves Portieria hornemannii
36 Plants strap-like or pinnate, with medulla consisting of periclinally directed irregularly arranged filaments joined to a subcortex of colorless stellate cells and 4-8 pigmented rows of small cortical cells Grateloupia (3)
36 Plants bladelike (=flattened) or fleshy and erect, variously branched 37
37 In cross-section of erect or fleshy plants, medulla consisting of large rounded, colorless cells (pseudoparenchyma) 38
37 In cross-section of blades, medulla consisting of colorless filaments 41
38 Medullary layers restricted to lining of hollow segments or vesicles 39
38 Medullary layers composing bulk of cross-section 40
39 Vesicles resembling spherical to oval grapes, one to many on short stalks from erect, elongate stipes; medullary cells frequently with secretory cells along cavity Botryocladia skottsbergii
39 Segments tubular, terete or compressed, internally divided by one-layered septa; longitudinal filaments inward of medullary cells bearing small secretory cells adjacent to the cavity Champia (2)
40 Erect, fleshy to compressed, cells of medulla very large some more than 200 Ám in diam.; cystocarps conspicuous as protruding external bumps; spermatangia borne in species- specific internal arrangements Gracilaria (9)
40 Erect, wiry, elongate, most branching near apices where plants are usually bleached yellow; plants usually in continuous high-intertidal bands on basalt rock; cells of medulla small but densely packed; all reproductive structures internal and inconspicuous Ahnfeltiopsis (3)
41 Blades with simple, dissected, fringed, or ruffled margins; in cross-sections, medullary filaments typically cross the central space anticlinally from cortex to cortex; female reproductive structures borne in basket-like structures (ampullae) Halymenia (5)
41 Blades with simple or undulate margins; female reproductive structures not in ampullae 42
42 Plants subtidal only, 5-10 m depths, dark crimson, blades undulate, spreading, slippery; medullary filaments few, arranged randomly; attached by thickened lower edges Kallymenia sessilis
42 Plants high intertidal only; dark green to brownish-red, margins plane or ruffled; two cells thick in cross-section except where fertile; attached by thickened holdfast made up of rhizoidal extensions from lower cells Porphyra vietnamensis
43 Plants hair-like in width of primary axis (filamentous), with branches of nearly same width; vegetative cells mostly monosiphonous (one main axis, uniseriate) Ceramiaceae.. 44
43 Plants with primary axial cells forming one or more cell layers centripetally (polysiphonous; an initial single axis surrounded by tiers of cells (pericentral cells) of the same length, these in turn forming other cells in many species

Delesseriaceae / Rhodomelaceae.. 53

44 Plants dichotomously divided with lateral elongate branches alternate or pinnate in one plane, but central axis retaining its shape and dominance Aglaothamnion (2)
44 Plants irregularly dichotomously divided, laterals not alternate or pinnate so central axis not dominant 45
45 Prostrate portions anchored in tissue of other algae, erect axes from dorsal side of prostrate axes; reproductive structures lateral or terminating lateral branches Tiffaniella / Ptilothamnion (1+1)
45 Plants not divided between prostrate and erect portions, but divided throughout 46
46 Axial cells relatively large, often visible with the unaided eye 47
46 Axial cells relatively small, visible with low magnification 48
47 Tetrasporangial and spermatangial heads naked, borne singly on distinct one-celled pedicels Anotrichium (2)
47 Tetrasporangial and spermatangial heads surrounded by clusters of modified cells (involucres) Griffithsia (4)
48 Whorl-branchlets 2-4 per axial cell 49
48 Whorl-branchlets represented by periaxial cells forming cortication at each segment of axial cells 51
49 Whorl-branchlets 2 per axial cell; gland cells usually spanning two adjacent cells; branchlets of erect axes opposite each other Antithamnion (4)
49 Whorl-branchlets 3 or 4 per axial cell 50
50 Whorl-branchlets 3 per axial cell; plants with fused laterals forming a spongy network, common under ledges and shady places, mostly subtidal; tetrasporangia without involucres Haloplegma duperreyi
50 Whorl-branchlets 4 per axial cell; plants silky, plumose, in low indertidal; all reproductive structures with involucres Wrangelia (2)
51 Plants divided into naked internodes and corticated nodes; cortical (periaxial) cells forming centrifugally, in turn these forming secondary filaments in acropetal (toward the apex) or basipetal (toward the base) directions Ceramium (19)
51 Plants having axial internodes covered nearly continuously with acropetal and basipetal filament 52
52 Terminal branches frequently pincer-like; acropetal and basipetal filaments in straight longitudinal lines on axes and laterals Centroceras (3)
52 Terminal branches usually straight; only axial internodes corticated throughout; lateral branches corticated only at nodes Spyridia filamentosa
53 Plants initially uniaxial, some subsequently losing strictly apical growth and exhibiting marginal meristems; cell divisions in central axial filament forming four pericentral cells; all reproductive structures in sori Delesseriaceae.. 55
53 Plants retaining uniaxial nature even when this is obscure on maturation, axes monopodial or sympodial, branching radially, bilaterally or dorsiventrally organized 54
54 Axes sympodial in which initial apical cell is displaced by a lateral below it, which in turn is also displaced by others, giving zig-zag axis seen best near apex; pigmented, monosiphonous filaments in clusters covering plant; tetrasporangia and spermatangia in stichidia (specialized reproductive branches) Dasyaceae… 58
54 Axes monopodial, apical filaments persistent even though plants may assume large and complex morphologies; non- pigmented monosiphonous trichoblasts present in most genera, tetrasporangia borne in stichidia or in main and lateral branches Rhodomelaceae..   59
55 Plants 5 cells wide, flattened in those places, otherwise terete and polysiphonous, erect branches ending in 2-5 conspicuous, colorless terminal hairs Taenioma perpusillum
55 Plants more than 5 cells wide, blade-like 56
56 Forming small, delicate mostly lanceolate blades one to several cells thick at midribs and veins, one-cell thick near margins; commonly epiphytic on deep-water algae Hypoglossum (6)
56 Forming rounded clusters of iridescent blades, without midribs; some parts of blades netlike; reef crest or subtidal 57
57 Meshes of nets delicate, usually alternating with smooth portions near terminal margins of blades Martensia fragilis
57 Meshes of nets coarse, frequently occupying entire blade except for basal attaching region Neomartensia flabelliformis
58 Plants rose-colored epiphytes with fluffy laterals, or erect, wine-colored, with substantial stipe on rocks; most species corticated Dasya (5)
58 Plants pink with incurved laterals, the basal one straight, unbranched; uncorticated; ubiquitous on deep-water algae Heterosiphonia crispella
59 Fronds filamentous 60
59 Fronds fleshy, cartilaginous, or with leaflike blades clustered around branching "stems" 61
60 Filaments entirely erect, or prostrate and erect, with 4 to 16 pericentral cells; most Hawaiian species uncorticated; trichoblasts or their scar cells mostly on a 1/4 spiral pattern; gametophytes and tetrasporophytes usually needed for species recognition Polysiphonia (35)
60 Filaments mostly prostrate and creeping; erect determinate and indeterminate branches in a consistent pattern of occurrence one to the other; trichoblasts usually limited to terminal cells; tetrasporophytes needed for species recognition Herposiphonia (11)
61 Fronds fleshy from base to top; mostly erect 62
61 Fronds with inflexible, somewhat leathery to "woody" stipes (stems), branched or unbranched, with clusters or rosettes of dark lanceolate "leaves" having dissected margins Melanamansia (2)
62 Plants short separate bushes, branched from all sides, firm, many cartilaginous when dried, bases fibrous or with large coalescing holdfasts; ultimate branchlets mostly short, knobby, in paniculate or radially branched patterns; among most colorful of red algae in the field: tan, green, rose, pink, dark brown; very common in intertidal, few species in subtidal depths Laurencia (17)
62 Plants in inconspicuous masses or clumps 63
63 With few (2-3) orders of branching; ultimate branchlets as short spines alternating or radially attached; intertidal species rather uniformly dark red to chocolate brown; subtidal species more flattened, bluish iridescent, losing pigments quickly and turning a salmon color Acanthophora (2)
63 With 3-4 orders of branching; branches soft, shortened, with acutely pointed apices, with or without colorless hairs (trichoblasts); dark purple to brown; abundant epiphyte in certain localities, lacking in others Tolypiocladia glomerulata

Back to  Reef   Algae