Seaweeds

Lundy has a huge diversity of seaweeds. With just 15km of coastline over 317 different species have been identified around the island.

Depth is a very important factor for plants, as they must have light in order to live. Water absorbs light, and as a consequence less light penetrates into deeper water, so plant-life is restricted to shallower water. Around Lundy, plants can be found from the top of the intertidal zone to a depth of 15-20m.

Oarweed (Kelps)

© Keith Hiscock

Dark olive brown in appearance, it has a long flexible stem and branched holdfasts which are often seen washed up ashore after stormy weather.  Communities of small animals live in the holdfast, while colonial animals live on the flat blades.  Huge kelp 'forests' including other species of kelp provide a home for a wide variety of different animals and are special and thriving habitats on the island.

 

Wire weed - Sargassum muticum

© Keith Hiscock

Sargassum muticum, commonly known as wire weed, strangle weed or ‘Jap’ weed, is a perennial brown seaweed that grows just below the low water mark in the shallow sublittoral or in areas of standing water on the lower shore.

Sargassum muticum is a member of the fucoid family, but is distinctly different to the wracks found around the UK coastline. Normally brown, but colour varies to orange and light yellow with high light intensity and high surface water temperatures. S. muticum originates from the western Pacific, where within its natural range (south-east Asia to southern Russia), it is one of the smaller Sargassum species, growing to around 1m long.

Sargassum muticum is a non-native species that has been accidentally introduced to other parts of the world and once removed from the constraints of its native environment, it becomes an aggressive coloniser and grows larger. There are concerns that Sargassum muticum competes and displaces indigenous species of brown seaweeds, and on Lundy steps are being taken to remove it from the areas where it has colonised in the Landing Bay and Devil's Kitchen.

There are a number of other implications relating to recreational and commercial uses of coastal areas where dense growth occurs - floating mats of drifting Sargassum muticum can potentially affect water sports and sailing. It can foul fishing lines and nets and the steering gear of small boats as well as trapping marine debris and being cast ashore.

 

Knotted Wrack - Ascophyllum nodosum

© Keith Hiscock

Knotted wrack belongs to a group of seaweeds which often dominate the rocky shore around Lundy and various characteristics make it an easily identifiable seaweed of the shore. Attached firmly by a disc-like holdfast, several narrow long fronds arise which can grow to about one metre in length and longer in very sheltered situations.  At intervals along the frond single egg-like air bladders grow, and which gives rise to its other name 'egg wrack'.  The seaweed produces about one of these bladders every year which makes it possible to estimate the age of the seaweed by counting a series of bladders.  The bladder's main purpose is to hold the plant afloat in the water to gain maximum light.   In summer small reproductive organs grow on short side branches.

 

Sea Lettuce - Ulva lactuca

Sea lettuce is common worldwide on sea shores and shallow sublittoral areas growing in a wide range of conditions and habitats and Lundy is no exception.  Its frond is a bright green flat sheet, translucent in appearance and often split or divided with a wavy edge.  In general it is shorter on exposed shores and on sheltered ones can grow much longer.  Grazed by numerous animals on the shore it is often seen on the sea floor detached and in tatters providing other animals an opportunistic meal.  The sea weed reproduces by releasing gametes from some cells, and it can also spread vegetatively by regeneration of small fragments.

 

Channelled Wrack - Pelvetia canaliculata

© Keith Hiscock

This small and brown bushy seaweed derives its name from the channel-like gutters along its fronds and is attached to the rock surface by a small holdfast.  It is a common alga found at the highest level on the rocky shore and its position is a good indicator of the high-water mark.  However its position on the shore is inhospitable due to the short time covered by the tide and as a result the growth is quite small.  It has a number of remarkable adaptations that give it the chance of survival high up the seashore, not found in the algae lower down. Special adaptations include rolled fronds to reduce the water lost by evaporation; trapping water with the channels in the frond; a fatty (oily) layer over the cell slows the desiccation by stopping water evaporating and a thick cell wall which shrinks with drying.  The seaweed can lose 90% of its water and rehydrate very rapidly in about 20 mins of being covered again by the tide. 

When mature, the alga produces swollen tips to the fronds. It is these receptacles which contain the reproductive conceptacles.  Being hermaphroditic each plant has both male and female structures on the same frond. The conceptacles ripen in summer with the release of gametes in September to coincide with the high spring tides. Fertilisation is therefore external and is improved by having both male and female sexes in close proximity on the same plant as immersion by the tide is brief.

 

Coral Weed

© Keith Hiscock

Coral weed is a striking, characteristically hard structured pink-red seaweed, which belongs to the coralline group of seaweeds. The individual fronds of the seaweed are made up of interlinking ridged sections held together by flexible joints and can grow up to 12 cm long. It is chalky deposits in the seaweeds cell wall structure which gives them a hard structure, and is particularly easy to identify as the seaweed often covers large areas in deep rockpools and low tide areas of the rocky shore.