Rocky Sea Shore Report: Old Head
By Liam Loftus
Toothed/Serrated Wrack MĂorĂĄnach 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 CaisĂneach 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 CaisĂneach 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 oldfashioned 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 GarbhĂĄn 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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