I am familiar with the commonly occurring horizontal stripes of rocky shore zonation where organisms are distributed between the tide levels according to their tolerance of exposure to air – but I wonder what influences the distribution and arrangement of different species of seashore creatures to result in the irregular patchwork pattern as found on the intertidal rocks at Fall Bay in Gower. The sloping flat surfaces of the limestone strata can be covered with a complete encrusting layer of mussels, limpets, and barnacles, organised by colour, shape, and size to make a patterned carpet.
Limpets are tiny but they have a big bite. They have a tongue-like structure called a radula with rows and rows of replaceable teeth. The teeth are very strong indeed. Limpets use the radula to scrape micro-organisms from the surface of rocks for food. In the process they can also remove some of the rock surface itself where the rock is sufficiently soft. The quantity of rock which is removed in this way is small but, over great lengths of time, and given that there are so many individual limpets, it all adds up to a significant degree of wearing away of our coastal rocks. This type of coastal erosion comes into the category of bio-erosion.
Andrews and Williams (2000) describe work on the Upper Chalk on the East Sussex coast where limpets (Patella vulgata) living on the chalk shore platforms contribute to the down-wearing processes. In a series of experiments designed to estimate the rate of erosion by limpets, they found that adult limpets consume about 4.9 grams of chalk a year and that, overall, limpets were responsible for lowering the platform by an average of 0.15 mm a year. However, where limpets were particularly abundant, the rate might be as high as 0.49 mm annually. Taking into account all weathering and erosion processes, it is thought that limpets are responsible for an average of 12% of the total down-wearing in this geographical location but this can be as much as 35% of down-wearing in the areas where limpets live in higher densities. The figures obtained from this research have wider implications for the wearing away of other types of rock by limpets in other places.
The images in this post were taken on the beach at Seatown in Dorset, on the south coast of England. The rock to which the limpets are attached is calcareous mudstone that belongs to the Belemnite Member of the Charmouth Mudstone Formation of the Lias Group, and was deposited during the Jurassic Period . It comprises alternating light grey and dark grey layers which are full of trace fossil animal burrows and fossils such as belemnites and ammonites. The dark grey layers seem to be softer than the light grey ones and limpets live on both types, and on bedrock and boulders. The darker mudstone has a characteristic way of fracturing giving an almost polygonal pattern of cracks, from which small pieces easily break off, leaving regular-shaped shallow hollows across the surface. Limpets often settle in these natural hollows and further adapt them to suit their individual size and shape.
The destructive side-effect of the feeding activity of limpets is just one kind of bioerosion. Both feeding and resting habits of limpets can result in the wearing away of rocks. When the tide is in, limpets venture forth in their underwater world to feed mostly by scraping up microscopic food and sometimes by biting larger pieces from seaweed. When the tide goes out and limpets have to endure a dry world, they return to their home base to rest and batten down against moisture loss and desiccation. Limpets may take advantage of existing nooks, crannies and hollows to settle when exposed to air but, in order to ensure a secure fit as they clamp against the rock, limpets agitate and grind their conical shell into the rock surface, wearing it away to get an exact fit. Each circular home base is a depression that is custom-made for an individual limpet. When the home base is abandoned by a limpet, other younger limpets may take it over, either as individuals or in groups.
In the photographs you can see the places where the limpets have made their home bases in such hollows. There are unoccupied home bases, and re-occupied home bases as well. Surrounding many of the limpets and their home bases are typical patterns of grazing marks that trace the limpets’ foraging expeditions outwards from the base when water covers the rocks. It is also possible to see in some pictures the way that the radula teeth have actually carved into the mudstone. Burrows made by bristle worms living in mud tubes are an additional feature on the same rocks, and belemnite fossils lie close to the surface in places.
Follow in my footsteps with a virtual walk along beautiful Rocquaine Bay on the west coast of the Channel Island of Guernsey. It is protected by a long sea defence wall which has employed different construction techniques along its length; mostly using local stone but also with along stretch of reinforced concrete (probably originating from German occupation World War II fortifications). The beach is both rocky and sandy with some pebble patches. Seaweeds of every colour abound. Huge limpets with white shells cluster on the bright orange-spattered L’Eree granite bedrock while outcrops of monochrome microgranodiorite occur on the upper shore near Fort Grey. Marine worm casts cover the softer muddy sands. Streams flow across the shore, their clear shallow water reflecting sunlight from the ripple crests and creating shadow patterns. A small stone jetty looks marooned among the rocks and a multi-coloured carpet of weed. Small boats bobbing in the turquoise water, rusty buoys and chains half-buried in seaweed, and algae-encrusted mooring ropes add to the evidence for fishing and leisure boating activities.
Click on the first picture to view the images in the gallery in the sequence that they were taken during the walk.
The flat quarried limestone ledge on the water’s edge at Winspit in Dorset provides a slightly unusual substrate for seashore life. There are natural gullies and deep angular man-made inlets in the stone but the area is mostly characterised by an extensive network of very shallow rock pools. Although only capable of retaining a centimetre or two of salt water as the tide recedes, these shallow pans and the surrounding surfaces are intensely colonised by numerous marine organisms, The natural patchwork of seaweeds and seashore creatures resembles a vast multi-coloured carpet with predominating pink and green hues.
The depressions in the rock are caused by the differential erosion of the softer limestone and the more resistant black chert nodules liberally embedded in it. The chert is composed of hard quartz derived from the opaline silica of decomposing sea sponges millions of years ago. The exposed rock stratum belongs to the Portland Chert Member dating from the Jurassic Period. Physical wear and acid erosion affect the softer matrix by chipping away and dissolving the stone respectively. The result of these ongoing processes can be seen from the small pitting marks.
In addition to this, the colonising organisms contribute significantly to erosion processes. For example, encrusting lichens can penetrate the rock surface, and as limpets feed by scraping this and other types of biofilm from the surface, they incidentally remove minute particles of stone with the food. Over great periods of time this feeding behaviour, together with other natural phenomena, imperceptibly degrades and removes rock thereby increasing the depth of the depressions. Additionally, the limpets always return to a home base when the tide goes out, and the circular impressions left by the friction of the shell margin as the limpet suckers tight down to prevent moisture loss are evident everywhere. When a large limpet dies or is removed, the home base is frequently re-occupied by new generation small limpets.
The natural depressions retain water at low tide, sometimes just a few millimetres but enough to support continued activity and prevent dessication. The wet hollows and much of the surrounding rock are covered by a patchwork of black, green, pink and white encrusting lichens and algae with groups of sessile or acorn barnacles. Some of the encrusting algae are calcareous, and there are abundant short tufts of pink calcareous coral weed, branched and articulated. Soft, finely-branched and filamentous red algae also occur – sometimes amusingly attaching themselves like decorative plumes to the shells of living limpets which often provide a home for dark brown encrusting algae, while dark red, almost black, beadlet anemones also glisten in the water.
A cold and rainy day in March saw me exploring a beach on the west coast of Ireland in Galway Bay, between Galway City and Salthill. Braving the inclement weather were joggers, plugged-in to headphones and clutching water bottles, as they ran along the promenade at the top of the shore. One or two individuals strolled with raincoats flapping and umbrellas braced against the wind. I had the seashore itself more-or-less to myself.
It is a sheltered, gently sloping, sandy shore where coloured pebbles accumulate at the top of the beach. Line after line of boulders, like loosely constructed groynes, stretch from high to low water mark dividing the shore into sections. They remind me of the stone walls that seem to proliferate in countryside and hill slopes all along this coast. Each beach section is like a field where mid- to low-shore rocks anchor a crop of seaweed – a profusion of vegetation that drapes each boulder and spreads out to blanket the surrounding sand.
The cloud-filled sky and persistent rain make the beach seem, from a distance, dull, almost monochromatic and melancholic – but that is an illusion. Close up, the limestone and granite pebbles provide a mosaic of many colours, intensified by the wetness. The seaweeds are made up of many types with a range of hues. Golden yellow fruiting bodies, and fronds in shades of olive, mark out the dominant Egg Wrack (Ascophyllum nodosum). Finely-branched red Wrack Siphon Weed (Polysiphonia lanosa) contrasts with the Egg Wrack on which it grows epiphytically. Darker greens and browns are typical of the smaller Bladder Wrack (Fucus vesiculosus). Short curling clumps of greenish-yellow early-stage Channel Wrack (Pelvetia canaliculata) are distinct. Both limpet shells and mussel shells show patches of dark brown encrusting algae (probably Brown Limpet Paint, Ralfsia verrucosa). The seaweeds splash colour across rocks, pebbles and sand. – and the rocks themselves originate from different locations, sedimentary or igneous, with their own subtle colouring, texture and patterns.
How different this scene must look when the tide is in and the seaweed can float upright and sway in the waves. It really must look like an underwater field. The Egg Wrack (growing up to a metre and a half long) has egg-sized and egg-shaped air bladders, one formed every year along each frond, to aid buoyancy. The much shorter Bladder Wrack has small rounded air bladders in pairs either side of the midribs to help it float.
When the intertidal shore is submerged, acorn barnacles (Cirripedia) and edible mussels (Mytilus edulis) attached to the rocks can filter food particles from the water. The huge numbers of large limpets and common periwinkles living amongst and feeding upon the seaweed, and grazing red and yellow biofilms that encrust the rocks, can move far more easily and for greater distances when buoyed up by water and there is no danger of dessication – although they can be active when exposed to air at low tide if conditions remain cool and moist.
COPYRIGHT JESSICA WINDER 2014
All Rights Reserved
The fantastically sculptured Carboniferous limestone around the tidal island of Burry Holms, which lies at the northern end of Rhossili Beach on the Gower Peninsula in South Wales, provides a habitat for many intertidal species.
The exposed rocks between the highest and lowest tide levels are covered with a patchwork pattern of permanently attached dark mussels and pale acorn barnacles on which thousands of roaming dog whelks feed. Periwinkles and limpets graze on the algal films that cover the rocks and the shells. The curiously curving contours of the rocks supply numerous sheltered micro-habitats in the form of small hollows, crevices, gullies, overhangs, and pools.
Some of the pools are only just big enough to accommodate a couple of sea anemones and a few dog whelks. Some bigger pools are almost perfectly circular smooth basins dissolved into the stone, characteristically highlighted in summer by vivid green soft seaweeds concealing minute fish and multitudes of striped top shells and other gastropods. The occasional deeper pool becomes a safe haven for clusters of common starfish and small shrimps; while wet overhangs and clefts display numerous beadlet sea anemones in a vast array of colours from pale khaki to bright red, together with rounded mounds of orange sponge.
All the organisms that live on the rocks in the inter-tidal zone contribute to the process by which the rocks are shaped. Frequently, this is done in a slow, subtle, and imperceptible way by the actions of epilithic and endolithic micro-organisms such as bacteria, fungi, algae, and lichens, and by the way these microscopic organisms are scraped from the surface and surface layers of the limestone by grazing seashore creatures.
Sometimes, the erosion is visible to the naked eye – as in the circular “home bases” that limpets have created by the continual grinding and wear of their shells against the rock as they settled in the same place each time after foraging trips; together with acid dissolution of the stone by their waste metabolic by-products. Another easily observable kind of bio-erosion damage is the burrowing activity of marine polychaete worms and boring bivalved molluscs. These small holes in rocks are often clustered in a band immediately above and below the water line of pools but also in any continually wet or damp grooves and channels. The overall persistent erosional activity of marine invertebrate organisms on intertidal seashore limestone over thousands and even millions of years contributes to the creation of fascinatingly sculptured karst topography like that seen around the island of Burry Holms.
COPYRIGHT JESSICA WINDER 2014
All Rights Reserved