Untangling the literature on unattached Fucus (Phaeophyceae): an overview of current knowledge

The genus Fucus includes several ecologically significant species that are well-known as foundational to ecosystems on sheltered and moderately exposed rocky shores in the northern hemisphere. Less well-known are the perennial populations of unattached Fucus occurring in sheltered environments on any substratum. These are polymorphic and occur in a range of habitats including embayments, saltmarshes and estuaries, creating structurally complex habitats in areas generally considered atypical for Fucus. This review considers the literature on unattached Fucus from more than 200 years of research. Six main aspects of unattached Fucus are discussed: nomenclature, geographic distribution, morphology, reproductive mode, ecological significance and potential threats. Special focus is given to unattached forms within the Baltic Sea. At least six taxonomically recognized species are associated with unattached forms, classified at both inter- and intraspecific levels. Unattached forms have a wide geographic distribution, although the extent is likely to be currently under-represented. Both convergent and divergent morphologies are common within and among unattached Fucus species and several transitional series and morphological gradations are observed. Asexual reproduction is presumed to be dominant, although further investigation is needed to determine the sexual reproductive capabilities of unattached forms. Unattached forms play a part in structuring their habitats, support numerous ecological processes and facilitate high biodiversity. Unattached populations are exposed to anthropogenic threats due to their life histories as well as because their habitats are vulnerable. Overall, unattached forms are important distinct ecological entities that require management and conservation efforts.