Between the Tides, Part 2


Gut Weed covering the upper shore.

Living conditions and the resulting problems differ greatly throughout the intertidal zone and most plants and animals have adapted to a certain part or zone of the rocky shore habitat. The rocky shore can be divided in either biological zones or physical zones. Biological zones look at the resident flora and fauna and takes its nomenclature from the prevailing colour the rock has been clad in. The orange zone hosts lichen of the orange and yellow kind as well as salt tolerating plants, the black zone is dominated by black tar lichen, the grey zone can be identified by their barnacle and limpet populations and the brown zone is home to the various brown seaweed species.

Physical zones on the other hand are defined by their immersion or emersion time and are measured in meters above CD (chart datum) which is the waterline at the lowest spring tide.

The sublittoral zone sits at the bottom of the shore and always stays under water even at the lowest spring tides. The lower shore only gets uncovered at spring tides, the middle shore is always exposed at low tide and covered at high tide and the upper shore is only immersed at spring tides. The splash zone sits above the highest spring tide water mark and while plants and animals in this zone might get splashed with seawater this zone never gets covered with water.

The splash zone is a good spot to sit down and have a look over the rocky shore before exploring in more detail. In spring this spot is likely to display one of Ireland’s most delightful wildflowers. Thrift, also known as sea pink, is one of the most common coastal plants and can cover roadsides, cliff tops and dune edges in a sea of various shades of pink, some flowers can also go to extremes and be white or an almost crimson red. If necessary single plants are complacent with the tiniest amount of dirt and grow out of narrow crevices or shallow hallows in the rock and are also not an unusual sight on walls. Often close by but far less conspicuous stand rock samphire, the leaves of which have traditionally been pickled or used fresh in salads, and the scurvy grass with its tiny white flowers. The leaves of this perennial are high in vitamin C and before citrus fruit were widely available scurvy grass was a main stable on ships to prevent the disease that gave the plant its name.

Rock Samphire (Crithmum maritimum)

Rock Samphire

The rocks those flowers are surrounded by are often covered in a colourful and intriguing mantle. Lichens are curious life forms that have been around for some 400 million years. They have been and still sometimes are classified as plants but lichen should rather be described as a community of certain life forms than an individual organism. For a long time it was though that lichens are a simple symbiosis between a fungus and an algae where the fungus provides housing and water for the algae and in return receives food in form of sugars produced by the algae through photosynthesis. Only in recent years it became clear that at least some lichen species consist of more than just one fungus and one algae. Some lichen have a 2nd fungal partner, which is often a yeast, that is thought to be responsible for the structure of the lichen. Many lichen also have cyano- or other bacteria as part of their community for functions like nutrient transfer between the symbionts or defense against outside threats.

Lichen grow very slow but can live a very long time, 100 years or even older is no rarity, and survive in extreme environments. A lichen that was brought to the international space station survived for 15 days in the vacuum of space and regular temperature swings from -12 to +40 degree. Life on a rocky shore must be a holiday in comparison.


A selection of lichen.

One of the most beautiful sights in the splash zone is sea ivory, a fruticose lichen (lichen with a bushy growth structure), growing alongside the yellow, foliose lichen (lichen with a lobed or leaflike shape) Xanthoria which is commonly known as orange or sunburst lichen. These lichen can cover vast areas of rock and often intermingle with white lichen like Lecanora gangaleoides and  Ochrolechia parella. Telling the different but often very similar looking lichen species apart is more than difficult and more often than not the use of a hand lens or even a microscope is necessary. Further down the shore, on the upper shore and even parts of the middle shore, the black tar lichen and green tar lichen are very common. Both can tolerate some immersion in salt water and often thrive side by side with seaweeds.

Seaweeds are the most obvious rocky shore dwellers and inhabit the complete intertidal zone as well as the shallow sublittoral zone. Seaweeds are part of the algae family which in the widest sense is part of the plant world. Some microalgae, microscopic algae that exist as a single cell or string of cells, are classified as protist which is a fancy way of saying they can’t be put in either the plant or animal kingdom. Seaweeds are classified as macroalgae and while many seaweed species share only a very distant relationship to the land living plants they all share one characteristic: They produce the nutrients they need to grow and survive mainly through photosynthesis.



A selection of seaweeds.

This photosynthesis can only take place, as already mentioned, when the seaweed is immersed in water. To catch enough light however most seaweed species have to stay more or less close to the surface which is the reason most seaweeds thrive only in the intertidal or shallow sublittoral zone. Only few species can be found free-floating in the open seas. In order to photosynthesize all seaweeds have chlorophyll in their cells, just like their cousins on dry land, but many seaweeds don’t appear green and some don’t even look anything plant-like. To stay in place most seaweeds anchor themselves with a holdfast, a structure resembling a small plate often with short, root-like fingers. In addition to this mechanical structure seaweeds also secrete an adhesive compound made of polysaccharides and proteins.

As seaweeds are the only plants around they are the only food choice for all the grazing animals on the shore. To avoid being constantly nibbled on some seaweeds have developed chemical warfare and can produce anti-grazing compounds like tannin and terpenes which makes them unpalatable. But not only that. Seaweeds can also warn their neighbours of potential attacks by periwinkles and other grazers. Like trees and other land based plants seaweeds also can communicate with each other.

Seaweeds are separated into three groups: Red seaweeds, green seaweeds and brown seaweeds. The red seaweeds can be found mainly on the lower shore and can be traced back some 1.2. billion years which makes them the oldest seaweed group around. Their red colour comes from the pigments phycocyanin & phycoerythrin which allows them to photosynthesize at the low light levels that regularly occur on the lower shore. Common species are the Irish moss, also known as carragheen and the pepper dulse. Also a part of the red seaweeds are the corallines like the coral weed and pink paint weed. Both integrate calcium carbonate into their cell walls which hardens their structure. While coral weed still has a seaweed-like appearance the pink paint weed is an encrusting seaweed and forms a continuous and colourful cover on the rock surface which can range from almost white to a deep pink.



Green seaweeds have chlorophyll B as their second pigment which gives them the green colour we expect from plants. It is not surprising then that green seaweeds are very closely related to land plants and most species thrive in freshwater far away from the coast. The most common of the few marine species are the various sea lettuce species and gut weed which are both the main food source of the rocky shore grazers. Unlike their red and brown cousins green seaweeds don’t produce any ant-grazing compounds which makes them an easy meal.

The youngest of the seaweeds, they only developed some 200 million years ago, are the brown seaweeds. They contain fucoxanthin and chlorophyll C to give them their brownish appearance. The brown seaweeds are the big seaweeds, the likes of the wracks and the kelps that mainly thrive in colder and nutrient rich waters like the north Atlantic.

Apart from their size and colour the brown seaweeds show a major difference to the reds and greens on a cellular level. The chloroplast is the part of a plant cell that conducts photosynthesis. It is thought that once upon a time an early plant cell engulfed a free living cyanobacterium which then became the host’s chloroplast, a partnership known as endosymbiosis. The chloroplasts of the red and green seaweeds have two membranes. The chloroplasts in brown seaweed however have four membranes, two from the original cyanobacterium and another two from the host cell. Brown seaweeds must at some stage of their evolution have engulfed a cell that already contained a chloroplast, for example a red or green seaweed cell.

The best known of the brown seaweeds are the various wrack species. The previously mentioned channel wrack spends the majority of its life out of water and has a fungal partner to help tolerate this emersion. Bladder wrack adapt their bladders, which helps the seaweed float upright underwater, helping them exchange gases and absorb nutrients when submerged, to the living conditions on their particular shore. Bladder wrack on exposed shores with strong wave action have smaller and fewer bladders than their relatives on sheltered shorelines. Egg wrack is one of the longest living seaweeds and reach an age of up to 25 years. Other brown seaweeds common on rocky shores are spiraled wrack, serrated wrack, oarweed, thong weed and sugar kelp which gets its name from the sugars forming its surface when being dried.

Although seaweeds have been used as food and for medicine since the stone age, kelp for example has long been used as source for iodine, carragheen is known for its anti-viral properties and use as a thickening agent and dried pepper dulse is widely used as a spice in Scotland, they only recently regained their recognition as superfood in the modern western society.

Ross Tidal Pools

Common Limpets

While some rocky shore inhabitants are happy to seek shelter in and under the carpet of seaweed others feel safer in a rock pool, an indentation in the rock surface that holds seawater and the inspiration for Phillip Henry Gosse (1810-1888) to build the first seawater aquarium. The size of these pools varies considerably, some are mere puddles, others can be the size of a swimming pool. Living conditions in this micro habitat resembles that of the lower shore and sublittoral zone. There is however one major difference. Living conditions in a rock pool can change rapidly. As already mentioned the often considerable variations in salinity can prove deadly for some animals. The salinity fluctuations often go hand in hand with rising water temperature and changes in the PH level of the water, all adding to the struggle of the rock pool dwellers. The normal PH of sea water is between 7.6 and 8.4. If there are seaweeds present in the pool they, during the day, use the CO2 in the water and exhale O2 which increases the PH level. In shallow pools with a lot of seaweed this process becomes visible in the form of bubbles, the O2 the plant produces during photosynthesis. In the night the process reverses, the seaweed expires CO2 and the PH goes back down to normal levels. While this PH shift rarely has any life altering consequences in the rock pools because it balances itself out we currently see a similar process in the oceans as a reaction to the ever increasing CO2 levels in the air. Much of the CO2 in the air is being absorbed by the oceans and reacts with the seawater to form carbonic acid which lowers the PH and in extreme scenarios would render the oceans’ PH neutral or slightly acidic. This ocean acidification unfortunately has some dire effects on calcifying organisms. These plants and animals will no longer be able to build and maintain their shells or skeletons and eventually perish. The first signs of this process have been seen in recent years as the coral bleaching events in the tropical zones around the equator. Should this process continue it is not unlikely that Ireland’s coralline algae, the vast shellfish population and any organism that relies on the production of calcium carbonate structures will also be affected.

Rocky Shore

Common Limpets, Periwinkles and other members of the shore community.

One of those shellfish and part of the mollusk family that face an uncertain future is the ubiquitous common limpet. Limpets, like many of their relatives, are made of a head that contains tentacles, eyes and the radula, a muscular foot and a visceral mass that houses the animals organs and is covered by a mantle which is not only connected to the shell but also grows it. The most interesting of those body parts is the radula, effectively a ribbon-like tongue spiked with countless teeth made of goethite, an iron-based mineral and one of the strongest materials on earth. Limpets and the majority of the rocky shore molluscs are grazers. This doesn’t mean they are munching on seaweeds; they are rather scraping up the biofilm that covers the rock and which consists of micro-algae, algae spores and cyanobacteria. The main grazing time for limpets is at night when the tide is out. During the day and at high tide limpets stay put in their personal parking space. This spot is tailormade to fit the limpet’s shell. To get the perfect fit the limpet either grinds away at the rock until it fits its shell or grows its shell to fit the rock surface. Having this home base means the limpet has to return to this particular spot after every grazing session. For a long time it was thought that limpets just follow their own trail back to get home. Studies however have shown that this is not the case. Limpets have no fixed grazing pattern and take different routes every day. Experiments have shown that they even find their way home when the surface around their home base is being changed during their absence which speaks for a certain amount of topographical awareness. A surprising ability considering limpets don’t have a brain.

Common Limpet (Patella Vulgata), County Clare, Ireland, August

What limpets are well known for is their ability to cling on to the rocky surface of their habitat. This is done with the beforementioned muscular foot and a very special mucus compound that can be switched from lubricant (when on the move) to superglue (when in danger from predators) in seconds. When clinging on isn’t enough limpets have another, rather unexpected defense that works especially well against starfish. This technique is known as mushrooming: The limpet brings up its shell and at the right moment, when the starfish is trying to get to the limpet’s body, the shell is brought down hard on the starfish’s arm or arms. This hurts and in most cases scares the predator away.

Another member of the limpet family that is often overlooked due to its size is the blue-rayed limpet. This tiny animal lives exclusively on kelp on which it also feeds. What makes it stand out is its unique colouring: The shell is somewhat kelp-coloured with a translucent quality and features kingfisher-blue, broken, parallel lines along its back.



Images & Text Copyright by Carsten Krieger

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