Nutrition of Snails

Salad destroyer in action...

What do snails eat? Different kind of food in the terrarium test.

Snails and slugs, also known as gastropods (Gastropoda) are a very diverse systematic group. In the course of millions of years they managed to colonize almost every habitat on earth. There are snails in the sea, where they originally come from, but also on land and in fresh water. Under all the adaptations, that had to be overcome by gastropods during their evolution, that of feeding methods and nutrition resources remains among the most remarkable.

Terrestrial snails


The Spanish slug (Arion vulgaris) drives gardeners mad: No
cabbage head is safe. Picture: Andreas Heidl.
 

Thinking of a snail feeding, the first thing to jump into one's mind is the common slug destroying all of a garden's cabbages. Not only one of those slug species, such as Arion vulgaris, the much dreaded Spanish slug, can become a garden pest, when it occurs in large numbers.

But how does the Spanish slug manage to inflict such damage to gardens, as the desperate gardener can see in the morning after, observing the feeding traces in his cabbage?

Snails do not possess any mouthparts, such as a crab's mandibles; neither does it possess a set of teeth like a mammal. Instead, snails have got a specialized food processing organ, common to all molluscs: A rasping tongue or radula. Using it, snails are not able to bite of their food, but to rasp it down. The amounts of cabbage to be destroyed in one night by a sufficient number of slugs bear witness to this organ's efficiency.

A snail's rasping tongue basically resembles a miniature bucket-wheel excavator: An elastic band is moved over a gristle core. Toothlets on the band move through the food and doing so, they rasp particles away and move them to the rear, into the snail's gullet. But a snail does not only use its radula to process food: The radula is also used to clean the shell from rests of dried mucus. When the snails do that, a distinct rasping sound can be heard, if one pricks one's ears.

Radula function in snails (Gastropoda).

  Roman Snail (Helix pomatia) a Roman Snail (Helix pomatia) b Lusitanian Slug (Arion lusitanicus) c
Electron microscope pictures of different terrestrial snails' radulae.
From left to right: a, b: Roman snail (Helix pomatia); c: Lusitanian slug (Arion
vulgaris
). Source: Salzburg University (Josef Ramsauer).

Movie: A snail (Cornu aspersum) feeding on a slice of cucumber. [RN] MOV-File, ca. 4,5 MB.


Radula of Daudebardia rufa.
Source: FALKNER (1990).
 

As taken during a project at Salzburg University, an electron microscopic picture is needed to show that a snail's radula toothlets are by no means simply built, but that they show a quite intricate fine build. Besides, not all radula toothlets are the same, as can especially be seen in the enhanced picture of a Roman snail's radula: The so-called rhachis teeth in the middle are built differently from the lateral teeth to the left and right of them.

Besides, in spite of all resemblance, the radulae of Helix pomatia (the Roman snail, an exclusive herbivore) and Arion vulgaris (the Spanish slug, a facultative omnivore, see also: Snails as garden pest), there are distinct differences between both radula types. While the Roman snail's radula toothlets are rather broad, the slug's toothlets are longer and pointed.

More distinctly different a carnivorous snail's radula would be, such as Testacella haliotidea or Daudebardia rufa:

 
Reddish Daudebardia (Daudebardia rufa), a carnivorous snail,
feeding on earthworms. Source:  biolib.cz (Jiří Novák).

In spite of the general opinion that snails mainly live on cabbage, there are definitely numerous carnivorous snail species. Those common in Europe usually are shell slugs living underground, hunting for smaller snails, insect larvae and earthworms, which they catch with their long, sickle-shaped radula teeth. As the earthworm usually is longer than the snail, it is digested on one end, while the other still looks out of the snail's mouth.


Sicilian predator snail (Poiretia dilatata) with a
freshly caught Pomatias elegans.

Pictures left and lower right: Fabio Liberto, Na-
tura Mediterraneo
.

Pomatias elegans. Below: The
traces of a Poiretia attack.

 

Another interesting hunting method can be observed in the Dalmatian predator (Poiretia cornea), a Mediterranean species occurring on the Adriatic coast from Monfalcone as far as Albania. This species' preferred prey are operculate snails, such as the round mouthed snail, Pomatias elegans.

When Pomatias withdraws into its shell, the shell mouth is closed by a shell lid (operculum). So the predator snails is unable to enter the prey's shell by this way. Instead, it has developed an acid gland in the foot, but the means of which it dissolves the operculate snail's shell wall. Then it is able to feed on its prey without any possible means of defence.

Systematically, Poiretia belongs among the Oleacinidae family, an exclusively predatory family of terrestrial snails living mainly in the Neotropic, for example in Southern America (see: Faunal provinces). Another well known member of this family is the rosy wolf snail (Euglandina rosea), originally occurring around Florida.

Taxonomically the family makes a superfamily called Oleacinoidea, together with the already mentioned Testacellidae.

 
Rosy wolf snail (Euglandina rosea). Picture: Bill Frank, Jacksonville Shell
Club.

The rosy wolf snail (Euglandina rosea), another family member, in spite of its flowery name, is not a friendly creature. Its lips are prolonged to resemble a third pair of tentacles, so the snail is able to pursuit its prey, smaller terrestrial snails, along their slime trace. In doing so, Euglandina will not even stop at water and will even follow its prey up trees. The juveniles already feed on their smaller siblings and so become stronger themselves.

On the islands of French Polynesia, the wolf snail, originally not at home there, has been introduced by man to overcome the giant African land snails, also not at home there. Instead the wolf snail tended to hunt for smaller tree snails, instead of attacking the giant African snails. As a consequence, several species of small endemic tree snails (for example from the Partula genus) have gone extinct by today.

Instead of those examples of predatory behaviour described so far, most terrestrial snail species seem more likely to feed on plant matter. Many slug species, such as the leopard slug (Limax maximus) also feed on mushrooms. There are also groups like the true glass snails (Zonitidae), that feed as well on plant matter, as on carrion and on living animal prey, and in whose systematic environment species living exclusively predatory (such as, in this case, the Daudebardiinae family) have evolved.

Johnathan Wojcik: Ghastly Gastropoda: Top Ten Predatory Slugs and Snails.

Many snail species also have a part in distributing plant species. This process called endozoochory for example occurs in snail species feeding on lichens. Those snails usually living on tree trunks rasp down the lichens and swallow them. but always some fragments of the lichen manage to pass the snail's digestive system unharmed and are defecated. From those cells then new lichens can develop and have also been distributed by the snail. Among snail species feeding on lichens are many door snail species (Clausiliidae), but also bulin snails (Enidae) and cheese snails (Helicodontidae).

Boch, S.; Prati, D.; Werth, S.; Rüetschi, J.; Fischer, M. (2011): Lichen Endozoochory by Snails. PloS ONE 6 (4). (Link).

Sea snails

Many sea gastropods, however, such as the common whelk (Buccinum undatum) displayed in the picture on the left, to a larger extent than their terrestrial relatives, are predators and carrion eaters. The whelk, for example, attacks mussels and pushes its foot between their shell halves, before the surprised mussel is able to close up. Afterwards the whelk feeds on its prey without the latter being able to help itself. With its siphon erect, containing many olfactory sense cells, the whelk afterwards crawls its way on the search for more prey, which it is able to smell from the water it breathes.

  Sipho of a common whelk (Buccinum undatum)
Sipho of a common whelk (Buccinum undatum).
Picture: Peter Jonas, Unterwasser-Welt Ostsee.

Other sea gastropods have specialized in drilling a hole in their prey's shell to feed on it afterwards. Many herbivorous sea snails have developed very large and thick shells to protect themselves against predators. Mussels try to tie up attacking snails with their byssus threads. Limpets (such as Patella vulgata) finally try to clamp their attacker's foot with their shell. If it works, this is a very effective way of defence, as a limpet's foot is very strong.

Not nearly all sea gastropods are carnivores. Especially the lastly mentioned limpets have specialized in grazing of algae growing on rocks in the tidal zone. Not to be washed off by the surf, limpets have got a cup-shaped shell and a very strong foot, so they almost cannot be loosened from their place without tools.


A limpet's (Patella rustica) radula.
Picture: Salzburg University, with friendly
courtesy.
 

A limpet's radula is adapted to their way of feeding: It is built like a rigid band. The toothlets are built in a very similar way, so they can rasp algae from the rock in a characteristic fashion. As can be seen in the picture on the right, the limpet removes competing barnacles from its rock like a bull dozer. Where it has crawled along during the day, so can easily be seen by a limpet's characteristic feeding path.

Based on the diverse food available and the often fittingly very much specialised ways of feeding, among sea gastropods a variety of different radula types has evolved. Their construction so is of a distinct systematic importance. The rigid radula form of a limpet scientifically is called a docogloss radula (beam tongue).

A special case, even for the variable gastropods, are the cone shells (Conidae) living in the sea. While smaller species, such as the Mediterranean cone (Conus mediterraneus) hunt for marine worms (Polychaeta) and other molluscs, larger species, such as the textile cone (Conus textile) even attack small fish.

The hunting method of cone shells is remarkable:

 
Conus marmoreus feeding on a cowry shell (Cypraea caput-
serpentis
). Source: James McVey, NOAA Sea Grant Program.
 
Radula tooth of a cone shell. Source: Underwater pictures
by D. and W. Fritz.

In most species the prey is first immobilized with a fast-acting neural toxin and afterwards swallowed whole. To apply the toxin, the cone shell has got only a small number of radula teeth left. Those are developed into a hollow needle at the end of which there is a toxin gland, from which the snail injects its prey with the toxin. Especially the larger species' toxins can even be harmful to humans. The Mediterranean cone feeding exclusively on worms and molluscs is not dangerous to humans.

The cone shells' radula fittingly is named toxoglossan (poison tongue).

Radula Types.

Further information: