Easily overlooked among the more commonly-occurring seaweeds, Grapeweed, Mastocarpus stellatus (Stackhouse), is an unusual-looking red seaweed. I discovered it for the first time on the rocky promontory called Frenchman’s Ledge at Osmington Bay, Dorset. Initially, the impression is that the ledge has a wide strip of seaweed mostly occupied by a thriving bed of the olive green Toothed Wrack, Fucus serratus Linnaeus – one of the brown seaweeds or Phaeophyceae which I’ve described in earlier posts.
This is a locational shot showing the view from Frenchman’s Ledge looking west across Osmington Bay, Dorset.
This photograph shows the view looking southwards out across the English Channel, and along the gently curving length of the rocky promontory called Frenchman’s Ledge. You can see a long thin strip of dark seaweed against the lighter coloured rock along its entire length – but restricted to just one side of the ledge.
Getting up closer, it looks as if most of the seaweed is Fucoid or brown seaweed with a few patches of brighter green and red seaweed visible at the edge but possibly extending in small numbers under the brown weed.
Closer still, it is easy to identify the main component of the seaweed bed as Toothed Wrack, Fucus serratus Linnaeus.
However, at the western edge of the Toothed Wrack individual plants of both red and green seaweeds are visible. The larger red/brown/purple seaweed in the picture above is Grapeweed, with the extremely long Latin name of Mastocarpus stellatus (Stackhouse) Guiry (Gigartina stellata).
As with the Pepper Dulse described in yesterday’s post, the colour of the Grapeweed varies with shore level, being more purple-brown or even blackish lower down and greener at higher shore levels. The fronds can be up to 13 cm long; and the upper fronds displayed in the above picture are mostly lacking reproductive bodies or these are in the early stages of development.
The outer edges of the flat fronds tend to roll or curve inwards forming a shallow gutter or groove (see the picture above). As in Channelled Wrack, the fronds lack a midrib. However, Grapeweed is a red alga compared with Channelled Wrack which is a brown seaweed.
On both surfaces of the tips of most fronds in Grapeweed are numerous small elongate reproductive bodies which superficially resemble grape pips – hence the common name of Grapeweed for this red alga (see the picture below). This feature also differentiates Grapeweed from Channelled Wrack in which the reproductive bodies occupy the entire swollen bifid frond tip.
On an exploitation note, Grapeweed is sometimes used as a substitute for agar production.
Revision of a post first published 16 June 2009
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