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The Rocky Sea Shore - Habitat Survey
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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.
Websites
Oracle Think Quest – Barnacles, online, retrieved October 2010, from
http://library.thinkquest.org/J001418/barnacles.html
Encylopaedia.com – Barnacles, online, retrieved October 2010, from
http://www.encyclopedia.com/topic/barnacle.aspx
Helium – Interesting Facts about Barnacles, online, retrieved October 2010, from
http://www.helium.com/items/1271747-what-are-barnacles
The Seashore – Channel Wrack, online, retrieved October 2010, from
http://www.theseashore.org.uk/theseashore/SpeciesPages/Additional%20Species/Pelvetia.jpg.html
Wild Ocean Photography: Marine and Ocean Photography – Channel Wrack, online, retrieved
October 2010, from http://www.wildoceanphotography.com/photo1762904.html
The Seashore - Topshells, online, retrieved October 2010, from
http://www.theseashore.org.uk/theseashore/SpeciesPages/Topshells.html
Forafood.com – Topshells, online, retrieved October 2010, from
http://www.forafood.com/topshell.htm
BBC Home – The Blue Planet: A Natural History of the Oceans – Serrated Wrack, online, retrieved
October 2010, from
http://www.bbc.co.uk/nature/blueplanet/factfiles/plants/serrated_wrack_bg.shtml
Oracle Think Quest – Serrated Wrack, online, retrieved October 2010, from
http://library.thinkquest.org/06aug/00438/mandy/serratedwrack.html