Between the Tides: Rocky Shores
I am standing on the cliff top overlooking the small cove. Its western end is protected by towering rock sheets that, over a few million years, have been raised to an almost vertical position. Parts of the sheets have been torn open more recently by the forces of the Atlantic Ocean and now a small sea cave extends into the rock. The eastern end of the cove seems to have been tortured considerably more by the waters of the Atlantic. Low lying rock platforms are overlooked by narrow rock towers and twisted cliffs. Caves, tunnels, blow holes and deep pools have been sculpted into these cliffs. The petrified remains of sand volcanoes and ripples that flowing water left on the bottom of an ancient river bed reveal the ancient origins of the rock.
A path and an artificial ramp, built for the filming of the Hollywood classic Ryan’s Daughter, lead to the bottom of the cove. Here it becomes clear that the cliffs that encircle the head of the cove are not made of solid rock but consist of clay and rocks of various sizes. Lumps of clay at the bottom of the cove and sagging parts of the cliff top state the obvious: These soft cliffs are being eroded away with every spell of rain and every gust of wind that comes in from the north Atlantic. Just below the cliffs the eastern half of the cove is covered with a boulder beach. Hundreds of hefty rocks, rounded and smoothed by water, cover the underlying rock platforms that stretch out into the bay and become visible where the boulder beach ends. On the other side of the cove the boulders are missing and sculpted and terraced rock dominates the scene. In the far eastern corner the Atlantic has managed to drill a canal into the rock that disappears under the soft cliffs und underlying rock only to reappear a few meters inland where a sinkhole has formed.
The rock platforms in the center of the cove are covered in seaweeds and dotted with rock pools of different size and depth. Massive rocks dislodged from the cliffs at the entrance of the bay and thrown onto the shore are a brutal reminder of the violent storms that hit this coast on a regular basis. It is a place that never fails to amaze me. First it’s only a kaleidoscope of colours, shapes and textures, beautiful in its own right. A closer look reveals a microcosmos of life and death, struggle and survival. It is Ireland’s most fascinating habitat: The rocky shore.
If there is one truly wild place left in Ireland it is its coast. Ireland boasts over 3000 kilometers of coastline, the majority of which is made of rock, mainly shale, sandstone and limestone. This rock comes in a variety of shapes. The most eye-catching of those are the sheer cliffs that rise vertically out of the sea, can reach heights of several hundred meters and in places have been sculpted into pillars, arches and caves. As a habitat these rock faces are home to a variety of seabirds. Over the summer months these colonies with many thousand individuals transform the rock into a busy bird metropolis. In autumn the birds leave and the cliffs return to their quiet and barren state. Other varieties of the rocky shore however are keeping busy all year round. Where the rock is gently sloping into the sea or low lying rock platforms are regularly engulfed and released by the tides life is teeming under and above the water. These kind of rocky shores are one of the most crowded habitats imaginable. Especially the intertidal zone hosts a staggering variety of plants and animals with a population density comparable to the tropical rainforest.
This is an ever changing and rough environment full of challenges. The first challenge the rocky shore inhabitants face is the power of the waves when they meet the shore. The force of this wave power depends on a number of factors like the depth of offshore water, fetch (the expanse of open water before it meets a shoreline), aspect (the direction the shore faces), and position (the layout of coastal features). The worst case scenario in Ireland is a west or north facing shore without any protecting headland. The vast extend and the deep waters of the Atlantic Ocean combined with regular low pressure systems can create massive waves that can hit the shore with a power of up to 30 tons/square meter. On the other end of the scale are the rocky shores of the east coast, ideally situated inside a bay which is protected by headlands or a reef. The variations between these two extremes of an exposed and a sheltered rocky shore are of course numerous. The prevailing fauna and flora has adapted to these variations and has come up with tailormade and very interesting solutions to avoid being crushed to death on the rocks or being swept away by waves.
An even greater danger than crashing waves however comes with the tides. Twice each day the inhabitants of any rocky shore are being exposed to sun and wind and then being covered again by seawater. The movement of the seawater is of course mainly caused by the moon with the further away sun lending a helping hand. The gravitational forces of the moon and sun pull on our planet’s oceans and move these vast amounts of water. Roughly twice each month, when the moon is full or new and aligned with the sun, spring tides occour. At spring tide’s high water is higher and low water is lower than the remainder of the month and subsequently a wider area of the shore is exposed at low tide and covered at high tide. The spring tides are at their greatest at the equinoxes in March and September and at their weakest at the solstices in June and December. The ‘spring’ in spring tide has nothing to do with the season but has its origin in the old Anglo-Saxon language where the word springam meant ‘to rise’. The rest of the month the moon and sun sit at approximately right angles and their gravitational forces oppose each other causing smaller tides known as neap tides.
Most rocky shore dwellers are built for life under water, they breathe through gills and the concentration of their internal fluids is the same as that of the seawater surrounding them. Water can pass freely in and out and all is good as long as the plant or animal is immersed in water. The moment the plant or animal is emersed it loses water through evaporation and is in danger of dying from dehydration and, if it is not able to extract oxygen from the air, suffocation. The latter obviously doesn’t apply to plants. Those however face another problem: The plants of the intertidal zone, the seaweeds, can only photosynthesize when they are covered with water. Yet another problem that comes with immersion and emersion are temperature fluctuations. Under water the temperature is more or less constant no matter the time of day or year. Out of the water however temperatures can reach boiling point on a dark rock surface on a clear summer day or can go below freezing in winter.
One solution to the problem of dehydration, and the one that most rocky shore inhabitants capable of locomotion use, is to hide under seaweed, in rock crevices or rock pools, anywhere some moisture and shade remain after the tide has gone out and wait for the water to return. The safety of a rock pool is however deceptive. Variations in water salinity can be substantial. Average sea water salinity along Ireland’s shores is 3.5%. In rock pools this can rise to around 9% when wind and high air temperatures increase evaporation or drop to around 0.5% when rain or run-off from nearby fields add freshwater to the pool. Osmoconformer, animals like the acorn barnacle or common mussel that can control their internal fluids, are able to cope with these fluctuations. For stenohaline animals like anemones that have no control over their internal fluids, this can be lethal. An anemone stranded in a pool with decreasing salinity will involuntarily take on water and eventually burst.
Another solution to the dehydration problem is having a shell. A shell protects from evaporation and the space between the body and the shell, known as mantle cavity, can be used to store water which keeps the body moist and is a source of oxygen. Seaweeds unfortunately have no protection mechanisms against temperature fluctuations and evaporation and simply have to cope with the fact. Some do this better than others, the channel wrack for example can tolerate 95% of water loss and fully recover after 20 minutes immersion.
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.
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.
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.
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.
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.
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.
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.
The biggest group of shelled molluscs on the rocky shore are the topshells and periwinkles. Three of Ireland’s four topshell species, toothed topshell, purple topshell and grey topshell, have a similar appearance and it takes a close look to distinguish them. Very easy to recognize however is the painted topshell. This is not only because of its beautiful cone shape and colouring that consists of a creamy white with crimson streaks but also because it looks rather clean compared to other shells. The painted topshell is indeed the cleanest shell around. The reason for this is a habit known as shell-wiping. The animal regularly extends its foot all over the shell and wipes it down immaculately. This not only has the obvious cleaning effect, the painted topshell also gains 20% of its daily food requirement from its cleaning action.
The biggest of the periwinkles is the edible periwinkle. As the name suggests the edible periwinkle has been used as a food source for a long time and even today bags of the boiled periwinkles are being sold as a snack at many seaside resorts. Like other grazers this periwinkle feeds on the biofilm that covers the rock. The edible periwinkle however ads a little twist to its eating habits: It excretes its food in the shape of pellets and lets bacteria work in it for a while before it devours its pellets again.
Most of the periwinkles, edible periwinkle, small periwinkle and flat periwinkle, the latter comes in a range of colours like yellow, orange, green and brown, live on the middle shore. The rough periwinkle however has developed a trick that allows it to live on the upper shore and even venture into the splash zone. The rough periwinkle has only very small gills, so small in fact that it wouldn’t be able to survive under water for too long. Instead it uses its shell cavity as lung. Air is stored in the shell cavity and from there oxygen diffuses directly into the tissue of the animal. The advantage for the rough periwinkle is clear: It has the upper shore and splash zone to itself with no other grazers it would have to share food with.
The easy to identify dogwhelk is the only carnivorous shell. On top of its menu are acorn barnacles and the common mussel but if necessary the dogwhelk also eats other shells including other dogwhelks. Its eating process is rather interesting as well as time consuming: Dogwhelks have a so called boring organ which secrets an enzyme that softens the prey’s shell. Once enough shell has being softened the hunter uses its radula to remove the shell fragments and so on. Once the dogwhelk has bored through the shell of its victim, it first injects a narcotic to paralyze the prey and then digestive enzymes which will transform the prey animal into an easy to slurp soup. The whole feeding process takes time, a day for a barnacle and around a week for a mussel.
Dogwhelks have literally left their mark in Irish history. The purple dye that can be extracted from the animal has been used at early Christian monasteries. The monks used the striking colour to decorate their manuscripts. The dye was in such demand that in the 7th century a proper dogwhelk dye industry developed on Inishkea Island off the County Mayo coast.
One of the dogwhelk’s favourite food, the common mussel, is a member of the bivalve family, molluscs with a two-parted, hinged shell. Bivalves are mostly stationary filter feeders and anchor themselves into place with the help of a byssus, strong filament bundles. The common mussel also uses this byssus to bind attacking dogwhelks which are, if caught, unable to move and doomed to starve to death.
While the molluscs for most part prefer an understated appearance there are some animals that stand out. Sea anemones are one of them and the most common species on rocky shores is the beadlet anemone. This beautiful animal comes in a variety of colours like red, green and brown. The very similar looking strawberry anemone is a different species and there is increasing evidence that the brown and green beadlets might be as well.
Despite its pretty looks the beadlet anemone a ferocious hunter and very territorial. Should another anemone come too close a headbutting fight takes place with the goal to get close enough to deliver blows with the blue stinging cells. These battles can take many days until one anemone finally has enough and retreats. The main use of the stinging cells is however to stun prey which can be crabs, prawns or jellyfish. The stunned prey is then directed by the tentacles to the mouth which sits in the center of the oral disc. Once the prey has been digested its remains take the same way out as they had come in. Most beadlet anemones reside in rock pools but they can survive outside water for a certain time by retracting their tentacles and closing up their mouth and oral disc to reduce the evaporation rate. Beadlet anemones live up to three years in the wild but in captivity they can get much older. Granny, a beadlet anemone that spend part of its life in an aquarium at the Royal Botanical Gardens in Edinburgh became a bit of a celebrity by reaching the age of 50.
While the beadlet anemone is a bit of a loner the snakelocks anemone prefers company and lives in colonies. One reason for this is the preferred reproduction method of the snakelocks: They simply split in two, a process known as longitudinal fission. This means any snakleocks anemones that sit very close together are most likely clones.
The snakelocks come in two varieties, a brown coloured one and a green version with purple tentacle tips. The latter one hosts a symbiotic algae that supplies, apart from the green colour, additional food for the host and in return the anemone provides shelter and certain nutrients to the algae. Naturally this version of the snakelocks anemone prefers shallow waters and sunlit pools to allow the algae to photosynthesize. To avoid any damage from sun exposure the sun loving animal produces fluorescent proteins that act as a sunblock and is responsible for the purple colouring.
Another beautiful member of the anemone family is the dahlia anemone which can on occasion be found in big and deep pools. In general however this and other species prefer the sublittoral zone.
Urchins, especially the purple sea urchins, can often be found in rock pools together with beadlet anemones. These animals belong to the echinoderms, meaning spiny skinned, an animal group that can be traced back some 500 million years. The urchin’s body is shaped from fused calcareous plates that are covered in numerous sockets and contains the internal organs. The outside of this shell, known as test, is covered in a thin skin, spines that sit in the beforementioned sockets and are used for protection, tube feet for locomotion, feeding and respiration and pedicellariae, mini-pincers that are used to keep the animal tidy. Urchins are grazers and use their mouth, known as Aristotle’s Lantern, a very effective contraption of plates, muscles and chisel-like teeth to scrape the rock clean from any edible material. Purple sea urchin can sometimes be seen with limpet shells or seaweeds on them. This is no accident. These urchins deliberately place shells and floating seaweed on them as sun protection. The edible sea urchin is considerably bigger than the purple sea urchin, has shorter spines and comes in a colour range from white to red. This species is only a rare and unintentional visitor to the intertidal zone but from time to time an animal gets trapped in a rock pool and has to wait for the next high tide.
Another member of the echinoderm family are starfish. Their plates are less rigid than those of sea urchins and are held together by connective tissue which gives the starfish a greater flexibility. Just like the edible sea urchin they are no regular inhabitants of the intertidal zone of rocky shores but sometimes get stranded in rock pools or under rocks. The most common along the Irish coast are the seven armed starfish, obviously named after the unusual number of its arms (most starfish have only five arms), the spiny starfish (a big, greenish starfish that lives up to its name) , bloody henry and the common starfish. The latter feeds mainly on mussels and is the most likely one to appear outside the sublittoral zone. Once it has targeted its prey it uses its tube feet to force the two halves of the mussel apart and once it has a good grip it inserts its stomach right into the mussel. This appetite got the common starfish in trouble with fishermen and an often told fable tells of fishermen ripping the starfish in half to protect their shellfish stock. Only a few days later however the fishermen saw themselves confronted with even more starfish. Reason for that are the unusual regeneration abilities of all starfish. One arm and part of the central body are enough to regrow a complete animal.
All these animals are only a tiny percentage of all the life that thrives on rocky shores. Many of the rocky shore inhabitants are tiny, barely visible to the naked eye like shrimps, sea spiders midge larvae, marine springtails and minute brittle stars. Others are masters of disguise and hiding like various fish species, worms and crabs, including the hermit crab, a shy but curious animal that uses discarded shells of periwinkles or topshells as their home. Other life forms are hardly recognizable as such: Squirts, mats and sponges. The latter are among the most primitive life forms on the shore. They can best be described as cell colonies where each cell, in the absence of organs, has a particular function. The cells are held together by silica or calcium spicules that form a skeleton around which the cells are arranging themselves. In a classic experiment the sponge will be put through a sieve or be otherwise liquidized. If the resulting mass is being left to its own devices it will re-assemble and take on its previous form. Sponges are filter feeders. Seawater is taken in through tiny pores, known as ostia, and pushed out again through bigger openings known as oscula. In this way a small sponge can filter up to 20 liter / day.
It’s a wonderous world the no-men’s-land between the low tide and high tide mark with its many yet undiscovered secrets. Unfortunately environmental threats loom here as well. Apart from the problems climate change will bring, warmer and more acidic water with all its consequences, and more frequent and more violent storms there is also the constant attack from the landward side: Untreated sewage, run-off from heavily fertilized fields and the general ignorance of visitors. Finally there is plastic that chokes the shores from all sides. Thoughtlessly dumped bottles and wrappers from one side and the constant supply of flotsam and jetsam that the tide brings in. It is not only the big pieces that can entangle bigger animals. The big threat comes from the small plastic particles, some so small they can be ingested by plankton. These are in the best case of no nutritional value, toxic in the worst case and accumulate throughout the food chain to a most likely bitter and deadly end.
Carsten Krieger, Spring 2019