Brown seaweeds key

How to use the key


Thallus forms crusts on rock or other substratum 2

Thallus different3


This group includes several genera, and species are difficult to identify. Two species are relatively easy to identify:

Pseudoralfsia verrucosa: brown crusts with a rounded outline and light edge, common in rock pools.

“Aglaozonia”: relatively loosly attached to the substratum, rounded lobes at the edge.
The sporophytic stage of Cutleria multifida


Thallus either like a button (flat disc) or with an irregularly spherical outline 4

Thallus different 6


Vegetative thallus like a button (flat disc), intertidal, long dichotomously divided receptacles when fertile Himanthalia elongata

Thallus with a irregularly spherical outline 5


Thallus becomes hollow, parenchymatic tissue, surface covered with small cortex cells Colpomenia peregrina

Thallus with pseudoparenchymatic tissue (rows of cells sliding away from each other under pressure), surface covered with 2-5 celled paraphyses Leathesia marina


Uniseriate and branched, but axes so strongly intertwined that they appear as compact axes (“dreadlocks”). Short, hook-formed branches Spongonema tomentosum

Thallus different 7


Thallus small (often < 5 cm), having narrow and compact axes, often flattened with an indistinct mid-rib, irregularly branched. Individuals grow intertwined in turf high in the intertidal at special localities, rare Fucus cottonii

Thallus different 8


Thallus like unbranched axis, sometimes with hairs (which may be pigmented) or very short side branches 9

Thallus different 14


Thallus small (< 10 cm) and epiphytic 10

Thallus large (> 20 cm and up to > 1 m) 13


Erect part of thallus like uniseriate small (max 2 cm) axes, growing together in brush-like tufts epiphytically on Fucus Elachista fucicola

Mature thallus pluriseriate 11


Thallus covered with short side branches and hairs Myriotrichia clavaeformis

Thallus not with short side branches 12


Thallus pluriseriate and with hairs Litosiphon laminariae

Thallus pluriseriate and lacking hairs Pogotrichum filiforme


Long thallus > 1 m in summer, colourless hairs, old individuals hollow Chorda filum

Short pigmented assimilation hairs starting some distance from the base, only in spring in South Norway Halosiphon tomentosus


Thallus hollow 15

Thallus not hollow (or only in part of axes in old individuals) 20


Hollow in most parts, branching from main axes (often oppositely branched) Striaria attenuata

Thallus not branched 16


Intertidal, mature individuals with regular constrictions, several thalli originating from a common attachment point, spring (small individuals may appear compact) 17

Mature thallus hollow and inflated but not with regular constrictions 18


Up to 50 cm tall, plurilocular gametangia (uniseriate) when fertile but lack paraphyses, outer cortex layer 2-3 cells, medulla 4 cells thick Scytosiphon lomentaria

Up to 35 cm tall, plurilocular gametangia and unicellular paraphyses when fertile,
outer cortex layer 2 cells, medulla 2-3 cells thick Scytosiphon promiscuus

(In addition an unknown Scytosiphon taxon has been recorded on the southwest coast of Norway. For proper identification of species sequencing is advised)


Thallus hollow but dorsiventrally flattened Asperococcus ensiformis

Thallus not dorsiventrally flattened 19


Thallus elongated and up to 30 cm long, inflated and bulbous, summer Asperococcus bullosus

Smaller, thalli like narrow tubes, often epiphytic Asperocuccus fistulosus


Thallus flat and branched or with a blade-formed part 21

Thallus with round or rounded axis in cross section and branched 44


Thallus like an unbranched blade or with a stipe carrying a blade-formed part which may be divided (like fingers on a hand) 22

Thallus with flat or flattened branches 33


Thallus big and having a conspicuous stipe and blade, the blade can be plain flat, divided like fingers on a hand or have a mid-rib 23

Thallus smaller (< 20 cm) and with a small or inconspicious stipe 29


Lamina divided in broad parts, like fingers on a hand 24

Lamina not divided 26


Thallus with a round (or rounded) stipe, attached by branched hapteron 25

Lamina with hair groups, thallus with a broad, flat, twisted or frilled stipe and unbranched, knobby (in large individuals) holdfas Saccorhiza polyschides


Thallus with stiff and long stipe, subtidally and wave-exposed Laminaria hyperborea

Thallus with a flexible stipe, forming a zone in the upper subtidal, wave-exposed Laminaria digitata


Long and narrow lamina with mid-rib Alaria esculenta

Lamina without conspicuous mid-rib 27


Lamina with very frilled or wavy edge and often undulated middle parts Saccharina latissima

Lamina plain and flat without frills or undulations 28


Plain lamina with hair groups, North Norway only Saccorhiza dermatodea

Big and non-sectioned flat lamina: forms of [speceis]Laminaria hyperborea[/species] and L. digitata which develop at sites with little water movement from waves and currents


Attached by a small disc 30

Attached by a small hapteron: juv. Laminaria sp. or Saccharina latissima


A small “ring” at the bottom of the stipe, hair groups on lamina: juvenile Saccorhiza polyschides

Not a small “ring” at the bottom of the stipe, very short stipe 31


Thallus leaf-formed and narrow, growing in bundles, cross section shows small cortex cells and elongated inner cells, intertidal and wave-exposed 32

Thallus leaf-formed and narrow, cross section shows cells with more or less same size and form Punctaria*

* Punctaria plantaginea common during spring, blade 4-6 cells in cross section


Blade > 2 mm and up to 5 cm wide, growing in small bundles intertidally during spring, wave-exposed Petalonia fascia

Blade very narrow (< 2 mm), growing in small bundles intertidally during spring, wave-exposed Planosiphon zosterifolius


Thallus lacking a mid-rib 34

Thallus with a mid-rib (may be difficult to see in some) 38


Thallus not dichotomously branched 35

Thallus dichotomously branched, but may have additional side branches 36


Thallus flat and oppositely branched, side branches narrowing towards base Desmarestia ligulata

Thallus irregularly divided in narrow parts, each ending in a small hair tuft Cutleria multifida


Thallus dichotomously branched and with additional side branches, axes flat and strap-like with relatively large gas vesicles Ascophyllum nodosum

Thallus dichotomously branched only 37


With half-piped formed branches, growing in uppermost part of the intertidal, sheltered Pelvetia canaliculata

Thallus with thin, flat branches, growing submerged Dictyota dichotoma


Thallus with round gas vesicles Fucus vesiculosus

Thallus without such round gas vesicles 39


Thallus with a notched (serrated) edge Fucus serratus

Thallus with a smooth edge 40


Brackish species, often in river outlets, thallus relatively thin and flexible and often with inflated areas, and often the same individual can have varying width of branches Fucus ceranoides

Stiffer thallus, not growing exclusively in brackish sites 41


Thallus with an indistinct mid-rib and having caecostomata 42

Thallus with a clearly visible mid-rib 43


Thallus to 10 cm, growing only in strongly wave-exposed intertidal, long swollen receptacles Fucus distichus

Larger thallus, often growing in harbour areas, long and swollen receptacles Fucus distichus subsp. evanescens


 Thallus stiff and flat, elongated receptacles, dioecious, semi-exposed intertidal Fucus vesiculosus f. linearis

Thallus often twisted, during summer with round receptacles hermaphroditic, upper intertidal, sheltered Fucus spiralis

Thallus branched and with round or rounded axis in cross section:


Thallus fully or mainly uniseriate 45

Tallus pluriseriate in all parts (except apical cells or short side branches) 57


Erect part of thallus unbranched, brush-like tufts epiphytically on Fucus Elachista fucicola

Thallus branched in all parts 46


Thallus pluriseriate in basal parts or with some cells longitudinally divided 47

Thallus uniseriate in all parts 51


Thallus with mainly opposite branching from main axes 48

Thallus not oppositely branched, or only occasionally 49


Thallus pluriseriate in basal parts, oppositely branched or branches in whorls, or with unilocular sporangium opposite of branch, often epiphyte on Cladophora rupestris Isthmoplea sphaerophora

Thallus may be pluriseriate in basal parts, is mainly oppositely branched from main axes, with conspicuous reproducing cells (1-2) placed intercalary in side branches Tilopteris mertensii


Thallus may have round monosporangia with a dark brown content, normally on short branches Haplospora globosa

Thallus with intercalary sporangia and gametangia, forming cell rows 50


Intercalary sporangia or gametangia, disc-formed chloroplasts, may have some opposite branches and some cells longitudinally divided Pylaiella littoralis

Loose-lying, short side branches and terminal sporangia common, rare Pylaiella varia


With occasional opposite branching, Intercalary sporangia or gametangia,
forming cell rows, disc-formed chloroplasts 52

Sporangia and gametangia as side branches, elongated or cone-shaped 53


Intercalary sporangia (unilocular or plurilocular), disc-formed chloroplasts, may have some opposite branches and some cells longitudinally divided Pylaiella littoralis

Loose-lying, short side branches and terminal sporangia common, rare Pylaiella varia


Densely twisted axes (“dreadlocks”) and with small hook-formed side branches, often epiphyte on Fucus Spongonema tomentosum.

Not with small hook-formed side branches 54


Stunted sporangia without stalk, often in series, disc-shaped chloroplasts 55

Strongly elongated plurilocular sporangia often with a short stalk, chloroplast ribbon-shaped 56


Key for the genus Hincksia:

Species Size Branching Sporangia Comment
H. hincksiae To 10 cm Branches often to one side in upper part Adaxially in series, so dense that the sides meet Often epiphyte on Sacchoriza polyschides
H. sandriana To 10 cm Branches in series to one side Adaxially in series, not so dense that the sides meet May also have unilocular sporangia
H. granulosa To 15 cm Branching often opposite Sporangia round or rounded, often somewhat curved
H. ovata To 4 cm Opposite or sparsely Sporangia often on a one-celled stalk


Brances in bundles especially in upper part, plurilocular sporangia often in rows adaxally on branches, not ending in a pseudohair Ectocarpus fasciculatus

Plurilocular sporangia elongated and often terminating in a long pseudohair Ectocarpus siliculosus


Main branching irregular, in addition axes are densely set with short branches of determined growth 58

Axes not densely set with short side branches 59


Short, uniseriate side branches densely set and whorls not clearly separated, normally attached, often in rock pools Cladostephus spongiosus

Short side branches in whorls (verticillate) and separated by internodes (except in tips), normally loose-laying Cladostephus hirsutus


Thallus is small (< 5 cm) and with thin axes. Below top cells the axes are pluriseriate, and clearly divided in segments of cells with longitudinal divisions (seen in microscope, young shots). Axes may be covered with rhizoidal cortex 60

Axes not clearly divided in such segments in any parts 61


Key for Halopteris, Sphacelaria, Chaetopteris, Battersia, Sphaceloderma, Protohalopteis in Norway:

Species Branching Rhizoidal cortex Cross div. of cells in segments Propagule and zoidangia Dimentions Comment
Halopteris scoparia Alternating from main axes, last order branch very pointy No To 4-5 cm (in Norw.) Attached form rare, loose-laying form in sheltered bays
Sphacelaria plumula Feather branching No No Propagule w. 3 short horns (branches) To 2 cm
S. cirrosa From main axes No No Propagule w. 3 horns and 1 hair To 3 cm, axes to 100 μ wide Often epiphytic (mainly Halidrys siliquosa)
S. rigidula Irregular, thalli in small bushes No (no) Propagule with 2-4 horns. Uni-locular zoidangia on a curved cell To some cm Stolon growth.
Branches of propagule not constricted at base
S. triluboides Absent or sparsely No Rare Propagule w. 2 short horns Erect axes to 55 μ wide Solitary hairs frequent. Rare species
Chaetopteris plumosa Apical feather branching Yes from cells of segment in all planes To 10 cm Basal parts dark and coarsely branched. Common
Battersia plumigera Apical feather branching Yes from cells in the same plane as side br. Presence in Norway uncertain
B. arctica Irregularly feather br. On main axes, sparsely Yes Rare in Norway
B. racemosa Irregular, sparsely No Yes Zoidangia in groups, no propagule To 2 cm, axes to 55 μ wide Grows in sandy rock pools
Sphacelo-derma caespitulum Sparsely No Yes Pluriloc on 3-5 celled stalk, no propagule To 1 cm, axes to 35 μ wide Small erect axes from a thick basal crust
Sphacelorbus nanus Sparsely No yes Uniloc. on a short stalk, pluriloc. on separate plants. No propagule < 1 cm, filaments to 23 (30) μ wide Rare in Norway, Stolon growth, forms mats
Protohalop-teris radicans Irregular, sparesly No Yes Uniloc. often sessile, pluriloc. on separate plants. No propagule To 2 cm, filaments (upright) to 60 μ wide Stolon growth, forms dense mats


Thallus with gas vesicles (round or elongated like pea pods and ending in a narrow tip) on short stalks 62

Thallus without gas vesicles (or with very few) 63


Thallus with small leaves and small, round gas vesicles Sargassum muticum

Gas vesicles are elongated with compartments (“pea pods”), main axes are flattened Halidrys siliquosa


Thallus with a consistent regular branching pattern or partly so 64

Thallus with irregular branching 66


With long, dichotomously branched receptacles attached to a button-formed vegetative part Himanthalia elongata

With opposite or whorl branching 65


Consistently oppositely branched Desmarestia viridis

Commonly with opposite or whorl branches, hollow in most parts Striaria attenuata


Surface covered with paraphyses or assimilation filaments (seen in microscope) 67

Surface with angular cortex cells 71


Thallus slimy and gelatinous 68

Tallus different 70


Thallus to 5 cm, with short branches and epiphytic on Zostera Cladosiphon zosterae

Thallus larger and with relatively long branches 69


Thallus with relatively long 1st order branches and short 2nd order branches, branches of equal thickness: Eudesme virescens or Eudesme borealis (very similar, in E. borealis axes may be thicker in lower parts and tapering upwards)

Thallus with branches of unequal thickness, assimilation filaments with barrel shaped cells at basis Mesogloia vermiculata


Alternately branched from one main axis, almost only branches of 1rst order, thallus not hollow, low intertidal during summer Chordaria flagelliformis

Long 1rst order branches and short 2nd order branches ending in a hair, axes hollow in lower parts, unilocular sporangia Acrothrix gracilis


Fertile structures in conspicuous sori on surface of axes, consisting of sporangia, paraphyses and hairs 72

Thallus not with such sori 73


Pale yellowish, uneven and knotty surface with darker spots (sori), loose-laying in sheltered areas during summer, irregularly branched and axes with one central cell row Spermatochnus paradoxus

Similar to S. paradoxus, but with 4-5 central cell rows, growing epiphytically on Fucus during summer Stilophora tenella


Thallus irregularely branched, with round to flattened axes (2-3 mm thick), laying loose in intertidal in sheltered bays. Ascophyllum nodosum f. scorpioides, see photos under Ascophyllum nodosum

Thallus with thinner axes and sublittoral 74


Irregular branching, dark and coarse branches set with small “spikes” during winter, new growth in spring seen as light brown shoots set with small bundles of hairs Desmarestia aculeata

Thallus not with small “spikes” 75


Growing shoots ending in a hair, axes with rectangular cortex cells covering large inner cells (microscope) 76

Growing shoots not ending in a hair, rounded cortex cells, often epiphytic on Chordaria flagelliformis Dictyosiphon foeniculaceus


Rectangular cortex cells covering elongated inner cells (not always easy to see) Stictyosiphon tortilis.

Rectangular cortex cells covering large, round inner cells (easy to see), small branches often uniseriate Stictyosiphon soriferus