Seaweeds growing on the lower shore in north Devon.
Seaweeds are the most important plants on the seashore, and on sheltered coasts with suitable substrates they come to dominate the scene. Each species is adapted to rather precise degrees of exposure to waves and air, and to tidal influence, the result being a huge diversity around the British coast as a whole.
WHAT ARE SEAWEEDS?
Many seaweed species reach a considerable size and complexity of form, giving them more than a passing resemblance to flowering plants. However, despite these superficial similarities, seaweeds are algae – simple and relatively primitive plants in an evolutionary context, which have more in common with the minute planktonic algae that colour the water column than higher plants.
Apart from the fact that they do not reproduce by flowering, seaweeds differ from higher plants in the simplicity of their internal structure: the complex vascular system of flowering plants is absent, and what appear to be ‘roots’ (the holdfasts with which many seaweeds attach themselves to rocks) merely serve to anchor the plants; they do not take up nutrients and water.
Seaweeds do have one thing in common with other plants, flowering ones included – they produce their own food by photosynthesis. This complex internal process involves the use of sunlight energy, trapped by a pigment called chlorophyll, to convert carbon dioxide and water to glucose, with oxygen given off as a by-product. Chlorophyll is green, and indeed many seaweed species are greenish in overall appearance. But others are brown or red, the green pigment being masked by others of a different hue.
Seaweed reproduction is surprisingly complex for such relatively simple organisms, and the detail is beyond the scope of a book of this nature. However, in simple terms there are two stages in the life cycle, which involves a process called alternation of generations. In many species, one stage is minute (in some cases microscopic) and what we recognise as ‘seaweeds’ are in fact the second, spore-producing stage in the life cycle.
SEAWEED CLASSIFICATION
Seaweeds growing on the lower shore in north Devon.
The modern view of seaweed classification holds that the three main groups – all easily recognised as seaweeds of some kind or another – are not closely related. Colour is a useful guide for separating the groups in the field: the Chlorophyta, or green seaweeds, are considered to be true plants and their chlorophyll is not masked by other pigmentation; the Rhodophyta, or red seaweeds, are also regarded as true plants (some species are chalky and coralline in appearance), their green chlorophyll being masked by red phycoerythrin pigment, and sometimes bluish phycocyanin pigments too; but the Phaeophyceae, or brown seaweeds, are not considered to be plants in the strict sense, and their green chlorophyll is masked by brown fucoxanthin pigment.
The structure and form of British seaweeds varies tremendously between species, each one having evolved to cope with factors affecting them on a daily basis: exposure to air, depth and length of immersion in sea water, and damaging wave action, for example. Recognising the distinct structural differences can also be useful in identification.
Seaweeds attach themselves to a substrate (usually rock, but sometimes shells or buried stones) using a holdfast; this can range from simple in appearance to branching complexity, depending on the species. The rest of the seaweed is called the ‘frond’ (usually linked to the holdfast by a stem-like ‘stipe’), of which three basic forms can be recognised: a slender filament; a network of slender branches; or a broad, flat ‘lamina’. In terms of the way any given seaweed species divides, the branches may be simple and aligned in opposite pairs; they may alternate along the stipe; they may be arranged in whorls; or the stipe and frond may divide dichotomously (equally) throughout.
ZONATION
The influence of tides on British shores, caused by the gravitational pull of the Moon, is profound. For many seashore creatures this means that every six hours or so they will either be exposed to air or covered by sea water. All seashore life is affected by these daily changes, but on rocky shores in particular, seaweeds demonstrate the effect better than any other group. Each species has evolved to cope with differing degrees of exposure to air or inundation by the tide, and to differing degrees of exposure to waves. As a result, the position on any given shoreline where a particular species can grow is limited; typically, this leads to the creation of obvious zones that are identified by the colour of the seaweed in question. The effect is most noticeable on the west coast of Britain, where the tidal range is large and where rocky shores are often rather steep.
Sea Lettuce Ulva lactuca Length to 40cm
Membranous green seaweed. Grows attached to rocks on protected shores; often in rock pools on upper and middle shores. FRONDS are irregular in outline; often tattered, so exact shape can be difficult to determine. STATUS Widespread and locally common.
Gutweed Ulva intestinalis Length to 75cm
Aptly named green seaweed of sheltered estuaries, brackish lagoons and upper shore rock pools. FRONDS comprise variably constricted membranous tubes that inflate with oxygen in sunshine. STATUS Widespread and locally common.
Chaetomorpha linum Length to 10cm
Chaetomorpha linum (highly magnified)
Delicate, filamentous green seaweed. Grows in tangled tufts in sheltered sandy bays and in rock pools where sand collects. FRONDS consist of chains of single cells, arranged like a string of beads. STATUS Widespread and locally common.
Velvet Horn Codium tomentosum Length to 20cm
Distinctive green seaweed. Grows attached to rocks on mid- to lower shore. FRONDS are dark greenish and branch dichotomously, ending in rounded tips. Texture is rather felt-like. STATUS Widespread and locally common.
Cladophora Cladophora rupestris Length to 20cm
Delicate green seaweed that looks matted when out of water. Grows on rocks and in pools on middle shore. FRONDS comprise much-branched tufts of fine strands. STATUS Widespread and locally common. SIMILAR SPECIES Hen Pen Bryopsis plumosa (length to 10cm) is green and feathery or fern-like. Grows on sheltered rocky shores and on stones in pools, from mid- to low-tide mark. Widespread but scarce.
Rhizoclonium riparium Length to 25cm
Thread-like seaweed that forms dense mats (like tangled wool) that ‘spring back’ when squeezed. Found on mid-shore on sheltered coasts, often in areas of freshwater run-off, and brackish areas of estuaries. FRONDS are slender, green and undivided. Root basally. STATUS Widespread and locally common.
Mermaid’s Tresses Chorda filum Length to 7m
Long, rope-like yellowish-brown seaweed. Grows attached to rocks by a basal disc on sheltered shores, at or below low-tide mark. FROND is slender and unbranched. STATUS Widespread and locally common.
Dictyota dichotoma Length to 25cm
Rather delicate, flat yellowish-brown seaweed. Grows on rocks and in pools on the mid- to lower shore. FRONDS are rather ribbon-like and divide dichotomously; tips are indented. STATUS Widespread and locally common.
Petalonia fascia Length to 30cm
Distinctive yellowish-brown seaweed. Grows attached to stones and rocks; lower shore and sub-littoral zone. FRONDS are elongate and rather thin; seldom appear flat, because of ruffled margin and tendency to twist and contort spirally. STATUS Introduced; now widespread but only locally common.
Peacock’s Tail Padina pavonica Width to 10cm
Distinctive seaweed, typically with a slightly ‘chalky’ and hairy appearance. Found in shallow pools on sheltered rocky shores. FRONDS comprise fan-shaped lobes with concentric bands. STATUS Very local, mainly on S coasts from Devon to Hampshire.
Kelp or Oarweed Laminaria digitata Length to 1.5m
Kelp stem
Robust olive-brown seaweed. Often forms dense beds, with only floating fronds, not stipes, visible at most low tides. FROND comprises a tough, smooth and flexible stipe and broad blade, divided into strap-like segments; attached to rocks by branched holdfast. STATUS Widespread and locally common. SIMILAR SPECIES Golden Kelp L. ochroleuca (length to 2m) has a distinct yellowish colour. Grows in sheltered sites, attached to rocks on lower shore. Stipe is inflexible (snaps) and broad blade is divided into strap-like segments. STATUS Restricted to SW England and very locally common.
Cuvie Laminaria hyperborea Length to 3m
Cuvie stem
Impressive brown seaweed. Grows on rocky shores; mainly sub-littoral but sometimes just exposed at extreme low tide. FROND comprises a textured, inflexible stipe (snaps) and broad blade divided into strap-like segments; holdfast is branched. STATUS Commonest in W and N Britain; absent from the SE.
Sea Belt or Sugar Kelp Saccharina latissima Length to 1.5m
Distinctive brown seaweed of sheltered rocky shores. Grows on lower shore. FROND comprises a stout but slender stipe and strap-shaped blade with a crinkly margin. STATUS Widespread but commonest in the N and W.
Furbelows Saccorhiza polyschides Length to 2m
Furbelows holdfast
Impressive brown seaweed. Grows attached to rocks on lower shore; exposed only during low spring tides. FROND comprises a broad, flattened stipe and much-divided blade; base of stipe is twisted and corrugated, and holdfast is bulbous. STATUS Restricted mainly to W, SW and NW Britain.
Dabberlocks Alaria esculenta Length to 1.5m
Distinctive yellowish-brown seaweed. Grows on exposed rocks at extreme low water. FROND comprises a slender stipe and midrib with a delicate blade; usually looks tattered by late summer. STATUS Restricted to W and N Britain; absent from the SE.
Thongweed Himanthalia elongata Length to 1m
Thongweed ‘button’
Slender yellowish-brown seaweed. Grows attached to rocks on the lower shore, on rather sheltered coasts. FROND is strap-like and rather narrow (less than 1cm across); grows from centre of tough button-like structure attached to rock. STATUS Widespread and locally common in W and N Britain; absent from most of the SE.
Wireweed Sargassum muticum Length to 2m
Much-branched and rather bushy yellowish-brown seaweed. Grows on sheltered coasts, in rock pools and estuaries, usually attached to rock. FROND has slender branches with numerous round air bladders and irregular-oval branchlets. STATUS Introduced from Pacific, first recorded on the S coast in the 1970s; now widespread and invasive in S and W Britain.
Landlady’s Wig Desmarestia aculeata Length to 80cm
Brown seaweed that forms dense, tufted growths fancifully recalling a pantomime wig. FRONDS are slender and flattened, with regularly toothed margins. STATUS Widespread and locally common, except on the E coast.
Desmarestia ligulata Length to 1.75m
Flattened yellowish-brown seaweed. Grows in pools and in sheltered sites on rocky shores, often among coralline seaweeds. FRONDS are much branched, with lateral, narrowly tongue-shaped side divisions. STATUS Widespread, commonest in the SW.
Beanweed Scytosiphon lomentaria Length to 20cm
Delicate but distinctive yellowish-brown seaweed. Grows in pools and in sub-littoral waters on sheltered shores. FRONDS are slender and tubular, constricted at intervals and resembling strings of beans. STATUS Widespread and locally common.
Punctured Ball Weed Leathesia difformis Diameter to 5cm
Globular yellowish-brown seaweed. Grows attached to rocks and other seaweeds in pools and on sheltered shores. FROND is irregular in shape, solid when small but hollow when large. When crushed, creates a gelatinous pulp. STATUS Widespread, but commonest in the W and N.
Oyster Thief Colpomenia peregrina Diameter to 6cm
Globular yellowish-brown seaweed, similar to Leathesia. Grows attached to seaweeds on mid- to lower shores. FROND is hollow and irregular in shape. When crushed, breaks into strips or fragments, not a gelatinous pulp. STATUS Introduced. Widespread, but commonest in the W.
Egg Wrack Ascophyllum nodosum Length to 1.2m
Tough, leathery yellowish-brown seaweed. Often locally dominant on sheltered rocky coasts, around mid-tide level. FROND branches dichotomously and has numerous egg-shaped air bladders. STATUS Widespread and common on suitable coasts, least so in SE England.
Channel Wrack Pelvetia canaliculata Length to 12cm
Tufted greenish-brown seaweed. Forms a distinct zone on upper shore of sheltered rocky coasts. FROND is tough and leathery, and branches dichotomously; margins are usually inrolled, forming a channel. STATUS Locally abundant on suitable coasts, mainly in W Britain; virtually absent from the SE.
Serrated Wrack Fucus serratus Length to 65cm
Serrated Wrack growing on Dorset’s Jurassic coast.
Familiar and easily identified greenish-brown seaweed. Often dominant on lower shore on moderately exposed rocky coasts. FROND is flattened, with a distinct midrib and serrated margins. Swollen reproductive structures develop at frond tips in winter. STATUS Widespread and locally common except in the SE.
Spiral Wrack Fucus spiralis Length to 20cm
Aptly named greenish-brown seaweed. Grows on upper shore, below Pelvetia zone, on sheltered rocky coasts. Also found, unattached, in very sheltered sea lochs. FROND divides dichotomously and twists spirally along its length. Swollen reproductive structures form at frond tips. STATUS Widespread and locally common.
Bladder Wrack Fucus vesiculosus Length to 80cm
Distinctive greenish-brown seaweed. Found in a range of habitats but commonest on moderately sheltered rocky shores. FROND is broad and flattened, and divides dichotomously. Typically, has paired air bladders along its length; bladders can be absent in exposed situations. STATUS Widespread and locally common.
Estuary Wrack Fucus ceranoides Length to 40cm
Much-branched greenish-brown seaweed. Grows attached to stones in sheltered habitats where salt and fresh water mix, notably in estuaries. FROND divides dichotomously and is flattened, with a midrib and pointed frond tips. STATUS Locally common. SIMILAR SPECIES Two-headed Wrack F. distichus (length to 10cm) is a greenish-brown seaweed of exposed rocky shores in the N, mainly Shetland. Fronds divide dichotomously and lack air bladders.
Moss Wrack Fucus cottonii Length to 3cm
Moss Wrack, growing amongst Thrift
Tiny tufted brown species, barely recognisable as a Fucus seaweed. Grows as dense patches on mud, in sheltered spots on upper reaches of saltmarshes in NW Scotland. FROND is branched and moss-like. STATUS Very local.
Sea Oak Halidrys siliquosa Length to 1m
Much-branched greenish-brown seaweed. Grows on rocky shores in moderately exposed sites; sometimes found in pools. FROND divides alternately into narrow blades. Reproductive structures and air bladders are pod-like and stalked. STATUS Widespread and locally common, least so in the SE.
Cystoseira baccata Length to 1m
Rainbow Wrack
Bushy, much-branched greenish-brown seaweed. Grows on rocks; mostly sub-littoral but sometimes in pools. FROND branches alternately into narrow sections; has striking air bladders at base of side branches; tips are spiky. STATUS Locally common in SW Britain. SIMILAR SPECIES Rainbow Wrack C. tamariscifolia is bushy and iridescent blue-green underwater. Locally common in SW.
Furcellaria lumbricalis Length to 25cm
Tufted, much-branched reddish-brown seaweed. Grows attached to rocks with a small holdfast, on lower shore. FROND has stiff cylindrical stems; these taper towards tips, which often are bifurcate. STATUS Locally common except in SE.
Irish Moss Chondrus crispus Length to 20cm
Much-branched, slightly slippery-feeling seaweed. Usually reddish or purplish brown, but sometimes partly bleached greenish yellow. Grows on rocks on lower shore and in pools. FROND branches dichotomously towards tip, into broad, flat blades with rounded margins. STATUS Widespread and locally common.
Grape Pip Weed Mastocarpus stellatus Length to 10cm
Grape Pip Weed, close up
Tough, broadly branched reddish-brown seaweed. Grows attached to rocks on lower shore. FROND has slightly curled or channelled blades, with pip-like projections produced towards margins in female plants. STATUS Locally common, least so in the SE.
Calliblepharis ciliata Length to 30cm
Colourful, distinctive red seaweed. Grows on rocks; sub-littoral zone and lower shore. FROND is flattened and elongate, with frilly marginal outgrowths; fronds branch from basal holdfast. STATUS Widespread and locally common, mainly in the S and W. SIMILAR SPECIES Gelidium spinosum (length to 20cm) is a tough, branched red seaweed. Grows on rocks on lower shore and on kelp stems. Frond is flattened, with smaller, oval side branches and branchlets. Locally common, mainly in the W and SW.
Red Rags Dilsea carnosa Length to 45cm
Distinctive, bright red seaweed. Grows on rocks on lower shore and among kelps. FROND is broad, flattened and elongate, sometimes dividing or tearing towards tip. STATUS Locally common except in the SE.
Coral Weed Corallina officinalis Length to 10cm
Jania rubens
Calcified red seaweed with a pink chalky appearance. Grows on rocks, on lower shore and in pools, in moderately sheltered locations. FROND appears jointed and divides pinnately; overall shape is fan- or fern-like. STATUS Widespread and locally common except in E England. SIMILAR SPECIES Jania rubens (length to 20cm) is pink and coralline; forms branched, tufted mats attached to larger seaweeds and rocks. Locally common in the SW only.
Maerl Lithothamnion sp. and Phymatolithon sp.
Lithothamnion sp.
Phymatolithon sp.
Maerl is a term applied to several coralline seaweed species that grow unattached on sheltered offshore seabeds, forming gravel-like layers where undisturbed. Bleached maerl is washed up after gales. Lithothamnion sp. grow as branched, twiggy and coral-like structures; Phymatolithon sp. form more lumpy, nodular structures. STATUS Both genera are widespread, particularly in the W and N, but scarce or absent from the SE.
Lithophyllum incrustans Diameter to 10cm
Rock-encrusting pink seaweed that looks more like rock or coral than a plant. One of several similar (and hard to identify) species, many of which have different growths depending on where they grow. FROND is flattened, adpressed to rock on which it grows, and hard. STATUS Widespread and locally common in suitable habitats.
Pink Plate Weed Mesophyllum lichenoides Diameter to 15cm
Distinctive calcified seaweed that forms pink plates, recalling a terrestrial bracket fungus. Grows attached to Corallina on lower shores. FROND is lobed and fan-shaped, with a pale margin and darker concentric bands. Usually pink, sometimes bleached yellow. STATUS Locally common in the SW.
Bead-weed Lomentaria articulata Length to 10cm
Distinctive red seaweed whose form vaguely recalls structure of the saltmarsh plant Common Glasswort. Grows attached to rocks and larger seaweeds on lower shore. FROND is much divided and constricted along its length into oval bead-like segments. STATUS Commonest in the W and N; absent from much of the SE.
Dumontia contorta Length to 20cm
Long-stranded reddish-brown seaweed, attached to rocks on lower shore. FRONDS are solid when young, tubular to flattened-cylindrical when older and often twisted. STATUS Widespread and locally common. SIMILAR SPECIES Sea Noodle Nemalion helminthoides (length to 20cm) is gelatinous, tubular and reddish brown, resembling overcooked noodles. Locally common only in the S and W.
Pterothamnion plumula Length to 15cm
Branched, tufted red seaweed. Grows in pools on lower shore. FROND has whorls of divided branchlets along main branches. STATUS Widespread but local. SIMILAR SPECIES Plumaria plumosa (length to 12cm) is much divided and reddish brown. Grows on shaded rocks on lower shore. Frond branches in feather-like manner; main branches are alternate, smallest side growths are opposite. Locally common only in the W and N.
Ceramium virgatum Length to 25cm
Distinctive seaweed, the most widespread of several superficially similar species. Grows attached to Fucus and Laminaria stipes, on rocks and on Codium in rock pools. FRONDS branch dichotomously, narrowing to pincer-like tips; red, with a banded appearance. STATUS Widespread and locally common, least so in the SE.
Dulse Palmaria palmata Length to 40cm
Flat reddish-brown seaweed. Grows attached to rocks and stipes of kelps, especially Cuvie; favours moderately exposed sites. FROND is tough and membranous, usually divided into finger-like lobes towards tip. Holdfast is a small disc. STATUS Widespread and locally common, except in the SE.
Grateloupia subpectinata Length to 60cm
Distinctive seaweed, found on lower shore on sheltered coasts. FRONDS comprise a broad, flat blade from which narrow, tapering branches arise, mostly laterally. Reddish brown, bleaching yellowish in summer. STATUS Alien species, introduced from Pacific. Currently recorded from Hampshire, Isle of Wight and Dorset.
Harpoon Weed Asparagopsis armata Length to 25cm
Unusual seaweed with 2 distinctly different stages in its life cycle. Typically seen as sexual stage, attached to other seaweeds on lower shore. FRONDS of sexual stage comprise whorls of finely divided ‘frothy’-looking segments, and barbed, bare ‘harpoons’. Asexual stage resembles a red woolly ball and is sub-littoral. STATUS Introduced, now widespread in the SW and NW.
Sea Flaxweed Stypocaulon scoparium Length to 15cm
Much-branched brown seaweed. Grows attached to rocks in pools and on lower shore, often where sand collects. FRONDS comprise tufted bunches of hair-like growths. STATUS Locally common, mainly in the SW.
Cladostephus spongiosus Length to 15cm
Branched reddish-brown seaweed. Grows attached to rocks in pools on mid-shore. FROND is branched and slender, the branches with whorls of branchlets; the effect recalls a pipe cleaner. STATUS Widespread and locally common, except in the E. SIMILAR SPECIES Sea Mare’s-tail Halurus equisetifolius (length to 15cm) is red and branched, with whorls of incurved branchlets. Overall effect recalls freshwater plant Mare’s-tail Hippurus vulgaris. Locally common only in the SW. H. flosculosus (length to 20cm) is tufted and grows in pools on sheltered rocky shores. Fronds comprise stiff hairs, attached to substrate by fibrous base. Locally common in the W.
Bifurcaria bifurcata Length to 40cm
Tough yellowish-brown seaweed. Grows in rock pools and sheltered spots on lower shore. FROND divides evenly, the fork curved in a distinctive manner. STATUS Confined to SW Britain.
Pepper Dulse Osmundea pinnatifida Length to 20cm
Tough, rather cartilaginous reddish-brown seaweed. Grows, often in lush abundance, on rocks on mid- to lower shore. Tolerates moderately exposed sites. FROND is flattened and branched, the effect like a fern, notably a filmy-fern. STATUS Widespread and locally common except in SE England.
Phycodrys rubens Length to 25cm
Distinctive red seaweed. Grows sub-littorally, attached to rocks; sometimes washed up. FROND comprises a tough stem with membranous red blades; these recall oak leaves. STATUS Locally common only in W and N Britain.
Polysiphonia lanosa Length to 10cm
Polysiphonia sp.
One of several confusingly similar reddish seaweeds. Grows epiphytically on Egg Wrack; other species grow on rocks. FROND comprises branched, segmented stems, appearing as tufts; these collapse out of water. STATUS Locally common. SIMILAR SPECIES Several confusingly similar filamentous Polysiphonia species are also widespread, growing attached to rocks in pools and on lower shore.
Sea Beech Delesseria sanguinea Length to 20cm
Distinctive red seaweed. Grows attached to rocks; usually sub-littoral but sometimes in pools on lower shore. FROND comprises a tough midrib and lateral veins, and membranous blades; overall effect recalls a Beech leaf. STATUS Locally common only in W Britain.
Laver Porphyra sp. Length to 50cm
Represented by 5 superficially similar species. Most grow attached to rock, but some are epiphytic on larger seaweeds. FRONDS All are thin and membranous, ranging from red or purple to yellowish green. STAUS Widespread and locally common. SIMILAR SPECIES Nitophyllum punctatum (length to 45cm) is membranous, pink and fan-shaped; variably divided towards tip. Female plants are dotted with dark reproductive structures. Locally common only in W and NW Britain.