A spikey cockle with a red foot

A Prickly Cockle with Eelgrass stuck to its shell at Studland Bay, Dorset, UK - part of the Jurassic Coast (1)

The black strips of dead eelgrass, impaled on the white recurved spikes of the shell, drew my attention to this cockle on the Studland strandline. So pretty, I thought, as I picked it up; and then discovered that the shell was still occupied. I put it on the wet sand to get a better photograph in the slanting light of the late afternoon sun. It was at that moment that it started to move.

The two ribbed valves slowly parted to expose the animal within the shell. Nestled among the soft pale tissue of the mantle that lines the shell, a bright orange-red structure could be just seen. This was the foot which is resposible for moving the cockle into and out of the wet sediments of the low shore. Gradually the foot unfolded itself by a combined effort of contracting and relaxing layers of circular, longitudinal and cross muscles that surround a blood-filled space.

Bit by bit, the foot lengthened and straightened in an attempt to make contact with the surface of the sand. Normally, the cockle engages in a “digging cycle” where the pointed tip of the extended foot first pushes down into the wet sediments before swelling and forming a pedal anchor.  The shell valves would then close – at the same time that foot muscles contract and shorten - so that the cockle can be dragged downwards and below the surface. Next, the two shell valves open to this time create a shell anchor that holds the cockle in position, while the foot again lengthens and repeats the process. 

In this instance, the cockle had been placed with the hinge lowermost so that, despite repeated extension attempts, twistings, and manoeuvrings, its foot was unable to make that important first contact with the sand to start digging itself in – until I returned the mollusc to the water.

A Prickly Cockle with the red foot just visible within the gaping shell at Studland Bay, Dorset, UK - part of the Jurassic Coast (2)

A Prickly Cockle with the red foot just beginning to emerge from the gaping shell at Studland Bay, Dorset, UK - part of the Jurassic Coast (3)

A Prickly Cockle with the red foot unfolding and protruding from the gaping shell at Studland Bay, Dorset, UK - part of the Jurassic Coast (4)

A Prickly Cockle with the red foot partially unfolded and extending from the gaping shell at Studland Bay, Dorset, UK - part of the Jurassic Coast (5)

A Prickly Cockle with the red foot arching out of the gaping shell at Studland Bay, Dorset, UK - part of the Jurassic Coast (6)

A Prickly Cockle with the red foot reaching out of the gaping shell to find wet sediments in which to rebury itself at Studland Bay, Dorset, UK - part of the Jurassic Coast (7)

A Prickly Cockle with the red foot fully extended prior to retraction back into the gaping shell at Studland Bay, Dorset, UK - part of the Jurassic Coast (8)

Revision of a post first published 24 May 2010

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A spikey cockle at Weymouth

A living cockle with spines on the shell from Weymouth Bay, Dorset, UK, part of the Jurassic Coast (1)

I found this spikey-shelled cockle on the beach at Weymouth. The tide was out and had left a series of shallow sandy ripples. Bait diggers had created some deep holes and I think that was how the cockle came to be lying on the surface in a ripple valley. It was sucking in water and periodically squirting it out.

I am not sure of its identity. It looks a lot like the Spiny Cockles (Acanthocardia aculeata) I find on Gower beaches but its overall appearance, size, and the sharpness of the spines make me think it is either a Prickly (Acanthocardia echinata) or a Rough Cockle (Acanthocardia tuberculata). Because it was alive and I returned it to the sediments, I could not examine the teeth in the hinge area or the inside of the shell, which would have provided vital clues. I’ll have to look out for empty Acanthocardia shells on the beach when I next visit Weymouth.

Click here for more pictures and information about Spiny Cockles in Jessica’s Nature Blog.

Another view of a living cockle with spines on the shell from Weymouth Bay, Dorset, UK, part of the Jurassic Coast (2)

A different view of a living cockle with spines on the shell from Weymouth Bay, Dorset, UK, part of the Jurassic Coast (3)

Another aspect of a living cockle with spines on the shell from Weymouth Bay, Dorset, UK, part of the Jurassic Coast (4)

Detail of the ribs and spines on an undetermined species of cockle shell found at Weymouth Bay, Dorset, UK, part of the Jurassic Coast (5)

Detail of the ornamentation on an undetermined species of cockle shell found at Weymouth Bay, Dorset, UK, part of the Jurassic Coast (6)

Detail of the umbones and external hinge area of an undetermined species of cockle shell found at Weymouth Bay, Dorset, UK, part of the Jurassic Coast (7)

Detail of an undetermined species of cockle shell found at Weymouth Bay, Dorset, UK, part of the Jurassic Coast (8)

Detail of the crenellated valve margins of an undetermined species of cockle shell found at Weymouth Bay, Dorset, UK, part of the Jurassic Coast (9)

A living cockle with spiny shell lying on wet sand ripples at Weymouth Bay, Dorset, UK, part of the Jurassic Coast (10)

View of Weymouth Beach at low tide showing wet sand ripples where the spikey-shelled cockle was found alive, Dorset, UK, part of the Jurassic Coast (11) 

Revision of a post first published 1 February 2010

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Banded Wedge Shells at Rhossili

Banded Wedge Shells: Paired empty valves of Banded Wedge Shell on sand at Rhossili, Gower, South Wales, U.K. (1)

Like colourful butterflies on the sand, the paired valves of the Banded Wedge Shell are one of the common and attractive finds on Rhossili beach, Gower. On the outer surface they are often bright yellow while the inner surface is frequently a lovely lilac or purple colour.

Banded Wedge Shells: Inner surface of paired valves of Banded Wedge Shell on the sand of Rhossili beach, Gower, South Wales.

The empty shells can occur in large numbers on the strandlines but, if you look carefully underfoot at low tide level, you can find the animals alive in the wet sand. Typically, part of the shell protrudes above the surface; it may be obscured by sand.

Banded Wedge Shell: A living Banded Wedge Shell part-buried in wet sand at low tide level at Rhossili, Gower, South Wales.

The texture of the sand varies from fine and compact with a smooth surface texture to patches where the sand grains appear coarser and the surface is an uneven texture looking a bit like lumpy porridge.

Living Banded wedge Shell: Living specimen of Banded Wedge Shell (Donax vittatus da Costa) protruding from coarse wet sand at low tide level at Rhossili, Gower, South Wales, U.K. (4)

Occasionally, the Banded Wedge Shells are found entirely exposed, lying in the surface water. In the picture below you can see the soft translucent fleshy foot and the siphons partially extended between the hard shiny shell. The large muscular foot is used to rapidly draw this bivalve down into the safety of the wet sediments if it is disturbed – either by the incoming surf or passing feet.

Live Banded Wedge Shell on sand: A living Banded Wedge Shell, Donax vittatus (da Costa), with fleshy foot and siphons partially extended, in surface water on sand at Rhossili, Gower, South Wales, U.K. (5)

If you wait patiently and watch a partially buried Banded Wedge Shell, you may witness the cyclical retraction and subsequent extension of the paired tubular siphons as they squirt out water. In the photograph below the siphons are fully extended and have just evacuated.

Banded Wedge Shell alive showing siphons: Living Banded Wedge Shell with the siphon tubes fully extended on the lower seashore at Rhossili, Gower, South Wales, U.K. (6)

Revision of a post first published 29 August 2009

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Toothed Top Shell at Ringstead Bay

Toothed Top Shell, Monodonta lineata (da Costa), the animal emerging from its shell, from Ringstead Bay, Dorset, UK - part of the Jurassic Coast (1)

Say “Hello” to the occupant of the Toothed Top Shell, Monodonta lineata (da Costa). Looking a bit like an alien but a lot friendlier, this animal was not at all shy about coming out of his shell. As soon as you pick up a living gastropod mollusc it usually retreats to safety – but not this one.

What you see here is mostly the shiny orange head of the mollusc. A large black wrinkled snout is in the middle. Unseen, the mouth is inside the snout along with the radula – the tongue-like structure bearing rows of teeth that it uses to scrape algal films from rocks. Small black eyes sit at the tip of a blunt orange stalk on each side of the head. Next to the eye-stalks are the long, thin, tapering cephalic tentacles. The glossy tentacles are ringed by black zig-zag patterns along their length. The animal even seems to have greenish coloured eye-brows!

Toothed Top Shell, Monodonta lineata (da Costa), at Ringstead Bay, Dorset, UK - part of the Jurassic Coast (2)

The shell itself doesn’t look anything special. It is like a large blunt common winkle and rather thick. It has a slightly convex profile and the five or six whorls that make up the 30mm high cone are not very pronounced.

Toothed Top Shell, Monodonta lineata (da Costa), at Ringstead Bay, Dorset, UK - part of the Jurassic Coast (3)

Shown here in profile, the shell is cream coloured with a design of reddish brown or purple zig-zag stripes. The surface texture of the shell is normally dull and smooth but in younger specimens there may be some fine spiral grooves and ridges. The colouring of the shell is often obscured by silt and algae so that it just looks mud coloured and undistinguished. This specimen seems to be quite old, maybe six years old, because it has a number of clear growth stages in the shell visible near the shell opening.

Toothed Top Shell, Monodonta lineata (da Costa), at Ringstead Bay, Dorset, UK - part of the Jurassic Coast (4)

Looking down on the top of the shell in the above picture, you can see that the tip of the shell spire tends to get worn so that the inner, mother-of-pearl layer is exposed.

Toothed Top Shell, Monodonta lineata (da Costa), the living animal, from Ringstead Bay, Dorset, UK - part of the Jurassic Coast (5)

The large dark muscular foot on which the mollusc crawls over the rocks is clear to see in this photograph. Most of the organs such as the digestive system are inside the shell cavity.

Toothed Top Shell, Monodonta lineata (da Costa), the living animal, from Ringstead Bay, Dorset, UK - part of the Jurassic Coast (6)

Just another view of this remakable seashore creature. It responded to the strange circumstance of being lifted from the rocks by continually moving and gyrating in an attempt to re-establish itself in a safe place with plenty of food. This behaviour would be a survival instinct for the occasions when it was accidentally dislodged by waves or, perhaps, birds. I replaced this animal in a cool, shaded rock pool after photographing it.

Toothed Top Shell, Monodonta lineata (da Costa), showing the living animal withdrawn into the shell and the operculum with its pattern of concentric circles, from Ringstead Bay, Dorset, UK - part of the Jurassic Coast (7)

The operculum, a brown horny lid that is attached to the back of the foot, and with which the mollusc closes the aperture when it retracts into the shell, is round in outline and has a continuous circular growth line starting in the centre and extending outwards.

The last couple of features by which to recognise this seashell are not easily seen in the living specimen – so I have photographed an empty shell from the beach, below.  The columella is a thick smooth, shiny, white structure at the core of the shell. At its base, on the outside of the shell, next to the mouth or opening, is the shallow cleft or dimple marking the lowermost point of the columella. The shape is characteristic for the Toothed Top Shell. The blunt protruberance or ‘tooth’ from which this species gets its name can be seen on the inner edge of the mouth of the shell.

Toothed Top Shell, Monodonta lineata (da Costa), showing the 'tooth' (bordering the inner edge of the aperture) from which it is named. Ringstead Bay, Dorset, UK - part of the Jurassic Coast (8)

Revision of a post first published 21 June 2009

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Parts of the shell – Common Whelk

It is useful to know a few simple terms that describe the different parts of seashells. This special terminology defines the particular features of a shell that distinguish one species from another. It also helps to know these words in order to understand accounts of the development of the creatures, their biology and life history. 

I am starting with a few names of basic parts in a gastropod mollusc shell. The example illustrated is the Common Whelk – Buccinum undatum Linnaeus. A gastropod shell is like a long, tapering tube or cone that has been coiled up.  The top of the coiled shell, where the diameter of the tube is at its smallest, is called the apex of the shell.

Each complete turn of the shell tube, each coil, is known as a whorl. The areas where the whorls touch and join each other are called sutures. As the living animal grows, the diameter of its shell increases. Most of the body of the animal is housed in the last large whorl, the body whorl, leading to the mouth or aperture of the shell. Typically the snail can protrude from the aperture or withdraw into it. The whorls above the body whorl are termed the spire. The spire can be short and squat (like in a Necklace Shell) or drawn out to form a point (as in the Common Whelk).

The flesh of the animal is covered by a layer of tissue called the mantle. The mantle makes the shell. The shell is made up of three layers.  The inner and middle layers of shell are composed of calcium carbonate crystals arranged on a framework of an organic, protein-like material called conchyolin.

The smooth inner or nacreous layer of the shell is continually secreted by the overall surface of the mantle with which it is in continuous contact. In some species of mollusc this is a layer of iridescent ‘mother of pearl’.  The thin sheets of crystalline calcite that make up this inner layer lie more or less parallel to the surface. The inner layer grows in thickness throughout the life of the animal.

Around the aperture, the edges of the mantle produce the middle layer of the shell which comprises crystals of aragonite lying at right angles to the surface. This layer stops getting bigger when the mollusc becomes adult.

The outermost layer of the shell is the periostracum. This is also formed by the edge of the mantle and stops growing on sexual maturity. It has no calcium carbonate crystals and is entirely made of conchyolin. It is thin and easily worn away and rarely seen in mature gastropods.

The outer calcareous layer of the shell has patterns and textures that are typical of the species. Spiralling around the shell of the whelk, along the length of the ‘tube’ as it coils, is a series of narrow ridges and grooves called striations. Running diagonally across the striations, and curving right over each whorl, are thick ribs. On a much finer level, very fine growth lines loop the circumferance of the ‘ tube’ of the shell, and intersect the striations.

The next posting on Jessica’s Nature Blog will show photographs and detailed close-ups of the patterns and textures made by growth lines and sculpturings on the outer surfaces of  Common Whelk shells. It will also further illustrate some of the features mentioned here.

 

Revision of a post first published 6 April 2010

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A Common Starfish eating a Spiny Cockle

Starfish eating a cockle: A Common Starfish (Asterias rubens Linnaeus) preparing to eat a gaping Spiny Cockle (Acanthocardia aculeata (Linnaeus), at Rhossili, Gower, South Wales, UK (1)

Well, the Common Starfish is not actually eating the Spiny Cockle just yet – but it is moving in for the kill!

A whole load of marine creatures was washed up after a storm on the causeway between Burry Holms and Spaniard Rocks at Rhossili, Gower. The orange-skinned starfishes (Asterias rubens Linnaeus) were making the most of the situation by homing in on the distressed bivalved molluscs. There was so much debris of one sort or another that the cockles, razor shells and other bivalves could not easily re-bury themselves. The Spiny Cockle shells - Acanthocardia aculeata (Linnaeus) - were gaping wide open while they repeatedly reached out with their large, glistening, red muscular foot to gain a purchase on the wet sediments to pull themselves back down into underground safety. Mostly they were unsuccessful.

The starfishes were taking advantage of the opportunity provided by the mass stranding. They inched forward and over the shells ready to eat the easy pickings. Normally, they would have had to wrap their five arms around the mollusc and sucker on tight with their multitudes of tube feet. By exerting a continuous steady pull, the starfish would aim to tire the muscles that hold the two hinged shells tight shut. Eventually the cockle would become exhausted and gape open, allowing the starfish to eat its prey. 

In this dramatic scene of invertebrate carnage, thousands of small seashore creatures died or were dying. However, for some organisms like the starfishes it was a bonanza.

Spiny Cockle at Rhossili Bay: A Common Starfish (Asterias rubens Linnaeus) preparing to eat a gaping Spiny Cockle (Acanthocardia aculeata (Linnaeus), at Rhossili, Gower, South Wales, UK (2)

Spiny Cockle (Acanthocardia aculeata Linnaeus) protruding its red muscular 'foot' while trying to escape from the Common Starfish (Asterias rubens Linnaeus) that is attacking it at Rhossili, Gower, South Wales, UK (3)

 Prey and predator on the seashore: Common Starfish about to eat a Spiny Cockle with shell gaping open at Rhossili, Gower, South Wales, UK (4)

Revision of a post first published 21 December 2009

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Black Whelk shells from Whiteford

These empty shells of the Common Whelk, Buccinum undatum L., were originally lying randomly on the sandy strandline at Whiteford in Gower. I photographed this arrangement on a layer of fine beach gravel because I liked the complementary colours and contrasting textures. The natural sculpturing of growth lines, ridges, and grooves on the whorls of this gastropod mollusc shell are emphasised by the dark hues. In natural sunlight the black staining looked almost navy blue. The dark colour has been acquired by a period of burial deep in the beach sediments.

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