Monday, 21 January 2013

Common seaweeds of the shore -Wracks (Part 2)

Below are images of brown seaweeds, other than fucoids that come under the generic term 'wracks'. There are more species than shown here, as only those for which I have photos are shown.

Thong weed (Himanthalia elongata) is  olive brown with long strap like dichotomous branching fronds. Himanthalia elongata occurs at the low water mark and into the shallow subtidal.

Thong weed (Himanthalia elongata)  fully submerged.

Early growth stages of Thong weed (Himanthalia elongata) from which the long fronds grow.  

Bifurcaria bifurcata is a brown seaweed with cylindrical dichotomous branching fronds. Bifurcaria bifurcata occurs in rock pools of the lower shore.

Sargassum muticum is a brown bushy weed with small spherical air bladders. It occurs in rock pools of the lower shore and shallow sublittoral. Sargassum muticum is an invasive species that often out competes other seaweeds and can block out the sun from rock pools.
Bifurcaria bifurcata  and Sargassum muticum can dominate lower shore pools.
Rainbow wrack (Cystoseira tamariscifolia)  is an iridescent bushy weed found in pools of the lower shore. There are a number of 'bushy' wracks belonging to the genus Cystoseira in the UK.
Sea Oak (Halidrys siliquosa) has alternate branching with oblong air bladders. It occurs in pools of the lower shore and shallow sublittoral.

Friday, 4 January 2013

Common Seaweeds of the shore - Wracks (Part 1)

Seaweeds are often the most conspicuous organisms of the rocky shore and are vital to the intertidal ecosystem, providing food, shelter and a substrate upon which to grow.

There are hundreds of species of seaweed in the UK and these are broadly divided into groups depending on their colour: reds (Rhodophyta), greens (Chloropyhta) and browns (phaeophyta).

The most familiar of the brown seaweeds are the wracks. Wracks can dominate the shore and are often used as the characterising species of a habitat.  For example LR.LLR.F Fucoids on sheltered marine shores.  Wracks can exhibit zonation where horizontal bands of the shore are characterized by a particular species of wrack, these patterns can be useful in inferring the hydrodynamic regimes of a shore and extent of exposure during low tide.

Below are images of wracks you may encounter whilst rock pooling in the UK.

Channel wrack (Pelvetia canaliculata), named after the furrow or, channel that forms as a result of the rolled fronds. Channel wrack is found in the upper shore of sheltered to exposed rocky shores.
Channel wrack (Pelvetia canaliculata)

Channel wrack (Pelvetia canaliculata)

Spiraled wrack (Fucus spiralis), is twisted as it hangs and has a midrib running down each frond. Spiraled wrack occurs in the upper shore below the band of channel wrack

Spiraled wrack (Fucus spiralis)

Bladder wrack (Fucus vesiculosus) has lots of swollen sacks formong pairs either side of its frond. Bladder wrack is found on exposed and sheltered coasts on the mid to lower shore.

Egg wrack (Ascophyllum nodosum) is long with a more rubbery texture than bladder wrack with oval to egg shapped bladders within its fronds. Egg wrack occurs on the mid shore and in sheltered locations can form huge expances.

Egg wrack (Ascophyllum nodosum)

Serrated wrack (Fucus serratus) has a toothed edge to its frond which is also hairy, it has no bladders and is found in the mid to lower shore where it can dominate.
Serrated wrack (Fucus serratus) dominates the lower shore

Serrated wrack (Fucus serratus), close up showing the hairy frond