Green seaweeds key

How to use the key


Thallus up to 30-40 cm, thick, spongy and round axes, and normally dichotomously branched Codium fragile

Thallus different 2


Thallus leaf or blade-formed 3

Thallus not leaf or blade-formed 9


Thallus very small (normally < 1 cm and max. 3-4 cm) and in high intertidal often on resting places for sea birds, cells arranged in lines 4

Thallus relatively big (several cm long) and not high in the intertidal 5


Thallus blade-formed, < 1 cm and narrowing to a short stipe, cells arranged in groups Prasiola stipitata

More rare, also terrestrial and also on additional substrate than rock:

– Thallus to 3-4 cm, often curly edge and lacking stipe Prasiola crispa

– Thallus smaller and rounded, stipe short or absent, blade curled at edge, cells arranged in groups, sometimes conspiciuous separation lines between cell groups Prasiola furfuracea


Distromatic blade (or partly hollow) 6

Monostromatic blade 7


Long narrow thallus with frilled and wavy edge, tapering in both ends, older individuals partly hollow Ulva (‘Enteromorpha’) linza

Thallus distromatic, 1 pyrenoid per cell Ulva fenestrata

(Ulva lacinulata probably under-reported, broad and undulated, margin with “teeth”)


Intertidal and with a very torn edge of thallus, spring, up to 6 pyrenoids in elongated long cells towards base Monostroma grevillei

Edge of blade not torn or only torn in outer parts, one pyrenoid per cell 8


Blade with elongated cells towards base, blade torn in outer parts and with rounded sporangia in groups Protomonostroma undulatum

In basal parts vegetative cells have elongated rhizoids, blade not torn and disintegrating in outer parts Gayralia oxysperma


Thallus hollow 10

Thallus not hollow 11


Key for tube formed taxa (Blidingia, Capsosiphon, some Ulva (‘Enteromorpha’)). Some important characters in bold

Species Habitat Habit Cell arrangement Pyren-oids
Capsosiphon fulvescens On rock in high intertidal, often in polluted sites To 20 cm, narrow, sometimes branched Characteristic pattern of 2 or 4 small cells (to 10-12 μm) together in
groups, separated by thick cell walls
1 per cell
Blidingia minima High intertidal To 10 cm long and 4 mm wide, often smaller. Several ind. from same attachm. disc. Small cells (5-10 μm), may be arranged in rows in the narrowest parts. 1 per cell
Blidingia marginata Brackish water Like B. minima, but more rare Small cells, always arranged in rows 1 per cell
Ulva linza Semi-exposed in rock pools or shallow Light green, flat with wavy edge, hollow margins Cells (15-20 μm) arranged in rows, rectangular with rounded corners Normally 1 per cell
U. intestinalis Most common Ulva, very common in rock pools Tubular and narrowing to the base. Can be compressed. Characteristic hood-shaped chloroplast, placed apically in cell.
Cells 8-20 µm diam.
1 per cell
U. compressa Variable, but not in low salinity Variable, but much branched and often com-pressed Characteristic hood-shaped chloroplast, usually placed apically in cell. Cells 15-30 µm 1 per cell
U. prolifera Abundant at eutrophicated sites Very variable, may be branched or not Cells to 18 μm in longest direction and can be placed in long rows 1 per cell centrally placed
U. flexuosa Variable Normally only first order branches Large cells (to 30 μm in longest direction) rectangular and arranged in rows (1)-2-(3) per cell
U. clathrata Variable Highly branched tubes Large cells (20-50 μm in longest direction), often in rows. Chloroplasts creating a
lattice-like pattern
Several per cell
U. criniata Closely related to U. clathrata Highly branched, with some short thorn-like branches Large cells (20-50 μm in longest direction), often in rows. Chloroplasts creating a
lattice-like pattern
Several per cell


Thallus branched 12

Thallus not branched (or with few, short (1-4 celled) branches) 18


Thallus siphonous 13

Thallus uniseriate 15


Thallus siphonous with main axes, side branches more or less in one plane, normally no cell walls 14

Thallus siphonous and small, irregularly branched, cell wall formes between main axis and side branch Derbesia marina


Side branches not regularly in one plane Bryopsis hypnoides

Branches regularly in one plane Bryopsis plumosa


Thallus lacks hook-formed branches and/or rhizoids (with little pigment) growing downwards along axes at the base, and have side branches always initiated from top part of a cell 16

Thallus with short hook-formed branches and/or rhizoids (with little pigment) growing downwards along axes at the base 17


Key for marine (and marine-brackish) Lychaete and Cladophora. Some important characters underlined:

Species Habit Branch system Cell shape and size Habitat
Lychaete pygmaea Diminutive; to 1.5 mm Irregular Barrel-shaped, discoid holdfast Sublittoral
Cladophora rupestris Stiff, broom-like tufts, dark green Irregular, 1-4 laterals per cell Main axes cell diam. 90-220 µm, length:width ratio 2-7. Thick cell wall Intertidal, common
C. albida Dense tufts with spongy texture Main axes with branches of different age, often bundles of flexed branches Main axes cell diam. 20-90 µm, l:w ratio 1.5-8. Apical cell rounded tip Intertidal, often in rock pools
C. dalmatica Light pale to grass green. Main axes ending in acropetal to irregular branching. Tips often bent (falcate) Apical cell diam. 13-55 µm. Main axes cell diam. 60-150 µm, l:w ratio 2-20. Cell walls thin, apical cell rounded tip Marine-brackish, rock pools to shallow subtidal
C. flexuosa Genetically close to C. sericea and C. albida
C. hutchinsiae Coarse tufts to 40 cm Younger branches between older ones (sometimes branches slightly acropetal in tips) Ap. cell diam. 90-195 µm. Main axes cell diam. 200-400 µm, l:w ratio 1-3.5 Littoral and upper sublittoral, rare in Norway
C. laetevirens Spongy tufts Branches often end with acropetal, sometimes flexed or falcate branching Ap. cell diam. 35-110 µm. Main axes cell diam. 100-260 µm, l:w ratio 2-10 Littoral and upper sublittoral, rare in Norway
C. sericea Light green, branching angle often < 45° Younger branches between older ones Main axes cell diam. 55-170 µm, l:w ratio 1-15. Apical cell tapering Marine to brackish, in the littoral zone
C. vadorum Akinets may occur: cells short and thick walled Branching with acropetal organization in attached individs Main axes cell diam. 120-200, l:w 2-11 Brackish localities
C. vagabunda Light to dark green Main axes ending in acropetal branching Main axes cell diam. 80-300 µm, l:w ratio 1.5-15 Marine – brackish, often loose lying in sheltered bays


Big, dark green thallus, some short pointed or spinous branches, grows in wave-exposed intertidal during spring Acrosiphonia arcta

Small, light green thallus, densly branched and with a spherical outline, wave-exposed intertidal during spring Spongomorpha aeruginosa


Thin entangled thalli, < 100 µm in diameter, need microscope to see each individual 19

Macroscopic, uniseriate thallus, > 200 µm in diameter 23


Thallus with a few 1-4 celled branches Rhizoclonium riparium

Thallus without a few short branches 20


Thallus biseriate, sheltered in high intertidal Percursaria percursa

Thallus uniseriate 21


Forming a green felt-like cover on rock in spring 22

Forms soft, light green ”bundles” of curly thalli entangled with other algae, cells 50-100 μm in diam Chaetomorpha ligustica.


Thallus with band-shaped, parietal chloroplast, in wave-exposed intertidal during spring Ulothrix sp.

Thallus not with such chloroplast, cells slightly barrel-shaped, often in association with Ulothrix Urospora sp.


Thallus dark green and with big cells (basal cell 1-3 mm long), grows wave-exposed in shallow subtidal Chaetomorpha melagonium

Thallus bright green, dense vegetation in rock pools during summer, or entangled in high intertidal, cells 200-300 μm in diameter Chaetomorpha linum (Chaetomorpha aerea)