Another Day and a new species. This time, I want to show you Helix pomatia LINNAEUS (1758); a snail from the Helicidae, which belongs to the Molluscs. In German, the species is known as “Weinbergschnecke“ (literally: vineyard snail) and in English as "Roman Snail", “Burgundy Snail” or “Escargot”.
1) Morphology & Anatomy
H. pomatia is a land living and air-breathing snail. It's one of Europe largest snails with a maximum length of nearly 10 centimeters (4 inches). As with the most snails, the body consists of four parts: head, foot, visceral hump and shell. Because of its size, the Roman Snail is also a model organism to explain the morphology and anatomy of the land snails.
H. pomatia - habitus
1.1) Head and Body
The head has four antennas, with the eyes located at the end of the upper antennas. They are well developed for molluscs and are able to see a black/white image. However, there are also many odor and taste sensors along the antennas. All these sensors give the Roman snail a good image of its environment.
H. pomatia - antennas with eyes (black spots; see circles)
The body merges into the large, muscular “foot” (Cephalopodium) with which the animal moves over the soil. This happens by muscle contraction, which creates wave-like movements. However, the edge of the body always remains on the soil to avoid injuries. Like many other slugs, H. promatia secretes slime by a large gland at the mouth. This slime reduces rubbing and facilitates locomotion.
H. pomatia - the snail clims over a leaf. Note, how the
cephalopodium has always contact to the substrate
In addition, the slime is also secretes by many, smaller glands over the whole body. It protects H. pomatia against dehydration and smaller enemies like ants. Another protection is the thick mantle (Pallium), which covers the whole, dorsal side. It's a distinctive feature of the molluscs and should not be confused with the shell, because molluscs without a shell have pallium, too.
H. pomatia - head from behind
The visceral hump on the back of H. pomatia contains all important organs like liver, kidney, stomach, heart and lungs. As with the most gastropods, the guts have performed an asymmetrical, 180-degree twist (torsion). As a result, the anus of H. promatia is up front. The torsion is a phenom, which happens to many gastropods. It reduces the needed space for the visceral hump and fit the guts to the shell.
1.2) Respiration and Bloodstream
As Member of the Pulmonata, H. pomatia has no gills and breaths through a respiratory pore (Pneumostom). Through this pore, air flows into a small space between mantle an shell, where the gas exchange (O2 for CO2) happens between the air and the lung (in this area, the mantle is only a very thin membrane). The lung isn't as complex as the lung of humans or mammals. It's more like many, small bubbles, which are crossed by veinlets, which transport the oxygen to the heart.
Shematic view of the circulatory system of Helix pomatia
a) = heart; b)= mantle; c)= lung; d)= Pneumostom e)= veins; f)= shell
blue: Oxygen; red: carbon dioxide
H. pomatia has an open circulatory system. The blood is directly pumped through to the tissues and flows back passively. Such an open circulatory system is typical for many molluscs and snails. In addition, the bloodstream acts as a hydrostatic system, which creates a back pressure against the shell.
The shell of H. promatia is very thick and protects the animal from enemies and other harm. It consists mainly of lime and has a normally a yellowish color with brown strips. However, different colors are possible.
H. pomatia - shell with apex (a) and umbilicus (circle)
The shell is formed by the mantle during the embryonic state. It secretes a calcareous slime, which hardens, while the young snail becomes adult. Newly hatched snails already have a thin, transparent shell.
H. pomatia - shell (side view)
In the most cases, the shell is tortuous right, so the umbilicus, the opening of the shell, is on the right side. However, some rare individuals have a left-tortuous shell with their umbilicus on the left side. In German, these rare mutants are called “Schneckenkönige” (snail kings).
Snails have a very interesting and unique mating behavior. H. pomatia is a hermaphrodite and has male and female sex organs. When two snails mate, both rear up and rub their bodies together as an act of stimulation. Sex between two snails takes several hours.
H. pomatia - body and head
An important role plays the love arrow (Gypsobelum) which increases the excitation. This “arrow” is between 10 and 3 millimeters long and is driven into the body of the sexual partner. It consists of lime and is formed by the sexual apparatus.
H. pomatia - antennas
After six weeks, a fertilized snail lays between 20 and 40 eggs in a hole. The young snails hatch and needs two or three years to get sexually mature. During this time, the shell is very thin and many young snails are killed by predators. So, only a few individuals survive their juvenile state.
H. pomatia is a herbivore and eats algae and smooth parts of plants like wilted leaves. The snail creeps over the substrate and graze it with its tongue (Radula), which is covered with over 40.000 small teeth.
2) Habitat and Distribution
H. pomatia is native to Middle-Europe and belongs to the largest snails in this region. The species prefers habitats, where it can absorb lime, which is needed for the shell. In regions with little lime, the snails have only a thin shell and perish very fast. So, you can find H. pomatia in regions on lime; especially in lime forests or old limestone quarries.
H. pomatia - the animal feels threatened and has
withdrawn into the shell.
In France and other regions (like Switzerland or Southern Germany), the Roman Snail is a delicacy and eaten with garlic. However, the species is protected by law and it's forbidden to collect wild animals for cooking. So, the snails of the dish are from helicicultures.