A Walk at Rocquaine Bay

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.

Velella velella (By-the-wind Sailor) at Rhossili

By-the-wind sailor (Velella velella Linnaeus) on the beach at Rhossili, Gower, South Wales.

By-the-wind Sailors (Velella velella Linnaeus) have been washing ashore at Rhossili Beach over the past week. The sandy strandline has been dotted with dozens of the dark blue jellyfish-like creatures with their characteristic translucent sails. They are not actually jellyfish but pelagic (free-floating) colonial hydroids. [See also the earlier posts about this marine invertebrate in Sailing by Rhossili Bay and Bright Blue Blobs on the Beach.]

Stones with holes made by Wrinkled Rock Borers (& other seashore creatures)

Beach stone with holes made by seashore creatures at Charmouth, Dorset, England.

Beach stones with holes in them excite the curiosity of most people. How did the holes get into the rock? There is no single answer but in many cases the holes in pebbles and beach stones have been made by various seashore creatures including several types of bivalve molluscs, marine worms, and sponges. The same creatures can also make holes in thick old seashells. There are  several earlier posts on Jessica’s Nature Blog describing how the holes are made, by piddocks, for example Pholas dactylus, sponges such as Cliona celata, and polychaete worms like Polydora ciliata and Polydora hoplura. Frequently, there is evidence for more than one type of organism occupying the same stone.

One of the bivalve molluscs that creates holes in stones and thick oyster shells is the Wrinkled Rock Borer Hiatella arctica (Linnaeus). Like the piddock, this species can actively excavate a burrow in soft stone for shelter and protection although unlike the piddock it can attach itself by byssus threads to the outside of solid objects or in cracks and crevices. However, once embedded in the stone it can no longer exit the burrow but obtains all it needs for sustaining life via the tunnel connecting it to the outside world. Wrinkled Rock Borers are smaller than piddocks, measuring no more than 3.8 cms in length when mature. The valves of the shell are thick and robust with distinct furrows, and the leading edges exposed to view in the burrow are rough and straight edged (truncate). Tebble (1966) says that it is not possible to distinguish between the different species of Hiatella in British waters but the descriptions apply to all the species ever recorded here. Hiatella arctica is common around the British Isles from the lower regions of the shore to considerable depths…… It has a wide geographical distribution in the northern hemisphere from the Arctic south through the Atlantic, Mediterranean and Pacific, but the particular limits of its occurrence are not known. It is almost impossible to remove empty Hiatella shells from the excavated holes without breaking them.

Hiatella holes in rock can be secondarily occupied by a similar but smaller bivalved mollusc called Irus irus (Linnaeus). This grows to about 2.5 cms in length. The protruding frill-like concentric ridges on the shell can be very distorted in shape if the shell is occupying a burrow that is too small to allow normal growth. I am not able to discount the possibility that it is Irus shells occupying Hiatella burrows in some of the beach stones illustrated here. Irus (also known as Notirus irus) occurs from low in the littoral zone to a few fathoms.

REFERENCES

Tebble, N (1966) British Bivalve Shells: A Handbook for Identification, published for the Royal Scottish Museum by HMSO, Second Edition 1976, [Hiatella p172-173 & Plate 7h; Notirus p124-125 & Plate 7g].

Hunter, W. R. (1949), The Structure and Behaviour of ‘Hiatella gallicana@ (lamarrck) and ‘H. arctica’ (L.), with special reference to the Boring Habit. Proc. Roy. Soc. Edin. B, 63 III (19): 271-289, 12 figs.

Another Pennant’s Swimming Crab

Close up of the small Pennant's Swimming Crab at Rhossili, Gower, South Wales June 2009.

A small sand-covered Pennant’s Swimming Crab, about 2 cm across, emerging from its burrow in the wet sediments at low tide on Rhossili Beach one June.

For more details of this little seashore creature, Latin name Portumnus latipes (Pennant), see the earlier post.

A Pennant's Swimming Crab emerging from the wet sand at low tide on Rhossili beach, Gower, South Wales.

 

Traces in the Sand at Fermoyle

Bird beak marks in the wet sand at Fermoyle beach

On the beach at Fermoyle on the Dingle Peninsula in Ireland, the sea sifts and sorts the sand grains into different weights and colours to arrange in sinuous ripple patterns across the shore. The red particles from nearby outcrops of Devonian rock make contrasting curved lines along the receding water’s edge. At low tide level, the wet sand has been punctured hundreds of thousands of times by the beaks of shore birds both large and small. I have never noticed that before. There must be something about the texture of the sediment there that preserves the shape of the bills when they are withdrawn from the spot during feeding activities. It is plain that this particular place must have a very rich in-fauna of small seashore creatures like crustaceans, worms, and maybe molluscs.

Coloured ripple patterns in the sand at Fermoyle

Coloured ripple patterns in the sand at Fermoyle

Coloured ripple patterns in the sand at Fermoyle

Coloured ripple patterns in the sand at Fermoyle

Coloured ripple patterns in the sand at Fermoyle

Coloured ripple patterns in the sand at Fermoyle

Bird beak marks in the wet sand at Fermoyle beach

Bird beak marks in the wet sand at Fermoyle beach

Bird beak marks in the wet sand at Fermoyle beach

Bird beak marks in the wet sand at Fermoyle beach

Bird beak marks in the wet sand at Fermoyle beach

Bird beak marks in the wet sand at Fermoyle beach

Bird beak marks in the wet sand at Fermoyle beach

More Rayed Trough Shells at Rhossili

Living Rayed Trough Shell in a shallow tide pool on a sandy beach

Following the line of the limestone cliffs towards Kitchen Corner as the tide receded, the tide pools and beach were littered with dozens of living Rayed Trough Shells (Mactra stultorum Linnaeus) as they popped up to the surface of the sand. I don’t quite know why they chose to do this but it afforded an opportunity to see the living animal as opposed to the dead ones and empty shells that wash up more frequently on Rhossili Beach.

Two pale fleshy tubes joined together were extended between the two hinged shell valves. One inhalent siphon for sucking water with suspended nutrients inwards, and one exhalent siphon for dispelling de-oxygenated water with bodily waste products. I was afraid that these bivalved molluscs would die while gaping and exposed to the air, so I picked up a few and put them in the water of the pools but they were not very lively and did not re-bury themselves. I was surprised that no-one else seemed to notice them. Even the dog that I saw appeared more interested in splashing in the pools than snacking on the free harvest.

Tracks and Trails on Whiteford Sands

Furrowed trails made by common winkles on wet beach sand

You don’t exactly have to keep your nose to the ground to see them but you do have to be a keen observer to notice all the different tracks and trails left on the soft wet sediments of the beach at low tide. Larger marks left by people and vehicles are the first ones you see. Bird footprints are every where. The birds are feeding on all sorts of invertebrate seashore creatures like worms, small crustacea and molluscs – all of which leave holes, burrows and furrows as they move in and out of the sand and across the surface. Some of the pictures shown here simply aim to give the general context for the area of Whiteford Sands that I was walking across. If you look closely the other images, you will see not only the ripples in the sand but also the intricate network of traces left by the virtually invisible organisms that inhabit this ecosystem. The larger furrows in photos 1, 12 and 13 are made by the common winkle (Littorina littorea Linnaeus). I cannot name each animal that is responsible for each of the other types of trace. However, I am sure that there will be some specialists out there who could, especially those researchers concerned with the interpretation of trace fossils (the ichnologists).

Click images to view full size.

View looking west towards the sea at Whiteford Sands

View looking north-east towards the dunes at Whiteford Point

Wet seashore sand with marine invertebrate and other tracks and traces

Wet seashore sand with marine invertebrate tracks and traces with bird footprints

View looking north-east over wet seashore sand ripples with marine invertebrate tracks and traces at Whiteford Sands

Wet seashore sand ripples with marine invertebrate tracks and traces

Wet seashore sand ripples with marine invertebrate tracks and traces

Wet seashore sand ripples with marine invertebrate tracks and traces

Wet seashore sand ripples with marine invertebrate tracks and traces

Wet seashore sand ripples with marine invertebrate tracks and traces

Furrowed trails made by common winkles on wet beach sand

Furrowed trails made by common winkles on wet beach sand