Abstract
BackgroundIn the brackish Baltic Sea, shedding of adventitious branches is central to asexual recruitment of new thalli in the brown algae Fucus vesiculosus and F. radicans. To test which factors influence the formation of adventitious branches in brackish and in more marine conditions, we sampled 29 Fucus sites in the Baltic Sea (salinity 3–11) and 18 sites from the Danish straits, Kattegat, Skagerrak, and the North Sea (salinity 15–35). Separately for each area, we used structural equation modelling to determine which of eight predictor factors (phosphate, nitrate, chlorophyll-a (as a proxy for turbidity), temperature, salinity, oxygen, grazing pressure, and thallus area) best explained observed numbers of adventitious branches.ResultsIn more marine waters, high yearly average values of phosphate, salinity and turbidity had positive effects on the formation of adventitious branches. In brackish-waters, however, high numbers of adventitious branches were found in areas with low yearly average values of temperature, salinity and oxygen. Grazing intensity had no significant effect in either of the two study areas, contrasting findings from studies in other areas. In areas with both sexually and asexually reproducing Fucus individuals, clones had on average more adventitious branches than unique genotypes, although there was strong variation among clonal lineages.ConclusionThis study is the first to investigate multiple potential drivers of formation of adventitious branches in natural populations of Fucus. Our results suggest that several different factors synergistically and antagonistically affect the growth of adventitious branches in a complex way, and that the same factor (salinity) can have opposing effects in different areas.
Highlights
In the brackish Baltic Sea, shedding of adventitious branches is central to asexual recruitment of new thalli in the brown algae Fucus vesiculosus and F. radicans
The adventitious branches resemble embryos, asexual reproduction of benthic thalli has not been observed in any Fucus species with one notable exception: in the Baltic Sea, detached adventitious branches produced by F. vesiculosus and F. radicans may reattach to the substratum by developing rhizoids, comparable to root hairs in vascular plants, and grow into new, seemingly functional male and female thalli, resulting in asexual recruitment of new individuals [15]
We show that the number of adventitious branches in natural populations of Fucus spp. are strongly affected by several environmental factors, in particular salinity, and that the effects of these factors differ between sites inside and outside the Baltic Sea: inside, the number of adventitious branches increased with declining salinity, whereas the opposite relationship was seen outside
Summary
In the brackish Baltic Sea, shedding of adventitious branches is central to asexual recruitment of new thalli in the brown algae Fucus vesiculosus and F. radicans. Species of Fucus are atypical in this respect: epidermal cells divide perpendicularly to the surface and grow outwards, such that they develop into distinct ‘embryos’, as described by McCook and Chapman [14], instead of forming lateral branches [14]. These ‘embryos’ are morphologically indistinguishable from sexually produced embryos (sporelings) at early stages of development [12]. The reattachment of loose adventitious branches in Baltic Sea Fucus spp. has resulted in clones spread over extensive geographic areas (550 km) [16, 17]
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