On the Rocks at Fall Bay

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.

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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Scenes at Blacks Harbour

Bay of Fundy shoreline with wide inter-tidal zone

The scenery around Blacks Harbour in New Brunswick, Canada, demonstrates the effects of the extreme tidal range and scour on the coastline. The shoreline as far as the eye can see is marked by a wide dark band immediately above the water line where the rocks are draped with olive brown seaweeds. Above this lower region of the inter-tidal zone is an upper drier inter-tidal band mostly comprised of bare bedrock. The rock is light in contrast to the wrack on the shore below, and appears pink in many places.  The rock belongs to the Late Devonian Perry Formation which is mostly a red and green conglomerate of angular fragments deposited in former fluvial environments – a rock type which it seems is easily eroded by wave action. Dense woodlands cloak the slopes above this lighter rock zone.

In parts of the Bay of Fundy there is a vertical tidal range of nearly 56 feet at certain times of the year: these tides are the highest tides in the world. [The second highest tides in the world are experienced on the Gower Peninsula in South Wales where I also like to photograph the seashores]. The extreme variations in sea level with the ebbing and flowing of the tides make it hard work for the fishermen to go about their business but, here at Blacks Harbour, the winch, the pontoon walkway across the shore, and the rafts permanently afloat (one even with a hut) to store lobster pots, nets, and other gear, help with the work in these conditions. Concrete mooring weights, rusty iron chains and seaweed-covered ropes are strewn on the stony beach with the weathered remains of former wooden pier pilings.

Earlier posts about Blacks Harbour on the Bay of Fundy:

Lichens at Blacks Harbour

Barnacles at Blacks Harbour

Rocky shore zonation on a Bay of Fundy shoreline

Shoreline and fishing activity at Blacks Harbour

Old wooden winch or crane on the beach at Blacks Harbour

Floating rafts with fishing gear in the Bay of Fundy

Mussel, periwinkle, and soft clam shells with beach stones

Concrete mooring weight with rusty iron ring on the shore

Empty scallop shells on beach stones

Rocky shoreline on the Bay of Fundy

Patterns in driftwood on a stony beach

Moss-clad rocks at the top of the inter-tidal zone

Exposed bedrock between seashore and woodland

Mooring chain and rope with seaweed amongst beach stones

Rusty mooring chain lying on a stoney beach

Mooring rope with attached seaweed on the beach

Floating raft with fishing gear

Bay of Fundy inter-tidal zone in New Brunswick

View of the inner harbour area at Blacks Harbour


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Green Leaf Worms at Spaniard Rocks, Rhossili

Green Leaf Worm, Eulalia viridis (Linnaeus), on mussels and barnacles encrusting limestone cliffs at Spaniard Rocks, north end of Rhossili Bay, Gower, South Wales.UK (1)

Bright green worms on mussel beds? I have never even noticed them before but I guess they are common from the numbers I found and by accounts I have now read in text books. This is the Green Leaf Worm, Eulalia viridis (Linnaeus). It was photographed writhing around with many others on the mussels and barnacles that were encrusting the vertical faces at the base of the Carboniferous limestone cliffs on the north side of Spaniard Rocks at Rhossili Bay, Gower.

The Green Leaf Worm is one of thirty species of marine polychaete worm belonging to the Family Phyllodocidae in Britain. Although not visible in the photograph above, the worm has a row of paddle-like appendages along each side of its body. These are very lively carnivorous worms that secrete loads of mucus which, no doubt, helps them to wriggle around the rocks at low tide looking for food.

The bright orange patch in the photograph above is encrusting sponge. The light green colour on the rock and the barnacles is a coating of microscopic surface algae. The deep pink tufts are red algae. There was a lot of this seaweed attached to the rocks here, often in a distinct band.

You can see from the pictures below how there is rocky shore zonation of the organisms colonising the limestone surface on Spaniard Rocks.  

Green Leaf Worm habitat - Spaniard Rocks, Rhossili, Gower, encrusted with red seaweed, mussels and barnacles in April 2009 (2)

Home of the Green Leaf Worms - rocky shore zonation of Carboniferous limestone at Spaniard Rocks, Rhossili Bay, Gower - showing successive colonisation of the surface by various organisms at different heights above sea level (3)

Green Leaf Worm, Eulalia viridis (Linnaeus), on mussels and barnacles encrusting limestone cliffs at Spaniard Rocks, north end of Rhossili Bay, Gower, South Wales.UK (4)

Revision of a post first published 8 May 2009


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Mussels on Spaniard Rocks, Rhossili

Common edible mussels, Mytilus edulis Linnaeus, at Spaniard Rocks, Rhossili Bay, Gower, South Wales, UK (1)

Dark glistening masses of common edible mussels, Mytilus edulis Linnaeus, smother the Carboniferous limestone of Spaniard Rocks and the island of Burry Holms at the north end of Rhossili Bay, Gower. This bivalved mollusc thrives here, particularly where the limestone has dissolved into peculiar shapes providing endless new facets for the mussels to colonise.

Common edible mussels, Mytilus edulis Linnaeus, growing at Spaniard Rocks, Rhossili Bay, Gower, South Wales, UK (2)

We tend to think of mussels as having blue/black shells – at least, that is what we normally see if moules mariniere is ordered, or we find shells on the beach. You can see from these pictures that the living mussels are quite varied in appearance. At least, the immature individuals show lighter colours and stripeyness. I also think the degree of exposure to wave action erodes the brown outer horny periostracum layer, and settlement intensity and competition for food may affect shell thickness and therefore colour. 

Common edible mussels, Mytilus edulis Linnaeus, growing at Spaniard Rocks, Rhossili Bay, Gower, South Wales, UK (3)

Where the mussels do not form a continous layer on the rocks, interesting patterns develop. Bare limestone can be coated with green surface algae. Cream coloured acorn barnacles compete for settlement space with the mussels. Where mussels and barnacles occur in abundance, dog whelks are sure to be found in vast numbers because they just love to eat these encrusting shelled animals. They often bore a hole into the shell to get at the meats.

Dog whelks, Nucella lapillus (Linnaeus), feeding on young mussels and barnacles on Spaniard Rocks, Rhossili Bay, Gower, South Wales, UK (4)

I am always amazed at the variety of colour and pattern in dog whelks. White or yellow through all shades to brown. Plain or striped. Thick shelled or thin. Pointed apices or blunt. Surprisingly, not edible like the mussels they feed on. Not having tried to eat them, I’m not sure why dog whelks are considered unpalatable.

Irregular patches of common edible mussels with acorn barnacles on Spaniard Rocks, Rhossili Bay, Gower, South Wales, UK (5)

The photographs show that the rocks are covered in several generations of mussels. You can tell this from the relative sizes. New seasons of young mussels prefer to settle and fix themselves by their fine byssus threads to surfaces where mussels already grow. Having said this, sometimes there are so many larval mussels that they settle on an incredible range of surfaces whether or not adult mussels are colonisers there. I have found hundreds of thousands of minute mussels attached to strands of green seaweeds round the corner from Rhossili in Broughton Bay.

Common edible mussels, Mytilus edulis Linnaeus, on Spaniard Rocks, Rhossili Bay, Gower, South Wales, UK (6)

Rock surfaces in the right zone for survival can be almost entirely covered with mussels. This is usually the lowest level on vertical or near vertical surfaces.

Common edible mussels, Mytilus edulis Linnaeus, colonising the lowest zone on the Carboniferous limestone at Spaniard Rocks, Rhossili Bay, Gower, South Wales, UK (7) 

On a final note, I will return to this subject as mussels are of great economic importance to the local economy, especially in years when, for one reason or another, the cockle populations that are normally commercially exploited off north Gower shores, fail.

Common edible mussels, Mytilus edulis Linnaeus, on Spaniard Rocks, Rhossili Bay, Gower, South Wales, UK (8)

Revision of a post first published 14 May 2011


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Zonation & Exposure on Burry Holms

 Rhossili Bay with the island of Burry Holms in the distance

Regular visitors to Jessica’s Nature Blog may already be aware from earlier posts that my interest in Rhossili Bay and Burry Holms goes back a long way. As an undergraduate I undertook a project on rocky shore zonation of marine invertebrates, seaweeds, and lichens in relation to exposure around the shores of this small island on the tip of the Gower Peninsula in South Wales.

I recently came across this dissertation and thought that it might be of interest to some readers as it still has relevance today for the understanding of seashore life in this location.

It was written in the days long before word processors and personal computers were available for the professional presentation of reports; so it was written in ink directly into a hard-backed notebook with accompanying hand-drawn graphs, tables, and sketches. Amazingly there are very few errors – just as well, as there was no correcting fluid then either. I have now scanned the original dissertation into a pdf file. [The result is mostly fine but with some poor quality where diagrams glued in as separate sheets have wrinkled or where sticky tape has deteriorated over time]. 

Click the link below for a copy of Zonation and exposure on the rocky shores around Burry Holms:


Burry Holms at low tide viewed from the south 

Zonation of seashore organisms on the exposed rocky shore of Burry Holms island at Rhossili Bay on the Gower Peninsula.


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Rocks at Burry Holms – Part 1

Rocks at Burry Holms: Rocks on the south west shore of Burry Holms island at northern end of Rhossili Bay, Gower Peninsula - Carboniferous limestone scoured almost bare by rough seas over winter (1)

Burry Holms is an island at high tide but accessible and joined to Spaniard Rocks and Llangennith Burrows when the tide is out. It is rarely possible to walk entirely around Burry Holms even when the tide is very low – although some people can remember doing so at least once in their lifetime. However, the south-west corner of Burry Holms island, at the north end of Rhossili Bay on the Gower Peninsula, is frequently exposed right down to the sand at low tides – and a wonderland of rock formations and rocky seahore creatures is revealed. Agile anglers are already familiar with this fairly inaccessible terrain as they hunt regularly in the nooks and crevices above the waterline for suitable bait before balancing precariously to fish. 

The whole of Burry Holms is composed of sloping strata of Carboniferous Limestone, laid down in oceans about 350 millions of years ago in the Palaeozoic Era. The island represents the remaining northern stump at the outer edge of a huge upheaval and folding of these rock layers. This anticline once covered the entire bay but the top of the curved mound has been worn away and flattened with time. At the south end of the bay, Worms Head and the Rhossili headland represent the remaining southern stumps of rock on the other side of the upfolding.

The rocks of Burry Holms look dark, jagged and stark from a distance but, on a closer examination, you discover that the rock is really pale gray in colour. For the most part, this light colour is obscured by different types of encrusting organisms. At low tide on the south-west shore, the bases of the rocky cliffs and outcrops are visible. Here the rock is bare: scoured clean and smooth by seas loaded with natural and man-made debris pounding away for thousands of years.  

Strange shapes, hollows and bowls have been dissolved and sculpted by nature in these lower rocks. It looks as if a giant spoon has been scooping the stone. Rock pools remain in the natural bowls, providing temporary havens for stranded marine animals, and permanent sheltered surfaces for small invertebrates that attach to rock or graze it. Here you can find rows of tiny barnacles newly settled in surface cracks, rounded clusters of glistening black juvenile mussels, and roving groups of dog whelks feasting on both young mussels and barnacles before laying masses of bright yellow eggs.

If you stand on the sand and look right up the steep cliff surfaces towards the sky, bands of differently-coloured encrusting organisms can be seen clinging to the rocks higher off the ground. The most obvious of these are the lower zone blackened by maturing mussels, and the greenish-cream zone above it predominently covered with mats of adult acorn barnacles.

Rocky shore zonation describes this phenomena where various seashore creatures, with different abilities to cope with and to survive exposure to the waves and the air, become naturally arranged on the shore at various levels above Chart Datum according to their lifestyle preferences and requirements. We see these bands of colonisation and colour particularly well on steep rock surfaces. There is of course a certain amount of overlapping between the zones but overall the colour bands are distinct.

There are occasional deep clefts in the rock faces, running across and through the original layers or strata, and extending vertically many metres from beach to cliff top. I wonder if they are natural faults? These narrow fissures can trap flotsam flung forcibly from the sea – as you can see in the final photograph showing a brightly coloured fishing float wedged high and tight in the cracked rock.

Click here for more posts about ROCK in Jessica’s Nature Blog

Mussels and Dog Whelks on rocks at Burry Holms: Rocks on Burry Holms newly colonised by young Common Mussels and Dog Whelks feeding upon them, Rhossili Bay, Gower, South Wales, UK (2)

Rocky shore zonation at Burry Holms: The rocky cliff face at Burry Holms, looking upwards, mussel zone below and barnacle zone above, Rhossili Bay, Gower, South wales, UK, (3)

Burry Holms rocks: Rocky hollows and pools dissolved into the Carboniferous Limestone on the south-west shore of Burry Holms (with mussels, dog whelks and the bright yellow mass of dog whelk eggs) Rhossili Bay, Gower, South Wales, UK (4)

Bare eroded rocks at the base of Burry Holms:Cliff rocks on the south west corner of Burry Holms island at Rhossili Bay, Gower, South Wales, UK (5)

Rock cleft at Burry Holms: Deep vertical cleft in the rock face at the south west corner of Burry Holms, Rhossili Bay, Gower, South Wales, UK (6)

Fishing buoy flotsam at Burry Holms: Fishing buoy and chain trapped in a rock cleft at Burry Holms, Rhossili bay, Gower, South Wales, UK (7)


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