PREMATURE CHROMOSOME CONDENSATION OF THE KARYOGAMY-BLOCKED SPERM PRONUCLEUS IN THE FERTILIZATION OF FUCUS DISTICHUS (FUCALES,PHAEOPHYCEAE)1

Mitosis of egg and sperm pronuclei of Fucus distichus subsp. evanescens (C. Agardh)Powell was examined by fluorescence and electron microscopy when migration of the sperm pronucleus and, as a result, karyogamy were blocked by colchicine treatment after plasmogamy. Chromosome condensation was obsewed in both pronuclei Microspectrophotometric studies after staining the nuclei with mithramycin A clearly showed that DNA synthesis ocurred in the egg pronucleus but not in the sperm pronucleus. This means that chromosomes condensed prematurely in the sperm pronucleus (premature chromosome condensation). In some cases, the egg chromosomes became arranged on a metaphase plate, whereas the sperm chromosomes lay scattered near the egg pronucleus. Immuno fluorescence microscopy using anti-β-tubulin antibody confirmed that a normal spindle was formed at the egg pronucleus. A pair of centrioles existed at the two poles of this spindle. The sperm nuclear membrane disappeared, and microtubules radiated to the sperm chromosomes from one pole of the egg spindle.     

SECRETION OF PHENOLIC SUBSTANCES INTO THE ZYGOTE WALL AND CELL PLATE IN EMBRYOS OF HORMOSIRA AND ACROCARPIA (FUCALES, PHAEOPHYCEAE)

We have used light and electron microscopy to study the secretion of phenolic substances into the primary walls and cross walls of zygotes and embryos of the brown algae Hormosira banksii (Turner) Decaisne (Hormosiraceae) and Acrocarpia paniculata (Turner) Areschoug (Cystoseiraceae). Phenolic compounds packaged within physodes are a major cytoplasmic component of brown algae. Physodes accumulate at the zygote periphery early in development and are secreted into the primary zygote wall. At germination, physodes accumulate at the rhizoid tip. The first sign of impending cytokinesis is the lining up of these phenolic vesicles in the plane of cell division. This precedes the arrival of other wall constituents. Physodes, together with other wall components, contribute to the development of the cell plate. We conclude that phenolic compounds play an important role in cell-wall construction in these species.     

GENETIC AND MORPHOLOGICAL ANALYSES OF THE SOUTHERN BULL KELP DURVILLAEA ANTARCTICA (PHAEOPHYCEAE: DURVILLAEALES) IN NEW ZEALAND REVEAL CRYPTIC SPECIES1

Many macroalgae exhibit considerable intraspecific morphological variation, but whether such variation reflects phenotypic plasticity or underlying genetic differences is often poorly understood. We quantified both morphological and genetic variation of 96 plants from seven field sites across eastern South Island, New Zealand, to assess genetic differences between morphotypes of the southern bull kelp Durvillaea antarctica (Cham.) Har. Consistent DNA sequence differentiation across mitochondrial, plastid, and nuclear loci was correlated with two broadly sympatric morphotypes: “cape” and “thonged.” These ecologically, morphologically, and genetically distinct bull‐kelp lineages were previously considered to be environmentally determined phenotypes with no underlying genetic basis. Interestingly, the sheltered “cape” lineage appears essentially genetically uniform across its South Island range, whereas the exposed “thonged” lineage exhibits marked phylogeographic structure across its range. Results suggest that D. antarctica in New Zealand comprises two reproductively isolated species.     

DISPERSAL OF HORMOSIRA BANKSII (PHAEOPHYCEAE) VIA DETACHED FRAGMENTS: REPRODUCTIVE VIABILITY AND LONGEVITY1

Drifting, fertile thalli are well documented to be the primary long‐distance dispersal vector for many marine macroalgae, but little information about reproductive viability of drift is known. This study examined the reproductive viability and longevity of floating fragments of the intertidal Australasian fucoid Hormosira banksii (Turner) Decne. Beach wrack surveys and field experiments were conducted to test the model that long‐distance dispersal is achieved in H. banksii via floating, fertile fronds. High densities of beach wrack fragments were evident during summer compared to autumn. The majority of beach wrack occurred on sandy beaches rather than rocky shores. Both male and female fragments were present in the beach wrack. Detached fronds were capable of releasing gametes up to 8 weeks after detachment. Beach wrack produced high fertilization rates and recruited successfully onto artificial panels. Results suggest that detached fragments are reproductively viable and that floating, fertile fronds may be an important mechanism for facilitating long‐distance dispersal in this species. Nevertheless, the frequency of fronds reaching a suitable habitat and contributing to gene flow between populations, or colonizing new populations, may not be proportional to the total density of beach wrack.     

RECENT VERSUS RELIC: DISCERNING THE GENETIC SIGNATURE OF FUCUS VESICULOSUS (HETEROKONTOPHYTA; PHAEOPHYCEAE) IN THE NORTHWESTERN ATLANTIC1

Fucus vesiculosus L. is one of the most widespread macrophytes in the northwestern Atlantic, ranging from North Carolina (USA) to Greenland (DK). We investigated genetic diversity, population differentiation, patterns of isolation by distance, and putative glacial refugial populations across seven locations from North Carolina (USA) to Cape Breton Island, Nova Scotia (Canada), with microsatellite analyses. Distinct northern versus southern (Delaware–North Carolina) populations were revealed by microsatellite data. Five of six microsatellite loci were fixed in populations in North Carolina, suggesting a recent founder event or a bottleneck, and the same homozygous genotype was found in herbarium materials collected on the North Carolina coast from more than 60 years ago. An additional set of individuals from the northern limit in Greenland was included in our analysis of mitochondrial intergenic spacer (mt IGS) haplotypes in the northwestern Atlantic. Remarkably, 184 of 188 F. vesiculosus specimens from North Carolina to Greenland shared the same haplotype. Recent colonization of the North American shore from Europe is hypothesized based upon the ubiquity of this common haplotype, which was earlier reported from Europe.     

ACROSYMPHYTON FIRMUM SP. NOV. (RHODOPHYTA,CRYPTONEMIALES), A NEW SUBTIDAL RED ALGA FROM NEW ZEALAND; DEVELOPMENTAL MORPHOLOGY AND DISTRIBUTION OF THE GAMETOPHYTE1

Acrosymphyton firmum sp. nov., is described from the northeastern coast of North 1., New Zealand. Gametophytes are spring–summer annuals which grow subtidally on cobbles. Thalli are uniaxial; each axial cell bears a whorl of four indeterminate and one determinate branchlets. Indeterminate branchlets are alternately arranged giving the thallus a distichous and feather-like appearance. Numerous corticating rhizoidal filaments are produced from the periaxial and lower whorl branchlet cells. These rhizoids entwine and obscure the main axis as the thallus develops until in the mature plant the axes have a firm consistency and lubricous texture. The carpogonial branch bearing short lateral filaments and auxiliary cell branch with terminal auxiliary cell place this new species in the genus Acrosymphyton Sjöstedt. Of the three described species in the genus, A. firmum is most similar to A. taylori. This is the first report of the genus Acrosymphyton and the only confirmed report of the family Dumontiaceae in New Zealand waters.     

THE EFFECT OF SULFIDE ON THE BLUEGREEN ALGAE OF HOT SPRINGS. I. NEW ZEALAND AND ICELAND1

The concentration of soluble sulfide (H₂S, HS^?, S⁼)² in hot springs of New Zealand and Iceland has a “species-determining” effect which appears to override the effect of all other chemical factors excepting hydrogen ion concentration. The cosmopolitan high-temperature form (HTF) of Mastigocladus laminosus (Ag.) Cohn appears to be sensitive to concentrations of S⁼ over 0.15 mg·l^?1 in New Zealand and 0.25 mg·l^?1 in Iceland. It was absent in thermal streams where S⁼ level was high enough to exceed this concentration at a point below its optimal temperature range (i.e., below ca. 50 C). In low or non-S⁼ springs in these regions this alga formed mats to an upper temperature limit of 63–64 C. In contrast, one bluegreen alga (Oscillatoria amphigranulata van Goor) occurred abundantly only in moderate to high S⁼ springs of New Zealand; this was a species with an upper temperature limit of about 57 C. Field ¹⁴C-HCO₃^? incorporation experiments with various levels of added S⁼ confirmed the sensitivity of HTF Mastigocladus and the great tolerance of O. amphigranulata. In the latter, the photosynthetic incorporation of ¹⁴C-HCO₃^? was sustained or enhanced by the additions of S⁼ in the presence of 5 × 10^?5m DCMU, an inhibitor of photoelectron transport on the reducing side of photosystem II. It is possible, but not proven, that S⁼ may act as a photoreductant in this species.     

ULTRASTRUCTURE OF SWARMERS IN THE LAMINARIALES (PHAEOPHYCEAE). II. SPERM1

The ultrastructure of sperm from 13 species in 11 genera of Laminariales collected in the northeast Pacific Ocean is unique in the brown algae. The sperm are elongate, and possess a nucleus, several mitochondria and two or three chloroplasts, but no eyespot. The anterior flagellum bears mastigonemes on the proximal half of its length; a distal “whiplash” portion lacks mastigonemes and is an extension of only the two central singlet microtubules of the axoneme. A peculiar feature of these sperm is the posterior flagellum, which is longer than the anterior flagellum and tapers distally as the doublet microtubules become singlets and decrease in number. This feature contrasts with the laminarialean zoospore, which possesses a short posterior flagellum with the usual “9 + 2” axoneme. The structure of these sperm differs from that reported for Chorda, the sperm of which resembles a primitive brown algal zoospore. The facts support the concept that Chorda is the most primitive member of the Laminariales.     

A REASSESSMENT OF SPECIES BOUNDARIES IN CYSTOSEIRA AND HALIDRYS (PHAEOPHYCEAE,FUCALES) ALONG THE NORTH AMERICAN WEST COAST1

Application of phylogenetic species recognition to morphologically recognized species in the genera Cystoseira Agardh and Halidrys Lyngbye on North American west coasts revealed little genetic variation despite a remarkable degree of morphological variation currently used to recognize and delineate species. Whereas morphological characteristics allow recognition of two genera, four morphological species and three informal forms, maximum genetic variation among them was similar to that characteristic of the intraspecific level in European congeners and other Fucales. Among morphological species and forms, nucleotide variation in a combined 26S (large subunit (LSU)) and internal transcribed spacer (ITS) ribosomal DNA analysis was below 3% while it was 1% or less for the RUBISCO spacer of the chloroplast DNA. Comparison of the LSU data to available data for European congeners showed that the genera Cystoseira and Halidrys are not monophyletic and that the previously recognized Cystoseiraceae should be included within the family Sargassaceae. These observations suggest that the current taxonomy for the Sargassaceae fails to reflect evolutionary history because Atlantic and Pacific Cystoseira and Halidrys appear to have arrived at similar morphologies independently. Our results indicate a comparatively recent establishment on the west coast of North America of a sargassacean progenitor whose descendant taxa have experienced limited genetic divergence and are characterized by a high capacity for phenotypic variation despite their overall genetic similarity.     

DISAPPEARANCE OF MALE MITOCHONDRIAL DNA AFTER THE FOUR‐CELL STAGE IN SPOROPHYTES OF THE ISOGAMOUS BROWN ALGA SCYTOSIPHON LOMENTARIA (SCYTOSIPHONACEAE,PHAEOPHYCEAE)1

Mitochondrial DNA (mtDNA) of the isogamous brown alga Scytosiphon lomentaria (Lyngb.) Link is inherited maternally. We used molecular biological and morphological analyses to investigate the fate of male mitochondria. Ultrastructural observations showed that the number of 25 mitochondria in a zygote coincided with the number of mitochondria derived from male and female gametes. This number remained almost constant during the first cell division. Strain‐specific PCR in single germlings suggested that mtDNA derived from the female gamete remained in the germling during development, while the male mtDNA gradually and selectively disappeared after the four‐cell stage. One week after fertilization, male mtDNA had disappeared in sporophytic cells. Using bisulfite DNA modification and methylation mapping assays, we found that the degree of methylation on three analyzed sites of mtDNA was not different between male and female gametes, suggesting that maternal inheritance of mtDNA is not defined by its methylation. This study indicates that the mechanism of selective elimination of male mtDNA is present in each cell of a four‐celled sporophyte and that it does not depend on different degrees of DNA methylation between male and female mtDNA.     

LIFE HISTORY OF COLPOMENIA SINUOSA (SYCTOSIPHONACEAE,PHAEOPHYCEAE) IN THE AZORES1

Colpomenia sinuosa (Mertens ex Roth) Derbès and Solier (Scytosiphonaceae, Phaeophyceae) is a common species on the rocky intertidal shores of the Azores, where reproductive gametophytes occur throughout the year. Life‐history studies of this species were carried out in culture, and both sexual and asexual reproduction were observed. Anisogamous gametes fused to form zygotes. The zygotes gave rise to a filamentous prostrate sporophyte generation bearing unilocular sporangia, under both short‐day and long‐day conditions at 15 and 22°C, and to both unilocular and plurilocular sporangia, under the lower temperature condition. Unispores developed into gametophytes, and plurispores gave rise to filamentous sporophytes. Asexual reproduction was carried out by unfused female gametes and asexual plurispores produced from the same gametophyte. Unfused gametes developed into filamentous prostrate sporophytes producing unilocular sporangia in both culture conditions, and unispores released from the sporangia gave rise to gametophytes. Asexual plurispores from field gametophytes, under both culture conditions, developed directly into new gametophytes. The species exhibited three types of life history: a heteromorphic, diplohaplontic; a heteromorphic, monophasic (both with alternation between the erect and filamentous prostrate thalli); and a monomorphic, monophasic.     

SHORT‐ AND LONG‐TERM EFFECTS OF ELEVATED CO2 ON PHOTOSYNTHESIS AND RESPIRATION IN THE MARINE MACROALGA HIZIKIA FUSIFORMIS (SARGASSACEAE,PHAEOPHYTA) GROWN AT LOW AND HIGH N SUPPLIES1

The short‐term and long‐term effects of elevated CO₂ on photosynthesis and respiration were examined in cultures of the marine brown macroalga Hizikia fusiformis (Harv.) Okamura grown under ambient (375 μL · L^?1) and elevated (700 μL · L^?1) CO₂ concentrations and at low and high N availability. Short‐term exposure to CO₂ enrichment stimulated photosynthesis, and this stimulation was maintained with prolonged growth at elevated CO₂, regardless of the N levels in culture, indicating no down‐regulation of photosynthesis with prolonged growth at elevated CO₂. However, the photosynthetic rate of low‐N‐grown H. fusiformis was more responsive to CO₂ enrichment than that of high‐N‐grown algae. Elevation of CO₂ concentration increased the value of K_1/2(Ci) (the half‐saturation constant) for photosynthesis, whereas high N supply lowered it. Neither short‐term nor long‐term CO₂ enrichment had inhibitory effects on respiration rate, irrespective of the N supply, under which the algae were grown. Under high‐N growth, the Q₁₀ value of respiration was higher in the elevated‐CO₂‐grown algae than the ambient‐CO₂‐grown algae. Either short‐ or long‐term exposure to CO₂ enrichment decreased respiration as a proportion of gross photosynthesis (Pg) in low‐N‐grown H. fusiformis. It was proposed that in a future world of higher atmospheric CO₂ concentration and simultaneous coastal eutrophication, the respiratory carbon flux would be more sensitive to changing temperature.     

INHERITANCE AND MEIOTIC ELIMINATION OF A VIRUS GENOME IN THE HOST ECTOCARPUS SZLZCULOSUS (PHAEOPHYCEAE)1

The marine brown alga Ectocarpus siliculosus (Dillwyn) Lyngbye is frequently infected by a latent DNA virus that multiplies in modified sporangia and gametangia of the host. We describe a polymerase chain reaction (PCR) procedure for the amplification and detection of viral DNA in Ectocarpus. PCR analysis of parents and progeny plants confirmed that virus DNA passes through meiosis like a Mendelian trait. An infected sporophyte produced equal numbers of gametophytes with and without the viral genome. Thus, meiosis in sexual populations of the host acts as a mechanism for the creation of virus-free progeny.     

TAXONOMIC REVISION OF SARGASSUM SPECIES (FUCALES,PHAEOPHYCEAE) FROM NEW CALEDONIA BASED ON MORPHOLOGICAL AND MOLECULAR ANALYSES1

Sargassum C. Agardh (1820) is a taxonomically difficult genus distributed worldwide and reported as the most species‐rich genus of the Fucales. It is especially abundant in the Pacific where decreasing species richness is reported to occur from west to east. New Caledonia has been recognized as one of the hotspots of Sargassum diversity; however, species lists available for this region are old and incomplete and have not yet been updated with regard to the latest taxonomic revisions published. This study aimed at revising Sargassum diversity in New Caledonia and to assess its geographic affinities with neighboring Pacific regions. We used combined morphological and DNA analyses on new collections and examined numerous type specimens. Although 45 taxa have been listed in the literature, most of them have been either transferred to synonymy since or misidentified, and in this study, only 12 taxa were recognized as occurring in New Caledonia. They belong to the subgenus Sargassum sect. Binderianae (Grunow) Mattio et Payri (2), sect. Ilicifoliae (J. Agardh) Mattio et Payri (2), sect. Polycystae Mattio et Payri. (1), sect. Sargassum (4), sect. Zygocarpicae (J. Agardh) Setch. (2), and subgenus Phyllotrichia (Aresh.) J. Agardh (1). New Caledonian Sargassum flora appeared as the second richest in the region after the Pacific coast of Australia, with which it has shown high similarity, and shared species with all neighboring regions. One species, S. turbinarioides Grunow, is considered as endemic to New Caledonia. The low genetic diversity detected among several polymorphic species belonging to sect. Sargassum is also discussed.     

A MORPHOLOGICAL AND MOLECULAR STUDY OF AUSTRAL SARGASSUM (FUCALES,PHAEOPHYCEAE) SUPPORTS THE RECOGNITION OF PHYLLOTRICHA AT GENUS LEVEL,WITH FURTHER ADDITIONS TO THE GENUS SARGASSOPSIS1

Sargassum subgenus Phyllotricha currently includes seven species restricted to Australian and New Zealand coasts. A recent study of Cystoseira and other Sargassaceae genera based on mitochondrial 23S DNA and chloroplast‐encoded psbA sequences resulted in the most widely distributed species of subgenus Phyllotricha, Sargassum decurrens, being transferred to the reinstated monospecific Sargassopsis Trevisan. The fate of the residual six Phyllotricha species, however, was not considered. The present study examines these Phyllotricha species, alongside other Sargassum subgenera, Sargassopsis, Sirophysalis trinodis (formerly Cystoseira trinodis) and the New Zealand endemic Carpophyllum Greville, using morphological evidence and the molecular phylogenetic markers cox3, ITS‐2 and the rbcL–S spacer. Our results suggest both the genus Sargassum and Sargassum subgenus Phyllotricha are polyphyletic as currently circumscribed. Four S. subgen. Phyllotricha species, i.e. S. sonderi, S. decipiens, S. varians and S. verruculosum, form a monophyletic group sister to the genus Carpophyllum, and S. peronii is genetically identical to S. decurrens with regard to all three loci. We propose the resurrection of the genus Phyllotricha Areschoug, with type species Phyllotricha sonderi, and include the new combinations Phyllotricha decipiens, Phyllotricha varians and Phyllotricha verruculosum. Sargassum peronii, S. heteromorphum and S. kendrickii are transferred to Sargassopsis and Sargassum peronii is considered a synonym of Sargassopsis decurrens.     

ISOLATION AND CHARACTERIZATION OF A DNA VIRUS INFECTING FELDMANNIA SIMPLEX (PHAEOPHYCEAE)1

We report on the isolation and characterization of a virus that is formed in modified zoidangia of the marine brown alga Feldmannia simplex (Crouan) Hamel (Ectocarpales, Phaeophyceae). Isolated virus particles had a buoyant density of about 1.35 g·mL^?1 in CsCl equilibrium gradients. They contained one major polypeptide (MW = 55,000) and at least six additional polypeptides (MW = 15,000–120,000). Four of these proteins were glycosylated. The viral genome consisted of double-stranded DNA and formed two freely migrating fractions in pulsed-field-gel electrophoresis, namely linear DNA with a size of 220 kilobase pairs, and fragments of 10–60 kilobase pairs. However, electron microscopic examination revealed that a substantial fraction of the viral DNA occurred as closed circles. We suggest that the viral DNA in native particles is circular but tends to break at random sites during the preparation.     

ANALYSIS OF SPATIAL AND TEMPORAL DIVERSITY AND DISTRIBUTION OF PORPHYRA (RHODOPHYTA) IN SOUTHEASTERN NEW ZEALAND SUPPORTED BY THE USE OF MOLECULAR TOOLS1

Molecular studies have shown that New Zealand’s rocky shores are a habitat for >30 species of Porphyra, but little is known of their seasonal and zonal distribution. The spatial and temporal distribution of bladed Porphyra gametophytes at Brighton Beach, southeast New Zealand, were monitored for 32 months. Molecular markers were used for species identification, and a total of nine species was observed as being present during this time. Two species, P. cinnamomea and Porphyra sp. “ROS 54,” were the most common, and both were present for most months, while the remaining seven species were present sporadically, for only a few weeks at a time. P. cinnamomea W. A. Nelson and Porphyra sp. “ROS 54” were most common in the midintertidal, and both showed a similar seasonality with the highest presence during spring. They also showed a similar trend of seasonal dieback resulting in at least 1 month (May) in two consecutive years when they were both absent. This is one of the few studies investigating spatial and temporal distribution within a genus and over a 3‐year period. Our results show no distinct intertidal zonation patterns within the genus, and we conclude that morphologically similar species in a similar habitat rely on physiological mechanisms for survival.     

ULTRASTRUCTURE OF SWARMERS IN THE LAMINARIALES (PHAEOPHYCEAE). I. ZOOSPORES1

Zoospores of 17 species in 14 genera of Laminariales, collected in the northeast Pacific Ocean, were studied by electron microscopy. These zoospores are unique in the brown algae in lacking both an eyespot in the single chloroplast and any associated swelling at the base of the shorter, posterior flagellum. Spores of all species examined possess a distal whiplash portion on the longer, mastigoneme-bearing anterior flagellum. This appendage may sometimes be as long as the mastigoneme-bearing portion of the flagellum, but it is only seldom preserved in the preparations for electron microscopy. A microtubular cytoskeleton is probably responsible for maintaining the shape of the spore. It consists of a short band of about 10 microtubules between the two basal bodies, scattered tubules converging at the anterior of the spore, a band of 7–9 tubules directed anteriorly from the anterior basal body, and a band directed posteriorly from the posterior basal body. These anterior and posterior bands may form one continuous band looping around the periphery of the spore. Variation with possible taxonomic significance was found in the ultrastructure of vesicles which apparently contain adhesive material, and which are extruded through the plasmalemma when the zoospores settle.     

GENETIC AND MORPHOLOGICAL IDENTIFICATION OF FUCUS RADICANS SP. NOV. (FUCALES,PHAEOPHYCEAE) IN THE BRACKISH BALTIC SEA1

Brown seaweeds of the genus Fucus occupy a wide variety of temperate coastal habitats. The genus is evolutionary dynamic with recent radiations to form morphologically distinct taxa. In the brackish Baltic Sea, fucoids are the only perennial canopy‐forming macroalgae. The most northern populations of Fucus occur permanently submerged in extremely low salinity (3–5 psu). These are currently referred to as Fucus vesiculosus L. but are morphologically distinct with a narrow frond without bladders. We report here that a population of this unique morphotype is reproductively isolated from a truly sympatric population of common F. vesiculosus and conclude that the northern morphotype represents a previously undescribed species. We describe Fucus radicans sp. nov., which is attached and dioecious with broadly elliptic receptacles, characterized by a richly branched narrow flat frond (2–5 mm), short thallus (<26 cm), and a high capacity for vegetative recruitment of attached plants. Analysis of five highly polymorphic microsatellite DNA loci showed genetic differentiation between sympatric populations of F. radicans and F. vesiculosus, whereas allopatric populations of the same species revealed a coherent pattern of genetic variation. Sequences of the RUBISCO region in F. radicans were identical to or differing at only one to two dinucleotide positions from those of F. vesiculosus, indicating a recent common origin of the two species.     

FREQUENT CLONALITY IN FUCOIDS (FUCUS RADICANS AND FUCUS VESICULOSUS; FUCALES,PHAEOPHYCEAE) IN THE BALTIC SEA1

Asexual reproduction by cloning may affect the genetic structure of populations, their potential to evolve, and, among foundation species, contributions to ecosystem functions. Macroalgae of the genus Fucus are known to produce attached plants only by sexual recruitment. Recently, however, clones of attached plants recruited by asexual reproduction were observed in a few populations of Fucus radicans Bergström et L. Kautsky and F. vesiculosus L. inside the Baltic Sea. Herein we assess the distribution and prevalence of clonality in Baltic fucoids using nine polymorphic microsatellite loci and samples of F. radicans and F. vesiculosus from 13 Baltic sites. Clonality was more common in F. radicans than in F. vesiculosus, and in both species it tended to be most common in northern Baltic sites, although variation among close populations was sometimes extensive. Individual clonal lineages were mostly restricted to single or nearby locations, but one clonal lineage of F. radicans dominated five of 10 populations and was widely distributed over 550 × 100 km of coast. Populations dominated by a few clonal lineages were common in F. radicans, and these were less genetically variable than in other populations. As thalli recruited by cloning produced gametes, a possible explanation for this reduced genetic variation is that dominance of one or a few clonal lineages biases the gamete pool resulting in a decreased effective population size and thereby loss of genetic variation by genetic drift. Baltic fucoids are important habitat‐forming species, and genetic structure and presence of clonality have implications for conservation strategies.