Pholas Dactylus
– Common Piddock –
| Kingdom: | Animalia |
| Phylum: | Mollusca |
| Class: | Bivalvia |
| Subclass: | Heterodonta |
| Order: | Myida |
| Family: | Pholadidae |
| Genus: | Pholas |
| Species: | P. dactylus |
| Binomial name |
|---|
| Pholas dactylus Linnaeus, 1758 |
Pholas dactylus, or common piddock, is a bioluminescent clam-like species of marine mollusc found on the coasts of the North Atlantic and the Mediterranean Sea. It bores into gneiss. It was once a highly esteemed food in Europe.[2][1]
It is sensitive to light, retracting into its shell when exposed to it.[3]
Description
The pholade will in principle rarely be observed by the diver, because of its way of life (see biotope).
In general, only the two siphons glued along their entire length, ivory-colored and which can measure up to twice the length of the shell , will be visible . The mantle of this mollusk presents, at night, a blue-green luminescence .
Most often, only the animal’s valves will be observed on the sea leash.
The shell of the common pholade is composed of two valves of equal size (so-called equivalent shell *), but of different appearance (so-called unequal shell *). It is thin , fragile, white , elliptical in shape and very elongated : its size reaches up to 120 mm , rarely 150. The anterior part of the shell is swollen while its opposite part is smoother.
It “ yawns ” clearly on both sides but more on the anterior side, which is also rostrated (it evokes a dolphin’s beak), which makes it easy to differentiate Pholas dactylusrelated species.
The surface of the valves is sculpted with prominent concentric ribs corresponding to the lines of growth. On the front side also appear radiating lines concentrated and decorated with small spines , evoking the surface of a lime.
Four small accessory calcareous plates are visible on the dorsal side when the animal is alive.
The umbo * presents on the outside of each valve a small elongated limestone structure (umbonal reflection) and separated from the main part by a partitioned space, which acts as a reinforcement.
The inner face of each valve shows on the impression of the palleal sinus * the presence of three muscles: two adductors and an abductor.
We can clearly distinguish an apophysis * in the form of a hooked finger allowing the insertion of the foot muscles.
Similar Species
Barnea candida Linnaeus, 1758, the white pholade, measures up to 60 mm. It is found in soft substrates such as chalk (Channel, North Sea, Baltic, Atlantic and Mediterranean). Small fold in the umbo instead of the external limestone structure. A single accessory plate.
Barnea parva(Pennant, 1777) measures up to 40 mm. The concentric lines are denser and the radiating lines are less marked. From the British Isles to the Mediterranean.
Zirfaea crispata (Linnaeus, 1776), the large rough pholade, measures up to 80 mm. This pholade is frizzy or rough, stocky and even more yawning. This species presents a groove connecting the umbos to the ventral edge. English Channel, North Sea, West Baltic and Atlantic.
Petricola pholadiformisLamarck, 1818, the false pholade, is a species that was introduced to the North Sea. Even if it looks more like Barnea candida , the resemblance is important and the risk of confusion exists, but we do not observe any external calcareous piece, neither folds, nor process.
Biotope
Pholas dactylus is a marine species that bores soft rock, digs galleries in which it lives. It is found up to 20 meters deep in various substrates such as soft limestone, clay, peat, clay, sandstone, etc … So, only its siphons which protrude are visible.
Food
The pholade is a filter feeder. It has two siphons, one inhaling for the capture of planktonic organisms and the other exhaling, for the discharge of waste.
Reproduction
The pholade is a gonochoric * species, that is to say that the sexes are separate: there are male and female individuals. The emission of gametes * and fertilization take place in open water between June and August. A larval stage follows (veliger larva *) and then fixation thanks to a byssus * on the substrate. This disappears as soon as the shell has reached a certain solidity and the mollusk begins to drill.
Her life expectancy is 14 years.
Various Biology
The foot of the pholade ends with a suction cup (this partly explains the yawning of the valves) which allows it to take support during drilling; at the same time the abductor muscle pushes the valves apart.
Pholas dactylus is luminescent. It emits a faint greenish-blue light that can only be seen in the dark. A hypothetical reason for this phenomenon would be that the light produced would attract the plankton on which the pholade feeds.
When the animal is alive the shell is covered with a pale yellow periostracum * only on the outside. It is eliminated quickly when the animal dies.
The valves do not have a hinge unlike the vast majority of bivalves. There is also no ligament. The valves are only attached to the muscles and are completely independent from each other.
Further Information
It is with the anterior part of the valves (front part swollen and concentrated in radiating wrinkles and thorns) that the animal drills perfectly round holes in all kinds of substrates: this ranges from wood to soft rocks which give it shelter from ‘where its two siphons protrude. The mollusk performs a very slow rotational movement which wears down the substrate (a few milliiimeters per month).
After its fixation, the young pholade begins its drilling only when its shell has hardened sufficiently. The more the animal sinks into the rock the more the hole widens, because throughout the drilling, it continues to grow. Thus he is totally prisoner in his burrow, sheltered from many predators except shore birds, such as gulls and oystercatchers ,
One hypothesis put forward by scientists is that Pholas dactylus secretes a chemical substance in addition to its rotational movement. This would partly explain why the fragile shell of the common pholade does not show significant signs of wear.
It is an edible and sought-after species, rarely encountered in the markets. It is also a good bait for fishing. These samples, which most often involve deliberate destruction of the rock, cause irreversible damage to its ecological niche.
The cavities left by naturally dead animals are all refuges for other organisms. It is common to find other molluscs, worms or crustaceans there. These cavities allow the development and maintenance of biodiversity.
Beware of the misuse of the term lithophagus, sometimes encountered in naturalist literature: the pholade does not consume the stone, it only hollows it out.
Regulations
Strictly protected species, Pholas dactylus has been listed in Appendix II of Annex II of the Bern Convention since December 6, 1996.
Ancient history
Pliny spoke of luminescence in the mouths of people who ate Pholas, the rock-boring shell-fish, and of such importance is this phenomenon that it is even said to have gained the first king of Scotland his throne.[4]
