Sea Creatures in St Fin Barre’s Cathedral

Mosaic of a lobster

In front of the altar of St Fin Barre’s Cathedral in Cork City, Ireland, is a wonderful mosaic floor made by craftsmen from Udine in the north-east of Italy, using marble from the Pyrenees. The mosaic represents a vision of heaven as described by St Matthew, likening it to a net that has been cast into the sea gathering every kind of creature. Here are a few photographs of the sea creatures depicted.

Mosaic fish

Mosaic fish

Mosaic seashell

Mosaic fish

The mosaic floor before the altar in St Fin Barre's Cathedral


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At Dogs Bay

Dogs Bay in Connemara has a wonderful white sandy beach composed of the tiny shells of microscopic one-celled creatures that live mostly on the mud of the ocean bed. These animals are called Foraminifera. When they die, millions upon millions of their calcium skeletons, bearing many chambers and holes, and not visible to the naked eye, wash ashore to form this unusual sand. This is such a rare occurrence that Dogs Bay beach is the only one composed of foraminifera in the northern hemisphere.

The bedrock of the land around this wonderful white sandy shore is made up of volcanic rocks including granite that has many different colour forms and patterns due to the different mineral crystals that it contains – if you get up really close to see it. The granite outcrops on the shores often have a rounded surface where ice sheets or glaciers passing over them have ground them smooth. The waterside rocks form attachments for a variety of seaweeds, along with many seashore creatures, particularly gastropod molluscs like periwinkles and limpets, whose brightly-coloured empty shells accumulate at the base of boulders low down in the intertidal zone.


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Striped shells from Studland

Striped seashells on a plate

Pictures of some seashells in a dish on my windowsill. I picked them up on the strandline at Studland Bay when I last visited because I liked the striped patterns. I think the concentric darker bands reflect the slower winter growth of the living mollusc while buried in deeper, mainly oxygen-poor, sediments offshore. This is where anaerobic bacteria thrive and their sulphur-rich waste products stain objects black.

Hayward (1994) says of this deeper sandy layer:

Below the boundary the black sand is essentially anoxic; free oxygen is totally absent, and the microfauna must survive through anaerobic processes. Bacteria still thrive but in the absence of oxygen use fermentation, or other chemosynthetic processes, to break down organic compounds. Many bacteria reduce sulphate, nitrate or carbonate ions to produce hydrogen sulphide. ammonia or methane, which give black sand the same unpleasant smell as sticky estuarine muds. The hydrogen sulphide reacts with iron in the sand to give black iron sulphides; as these are gradually carried to the surface by burrowing animals they are oxidised to ferric oxide, which imparts the yellow colour characteristic of the upper layers.


Hayward, Peter J. (1994) Animals of sandy shores, Naturalists’ Handbooks 21, Richmond Publishing, page 10, ISBN 0 85546 293 0

A heap of striped bivalve shells

Close-up of striped seashells from Studland


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Ringstead Bay Fossil Bivalve – Ctenostreon proboscideum

Most of the examples of this fossil bivalve, Ctenostreon proboscideum, were partial specimens embedded in the rocks at Ringstead Bay in Dorset, England. However, the large strongly-ribbed shell is unmistakable and easily recognised in the many boulders on the beach at the west end of the bay – at least they were easily seen when the pebbles had all been washed away after the storms. The photographs in the gallery above show Ctenostreon shells as they were found on the beach last week. The boulders had fallen from the Ringstead Coral Bed which is a narrow layer,  packed with fossils, of no more than 30 centimetres depth, and which can be seen in short lengths in the vertical section through the strata at the top of the beach.

The almost complete fossil specimen shown with the blue background (photographed at home) was found many years ago after similar severe weather. You can see that the two valves are still together and the space between them filled with marly limestone material, indicating that the original animal was already dead, with the two shells gaping open, when it was buried under new sediments.


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Common Piddocks – rock-boring molluscs

Common Piddock dorsal view

Here are some close-up photographs of the Common Piddock – Pholas dactylus Linnaeus (Mollusca; Bivalvia; Pholadacea; Pholadidae) showing details that are important for its specific identification. The specimen in the first three images still has the dead animal within the shells. This is one that I collected from those I found at Monmouth Beach in Lyme Regis (see the previous post) where a slab of shale, complete with the rock-boring molluscs still inside the burrows, had been thrown up on the shore by stormy seas. The empty shell shown in images 4 – 8 is a beach-worn specimen picked up on Knoll Beach at Studland a few weeks ago.

Although the length of the shell of the Common Piddock can be up to 15 – 24 cm or 6 inches, the examples shown here are smaller – with an  actual size of shell for the Monmouth Beach specimen of 50mm, and 108 mm in the shell from Knoll Beach.

Full details of the shell characters used for identification can be found in the references given below. This bivalve mollusc bores into sand, peat, marl, wood, shale, slate, chalk, limestone, red sandstone, schists, firm clays, and even thick old oyster shells, from low on the seashore to depths of a few fathoms. It occurs in The British Isles from Kent along the south and south-west coasts, including South Wales, and as far south as the Atlantic coast of Morocco. Of particular interest is the phenomena of phosphorescence or luminescence exhibited by the living animal which has has bioluminescent properties and glows with a blue-green light in the dark.

Earlier posts on Jessica’s Nature Blog that refer to the holes in rocks and pebbles made by piddocks and other seashore creatures include:

Rocks with holes made by Piddocks – Part 1

Beach Stones with Holes at Worms Head Causeway

Peat ‘pebbles’ with piddock holes

Pebbles with holes made by boring sponges

Pebbles with holes made by tube worms

Pebbles with holes made by sea creatures

Driftwood with holes made by Gribbles & Shipworms

Benjamin & the pebble full of holes

Shells with holes made by boring bivalves

A rocky beach near Portland Bill


Tebble, Norman (1966) British Bivalve Seashells – A Handbook for Identification, published for the Royal Scottish Museum by HMSO – Edinburgh, 2nd Edition 1976, ISBn 0 11 491401 X, pp 175 – 180. [Out of print but now available on CD from Pisces Conservation Ltd.]

Hayward, P. J., & Ryland, J. S. (Eds.) (1995) Handbook of the Marine Fauna of North-West Europe, Oxford University Press, 1998 reprint, ISBN 0 19 854055 8 (Pbk), pp 619 – 622. Still in print and available from Amazon and other booksellers.

MarLIN about Pholas dactylus

Wikipedia about Pholas dactylus

World Register of Marine Species about Pholas dactylus

Marine Species Identification Portal – about Pholas dactylus

Common Piddock right side view

Common Piddock ventral view

Common Piddock exterior view left valve

Common Piddock exterior view left valve

Common Piddock anterior end left valve

Common Piddock interior view left valve

Common Piddock shell showing umbonal reflection


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Rocks with holes made by Piddocks – Part 1

It is common to find pebbles and rocks which have holes in them when you walk on the seashore. These holes are frequently the result of various marine invertebrates that have burrowed into the rock. The larger, nearly round, holes about a centimetre or so across are often made by bivalved molluscs called Piddocks – these have a special shells and processes for mechanically boring into the rock.

Most commonly, you find stones, or shells, or bedrock, with holes – but the inhabitants have long disappeared. Other times, you glimpse the shell within the burrow but removing it is very difficult to do without totally destroying the shell. If you are really lucky, then you might spot a colony of living piddocks in their burrows at the lowest point of the tide where they will be sporadically squirting water out of their protruding siphons.

In any event, it is quite difficult to see all the details of the shells and thereby identify the creatures to species. Shells that occasionally get washed up on the beach may be abraded and worn, and vital parts for identification are always missing. So, I was delighted last Sunday to see slabs of broken shale, complete with piddocks in burrows (albeit dead specimens), washed up on the shingle of Monmouth Beach in Lyme Regis which is in Dorset, England.

This provided a wonderful opportunity to see what these boring bivalve molluscs and their shells really look like and observe the particular shell features that adapt the organism for this strange lifestyle – but which are normally missing from beach worn specimens on the strand-line.

As far as I can make out, all the specimens that I photographed on the beach were the Common Piddock (Pholas dactylus). I took a few home to clean them up and photograph them in better conditions. The light at the time was very poor, winds were very strong making it a problem to keep the camera still, and the air was laden with salty moisture that persistently misted the camera lens. I’ll show the photographs I took at home in the next post. Meanwhile this post will focus on the piddock shells in situ.

The shell of the Common Piddock is elongate, roughly elliptical and can grow up to 150 mm long – although the specimens I saw were a lot smaller. The anterior (front end) of the shell has a strange beaked appearance and surrounds a permanent pedal gape through which the muscular foot can be extended. The surface sculpturing at the front end of the shells consists of many short, sharp spines that develop at the junctions where the concentric ridges of the valves intersect with the radiating ribs. The spines enable the mollusc to use the shell like a rasp to file away the rock (or shell, or wood, or peat) as it bores down and creates its personalised living accommodation within the protective confines of the substrate.

This boring mechanism is aided by several extra features of the shell – accessory plates on the outside and a long curved process called the apophysis on the inside. The external accessory plates are the paired protoplax, the paired mesoplax, and the single metaplax. Rhythmic contractions of the muscles attached to these plates, and to the apophysis, enable the mollusc to perform a twisting action that aids the drilling process. The whole procedure is further enhanced by a forcing outwards of the two valves against the rock walls using hydrostatic pressure (sucking water in through the siphon and holding it  temporarily to create pressure).


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I really want you to have a look at the fantastic work of Thierry Alexandre. He not only paints and makes wearable art but also creates performance art. On his website you can see both still images and videos. The seashore provides him with much of the inspiration for the creations, often incorporating seashells and other flotsam for the costumes. The video at the top of this post shows a costume made with oyster, mussel, whelk and slipper limpet shells, adorned with cuttlefish bones and spider crab carapaces – to mention just some of the strand-line items. Incredibly innovative work. I particularly like the images where he is on the beach coated head to toe with glutinous clay to which gravel is adhering – rather like the clay ball from Ringstead Bay which I photographed some time ago.

Seashells on Normanby Island – Part 2

I enjoyed my first visit to Normanby Island so much that I went back a second time before I finished my Queensland holiday – and photographed some more seashells!

P.S. This is the 1000th Post I have published on Jessica’s Nature Blog.


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Seashells on Normanby Island – Part 1

A wonderful assortment of beautiful tropical seashells lies on the coral beaches around Normanby Island. Normanby Island is one of the Frankland Island group which lies on the Great Barrier Reef off the Queensland Coast of Australia. As it is a National Park, you cannot collect and take away anything from the island. All these photographs were taken on the beach during the visit. Access to Normanby Island is somewhat limited and only one company was running trips to the island when I was there. Most of the visitors sailing to the island from the Mulgrave River were intent on diving and snorkelling on the living coral reefs around the island, a pleasure that was denied me as a non-swimmer. I contented myself with exploring the shores and enjoying the fabulous picnic provided.


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Miscellaneous Seashells

Black-lined Periwinkle Seashells

Black-lined Periwinkle seashells (Littorina nigrolineata)

Banded Wedge seashells

Banded Wedge seashells (Donax vittatus)

Turban Top seashells

Top Shells (Gibbula spp.)

Variegated Scallop seashells

Small Scallop  seashells (Chlamys sp.)

Dog Whelk Seashells

Dog Whelk seashells (Nucella lapillus)

Common Cockle seashells

Common Cockle seashells (Cerastoderma edule)

Netted Dog Whelk seashells

Netted Dog Whelk seashells (Hinia reticulata)

Small Scallop seashells and Manila Clam

Small Scallop seashells (Chlamys sp) and Manila Clam

Common Whelk seashells

Common Whelk seashells (Buccinum undatum)


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