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

Clustered Periwinkles at Whiteford

Large groups of common winkles clustered around the base of large stones on the beach

At low tide many thousands of common winkles or periwinkles (Littorina littorea Linnaeus) seek shelter from dessication and predation by clustering together in the few hiding places available on the beach. At Whiteford Sands these niches include the overhung bases of larger stones, crevices in ancient timbers from the rapidly emerging submerged forest, and nooks and crannies in the recently exposed ancient peat. Alternating layers of peat and clay, overlain by rocks from glacial till, provide algae-covered surfaces on which gastropods can feed, and islands of low tide refuge in the vast expanses of sand on this sea shore.

Large groups of common winkles clustered around the base of large stones on the beach

Large groups of common winkles clustered around the base of large stones on the beach

Large groups of common winkles clustered around the base of large stones on the beach

Large groups of common winkles clustered around submerged forest wood on the beach

Large groups of common winkles clustered around submerged forest wood on the beach

Large groups of common winkles clustered around submerged forest wood on the beach

Large groups of common winkles clustered in exposed ancient peat beds on the beach

Large groups of common winkles clustered around submerged forest wood on the beach

Traces on the shore at Whiteford (1)

Sandgrain tubes of marine worms and sea shells

Fossils can include not only the actual preserved remnants of organisms from times long past (like shells and bones), and the replacements of such organisms (such as infilled moulds and casts), but also the evidence of their existence – for example their modified habitats, trails, burrows, tubes, marks and structures left by their feeding activities. Such evidence of long dead creatures is referred to as trace fossilisation; and the study of fossil traces is termed ichnology.

Part of the process of identifying the exact nature of trace fossils and reaching an understanding of their significance, involves making observations of the behaviour of related or similar present day organisms. This includes a study of their activities and the impact of them on their immediate environment in life. It also involves making records of the way in which the organisms change – what happens to them and their habitat after death (taphonomy).

Many of the trace fossils which I have discovered while walking along seashores and looking at cliffs, outcrops and boulders on beaches were made by various invertebrate seashore creatures in soft intertidal sediments that have later been buried, compressed, and hardened into rock. Recent marine worm tubes of various kinds are a fairly common occurrence on beaches and also as fossils in the sedimentary rocks of, for example, the Dorset coast in England. A short while ago I posted some photographs of fossil worm tubes that I found at Winspit.

For these reasons I was particularly interested to see the millions of sand grain tubes of marine polychaete worms on the low tide beach at Whiteford Sands on the north coast of the Gower Peninsula in South Wales. Many could be seen in situ, no doubt with worms in residence, projecting from the surface of the wet sand like some kind of stunted crop with seashells scattered among them. On the shallow bank created by a beach stream wending its way seawards, it was fascinating to see how far down into the sediments the tubes extended, a thick layer of unknown depth. It reminded me of a fantastic illustration in a book I recently acquired by Wilhelm Schäfer called Ecology and Palaeoecology of Marine Environments, originally published in Edinburgh by Oliver & Boyd in 1972. Copyright considerations mean the book is not old enough for me to reproduce the drawing here for you without written permission but Figure 190 on page 326 shows a cross-section through bedded sands and muds and shell deposits with many-branched and frequently extended dwelling tubes of Lanice conchilega cutting across the layers – just like the deposits on Whiteford Sands. Even though the book is old it is still widely available secondhand and is a wonderful repository of information.

It looks like snot but it’s not!

Sea squirts on an old lobster pot

I thought I would share these photographs with you. People usually like pretty photo-shopped pictures and these are a bit gross, not very attractive….. but interesting none the less. They look like snot but they are not. Most of these glistening gelatinous masses, the ones with the patterns (whether they are green, yellow or white) are Ascidians from the family Styelidae called Star Sea Squirts (I think two species are represented: Botryllus schlosseri and Botrylloides. leachii). I found them along with other sea squirt species which I am unable to identify on a quayside lobster pot. This marine invertebrate species is a primitive chordate and therefore related to us Homo sapiens.

Sea squirts on an old lobster pot

Sea squirts on an old lobster pot

Sea squirts on an old lobster pot

Sea squirts on an old lobster pot

Sea squirts on an old lobster pot

Shallow Rock Pools at Winspit

Seashore life in shallow rock pools on a limestone ledge

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.

Mauve Stingers at St. Peter Port

Mauve Stinger jellyfish on the beachJellyfish like small bright jewels littered the strand-lines along the St Peter Port shore in Guernsey last September. At first the bright pink parts in the clear jelly made me think they were immature Moon Jellyfish (Aurelia aurita) but a closer inspection revealed they were very different and not something I had encountered before. They were Mauve Stingers (Pelagia noctiluca) all about 5 centimeters diameter (2 inches) across the bell; mature specimens can reach 10 centimetres in diameter. The pinky-mauve features are the unbranched gastric pouches and the four frilled oral arms surrounding the mouth. There are also numerous tiny purple spots grouped within the transparent jelly.

There are sixteen lobes around the margin of the bell. The bell or umbrella when supported by the water column would look quite deep compared with the flattened stranded examples washed ashore and shown in these photographs. Eight long thin marginal stinging tentacles trailed from the jellyfish and adhered to adjacent pebbles where they lay on the beach. In this species it is not only the tentacles that are dangerous but also the entire outer surface of the bell (exumbrella surface) which has a characteristic bubbly texture created by nematocyst-bearing warts. The projectile stingers within the warts are triggered by touch.

Mauve Stingers are unusual in not having a sessile stage. The adult releases miniature medusae in the autumn, and the size of these increases until the following late summer. They feed on free-floating ascidians (sea squirts) and maybe other small jellyfish.

REFERENCES

Hayward, P., Nelson-Smith, T. Shields, C. 1996. Sea Shore of Britain and Europe, Collins Pocket Guide, ISBN 0 00 219955 6, p 48.

Hayward P. J. and Ryland, J. S. 1995 Handbook of the Marine Fauna of North-West Europe, Oxford University Press, ISBN 0 19 854055 8, pp 65-67.

Mauve Stinger jellyfish on the beach

Mauve Stinger jellyfish on the beach

Mauve Stinger jellyfish on the beach

Mauve Stinger jellyfish on the beach

Mauve Stinger jellyfish on the beach

Mauve Stinger jellyfish on the beach