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Many of the marine species that were introduced to the Baltic Sea during the Littorina stage (c. 8500–3000 years BP), e.g.Fucus vesiculosusandF. serratus, have adapted to the present low salinity. These marine species have gone from marine conditions into lower salinity environments. In this paper we ask why the recently discovered endemic brown algaFucus radicansshows the opposite pattern.Fucus radicansis only present in the northern parts of the Baltic Sea, the low salinity Bothnian Sea (4–6 psu). Potentially, the fitness ofF. radicansmight be reduced in higher salinities if it is better adapted to brackish conditions. We hypothesize, however, that the southern distribution limit ofF. radicansis set by biotic factors, e.g. competition withF. vesiculosusand higher grazing pressure byIdotea balthicaand not by salinity. Our results show that the reproductive output ofF. radicansis limited by low salinity (4 psu) but increases in higher salinities. However, the southern distribution limit, i.e. the northern Baltic Proper, is regulated by biotic factors, where the additive effects from shading by tallerF. vesiculosusthalli and grazing onF. radicansby the isopodI. balthicalimit the biomass production ofF. radicans. We suggest thatF. radicansstill maintains marine traits due to its ability to propagate clonally and is restricted to the Bothnian Sea by interactions withF. vesiculosusandI. balthica. We also propose that increased precipitation due to climate change might affect the northern range limit and that the distribution ofF. radicanscould be expected to shift further south into the Baltic Proper. [ABSTRACT FROM AUTHOR]

Parallel evolution has been invoked as a forceful mechanism of ecotype and species formation in many animal taxa. However, parallelism may be difficult to separate from recently monophyletically diverged species that are likely to show complex genetic relationships as a result of considerable shared ancestral variation and secondary hybridization in local areas. Thus, species' degrees of reproductive isolation, barriers to dispersal and, in particular, limited capacities for long-distance dispersal will affect demographical structures underlying mechanisms of divergent evolution. Here, we used nine microsatellite DNA markers to study intra- and interspecific genetic diversity of two recently diverged species of brown macroalgae, Fucus radicans (L. Bergström & L. Kautsky) and F. vesiculosus ( Linnaeus), in the Baltic Sea. We further performed biophysical modelling to identify likely connectivity patterns influencing the species' genetic structures. For each species, we found intraspecific contrasting patterns of clonality incidence and population structure. In addition, strong genetic differentiation between the two species within each locality supported the existence of two distinct evolutionary lineages ( FST = 0.15-0.41). However, overall genetic clustering analyses across both species' populations revealed that all populations from one region ( Estonia) were more genetically similar to each other than to their own taxon from the other two regions ( Sweden and Finland). Our data support a hypothesis of parallel speciation. Alternatively, Estonia may be the ancestral source of both species, but is presently isolated by oceanographic barriers to dispersal. Thus, a limited gene flow in combination with genetic drift could have shaped the seemingly parallel structure. [ABSTRACT FROM AUTHOR]

Reproduction of attached large brown algae is known to occur only by sexual zygotes. Using microsatellites we show evolution of asexual reproduction in the bladder wrack promoting population persistence in the brackish water Baltic Sea (<6 psu). Here a dwarf morph ofFucus vesiculosusis dominated by a single clone but clonal reproduction is also present in the common form of the species. We describe a possible mechanism for vegetative reproduction of attached algae, and conclude that clonality plays an important role in persistence and dispersal of these marginal populations, in which sexual reproduction is impaired by low salinity. [ABSTRACT FROM AUTHOR]

The reproductive state of Ceramium tenuicorne (Kütz.) Wærn was studied in a salinity and latitudinal gradient near its inner distribution limit in the brackish Baltic Sea, and in laboratory growth experiments on the gametophytes and tetrasporophytes.Along the gradient,a strong reduction in sexual reproduction and an increased tetrasporophyte dominance were observed. The observed changes may not only be explained by salinity constraints. Growth occurred between 1 and 24 PSU and the optimum range for growth corresponded well to the salinity regime of the area studied. Field observations showed that sexual reproduction is at least occasionally possible in salinities around 4 PSU. It is likely that severed climatic conditions constrain sexual reproduction towards the north,but C.tenuicorne remains an ecologically dominant species in large parts of its distributional area.Viable populations are maintained essentially by vegetative reproduction,such as fragmentation and rhizoidal re­attachment.Observations on vegetative propagules were sparse,but these may serve as an important physical escape from temporarily unfavourable conditions in areas where sexual reproduction is unreliable. [ABSTRACT FROM AUTHOR]

The spread and establishment of the exotic species Fucus evanescens in öresund, Southern Sweden was documented through inventories in 1966–72 and 2000. The species spread fast from the first known introduction in 1955, colonising harbours along the coast. After this expansion the further spread has been limited and the species is still largely confined to harbour areas, where it occurs in the same depth zone as its native relatives, F. vesiculosus, F. serratus and Ascophyllum nodosum. It is more common in the northern part of the area and has not expanded into the Baltic Sea proper. Tests of the attachment strength of F. evanescens plants suggest that the restriction to harbours is not due to the direct effects of wave exposure on adults. In the laboratory, reproductive success of F. evanescens decreased from 99% in 24 psu to 12% in 10 psu and at lower salinity reproduction failed. Growth of embryonic recruits was similarly affected by salinity. Hence, low salinity explains the limited success of F. evanescens in southern Öresund, where the salinity is low and fluctuating, and its failure to colonise the Baltic Sea proper. [ABSTRACT FROM AUTHOR]

This paper describes the large-scale geographical distribution, population structure, and reproductive season of the brown algae Fucus serratus L. in the Baltic proper. We have also experimentally tested fertilisation success and germination of F. serratus zygotes in a salinity gradient with the aim of finding out whether an adaptation to reduced salinity has occurred. Fucus serratus was found to be belt-forming along the shores of southeastern Sweden and scattered populations were found up to the middle of the Baltic proper. The ratio of non-reproductive (juvenile) plants per reproductive (adult) plant decreased significantly from 37 ± 8 S. E. in the south to 0.4 ± 0.2 in the north. Fucus serratus was reproductive during October to November on the mainland coast, while the populations on the southeast coast of Öland were reproductive during June to July. Laboratory experiments showed that successful fertilisation of Baltic F. serratus zygotes decreased drastically from 86.9 ± 1.0% S. E. at 9 psu to 5.5 ± 0.5% S. E. at 6 psu. Fucus serratus zygotes from the western coast of Sweden needed a significantly higher salinity for successful fertilisation than the Baltic gametes did, and at 8 psu, only a few eggs were fertilised (5.5 ± 0.5% S. E.). Since a relatively high proportion (35%) of the Baltic F. serratus zygotes germinated in 7 psu, (the salinity at the northernmost populations) we discuss other factors such as ice-scraping and eutrophication as possible factors limiting the geographical and vertical distribution of the species in the Baltic Sea. [ABSTRACT FROM AUTHOR]

Geographical morphological variations in Fucus vesiculosus populations were studied along the coast of Iceland. Principal component analysis (PCA) based on 11 morphological characters clustered the 26 sampling sites into four different morphological types as follows: (1) a morphological type found along the S-SW-W coast; (2) an intermediate form comprising only two populations in the NW; (3) a type found along the N-NE-E-SE coast; and (4) a type found independent of geographical area, in estuaries and at sites influenced by large freshwater outflows. Populations influenced by low salinity had significantly (P ≤ 0·001) shorter thalli, a shorter distance from the holdfast to the oldest dichotomy, smaller fronds, narrower stipes and midrib width compared to the morphology of all other more saline populations. No significant difference in frond width was found between the S-SW-W and the N-NE-E-SE populations. However, significant (P ≤ 0·001) morphological differences between them were observed, the former having shorter thalli, a greater distance from the holdfast to the oldest dichotomy, narrower stipes and smaller midrib width compared to the latter. [ABSTRACT FROM PUBLISHER]

An abstract of the article "Rapid Speciation of a Brown Alga in the Baltic Sea ," by R. T. Pereyra and colleagues is presented.

An abstract of the article "Reproductive Barriers in the Sympatric Seaweeds Fucus Radicans and F. Vesiculosus (Phaeophyceae)," by A. H. Forslund and L. Kautsky is presented.