Pholadidae P Pholadidae Pholadidae, known as piddocks or angelwings, are a family of bivalve mollusc similar to a clam ; however, they are unique in that each side of their shells are divided into 2 or 3 separate sections. Furthermore, one of the piddock's shells has a set of ridges or "teeth", which they use to grind away at clay or soft rock and create tubular burrows. The shape of these burrows is due to the rotating motion of the piddock as it grinds the rock to make its home. The piddock stays in the burrow it digs for the entirety of its eight year lifespan, with only its siphon exposed to take in water that it filters for food. When the piddock dies and leaves an empty tubular burrow, other marine life such as sea anemone , crabs and other molluscs may use the burrow. Some species of Pholadidae may reach up to seven inches. Their coloration is typically white, though through consumption of red tide algae some may develop a pink coloration.

The angelwing species Cyrtopleura costata has approximately 26 radiating ribs. Growth lines run horizontally over the surface of the shell. Angelwings have a spoon-shaped brace under the beak of the shell, called the apophysis, where the mollusc's foot muscles are attached. Cyrtopleura costata possesses long siphons which protrude from its burrow and circulate water as the source for its food supply. It cannot retract its siphons into the protection of its shell, so the two valves can never shut completely. The muscles fusing the shell's valves together are weak, making it rare to find angelwings with both halves still intact. Some shell hunters dig for the living clam, and if dug up, the fragile shell must be placed immediately into a container of water or it will close and shatter. The angelwing's shell is popular with collectors, as well as a delicious food staple. The angelwing lives offshore and in estuaries, sometimes as much as three feet deep in the mud or clay.

The Atlantic mud-piddock, Barnea truncata, often referred to as the fallen angelwing, is classed among the angelwing varieties, growing up to 2¼ inches and is similar to other angelwings but with weaker sculpture. One end is squared off and the other end pointed. Loose accessory plates are located above the hinge on live specimens. It possesses a white exterior and interior. It burrows into mud, clay or softwood. Occasionally it is washed onto sounds and ocean beaches, and has a habitat range from Maine to Brazil . This fragile shell is rarely dug from mud without breaking. It burrows deeply and has long, united siphons. The false angelwing Petricola pholadiformis, is also classed among varieties of angelwing, growing up to 2¾ inches. It has a thin, elongate shell resembling a small angelwing but lacks the rolled-out hinge area. Its beak is at one end of the shell with strong radial ribbing on the beak end. Teeth are located on hinge and a deep pallial sinus and partially united siphons. It has a white exterior and interior.

It typically burrows into hard surfaces such as clay or peat in intertidal zones. It is commonly found on sounds and ocean beaches with a range from Canada to Uruguay . The common piddock (Pholas dactylus) is known for its bioluminescence and was investigated by Raphaël Dubois in his 1887 discovery of luciferin . Genera and species A biomechanical model of rock drilling in the piddock Barnea candida (Bivalvia; Mollusca) | Journal of The Royal Society Interface A biomechanical model of rock drilling in the piddock Barnea candida (Bivalvia; Mollusca) Ralf Nederlof, Mees Muller Published 13 June 2012.DOI: 10.1098/rsif.2012.0329 Ralf Nederlof PDF Abstract The bivalve Barnea candida (Pholadacea) makes its burrow in clay, soft rock and peat. Barnea has developed a number of adaptations to accommodate this lifestyle. Four muscles enable burrowing.

These are situated around a dorsal pivot in such a way that the piddock is able to rotate the shells around two approximate orthogonal axes. The anterior adductor muscle anterior (AAM-A) and the posterior adductor muscle rotate the shells around a dorso-ventral axis; the anterior adductor muscle posterior (AAM-P) and the ventral adductor muscle rotate the shells around an antero-posterior axis. The AAM-A and the AAM-P have evolved from a single anterior adductor muscle and are attached to a piece of the shell that is folded inside out, the umbonal reflection. At the dorsal side of the piddock, the shell margins are reduced. This prevents collision of these margins during movement. Electrical stimulation experiments revealed that the opening of the antero-ventral side of the piddock is faster than its closure. These results were incorporated into a computer model that could simulate shell movements. The computer model allowed predictions about the shapes of burrows and scrape marks. As in Nature, simulated burrows had a long droplet shape with straight scrape marks. 1. Introduction Some species of bivalves have adopted a rock- or wood-boring lifestyle.

They make their burrows either by using acid to dissolve the substratum or mechanically, scraping at the substratum. Boring species occur in eight different superfamilies: Pholadacea, Adesmacea, Myacea, Mytilacea, Veneracea, Cardiacea, Gastrochaenacea and Hiatellacea. The shipworms Teredo (Pholadacea) are wood-boring bivalves [ 1 ]. Barnea (Pholadacea) [ 2 , 3 ], Zirphaea (Adesmacea) and Petricola (Veneracea) [ 4 , 5 ] are examples of species that make their burrow in clay, soft rock and peat. Martesia striata (Pholadacea) is able to perforate PVC pipes [ 6 ], which gives it an economic significance. Several cases of this type of biodeterioration in eutrophic coastal development areas are reported. Severe damage can occur within 1 year. Boring bivalves dating from the Cretaceous period have been recorded.

Fossils of Pholadidea species in Cretaceous layers have been found and described by Crampton [ 7 ] and Kennedy & Armentrout [ 8 ]. The boring habit is thought to have evolved along two different paths [ 9 ]. In one path, the bivalves were attached to hard substrata by means of byssus threads, but could still move using their foot. Thus they scraped at the substrata with their shell. In the other path, they evolved from species that burrowed in soft substrata such as sand. During the course of evolution, they gradually started to burrow in harder substrata. In this research, Barnea candida (Linnaeus, 1758), commonly known as the white piddock, was used as a study object. Barnea candida is a member of the family Pholadidae. It lives (among other places) in the English Channel, where it makes its burrow in soft rock, peat and clay. It makes its burrow mechanically. During the course of evolution, Barnea has developed a number of adaptations to accommodate its rock-boring lifestyle.

For instance, on the dorso-anterior side of the piddock an accessory piece of shell, the protoplax, is found. It covers and protects the anterior adductor muscle anterior and the anterior adductor muscle posterior (AAM-A and AAM-P, respectively) [ 2 ]. The AAM-A and AAM-P are attached to the umbonal reflection, a part of the shell that is folded inside out. These anterior abductor muscles start out as one anterior adductor muscle in the juvenile stage, but are split in two and migrate from the inside to the outside of the piddock during growth. This transition occurs during the formation of the umbonal reflection [ 3 ]. Together with this formation, a dorsal condyle is also created [ 2 ], which functions as a pivot for rotating the shells. There are two other muscles for moving the shells, the posterior adductor muscle (PAM) and the ventral adductor muscle (VAM; figure 1 ). Download powerpoint Figure 1. Microscopic view of Barnea candida. AAM-A, anterior adductor muscle anterior; AAM-P, anterior adductor muscle posterior; PAM, posterior adductor muscle; VAM, ventral adductor muscle.

The AAM-A and the PAM can rotate the shells around the dorso-ventral axis, the AAM-P and the VAM can rotate the shells around the antero-posterior (longitudinal) axis. The axes are indicated. The shells always touch at one point only, the pivot, which is the centre of rotation. Scale bar, 10 mm. The formation of the umbonal reflection, the migration of anterior adductor muscles, the protoplax and the dorsal condyle could be considered as a ‘key innovations’ in evolution [ 10 ], leading to the different way in which these bivalves move their shells. Only when a part of the original adductor muscle has passed the pivot is a wriggling movement for drilling enabled. This implies a step-like transition in the ability to exploit a new niche. This makes the piddock an ideal subject for the study of evolution. Until now, no research has been carried out on the biomechanics of the boring action of B. candida. It is difficult to observe piddocks in situ, because they are hidden a few centimetres deep in the substrate. Some observations of boring behaviour were carried out by Ito [ 3 ].

Additionally, Barnea seems to open its shells actively with its muscles instead of using a passive ligament. In light of this, experiments were carried out to find out how the boring cycle works exactly. To answer these questions, the shape of the shells, position of muscles, burrows and scrape marks were studied, alongside experimental work (in vitro) and modelling of the movements of the shells. 2. Material and methods 2.1. Animals For this research, live and dead specimens of the white piddock, B. candida, were studied. Photographs were taken of piddocks in clay, their burrows, their scrape marks, their shells from different angles and transverse sections of the shells. Additional information was obtained by studying microscopic sections and by doing experiments. For the experiments, piddocks were collected at Wimereux and Ambleteuse, Pas-de-Calais, France. The specimens were 30–50 mm in length. In Wageningen, The Netherlands, they were kept in aquaria containing seawater (30 ppt), at a temperature between 10°C and 14°C. They were fed Artemia twice a week.

The experiments were conducted in a different tank (l = 0.5 m, w = 0.3 m, h = 0.12 m), under the same conditions. 2.2. Experiments Two experiments were carried out. In the first experiment, muscles of 10 piddocks were alternately electrically stimulated by a specially designed pulse generator. In the other experiment, artificial scrape marks were made and photographed. During the first experiments, the piddock was suspended in a hole in a rubber membrane. The membrane was made of a piece of a surgical glove; the hole was made using a punch stamp. The membrane was stretched over a plastic ring and clamped in a glass tube (r = 14 mm). The ring was missing a small section, and therefore could be compressed; so it could fit in the glass tube. In the tube, it clamped itself tightly against the tube wall. The membrane now sealed the tube at one side, except for the small hole that was made in the middle. The piddock was suspended in this hole, where it could move its shells freely. Its siphon pointed to the middle of the tube, where it could be reached by a small flexible tube supplying air.

In this way, the piddock became more active and stayed in good condition during the experiments, which could take up to a day to conduct ( figure 2 ). Download powerpoint Figure 2. A schematic side view of the set-up of the electrical stimulation experiment. The piddock is suspended in a membrane stretched over a ring at the front of the tube. It is connected to a pulse generator via electrical wires. Some air is added with a tube to keep the piddock in better condition and more active. A video of the movements is made. Tank size: length 0.5 m, width 0.3 m, height 0.15 m. The diameter of the glass tube is 28 mm. Seawater is 30 ppt and 10–14°C. In this scheme, the AAM-P and VAM are connected. For the experiment, holes (0.2–1 mm diameter) were drilled carefully in the shells to enable positioning of the electrodes inside the muscles. The electrodes were placed on both sides of the muscle. In every experiment, an antagonistic muscle pair was connected to the electrodes. The electrodes were made of short platinum wires, which were connected to conventional insulated wires.

The connection was made by weakening the plastic insulation with acetone and inserting the platinum wires. The electrodes were inserted in the muscles and the wires were glued on the shell using cyanoacrylate. Any exposed parts of the wires were then insulated with nail polish. The electrodes were fed by a pulse train current (1 A, 50–100 Hz, 10–20 ms). A specified and constant current to the pulse train was provided by a Howland current source that we constructed. This device converts pulses from a pulse generator into stimulus pulses in such a way that one can specify the duration and voltage and choose the pair of electrodes that will be actively stimulated at any given time. A LED and sound indicated which muscle was stimulated. The pulse train was also connected to an oscilloscope, to visualize the voltage (and consequently the strength of the current). With a stopwatch, stimulus duration could be measured. A stimulation time of approximately 35 s per muscle was used, the members of each muscle pair being stimulated alternately. A digital camera mounted above the glass tube recorded the movements of the piddock during the stimulation. The video clips were transferred to a computer for analysis.

From the video clips, amplitudes and durations of the movements of the shells were measured. The amplitudes (in pixels) were indexed to the width of the tube in millimetres. The results were subsequently incorporated into a computer model (see below). 2.3. Scrape marks and burrow Artificial scrape marks were made in wax to investigate the morphology of scrape marks and to determine what size, i.e. depth and width, they have. In this experiment, two methods were used, a method in which a shell was pulled through the wax, and a method in which a shell was rotated through the wax. The pulling method gave insights into the depth and width of the scrape marks. The rotational method was applied to determine the role of individual denticles. The scrape marks of both methods were compared with real scrape marks. In the first method, an x–y recorder was used. A plastic stick, holding the piddock shell, was inserted in the penholder of the x–y recorder. The piddock was attached to the plastic stick via a cotton bud. The shell was glued on the cotton bud and laid in a groove, which was made in the plastic stick.

It was fastened by tape, which was wrapped around the stick and cotton bud a few times. The shell was attached with the denticles pointing down, so that it would make scrape marks when it was drawn through the wax. The wax was mounted on a piece of cardboard, which was curved upward and supported so it would not buckle during the experiment. This was done to let the wax roughly follow the curvature of the front end of the shell. This enabled all denticles to run through the wax. In the second method, the shell was also glued on a cotton bud. But this time, the cotton bud was attached on top of a plastic rod, again using tape. The ventral side of the shell, with the denticles, was situated a bit farther from the rotation axis than the dorsal side. The rod was hollow, so it could be slid over the shaft of a servomotor. This servomotor rotated the shell through the wax. The shell was rotated in the same direction as it would in Nature. The wax was melted and poured into a cup of a candle holder. It was roughly situated at one side of the cup and was curved concave—again, to enable all denticles to run through the wax.

2.4. Computer modelling Finally, a computer model was created incorporating the observations and the electrical stimulation experiments. In this computer model, the movements were simulated. From this simulation, the shape of the burrow could be derived. Super-imposing all steps of the movement created a burrow. The outer surface of the resulting image represents the wall of the burrow. In the same image, modelled scrape marks can be seen. Trajectories of points representing denticles resemble the scrape marks of the shell on the burrow wall. Scrape marks could be projected on both the burrow and the shell. The results of the modelled burrows, and the modelled and artificial scrape marks, were compared with the photographs of the burrows and scrape marks. For computer modelling, Matlab v. 7.0 was used. Programming was started by creating a simple ellipsoid, which was cut in half and mirrored to represent two simple shells. For simplicity, all lengths were set in a dimensionless unit length of 1. Obviously, this does not represent real piddock dimensions, but real dimensions could be added later if necessary. From this set-up, more detailed modelling was done concerning the shape and the movement of the shells.

The shells were transformed into a more realistic shape. In the dorso-ventral direction, the radius increases according to an exponential function [ 11 ], resulting in a spiral shape of the shell. In this case, the radius is r = r · eθcotα, where θ is the angle of rotation and α is the acute angle between the radius and the tangent of the spiral. The radii in the transverse plane were transformed according to this formula. The angle α was determined by making a fit of the photographed transverse cross sections of nine shells. In the anterior–posterior direction, the shells were elongated. The front half and the back half of the ellipsoid were elongated separately, according to measurements taken from real shells. This model does not have an umbonal reflection. An alternative shell model, not used in the simulations, generated the umbonal reflection. This is discussed in a separate section. A simple algorithm was added to move the shells around the antero-posterior (x)-axis and dorso-ventral (z)-axis. Points on the shells were followed during movements. The trajectories of these points represent scratch marks. Rotating the shell step by step over a period of time created rotational movement.

With every time step, the shells were rotated slightly around the antero-posterior axis and dorso-ventral axis. With a sine function, the tempo and amplitude of the rotation was coordinated. The rotations were made independent of each other, to allow for phase delays. 3. Results Barnea candida has two pairs of antagonistic muscles for moving its shells [ 3 ]. The AAM-A has an antagonist in the PAM, and the AAM-P has its antagonist in the VAM. On the inside of the shells, between the PAM and AAM-P, there is a dorsal condyle that functions as a pivot ( figure 1 ). The four muscles rotate the shells around two axes. The AAM-P and the VAM rotate the shells around the antero-posterior axis. The AAM-A and PAM rotate them around the dorso-ventral axis [ 3 ]. The dorso-posterior margins of the shells do not touch when the piddock is closed ventrally. When closed ventrally, there is a beak at the front. It is located roughly at the point where the foot emerges. When B.

candida has its foot attached to the substratum, it can move forward by contracting it. 3.1. The shell Barnea candida has a mainly white shell, which is elongated in a posterior direction. The outside of the shells is lined with ribs containing denticles. These denticles appear only on the anterior part of the shells. They scrape at the substratum during the boring cycle. The caudal part of the shell is 71 per cent of its total length. The frontal part has a sharp point, lying lateral to the antero-posterior axis (medial plane; figure 3 a–d). Going from anterior to posterior, the ventral part of the shell margin curves inwards from the frontal point to the medial plane. Download powerpoint Figure 8. (a) The umbonal reflection as created in a cross section of the alternative model and (b) in a cross section of a real shell. The edges of a small shell-shaped oval (black line); q and r are rotated around the origin o creating a spiral r = r eθcotα. The small spiral arm created by edge q will fold over the large spiral arm created by edge r, creating the umbonal reflection ur.

This model closely resembles a real shell, as can be seen in (b). The distance between q and o should not be too small and α not too large, so that the small spiral arm will fit inside the large spiral arm. However, the distance q to r in the model is very large in contrast to the real shell. The model picture is dimensionless; the graph applies to typical lengths for piddocks of 30–50 mm. Picture taken from Matlab. (c,d) Three-dimensional pictures of simulated umbonal reflection. In (c), the shell shape is clearly seen, with the umbonal reflection curling over. A close-up of the origin of the model shell and the umbonal reflection from behind is seen in (d). s, shell; r, ribs; ap, apophysis; d, dentricles; ur, umbonal reflection; o, origin; q, near edge; r, far edge. (b) Scale bar, 10 mm. 4. Discussion The model accurately predicted how the piddock made its burrow, resulting in model burrows and scratch marks closely resembling real burrows and scratch marks.

The shells were not exactly the same as real shells but possessed the general shape and the essential features of the real piddock shells, the posterior gap and the logarithmic spiral. Although the pivot was not drawn as part of the shell, it was modelled as a point in space. The ribs also did not correspond with real ones, but this did not interfere with the results. The denticles were represented by data points in the model. These data points were allocated to positions on the model shell that were similar to the position of the denticles on the shells. In the simulations, the shells were rotated backwards (contraction of PAM) around the dorso-ventral axis, opening up the anterior part of the piddock and closing the posterior, and then rotated forwards again (contraction of AAM-A). The shells could also be rotated upwards (contraction of AAM-P) around the antero-ventral axis, opening up the ventral side of the piddock and closing the dorsal side, and then rotated downwards again (contraction of VAM). After this, a new cycle could begin. The upward movement created the scrape marks.

The backward and upward movements could occur simultaneously (no phase difference) or with a slightly delayed upward movement (i.e. with a phase difference). Movement without a phase difference gave straight scrape marks; with a phase difference there was a wobble in the scrape marks. Alternatively, straight scrape marks could be created when the shells were rotated in sequence: first opened anteriorly, and then rotated about a perpendicular axis. The back end of the animal is then relatively stationary. Observations of the piddock in situ showed the following cycle. When not boring, the ventral side of the piddock as well as the posterior side seemed to be entirely open. In this configuration, the siphon has enough space to extend. When the piddock made a boring cycle, it first closed the ventral and posterior side of the shell by contracting the VAM and PAM. Then, it contracted the AAM-P making the boring stroke of the cycle, after which it opened up the posterior side of the shell to terminate the boring cycle. In this phase, the piddock seemed to strut the burrow by pushing its shells against the burrow wall. The same movements were observed by Ito [ 3 ]. This observation supports the results of the model.

To obtain straight scrape marks, the rotation around the two axes is done in sequence, and therefore there is no phase delay. The movements in the electrical stimulation experiment were induced and not natural. The durations of muscle stimulations were longer than one would expect for wild specimens. The piddocks did react quickly to the stimulus, but to allow them to remain in the resulting configuration a longer stimulation time was necessary. The duration of the movement in both the electrical stimulation experiment and the model did not represent real times. However, it gave us some insight into the relative speed of the different rotational movements. In absolute time, faster or slower movements will not give different outcomes of the model. In Nature, the piddock might move differently. The electrical stimulation experiment was done in water and not in clay. The piddock therefore has no resistance from a burrow wall. This might also influence the movements of the piddock. However, the piddock does not have a lot of freedom in its movements. It only has two rotational axes, and can move forwards by contracting its foot. In the initial model, the shells could move forwards and sideways. However, when these movements are added, the results were not satisfactory.

Therefore, in the rest of this study, only rotation around the pivot was assumed. Simulations with simultaneous forward movement and rotational movement did give different scrape marks, especially with bigger movements. Forward movement is therefore very small, if it occurs. The piddock may first move forwards and dig its anterior margin in the clay before rotation. Zirphaea crispata and Martesia striata, both closely related species, use their foot to draw the shells into the burrow before the boring cycle [ 5 , 12 ]. To clarify this, further study should be carried out on Barnea in their burrows. A better view of the piddocks in situ would give a better insight into the model. However, this is difficult. As mentioned in §1, the piddock sits a few centimetres deep in the substrate. Looking through the substrate with, for example, X-rays could be difficult. Impurities such as small grains of gravel could distort the image. This would not be useful for our research. When taken out of their burrow, it is difficult to let the piddock make a new burrow in a transparent medium.

It was easier to suspend the piddock in a membrane in a glass tube and film them when stimulated. It was also only possible to keep the piddocks alive for a few weeks. Although the movements in the artificial scrape mark experiments were not exactly the same as in Nature, they demonstrated how the scrape marks were made and what they should look like. The scrape marks made during the boring cycle are made by the most ventrally positioned denticles of the shells. These denticles are the last to scratch the surface of the burrow wall. They are the biggest and therefore will ‘overscratch’ previous scratch marks made by smaller more dorsal positioned denticles. Pre-scratching of the smaller more dorsal positioned denticles would also protect the ventral denticles against overload. These ventral denticles do not seem to have a lot of wear, and there are very few or none broken. This indicates that B. candida probably cannot abrade harder substrata such as chalk, but only soft substrata such as clay, peat or sandstone. The denticles on the ventral margin are bigger because there is more space.

The denticles stand on ribs and also stand in rows from dorsal to ventral ( figure 4 a,b). The same number of denticles stand on a longer rib and therefore can be wider and longer. Larger piddocks also need to abrade more from the burrow and thus also need larger denticles. The shape of the shells is vital for the boring cycle. If they had evolved differently, they would collide during movement, making it impossible for the piddock to make its burrow. This also indicates that this piddock (Barnea), which has muscles to open its shells, makes its burrow in an essentially different way from some other piddocks, which use a ligament to open their shell—for example, Petricola pholadiformis. Barnea has greater freedom of movement and better control over its shell movement, which gives it better opportunities to adapt its surroundings to create a better habitat, or possibly to live in slightly harder substrata than other piddocks. The changes in morphology of Barnea are rather simple. They occur during the transition from larval to adult state [ 2 , 3 ]. It turned out to be rather easy to mathematically generate the umbonal reflection.

Why and how these evolutionary important features—umbonal reflection, pivot, the split anterior adductor muscle and the posterior gap, and protoplax—emerged is an interesting subject for future study. Acknowledgments The authors thank Jacqueline Bastiaans for making the coupes of the piddocks, Henk Schipper for helping with imaging the piddocks, Corstiaen Versteegh for helping us with collecting the piddocks, Eric Karuppannan for making some equipment and Dr Jon Barnes (Centre for Cell Engineering, Glasgow University) for reading the manuscript and additional support. Appendix A A.1. The shell The programming in Matlab is started with an ellipsoid function to create a sphere ( figure 9 a). Each cross section consists of 50 points. The centre of the ellipsoid is centred at (x,y,z) = (0,0,0). Next, the ellipsoid is transformed into a shell-shaped form, with r the radius of the shell in the y–z plane and θ the angle of rotation, and Now, r can be transformed according to rs = r × eθcotα ( figure 9 b,c).

The ellipsoid is now halved owing to the squaring of y and z. After the transformation, the radius is translated back to y and z values, After elongation, a part of the shell is removed ( figure 9 f). A.2. The movement A function is added for rotational movement. The function was made by taking the first half of a cosine function with a period T1. Concatenation to this is the second half of a cosine function with a different period. The total period (Tper) is a concatenation of half period T1 with half period T2. The cosine with period T2 is created by multiplying the cosine T1 with a chosen integer (ratio), The entire function was raised by 1, the minimum value is now 0 and the maximum value 1. This function is used for movement around the x-axis (antero-posterior axis) and the z-axis (dorso-ventral axis). A phase delay can be introduced by letting one movement start later than the other. The movement is created by multiplying the value of the function on moment t with the maximum angle. A new angle is acquired, ranging from 0 on t = 0 to maximum on t = Tper.

This is done twice, once for the angle around the x-axis (a), and once for the angle around the z-axis (b). The values a and b are inserted into a rotation matrix. If the rotation around the z-axis is delayed, a function with phase delay is then used for the calculation for b. Rotation around the x-axis, Rotation around the z-axis, The shells can be shifted in the z-direction (and/or y-direction), to position the pivot at the correct height relative to the shell margins. For this shift, y2 and z2 are added with an appropriate value before rotating. Points can be allocated on the shell, which are followed during movement (scrape marks). The points are allocated on appropriate sites. A.3. Phase difference Phase difference is created by adding a phase angle to the cosine function T1. Owing to the concatenation of T2 to T1, the whole function will have a phase delay. This leads to a retardation time R (i.e. phase × (Tper/360)).

If the PAM and AAM-A muscles are denoted by pa and the VAM and AAM-P muscles are denoted by vp, the following rule will apply: if (T1,vp/2 + R) > Tper,pa, then pa will begin a new cycle before the vp cycle is finished. In other words, if the contraction time of the AAM-P muscle is delayed by a time R and gets bigger than the total period of the boring cycle of the PAM and AAM-A muscles, then the PAM and AAM-A muscles will start a new cycle before the VAM and AAM-P muscles are finished with their cycle. This leads to a wobble in the scrape marks, which becomes bigger with larger phase delays. Therefore, But ideally R should be somewhat smaller Received April 20, 2012.

This journal is © 2012 The Royal Society References Common Piddock | Jessica's Nature Blog Jessica's Nature Blog 5 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: 4 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).

COPYRIGHT JESSICA WINDER 2014 All Rights Reserved 89 Pebbles with holes in them are a common occurrence on the beaches along the Jurassic Coast in Dorset – and lots of other U.K. locations as well. Many of these tunnels, burrows or borings have been made by small marine invertebrate animals such as certain species of bivalved molluscs, polychaete worms, and even sponges. The arrangement of pebbles shown in the photograph above illustrates a selection of the different kinds of holes created by these animals in soft rocks. In this post I am just going to talk briefly about the larger tunnels which tend to be about a centimeter or more in diameter. These have been excavated by rock-boring bivalve molluscs. There are several types whose habits result in these borings and it is not easy to say which species has made which tunnel from the shape and size of the holes alone. Fortunately, the shells of these creatures often remain in the burrows. The appearance of the shells is diagnostic for each species. Unfortunately, the shells of some of these bivalves are both surprisingly fragile and wedged securely in the burrow so that they are difficult to extract.

The most commonly occurring rock-boring molluscs are the Piddocks and some other related species. From the Pholadidae family, for example, these include the Common Piddock Pholas dactylus Linnaeus and the White Piddock Barnea candida (Linnaeus). From the Hiatellidae family, these include the Wrinkled Rock-borer Hiatella arctica (Linnaeus). And from the Gastrochaenidae, the Flask Shell Gastrochaena dubia (Pennant). The softer rocks into which these bivalves bore are generally low on the shore and under water most of the time. When pieces of this rock break away, the stone becomes more rounded and worn and ends up as a pebble on the beach – like the one above which was seen at Charmouth in Dorset. The photograph below shows a flat platform of soft Blue Lias shale extending seawards at Charmouth. If you look closely at the near-vertical edge of the rock where it is lapped by the water, you may be able to see that the shale has many perforations caused by these rock-boring molluscs.

Below is a closer view of the seaweed free edge of the rock platform with the piddock holes. The next picture shows empty shells still in situ in the burrows. In future posts I will illustrate the actual shells of these rock-boring molluscs and will describe something of their life in such a unique habitat. I will also discuss at a later date the other smaller types of rock-borings made by worms and sponges; and show how they also colonise and leave physical evidence of their presence on mollusc shells. When this evidence is found in shells recovered from archaeological excavations, it provides clues to the environment that was being exploited for its marine resources by people in the past. Revision of a post first published 4 October 2009 COPYRIGHT JESSICA WINDER 2011 All Rights Reserved rock boring marine molusk « Beach Treasures and Treasure Beaches Posted by Jody on October 21, 2012 Beach rock with burrowing clam shell still inside. Have you ever found a rock on the seashore with holes bored into (or even through) it?

The first time I ever found one of these fascinating beach treasures, I had no idea what could have caused the small round cavities. After some research, I found that the little, artfully carved, natural sculptures are usually created by boring clams, members of the Piddock family (Pholadidae). I personally don’t find them to be the slightest bit humdrum! These burrowing bivalves are generally white in color and have brittle, often elongated, shells. Depending on the species, they are capable of penetrating wood, coral, or moderately hard stone! Holey moley! These interesting rocks are a fun addition to any beachcomber’s collection! Related links: The Most Interesting “Boring” Find Ever! Posted by Jody on January 26, 2012 While Greg and I were exploring the Pacific Coast of Northern California a few weeks ago, we headed out in search of Agate Beach County Park, located near the little coastal community of Bolinas. One might assume that a stretch of coastline named after ornamental stones would actually be a beachcomber’s paradise for finding said ornamental stones. One would be wrong. We didn’t find one agate!

Agate Beach, Marin County, California Agate Beach offers amazing views of a rocky shoreline and its stunning, though precarious, surrounding cliffs. The beach was nearly inaccessible during our visit because of the high tide, but the trip was well worth the drive and the very short hike in. This is where we spied the most interesting “boring” find ever! We happened upon an intriguing and beautiful sculpture, supplied by Mother Nature, herself. Sculpture by the Piddock Family (Marine Mollusks) This beach treasure is the perfect example of an artistic creation by marine mollusks; members of the Piddock Family (Pholadidae), to be precise. These bivalves are a group of relatively thin shelled, burrowing clams that, depending on the species, are capable of boring into firm mud or clay, soft to moderately hard rock, wood, and/or shells. (Think: apartment complex.) They can be found in temperate and tropical waters. There you have it! One of nature’s finest masterpieces crafted by members of the Piddock Family. This unique piece of art truly is our most interesting “boring” beach find ever!

FYI: During low tide, this beach has nearly two miles of exposed shoreline. Low tide is a great time to visit Agate Beach County Park, as it is renowned for its tidepooling opportunities. It is against the law to remove any type of tide pool life here. According to MarinCountyParks.org , “Agate Beach is part of the Duxbury Reef State Marine Reserve, which includes state and federal agency jurisdictions and signifies its status as a sensitive and highly protected resource area.” What is your most interesting beach treasure? We’d love to hear about it. Feel free to leave a comment. Thanks! *If you enjoyed this article, please share us with your friends. We’d appreciate it if you would “Like” us on Facebook , too!* Share this: Common Piddocks – rock-boring molluscs | Jessica's Nature Blog Common Piddocks – rock-boring molluscs by winderjssc 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: Angel Wing | SeaShell News SeaShell News SeaShell News, Shell Or Angel’s Wings By Darijus Strasunskas , Via Creative Commons. SeaShell News, 3-6-15, Angel Wings, Shelling, Sanibel Island. “Pholadidae, known as piddocks or angel wings, are a family of bivalve mollusc similar to a clam ; however, they are unique in that each side of their shells is divided into 2 or 3 separate sections. Furthermore, one of the piddock’s shells has a set of ridges or “teeth”, which they use to grind away at clay or soft rock and create tubular burrows.

The shape of these burrows is due to the rotating motion of the piddock as it grinds the rock to make its home. The piddock stays in the burrow it digs for the entirety of its eight-year lifespan, with only its siphon exposed to take in water that it filters for food. When the piddock dies and leaves an empty tubular burrow, other marine life such as sea anemone , crabs and other molluscs may use the burrow. Some species of Pholadidae may reach up to 18 cm (7″). Their coloration is typically white, though through consumption of red tide algae some may develop a pink coloration. The angelwing species Cyrtopleura costata has approximately 26 radiating ribs. Growth lines run horizontally over the surface of the shell. Angelwings have a spoon-shaped brace under the beak of the shell, called the apophysis, where the mollusc’s foot muscles are attached. Cyrtopleura costatapossesses long siphons which protrude from its burrow and circulate water as the source for its food supply.

It cannot retract its siphons into the protection of its shell, so the two valves can never shut completely. The muscles fusing the shell’s valves together are weak, making it rare to find angelwings with both halves still intact. Some shell hunters dig for the living clam, and if dug up, the fragile shell must be placed immediately into a container of water or it will close and shatter. The angelwing’s shell is popular with collectors, as well as a delicious food staple. The angelwing lives offshore and in estuaries, sometimes as much as a metre (three feet) deep in the mud or clay. Piddock clams. Interesting facts about rock boring mollusks - YouTube Piddock clams. Interesting facts about rock boring mollusks Want to watch this again later? Sign in to add this video to a playlist. Need to report the video? Sign in to report inappropriate content. The interactive transcript could not be loaded. Loading... Rating is available when the video has been rented. This feature is not available right now. Please try again later. Published on Mar 1, 2016 Penitella penita also known as angelwings or piddock.

For more information go to these webpages about piddock clams:

Question: The piddock (Pholadidae and 'angelwings') is a rock-boring?
Answer: Mollusc