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DescriptionHeterosiphonia plumosa is a red to deep crimson seaweed which appears black when dried. This species has a flattened, fern-like appearance with a hairy thallus growing from a discoid holdfast. The fronds are flat or slightly cylindrical up to 20 cm in length and 0.5 cm in diameter at the base, tapering towards the apex. The primary branching from the main frond occurs in a single plane, and is alternately, yet irregularly spaced with up to 1 cm between each branch. Each branch is bare at the base, with the rest bearing an irregular and alternately arranged series of smaller secondary branches. The secondary branches are progressively shorter towards the apex, and each branch bears numerous pointed branchlets giving an overall tufted and feather-like appearance.Heterosiphonia plumosa is a perennial (life cycle >2 years) species, reproducing mainly during the summer and autumn. The cystocarps of Heterosiphonia plumosa are ovate and borne near the base of the branchlets, as are the tetrasporangia in asexual individuals, which are lanceolate and stalked (Dickinson, 1963). Both can be easily distinguished under a hand lens.
Heterosiphonia plumosa has several recorded variations, most notably Heterosiphonia plumosa f. palens which is typically slender and up to 7.5 cm in length. The fronds are smooth with evenly spaced branches and commonly occurs in deeper coastal waters (Dickinson, 1963).