Between the Tides: Life on the Shore
Imagine the view from a cliff top in the west of Ireland. The vast expanse of the Atlantic Ocean stretches to the horizon where it merges with a misty sky. Gentle waves travel over this waterscape and disperse in millions of white droplets when they hit the rocky shoreline. The water doesn’t mind this measuring of forces as it is always the rock that loses out, one grain at a time. The water finds the weak spots in the rock and over time carves out tunnels, caves and pools.
At the foot of the cliffs, we are standing on lie flat sheets of rock that gently slope into the ocean. Parts of these rock platforms are thickly covered in seaweeds which are shimmering in various green, brown and red tones. Where the seaweeds couldn’t take hold ivory-coloured barnacles and black mussels are standing closely packed, giving the rock surface a chessboard-like appearance. Hollows in the rock, some shallow and others knee-deep, are filled with seawater that the retreating tide has left behind and on closer inspection reveal a myriad of shapes and colours. This is the intertidal zone, a strange world teaming with life that neither belongs fully to the sea nor the land.
This is a constantly changing and rough environment and full of challenges for its inhabitants. Over the course of a day, they are being beaten, suffocated, desiccated, baked and boiled, frozen, nibbled on and hunted.
The relentless power of the waves alone makes living at the shore difficult. The force of the wave power depends on several factors: The depth of the offshore water, the expanse of open water before it meets a shoreline (known as fetch), the direction the shore is facing (known as aspect) and the layout of coastal features (known as position). The worst-case scenario in Ireland is a west or north facing shoreline without any protective headland or reef. 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 shores of the east coast, ideally situated inside a bay which is protected by a headland or a reef.
The challenge for intertidal dwellers is to avoid being thrown around and eventually meeting an untimely end by being smashed against the rock or being sucked out into the open ocean. Body shape is one tool that allows animals and plants to deflect some of the wave power. Being round or flat reduces the impact area and leads the water around the body instead of offering resistance. A strong foothold is the other tool which is being achieved by suction or by secreting a glue-like substance and often both.
An even greater danger than crashing waves however comes from the tides. This regular movement of the oceans is 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 seas and manage to 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, while for the remainder of the month the moon and sun sit at varying angles to each other with their gravitational forces in opposition which causes smaller tides known as neap tides. The ‘spring’ in spring tide has its origin in the old Anglo-Saxon language where the word springam meant ‘to rise’. At spring tides high water is considerably higher and low water is noticeably lower than the rest of the month and subsequently a wider area of the shore is being 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.
This coming and going of water causes a number of problems. 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 stays immersed. The moment the plant or animal is out of the water it loses water through evaporation and is in danger of dying from dehydration and, if not able to extract oxygen from the air, suffocation. The latter obviously doesn’t apply to plants. These however face another problem: The plants of the intertidal zone, the seaweeds, can only photosynthesize when they are under 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 quickly reach boiling point in summer or can go below freezing in winter.
One solution to the problem of dehydration, and the one that most rocky shore inhabitants capable of moving around 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 there they wait for the water to return. Going into hiding also helps to avoid being picked up by a hungry predator.
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 also a source of oxygen.
Seaweeds can’t hide and don’t have a shell either, so they had to find other ways to cope. Most seaweeds have a leathery exterior which slows down water loss and some have become even more inventive. Spiral wrack curls up as it dries out to slow down evaporation, produces mucus to keep it moist and can tolerate up to 80% of water loss. Even more extreme is the channeled wrack, which has teamed up with a fungal partner although how they work together isn’t entirely clear yet, which can tolerate 95% of water loss and 8 hours out of the water. Both of these upper shore specialists fully recover after some 20 minutes of immersion.
Living conditions and the resulting problems differ greatly throughout the intertidal area 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 take their 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 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 is always covered with water even at the lowest spring tide. 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 gets 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 in water.
The splash zone is a good spot to sit down and have a look over the rocky shore before exploring it 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 even go to extremes and appear completely white or come in an almost crimson red. Single plants can be 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 stands rock samphire, the leaves of which have traditionally been pickled or used fresh in salads, and scurvy grass, a perennial with tiny white flowers and fleshy leaves. These leaves are high in vitamin C and before citrus fruit was 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 of lichen. Lichens are curious life forms that have been around for some 400 million years. In the past they have been classified as plants but today are recognized as a life form, or rather a complex community of life forms, in their own right. For a long time, it was thought that lichen is a simple symbiosis between one fungus and an alga where the fungus provides housing and water and in return receives food in form of sugars produced by the alga through photosynthesis. Only in recent years it became clear that at least some lichen species consist of more than just one fungus and one alga. 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 lichens 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 grows very slow but can live a very long time, 100 years or even older is no rarity, and survive in extreme environments. A sample 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 degrees.
One of the most beautiful lichen of 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 tricky 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 inhabit the complete intertidal zone as well as the shallow sublittoral zone and 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 plants living on land they all share one characteristic: They produce the nutrients they need to live and grow 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 close to the surface which is the reason most seaweeds thrive only in the intertidal or shallow sublittoral zone. In order to photosynthesize all seaweeds have chlorophyll in their cells, just like their cousins on dry land, but many seaweeds don’t display the typical green colour we associate with plants, some come in brown tones and others even take on a red colouring. 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 an obvious 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, just like trees and other land-based plants.
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 and phycoerythrin which allows them to photosynthesize at the low light levels that regularly occur on the lower shore. Common species are Irish moss, also known as carragheen and pepper dulse. Also a part of the red seaweeds are the corallines like the coral weed and pink paint weed. These 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 carpet-like cover on the rock surface, which can range from almost white to a deep pink.
Green seaweeds have the pigment chlorophyll B which gives them the green colour we are used to in plants. It is not surprising then that green seaweeds are very closely related to land plants and many species thrive in freshwater far away from the coast. The most common of the few marine species are sea lettuce 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 the colder and nutrient rich waters of 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 most common of the brown seaweeds are the various wrack species. There are the already mentioned spiral and channeled wrack, the egg wrack, which is one of the longest living seaweeds and can reach an age of up to 25 years, serrated wrack and bladder wrack. The latter can adapt its bladders, which allow the seaweed to float upright when submerged and help in the exchange of gases and absorption of nutrients, to the living conditions on their particular part of shore. Bladder wrack on exposed shores with strong wave action for example have smaller and fewer bladders than their relatives on sheltered shorelines. Other brown seaweeds are oarweed, thong weed and sugar kelp which gets its name from the sugars forming on its surface when being dried.
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. While seaweeds have played an important part in the diet of Asian countries for centuries, the western world only recently discovered the health benefits of seaweed. Seaweeds are rich in vitamins, minerals, antioxidants, fiber and polysaccharides which have various health benefits which range from preventing heart disease and diabetes to supporting weight loss and gut health.
While seaweeds provide sufficient shelter, moisture and food for some animals, many rocky shore inhabitants prefer to sit out low tide in a rock pool. The size of these pools, which were the inspiration for Phillip Henry Gosse (1810-1888) to build the first seawater aquarium, varies considerably. Some are mere puddles; others can be the size of a swimming pool and have a depth of several meters. Living conditions in this micro habitat resembles that of the lower shore and sublittoral zone with one major difference: Conditions in a rock pool can change rapidly. Salinity can fluctuate considerably, water temperature can rise or fall quickly, and the PH level of the water can change, 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 use up the CO2 in the water and exhale O2 which increases the PH level. In shallow pools with a lot of seaweed this process can become visible in the form of bubbles. During the night, when no photosynthesis can take place, the process goes the other way; CO2 accumulates in the water which lowers the PH level. While these PH shifts rarely have any life altering consequences in the rock pools because they balance themself out, the same process is currently happening on a much bigger scale 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 renders 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 already been seen in recent years as the coral bleaching events that occurred around Australia and other areas of the tropical zone. Should this process continue it is not unlikely that any marine plant and animal that relies on calcium carbonate, including Ireland’s coralline algae, echinoderms, and shellfish populations, will also be affected.
One member of the shellfish population that face an uncertain future is the ubiquitous common limpet, a member of the mollusk family. Limpets, like many of their relatives, consist of a head with tentacles, eyes and the radula; a muscular foot and a visceral mass that houses the animal’s organs which is covered by a mantle that is not only connected to the shell but also grows it. The most intriguing of the limpet’s body parts is the radula. It is effectively a ribbon-like tongue spiked with countless tiny teeth made of goethite, an iron-based mineral and one of the strongest materials on earth. Limpets and the majority of the rocky shore mollusks 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 walkabout. For a long time it was thought that limpets have a repetitive grazing regime and follow their own trail to get back home. Studies however have shown that this is not the case. Limpets have no fixed grazing pattern, take different routes every day and rarely return home the same way they left. Experiments have also shown that they even find their way home when the surface around their home base is being changed during their absence or when obstacles are being put in their way. This speaks for a certain amount of topographical awareness, a surprising ability considering limpets don’t have a conventional 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 lifts its shell up and at the right moment and when the starfish is trying to get to the limpet’s exposed body, the shell is brought down hard on the starfish’s arm. This hurts and in most cases scares the predator away.
An often-overlooked close relative of the common limpet is the tiny, blue-rayed limpet. This 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, parallel, broken lines along its back.
The biggest group of shelled mollusks 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 tell them apart. 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. Indeed the painted topshell is the cleanest shell around and 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 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 on them for a while before it makes another meal out of them.
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 a lung. Air is stored in the space between body and shell 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 around 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 the dogwhelk doesn’t shy away from eating other shells, including other dogwhelks, either. Its eating process is rather fascinating 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 drilled 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. Not only their eating habits are fascinating, but dogwhelks have also literally left their mark in Irish history. The purple dye, known as purpurin, that can be extracted from the animal has been widely used at early Christian monasteries. The monks used the striking colour, among other naturally sourced dyes, 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.
Acorn barnacles, which are often close neighbours of the common mussel, don’t have any defense against predators. These shell-like animals are crustaceans, same as lobsters and crabs, and have evolved to lead a stationary life. Before they settle however, they spend some time floating around. In this larvae stage they are part of the countless living beings that roam the sea as plankton, microscopic plants and animals that form the basis of the oceanic food pyramid. It is not surprising that only very few of the young barnacles make it to adulthood. Most are being eaten straight away or swept out onto the open ocean where they also will be eaten. The ones that make it go looking for a suitable dwelling place and they are rather picky. The rock or any other underground that will become their permanent home has to be smooth, should not be covered in too much biofilm and should be in a location that provides floating food. Once a place has been selected the barnacle attached its head to the underground and starts building its cone-shaped house which consists of six overlapping calcareous plates that leave a diamond shape opening on the top. When the tide is out this opening is sealed by two more calcareous plates, when the tide is in the barnacle extends its feet out of the opening to catch any floating, edible debris.
While most shore dwellers prefer to blend in and therefor display a rather understated appearance there are other animals that immediately stand out. Sea anemones are one of them and the most common among them 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, flower-like looks the beadlet anemone is a ferocious hunter and very territorial. Should another anemone come too close a headbutting fight breaks out with the aim to get close enough to deliver painful blows with the blue stinging cells which are known as nematocysts and act as miniature, poison-filled harpoons. 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 a 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 snakelocks 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 alga 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 alga to photosynthesize. To avoid any damage from sun exposure the anemone produces fluorescent proteins that act as a sunblock and is responsible for the purple colouring.
One of the most beautiful members of the anemone family is the dahlia anemone which can on occasion be found in the bigger and deeper pools. This colourful anemone can reach a diameter of up to 15cm and form large colonies that transform the rock into a deadly flower garden.
Urchins are a common sight in rockpools. 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 contain the internal organs and which are covered with numerous sockets that hold protective spines. The outside of this shell, known as test, is enveloped in a thin skin, pedicellariae (mini-pincers used to keep the animal tidy) and tube feet which are responsible for locomotion, feeding and respiration. Urchins are grazers and use their mouth, known as Aristotle’s Lantern, a very effective contraption of plates, muscles and chisel-like teeth that allow the urchin to scrape the rock clean from any edible material.
Black sea urchins, also known as purple sea urchins, often form colonies in shallow rock pools and can sometimes be seen with limpet shells or pieces of seaweed on them. This is no accident. These urchins deliberately place shells and floating seaweed on them as sun protection. Black sea urchins are also known to scrape out shallow burrows from the rock which they use as a resting place which lead to their other name ‘rock-boring urchin. The black sea urchin is a prime example of what overfishing can do. In the second half of the 20th century black sea urchins were harvested and exported from Ireland on a major scale. In 1976 350 tons were landed, ten years later this was already reduced to 48 tons and another 10 years on only 6 tons could be harvested. A few years after that the black sea urchin had virtually disappeared from the Irish coast. Since then the animal has made a slow recovery and is recolonizing the west coast but its abundance is still a far cry from the numbers that could be seen on Irish shores 50 years ago.
An occasional visitor to the intertidal zone is the edible sea urchin. This urchin is considerably bigger than the black sea urchin, has shorter spines and comes in a colour range from white to red. This species lives in the deeper waters of the sublittoral zone and its visits to the middle and upper shore are rather unintentional, but from time to time an animal gets trapped in a rock pool and has to wait for the next high tide.
Starfish are also members of the echinoderm family. 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 starfish 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 to see themselves confronted with even more starfish a few days later. The 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 like numerous 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 liters / 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 the communities living here face not only the problems waves and tides bring anymore, other threats have crept up on them from all sides. Climate change will bring warmer and more acidic water and more frequent and more violent storms. From the land the intertidal zone faces untreated sewage, run-off from heavily fertilized fields and the general ignorance of visitors. Plastic 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 from the other. It is not only the bigger pieces that can entangle animals or are mistaken as food and being ingested by birds, fish, mammals and other animals, eventually killing them. The greatest threat comes from the barely visible plastic particles, so-called micro-plastics, some of which are small enough to be devoured by plankton. These accumulate throughout the food chain, slowly releasing their toxic components. The old saying “what goes around comes around” has never been illustrated more literally and makes me wonder what Ireland’s shores will look like a few decades from now.
Carsten Krieger, August 2021