Shy Shell Dweller

Rock Pools at Ferriters Cove

Rock pool with encrusting algae

Location of the rock pools at Ferriters CoveThere are some unusual mini-habitats in the shallow pools that occupy the cracks and crevices between bedding planes of the Silurian strata at Ferriters Cove. Only a few seashore creatures are seen grazing within them. A few common periwinkles (Littorina littorea) and limpets (Patella sp.) are the most frequent inhabitants. The pools have a few isolated branching seaweeds – some miniscule fine branching greens but mostly tiny clumps of Coral Weed (Corallina officinalis).

However, the rock surfaces beneath the water are also covered with a continuous coating, or small individual patches, of a variety of coloured algae such as the chalky red encrusting seaweeds like Pink Paint Weeds (Corallinaceae crusts); dark red non-calcareous encrusting seaweeds (Hildenbrandia rubra and Peyssonnelia sp. are examples in this group but not necessarily present at this location); encrusting brown seaweeds (Aglaozonia sp. and Ralfsia verrucosa would be examples of the type algae in this group); and the commonly occurring bright green algal films. Less obvious to the naked eye but probably also present in this kind of habitat would be the biofilms created by microscopic cyanobacteria, fungi, and lichens.

It is difficult to identify the species or even genera without taking samples to section and examine under the microscope. A complication with the identification or classification of these encrusting seaweeds, particularly the dark reds, is that the crustal form may represent a true species in its own right, or it may simply be only one life stage (tetrasporophyte phase) in the life cycle of a more familiar-looking foliose (branching) seaweed, or even an extensive attachment disc for a foliose alga.

Rock pool with encrusting algae

Rock pool with encrusting algae

Rock pool with encrusting algae

Close-up of rock pool with encrusting algae

Rock pool with calcareous encrusting and foliose algae

Rock pool with calcareous encrusting and foliose algae

 Rock pool with encrusting algae

RocRock pool with encrusting algae

Close-up of rock pool with encrusting algae

Rock pool with encrusting algae

Rock pool with encrusting algae

Rock pool with encrusting algae

Rock pool with encrusting algae

Close-up of rock pool with encrusting algae

Ventry Beach on the Dingle Peninsula

Just a few pictures from my visit to Ventry Beach on the Dingle Peninsula on the west Coast of Ireland yesterday selected from the 480 photographs I took! The weather was amazing for September and I was able to spend five hours on the sand enjoying the sights and sounds as well as the atmosphere of this old pilgrims’ path.

Don’t forget, you can click on any photograph to enlarge the image!

The Barnacle Zone on Waterfront Structures

Barnacles and mussels above the waterline on a wooden pier piling.

Barnacles often settle higher on the shore than most other organisms. They are adapted to live part of their life, sometimes most of it, actually out of water – being able to get by on splashes of water that extend beyond the high-tide line.

The way that animals and plants are distributed across the shore is known as zonation. Zonation is generally accepted as meaning a vertical separation of different groups of organism, often into distinct bands of different colour when living on hard substrates, resulting from the tolerances of individual species to dessication, temperature, and wave action – otherwise termed ‘exposure’. The barnacles and mussels occupy the mid-shore level. Around the world, although the species differ, the same phenomenon is found, with zonation more clearly visible to the casual observer on steep exposed rocky shores.

An extreme example of this zonation can often be seen on the artificial structures of a waterfront harbour where wooden wharf-sides, timber pier pilings, and metal revetments substitute for rock surfaces on which organisms can settle. Many of these artificial substrates are vertical and therefore the zoning of the organisms may be exaggerated and clearer to see.

The pictures in this post show a pale band or stripe, made up almost entirely of cream-coloured sessile or acorn barnacles, naturally cemented onto harbour-side structures, sometimes wholly encircling them. A few common periwinkle gastropod molluscs move around the barnacles, feeding on the bio-film that accumulates on their shells.  Fronds of spiral wrack and sea lettuce type of seaweed, both also fairly tolerant of exposure out of water, are sometimes scattered over the barnacle zone. The barnacles have special adaptations that allow them to survive dehydration at low water but they are none-the-less vulnerable to predating dog whelks at all stages of the changing tides.

Below the barnacle zone, a darker, almost black band, is composed of edible mussels attached by byssus threads. Mussels are less tolerant to air exposure than the barnacles so they survive best lower down where they are not out of the water for so long. They are a sitting target, though, for starfish which use their tube feet to sucker onto these bivalves, forcing them to open, and then everting and inserting their starfish stomach into the mollusc so that they can feed upon the living contents.

All these photographs were taken on the waterfront in Halifax, Nova Scotia. Most of the piers and jetties are still traditionally made of timber because it is such an abundant commodity in Canada. There are generations of timber structures: new; old and decaying; and derelict examples. All of these show the barnacle banding. So do the more recently built rusting metal revetments to the edges of the renovated wharves in the more developed areas.

Interestingly, many modern high-rise buildings in that location have been constructed right on the water’s edge where they are supported by foundations of steel piles driven deep down into the very hard metamorphosed bed-rock. The pilings can be seen projecting below the buildings on the waterside elevations, disappearing into the harbour water. Each white-painted column displays at its base a lower ring of black mussels and a higher ring of paler barnacles – the structures themselves being reflected in the seawater with an odd abstract effect.


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A Galway Shore – Part 1

A seaweed covered shore on the edge of Galway Bay

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.


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Rocks & Pools on Worms Head Causeway

Looking down to the Worms Head at high tide

It was a sunny day with the prospect of a very low tide – just right for exploring the rocky causeway that links the Rhossili headland to the tidal island of Worms Head. I was really looking forward to it. The times when it is safe to venture out on the Causeway are always clearly indicated; and it is assuring to know that there is almost always a Coastguard Lookout monitoring the area through binoculars to render assistance if anyone gets into trouble out on the rocks. Most people seem merely to cross the causeway by the quickest route to reach the Worms Head. However, the Causeway itself is a source of great fascination for anyone like me and interested in natural history.

You have to be fairly fit to get down on the Causeway and need to have sensible footwear. The descent from the red-rimmed turf platform at the base of the headland, and the initial scramble over the tall projecting limestone strata of the first 50 metres or so, can be a challenge for some. However, it is well worth the effort because it is a different world out there. It is a unique experience. An alien landscape full of surprises awaits you.

When you look down on the Causeway from the headland , it might look a rough and barren expanse of dull buff and grey rocks. Boring, even. Once down of the rocks, a closer perspective reveals a wealth of detail with hidden pockets of colour, variations in texture and topography, strange natural sculpturings, ancient rocks with complex geological histories, embedded fossils such as entire Sea Lilies, tidal pools of every size, deep water gullies, multi-coloured seaweeds and myriads of seashore creatures. The variety and complexity of this beachscape might be perplexing but it is none the less inviting and exciting.

On this particular visit, I aimed for the central part of the Causeway that I hadn’t investigated before, and then slowly veered round in a more easterly direction before returning to base. I was interested not only in the geology and the seashore life as entities in their own right but was also intrigued by the way each component of the shore is influenced by the other – the way everything interacts. How the geology and landscape affect and facilitate the living organisms; and how the living organisms affect the landscape.

Once away from the very landward edge of the Causeway where the rocks can sometimes seem to be completely devoid of animal life, almost every rock surface is covered to a varying degree by small acorn barnacles of different types. The common mussel is abundant but not growing in such profusion as in previous years. Not many dog whelks were feeding on the barnacles and mussels along the route I was taking but, no doubt, they lurk in other lower shore areas. Large limpets cling to surfaces both wet and dry. Common Periwinkles and striped Top Shells are common. Even the smallest pool is home to red Beadlet and pink-tipped Snakelocks Anemones. Small fish and shrimps dart through the pools and hide beneath the seaweed. Large Balanus perforatus grow on the lower shore  – instantly recognisable with their volcano-shaped shells and beaky opercular plates.

While I was sitting eating my lunch, a large Common Green Shore Crab ventured out of the water right by my feet but soon made a hasty retreat. I made a little movie of him scuttling around the pool.

Even the most exposed rock surfaces out on the centre of the Causeway have some seaweeds growing on them. Bright green soft weeds of the Ulva species (like Sea Lettuce and Gutweed) seem to tolerate the dry rock as well as the pools. Red branching seaweeds make a dramatic colour counterpoint to them: they often grow together. Calcareous red seaweeds like the branching Coral Weed grow extensively, and patches of flat Corallinaceae crusts like Pink Paint Weeds line water-filled hollow basins and dips, and coat the water-line of large boulders in the gullies. The familiar Brown Fucoid seaweeds like Bladder Wrack and Toothed Wrack make an appearance further down the shore, while the large kelps such as Oarweed occupy deeper waters right on the shoreline and below. One interesting new alga that I spotted is an encrusting brown paint-like species covering the shells of limpets (probably Brown Limpet Paint, Ralfsia verrucosa).

The strange curvilinear shapes of some of the upstanding rocks, the deep gullies along bedding planes, and the numerous rounded hollows and depressions, are typical of coastal limestone karst topography. More extreme and more extensive examples can be seen elsewhere in Gower, such as around the tidal island of Burry Holms, and at Mewslade Bay and Caswell Bay. Many people assume that it is the impact of waves, acid dissolution by rain, and abrasion by sand-bearing winds, that are the combined means by which seashore rocks are worn away, slowly and steadily over the millennia. This is partly true; it does account for some of the erosion. However, there is another aspect to the erosion of seashore rocks which is equally, maybe more, important: bio-erosion.

It all starts in the smallest of ways on a microscopic level with organisms like bacteria, algae, fungi, and lichens – especially those that are capable of not only colonising the surface of the rock (endolithic organisms) but also of penetrating it (epilithic organisms), even if that is only to a depth of a few millimetres. The general effect of the rock penetration is a weakening of the substrate so that when grazing molluscs like periwinkles and limpets come along they can easily remove not only the bio-film on the surface but can also scrape off some of the surface rock as well.

For example, analyses of the gut contents of limpets shows that small particles of rock are ingested along with the food they obtain. Limpets also alter the rock in another way. They always return from foraging trips to the same position on the rocks – their home base. As a limpet adjusts its position on the home base, its shell mechanically grinds against the rock wearing away a circular depression; this depression is deepened and emphasised by the chemical effect of the limpet’s acidic waste products dissolving the rock. It has been calculated that over vast periods of time, the cumulative effects of limpets feeding on rocks can contribute the process by which they are reshaped or destroyed. Abandoned limpet home bases are common on the rocks of the Causeway where the animals have been dislodged by last winter’s stormy seas.

Another major bio-erosional component is the burrowing activity of marine polychaete worms, and of specially adapted rock-boring bivalved molluscs. It is amazing to see just how extensive is this kind of damage to the rocks on the Causeway. It is no wonder that there are so many pebbles and boulders with holes in them found on the shores all around the Gower Peninsula. Almost every damp patch, depression, hollow, pool, and gully has limestone riddled with these burrows. The burrowing activity of these marine invertebrates is made easier by the weakening of the rock by micro-organisms; and the burrows and holes then provide a greater surface area for the further colonisation by micro-organisms. The combined effects of all types of bio-erosion have a significant impact on the surface shape of the limestone and landscape.

The strata on the Causeway lie in parallel lines along an approximately northwest to southeast axis. Most of the rocks that you see are Black Rock Limestone Subgroup with some Shipway and Brofiscin Limestone. As you face the Causeway with your back to the Coastguard Lookout building on the Rhossili headland, then behind you and beneath the superficial loose deposits, lies first of all Gully Oolite and then High Tor Limestone as solid bedrock. In front of you, the Black Rock Limestone is bordered on the far side of the Causeway by strips of first Gully Oolite and then outermost High Tor Limestone solid bedrock. So there is a particular sequence to the layers of rock visible on the surface which reflects their history.

On the landward side of the Worms Head Causeway, the sharp projecting edge-on rock strata dip down and towards the Rhossili headland and lean at an angle in the direction of the open sea. On the seaward side of the Causeway, the lines of strata dip down and towards the open water with their free edges inclined in the direction of the land. Between these two areas of strata that point towards each other, there is a flatter, more eroded area, more severely cut away by wave action. The whole unit is the remains of an eroded geological feature called an anticline.

Imagine that the sedimentary rock layers were originally horizontal but later pushed upwards by earth movements into a mound or ridge; the resulting arched rock layers in the mound have been worn away by the elements over time until only the base of the mound remains with a characteristic layout in which stumps of the most recently formed younger rocks lie on the outside with the older layers on the inside of the feature.

You can visualise this process by thinking of a Swiss Roll. [If I am to persist with this analogy, perhaps we can go whole hog and imagine a chocolate one with cream filling?] If the cake were roughly cut  length-wise, the broken surfaces would have a pattern of longitudinal stripes with alternating sponge and cream. The layer which was original wrapped around the outside of the Swiss Roll cake would be seen as the two stripes of sponge on each side.

There is rocky shore zonation of the organisms that live on the Worms Head Causeway but this zonation is not so straightforward to recognise and interpret as on a normal stretch of shoreline. For a start, the Causeway is connected by beaches to the mainland at the Rhossili headland and the island at Worms Head. Elsewhere, the waters’ edge describes an irregular outline, the shape of which depends on the state of tide, and which more or less defies description. The surface is full of ups and downs on various scales.

Zonation is the way that organisms tend grow in associations on rocks depending on their tolerance for different degrees of exposure to the air – each type of organism having a physiological preference or need for more or less immersion in sea water. Typically, this zonation of organisms is seen on a rocky shore as different coloured bands – pale for barnacles, dark for mussels, yellow for lichens and so forth.

The best way to describe the zonation on the large scale out on the Causeway is by thinking of it radiating irregularly outwards, in a roughly concentric fashion, along a slight and highly disturbed incline from the highest to the lowest parts of the Causeway  – rather than a zonation with easily observable regular bands as on a normal rocky shoreline or cliff face. On a minor scale, there is zonation in the rock pools and in the water-filled gullies themselves.

At high tide the causeway is completely covered by the sea; sometimes the beaches are covered too. As the tide goes out, greater and greater areas of the causeway rocks are exposed to the air. It could be claimed that the water drains away from the perimeter and also from higher areas of the Causeway simultaneously. Water seems to continually make its way downwards from pools in the highest parts, through small cascades and gullies, to reach the sea. You can hear the continuous trickling sound of this water, merging with the noise of the wind, the breaking waves, the calls of birds. You are immersed in sound when you are out on the Causeway.

The tide seems to come in and go out in a very haphazard way. It is difficult for the occasional visitor to predict the direction in which the seawater will ebb or flow; or the speed with which it will rise and fall. This is what makes it potentially dangerous to be out on the Causeway when the tide is in flood – it could be difficult to decide which parts will be covered with water first, and therefore it is easy to get trapped by the water, with access denied to dry land. Swimming or even wading through the tide water is not a good idea because of the cross currents, water encroaching from three sides, and the hazardous sharp and barnacle-encrusted rocks beneath your feet.

Having kept an eye on the changing tide after spending a most enjoyable five hours out on the Causeway exploring and taking photographs, I looked for a safe, or easy way to get back to the Rhossili headland. The strata run in rows parallel to the headland and projecting higher and higher as you approach the beach. There are numerous pools between the layers of rock. This would make it hard work to traverse the last bit of terrain back to the beach from the location I was in. Luckily, north-south fault lines cross the rock layers. The areas of the fault lines tend to be worn down to lower levels than the surrounding projecting rocks because they are frequently filled with wide veins of softer white crystalline calcite and narrow veins of red haematite. Following these fault lines makes it much easier to negotiate a way back to the mainland.

The surfaces of these natural pathways are often worn smooth. Shallow streams of sea water flow along them and many small seashore creatures take advantage of the moist habitat they provide. The ‘stream’ beds and shallow pools along the fault lines are really colourful, often coated with a film of bright green microscopic algae that provides a vivid contrast to the red and white minerals, and to the purple striped Top Shells that love to graze there.

From out on the Causeway, not only can you view Worms Head from the most unusual angles and see it in a way that is completely different from the standard postcard perspectives – but there are also spectacular views of the Rhossili headland. The sixty metre high plateau of clearly stratified limestone is sketchily cloaked with turf which at its lowermost weathered edge reveals a vivid orange soil. This soil covers remnants of an ancient raised beach where seashells and pebbles from around 125,000 years ago, deposited in the Ipswichian Interglacial period, are cemented together by calcite and covered by glacial debris. The orange band contrasts dramatically with the bleached smooth pebbles and bizarre barren outcrops of the beach itself. This is the point to which I at last returned and was able to look back at the vast expanse of rocky causeway fully revealed by the now low tide. Next time I intend to venture out to the deep gullies of the far side of the Causeway and see what I can find there.

P.S. Don’t forget that you can click on any picture to enlarge it and see a description of the image


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