Fucus spiralis L.    
Spiral Wrack

Kingdom: Chromista
Phylum: Phaeophyta – brown seaweeds
Order: Fucales

British distribution: Around the entire coastline.
World distribution: ?

Fucus spiralis on rocky shore
Fucus spiralis on rocky shore
Fucus spiralis on rocky shore, Isle of Great Cumbrae, April 2009.

Fucus spiralis is one of our commonest and most familiar seaweeds. It typically forms a zone high on rocky shores, below Pelvetia but above the other large brown seaweeds (e.g. Ascophyllum nodosum, Fucus vesiculosus) that characterise the mid-shore.

Well grown plants are usually easily recognisable by the flattened, twisted, dichotomously branched thallus, lacking bladders, and the large, oval receptacles at the frond tips, each receptacle being surrounded by a narrow rim of vegetative frond. Nevertheless, younger plants are not always so easy to identify, and even mature plants can be confused with F. ceranoides or with bladderless forms of F. vesiculosus. Both of these species, however, have narrower, more pointed, rimless receptacles.

Life cycle
Like other members of the Fucales, Fucus species are gametophytic diploids, lacking any separate haploid generation. Arguably the gametophyte itself is merely a transient cellular stage within the tissues of the receptacle. Pits in the receptacle are the conceptacles, which release egg cells (female gametes) and spermatozoids (motile male gametes) into the sea (forced out by the secretion of mucilage). Spermatozoids are chemically attracted (i.e. by a pheromone) to the egg cells, the resulting fertilised zygote eventually forming a new diploid plant.
Since, unusually for Fucus, F. spiralis is monoecious (i.e. lacks separate male and female plants), the conceptacles are hermaphrodite.

A more detailed analysis of the sexual cycle of Fucus is beyond the aim of this page, but can be found in textbooks such as van den Hoek et al. (1995).

•   van den Hoek, C., Mann, D.G., & Jahns, H.M., (1995). Algae: and introduction to phycology. Cambridge University Press.

© A.J. Silverside
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