Rocky Sea Shore Report: Old Head

By Liam Loftus

Toothed/Serrated Wrack

Míoránach

Fucus serratus

Serrated Wrack is a shrubby, robust seaweed that grows in

high densities on the lower seashore. A member of the

brown algae group, it is olive-brown in colour and has a

flat, fan-shaped structure. The flattened blades have a

distinct midrib and are readily distinguished by the serrated

edge of the fronds. The entire plant can grow about five

feet long from a discoid holdfast and this allows it to attain

sufficient light to facilitate photosynthesis during high tide.

Serrated Wrack is found in dense populations on rocky substrates of sheltered areas of

coastline. It occupies the lowest area of the shore with Bladder Wrack Fucus vesiculosus just above

it and Egg Wrack Ascophyllum nodosum just below it. Serrated Wrack is unable to survive out of

water for long periods of time and the lower shore conditions require it to have a tough, flexible

frond in order to reduce damage from wave action, and a strong holdfast to prevent it from being

washed away in rough conditions.

It can support over 90 species of epiphytes (plants that grow on

other plants) as well as provide shelter for countless shore creatures that

can be found attached to its fronds. It is often covered by the tiny white

spiral tubes of the Coiled Tube Worm Spirorbis. Since many organisms

thrive on Serrated Wrack, the ability to grow quickly and reach a size

that prevents grazing is crucial for its survival.

Serrated Wrack is photoautotrophic and converts light into food

energy through photosynthesis. It is capable of absorbing carbon dioxide

and dissolved minerals from the water during high tide. It is single sexed and produces a large

percentage of germlings which provide food for grazing molluscs. Serrated Wrack has a low

dispersal rate and is very sensitive to changing conditions because of its lack of mobility at all life

stages. Its growth rate varies according to these conditions.

The distribution of Serrated Wrack extends along the entire Atlantic coast of Europe. It is

used in Ireland and France for the production of cosmetics and for thalassotherapy. In the Western

Isles of Scotland, it is harvested for use as a liquid fertilizer. This seaweed is also collected, dried

and used as a soil additive.

Channel Wrack

Caisíneach

Pelvetia canaliculata

Channel Wrack is a common brown algae found at the

highest level of the rocky seashore. It is highly tolerant

of desiccation and can survive out of water for several

days. It has a dark greenish-brown colour, a bushy

growth and channel-like gutters along one side of its

fronds. It contains no mid-rib and is anchored to the

substratum by a short holdfast (about l6 cm). Channel

Wrack is the only large algae growing on the rocks in

the upper littoral zone and it can spend as much as 90% of its time out of water while surviving

water loss of up to 65%. In this zone, fewer species can survive desiccation and as a result there is

less stress from competition.

Due to the inhospitable conditions of the upper littoral, the Wrack has adapted rolled fronds

that help it to trap the seawater and reduce water loss through evaporation. An oily layer over the

cell membrane also slows desiccation and a thick cell wall limits shrinking. It can survive lower

nutrient levels than other large algae and has a rapid recovery in

respiration and photosynthesis when the tide does return. It is a

very stress-tolerant alga and is well adapted to its niche at the

top of shore. Channel Wrack does however require periods of

exposure to the air and it will begin to decay if it is submerged

for more than six hours out of every twelve.

Channel Wrack is photoautotrophic which restricts it to

shallow, surface waters where there is an adequate amount of

light for photosynthesis. It is also hermaphroditic, having both

male and female structures on the same frond, which greatly increases the chances of fertilisation on

the upper shore. Its holdfasts support a rich diversity of fauna and it is often found alongside Egg

Wrack Ascophyllum nodosum. The lower limit of Channel Wrack is determined by its competition

with Spiral Wrack Fucus spiralis which is unable to tolerate the same desiccation and as a result is

found just below it. The physical factors associated with tidal emersion determine its upper limit.

Channel Wrack is found around the Atlantic shores, where it able to tolerate a wide range of

exposure conditions. In Ireland, it is known as Caisíneach and was collected as a source of

sustenance during the famine.

Flat or Purple Topshell

Faochán Mhuire Corcra

Gibbula umbilcalis

The Purple or Flat Topshell is incredibly widespread and

can be found worldwide. The shell is shaped like an old-

fashioned top; it is bluntly conical in shape, has a dull

greenish or creamy-yellow colour and is banded by broad

transverse stripes of reddish-purple. Topshells have a

hollow spiral through the centre of their shells and a

‘Mother-of-Pearl’ luster on their inside lip. Topshells are

molluscs, and have a flat, circular trap door called an operculum at the end of a muscular foot. This

provides them with protection whenever they withdraw inside their shells.

Flat Topshells are usually found under stones on sheltered rocky shores and extend from the

upper shore into the sublittoral. Their flatter shape allows them to withstand the stronger tidal

conditions experienced at lower levels. Topshells are also able to tolerate long periods of time out

of water. Their operculum can close across the shell opening to reduce water loss during low tide.

Purple Topshells are also more tolerant of raised

temperatures and are often found in abundance on the

seaweeds present in the middle and upper parts of the shore.

Topshells have separate and distinct sexes. Fertilization

takes place externally and they have an average life span of

about 10 years.

Flat Topshells are herbivores and graze the rocky

surfaces. In removing microalgae and detritus from rock

surfaces they play an important role within their environment. They remove algae and create a vital

space for Barnacles to thrive. They are primitive grazers and feed by rasping algae and small

detritus off rocky surfaces. All Topshells live in rocky seashore habitats and are widespread across

European coasts. Female Topsnails are larger than males and are a popular source of food in some

areas of the world. The commercial Trochus Shell Trochus niloticus is used to make pearlized

buttons for clothing sold throughout the world and is one of the leading industries in the Philippine

Islands.

The Common Acorn Barnacle

Garbhán Carraige Coiteann

Semibalanus balanoides

The Acorn Barnacle is a common and widespread crustacean

species found on large areas of the upper levels of the rocky

shore. They are protected by a hard, cone-shaped shell made up

of six plates; at the top of the cone is an opening which is

covered by four further plates. Barnacles are sessile suspension

feeders and grow their shells directly onto the substrate. They

obtain their food by filter-feeding or detritus gathering. When

submerged they extend their cirri to catch plankton and detritus from the water. Their feathery barbed

legs strain out plankton and absorb oxygen from the water because they have no gills. Most Barnacles

are hermaphroditic and are unable to leave their shells to mate. The males have extraordinarily long

penises to facilitate reproduction.

Barnacles live in tightly constrained locations within the intertidal zone and are usually visible

in great numbers at low tide. This can be seen as a distinct grey-white line that marks the upper limit of

the eulittoral zone. They are present particularly amongst Spiral Wrack Fucus spiralis and Bladder

Wrack Fucus vesiculosus. Barnacles are encrusters and permanently attach themselves to hard substrata

which afford them some protection against desiccation when exposed above the waterline. Barnacles are

able to attach themselves to the hard substrata by releasing a strong chemical solution. For this reason

they are used worldwide by industries in the production of one of the strongest known natural adhesives.

Barnacles are well adapted against water loss. Their

calcite shells are impermeable and they possess two plates which

they can slide across their aperture when not feeding. These

plates also protect against predation. Whelks Buccinum undatum

are their most common predators. They are able to grind through

the calcareous exoskeletons of barnacles and feed on the softer

inner parts. Mussels Mytilus edulis also prey on Barnacle larvae.

Predation sets the lower limits for Barnacles and competition with other species restricts them from

extending too far up the shoreline. In the intertidal zone Barnacles are in constant competition for space

with Limpets Patella vulgata. They employ two strategies to overwhelm competitors: ‘swamping’ and

fast growth. With swamping, vast numbers of Barnacles settle in the same place at once and thus cover a

large patch of substrate. Fast growth allows the suspension feeders to access higher levels of the water

column than their competitors and consequently to be large enough to resist displacement.

Bibliography

Books

Challinor, H. & Clark, J., 2003, A Beginner’s Guide to Ireland’s Seashore. Sherkin Island Marine

Station: Ireland.

Fish, J.D. & Fish, S., 1996, A Student’s Guide to the Seashore. Cambridge University Press:

London.

Gibbons, B., (1992). Seashore Life of Britain and Europe. New Holland, U.S.A.

Hawkins, S.J. & Jones, H.D., 1992, Marine Field Course Guide 1: Rocky Shores. IMMEL

Publishers: London.

Hayward, P., Nelson-Smith, T., Shields, C., 1996, Collins Pocket Guide: Sea Shore of Britain &

Europe. HarperCollins Publishers Ltd.: London.

Schaefer, L., (2003). Barnacles: Read and Learn Series Sea Life. Raintree, UK.

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