What Do Snails Eat? The Brutal Truth Of Gastropod Diets

Gastropod diets are as diverse as everything else about this amazing group.

The simple answer to the question “What Do Snails Eat?” is: nearly everything except living vertebrates (and there a few exceptions to that).

Gastropod feeding habits by category include: herbivores, carnivores, parasites, grazers and browsers, filter feeders, fungivores, omnivores and detritivores.

• • • Carnivores       –   feed actively on other animals that they seek out and attack
    • Browsers         –   feed on sessile animals such as bryozoans – they moves over the colony scraping up what is below them.
    • Grazers           –   feed micro-algae, diatoms and algal spores – they move over the substrate scraping up what is below them.
    • Herbivores      –   feed on larger plants which they seek out, or live on.
    • Filter feeders  –   feed by collecting small particles of food from the water current.
    • Detritivores     –   feed on dead material, either plant or animal, by crawling over the substrate until they find something to eat.
    • Fungivores     –   feed on fungi, either the mycelium or the fruiting bodies.
    • Omnivores     –   like us, as humans are omnivores, omnivorous gastropods eat across the board.
    • Parasites        –   feed of a single animal (it may be colonial) by living on, or inside, of it for their whole adult life.

The vast majority of gastropods are quite mobile, however some species, particularly limpets have evolved to live very sessile lives.  Again most gastropods feed using the radula to remove particles of food from whatever they are feeding on, but there are filter feeders as well.

Every species has its own specialized adaptations.  While as individuals their preference in food can be quite limited, as a group they eat almost everything that they can.  Despite the well known liking of terrestrial slugs and snails for garden vegies, a large number of gastropods, particularly marine species, are carnivorous. Many have even specialized in eating other gastropods.

The Lowly Herbivores (and Grazers)

Herbivory, eating plants, is one of the most common gastropod diets and is most likely what people first think of when the perennial question “what do snails eat” arises in conversation.  The majority of terrestrial slugs and snails are primarily herbivorous, although many herbivorous species will feed on dead and decaying animal matter if given the opportunity.  Similarly most freshwater gastropods are also herbivorous, many are classified as micro-herbivorous and/or micro-omnivorous grazers.

Their diet is hard to limit to a single category as they feed on bacterial films, algae and diatoms so they may be considered omnivores.  Technically they are often called grazers, as they eat whatever they are walking across.  Some marine species such as Limpets are grazers as well.

Other marine species are more typical herbivores feeding on seaweeds and marine grasses. Gastropod herbivores can show species preferences for particular algae, for example red algae over brown algae.  The Ragged Sea Hare Bursatella leachii feeds on the cyanobacteria that live on the sea floor.

Filter Feeding as a Gastropod Diet Choice

The filter or suspension feeding, while relatively uncommon in the gastropoda has evolved a number of times to be a successful gastropod lifestyle.  Food particles are either caught and retained in external mucus nets, ciliary tracts, or on the ctenidium. Filter or suspension feeding is a type of omnivory because the species involved can not be selective about what they catch, thus they may eat zooplankton, phytoplankton and detritus all in a single meal.

Suspension feeders occur in both freshwater and marine environments and filter feeding species may live attached to a solid substrate or buried in a muddy or sandy substrate.  While many suspension feeders live in relatively shallow water other can be found in the ocean depths, particularly around hydrothermal vents.  Some species even live commensally on or inside shells of other molluscs or hermit crabs.

Ciliary tract feeders which trap particles on ciliated fields on lobes of the foot. Once particles have become entangled in the mucus secreted by mucus cells interspersed between the ciliated cells, they are transported through ciliated grooves to the mouth. Ciliary feeding occurs in the families Ampullariidae, Cavoliniidae, Cymbuliidae and Lymnaeidae.

Ctenidial filter feeding is a derived feature and has evolved independently at least five times.  In all cases one of the ctenidia has been lost. The remaining  ctenidium is elongated, filling the entire length of the mantle cavity.  In most cases the triangular lamellae are modified into long filaments. A ciliated food groove on the floor of the mantle cavity meets the tip of the ctenidial filaments and conveys the food-mucus bolus to the left side of the head.

In many cases (seven out of eleven families) the stomach contains a crystalline style, a hyaline rod of mucoprotein which rotates and dissolves in the stomach. As it dissolves, it releases amylases that break down carbohydrates, and controls the stomach pH so that the mucus holding algal and detrital particles together is dissolved.  This system is very similar to the digestive process of bivalves.   Families of gastropods in which ctendial feeding has evolved include Bithynidae, Calyptraeidae, Capulidae, Neomphalidae, Siliquariidae, Struthiolaridae, Trichotropinae, Trochidae, Turritellidae, Verrnetidae and Viviparidae.

Mucous net feeding is less common and is only found in four families of gastropods i.e. Ampullariidae, Olividae, Trimusculidae and Vermetidae.  Mucous net feeders such as Trimusculus reticulatus trap food items such as diatoms and other tiny plants and animals from the surrounding water by secreting and then withdrawing thin filaments of sticky or adhesive mucous.

Strangely T. reticulatus is an sedentary air breathing species that must effectively hold its breath whenever it feeds.  It lives in tidal areas adhering to rocks and can survive long periods of time under water.

Detritivores; Nature’s Clean-up Crews

Detritivory is not seen as a noble life style by many but without the humble detritivores of the world we would be drowning in faeces, dead bodies, fallen trees and mountains of discarded leaves and flowers.

Detritivores are natures recycling agents and as such they set an example to the human race.

Globally, detritivores come in diverse shapes and form, fungi and bacteria are the heavy lifters in this scene, but the noble earthworm and many gastropods also work the floors of the world.

Detritivores exist in all three habitats, terrestrial, freshwater and marine.  In terrestrial habitats they feed mostly on leaves, some like the Flax Snails which belong to the genus Placostylus feed arborealy on algae that grow on the leaves of trees when they are small and as they get bigger they come down to the forest floor to spend the rest of their life feeding on fallen leaves.  Many freshwater snails are also detritivores, Neritina vespertina is a freshwater snail from Hawaii that feeds on leaves that have fallen into the streams it lives in.

Many detritivores derive most of the nutrition the acquire from their diet of dead leaves from the fungi and bacteria that have already colonized the dead plant material.

Farming Fungivores

Fungivory, feeding on fungi, is not usually listed as a dietary choice for gastropods and not a typical answer when looking at the question of “what do snails eat?”

Nevertheless anybody who goes out fungi foraging will know how common it is to find mature fruiting bodies already with a bite, or many bites, taken out of them – sometimes with the latest gourmand still present at the table.

The role of fungal fruiting bodies in feeding the gastropods of the world’s terrestrial biomes seems little studied.  However the iNaturalist group Molluscan Mycophagy notched up almost 2,000 observations of slugs and snails found feeding on wild mushrooms in its first four years.  These observations range across the whole world and include 106 species of gastropods.  Of the top 20 most reported species 17 were slugs.

One reason fungivory might not be considered a dietary category in its own right, as opposed to being part of omnivory, herbivory or detritivory, is the seasonal nature of many fungi.  While the fungi themselves are present all year round in the soil, leaf litter or fallen timber, many species only fruit at certain times of the year.

Fungi of course also occur in aquatic environments and in 2004 it was revealed that a true fungivore snail (Littoraria irrorata), actually farms the marine fungus it feeds on.  The snails in question live in fields of marsh grass and cultivates the local fungus by deliberately damaging blades of grass and defecating on the damaged areas to encourage fungal growth.  They then feed preferentially on the fungi by searching out fungal growths from previously damaged areas.  The snail’s actions ensure the fungus thrives and therefore the snail thrives, this is called “mutualism”.

A Carnival of Carnivores

While there are carnivores in both freshwater and terrestrial habitats the majority of carnivorous gastropods live in the oceans.  Here they feed on sessile creatures such as corals, slower moving creatures such as echinoderms and other molluscs (including other gastropods), many kinds of worms including annelids and even crustaceans if they can.  A few species cone shells even hunt up the evolutionary ladder by specializing in feeding on fish.

The Allied Cowries

The Ovulididae are also called Allied Cowries and False Cowries.  Each species has evolved to live with a specific host organism such as soft corals (Gorgonians) or sponges. Their adaptations to their host include their physical shape and their colour.  When on their host they are amazingly well camouflaged.  They have a soft mantle that covers the outside of their shell some species even have false polyps that they can put up, or withdrawn, to match their host’s behaviour.   As carnivores they feed on the host’s tissues, particularly mucus and polyps.  They also absorb pigments which they use to ensure their own coloration matches the host’s color.

Gorgonians (Alcyonacea, or soft corals) ,are generally unpalatable and are thus avoided by most predators. By closely resembling the gorgonian, the cowrie also gains protection from predators. The host animal continually regrows the lost tissue so the cowrie never runs out of food.  Because of this long term, living on, and feeding on, a single individual some people refer to them as ectoparasites.

The World’s Most Beautiful Carnivores

Nudibranchs are a large and successful part of the gastropoda, they number about 2,415 (COL) species and can be found wafting across the ocean floor from depths of around 30 meters or less to over 6,000 meters.   They are all carnivores, active, free swimming hunters.  Across their full range of species they feed on a wide variety of animals, however individual species can often be choosy and will eat only one or a few specific prey species.

The full range of creatures eaten by these mesmerizingly beautiful predators include: Anemones, Ascidian Tunicates, Barnacles, Bryozoans, Corals, Fish eggs, Gorgonians, Hydroids, other gastropods such as, Sea Hares (and other Nudibranchs), Sea slugs and finally Sponges. Nudibranchs acquire their wonderful colors from the organisms they eat.

• The Spanish Shawl  (Flabellina iodinea) feeds on a the hydroid  Eudendrium ramosum.
• The Striped Nudibranch, Armina californica feeds on species Sea Pens, such as the orange sea pen, Ptilosarcus gurneyiand.
• Marionia elongoreticulata feeds on species of Coral in the subclass Octocorallia
• Members of the large family Chromodorididae, such as Hypselodoris alboterminata feed exclusively on sponges.

Some nudibranchs, like the Blue Dragon (Pteraeolidia semperi), absorb the chloroplasts from the algal cells (zooxanthellae) of the corals they feed on and store them in their own cerata.  In this way they acquire nutrients resulting from the photosynthesis of these dinoflagellates.  These small plants are so effective at supplying their hosts with excess sugars that Blue Dragon Nudibranchs can remain healthy for months without eating.

Sea slugs (which are not Nudibranchs) demonstrate a variety of diets, some, such as species in the genus Elysia, are herbivores feeding on green algae, some species such as E. viridis and E. chlorotica hijack the chloroplasts for themselves. The chloroplasts end up lining the slug’s digestive tract, enabling the slugs to survive solely by photosynthesis for several months at a time.

Other Sea Slugs are carnivorous such as Forsskål’s Sea Slug Pleurobranchus forskalii which feeds on Ascidian Tunicates,  Gardiner’s Headshield Slug Philinopsis gardineri which feeds on polychaet worms and The Pleasant Headshield Slug Chelidonura moena which feeds exclusively on Aceol flatworms.

Heavyweight Hunters Down Under

New Zealand is a hotspot for biodiversity across the whole range of living creatures and the gastropoda are no exception.  New Zealand boasts an amazing 1,400+ species. In comparison, the UK, which has around the same land mass, can offer its gastropodaphiles a mere 112 native species.

The largest snails in New Zealand are in the genus Powelliphanta, these forest giants can have shells up to 10cm across and they are all carnivores with a preference for earthworm.

Gastropods Hunting Gastropods

The predatory Wolf Snail, or Cannibal Snail (Euglandina rosea) is a rapacious snail predator native to the southeastern United States. It is a predator that feeds exclusively on other molluscs and it is particularly fond of other gastropods.

E. rosea first locates its prey by following the mucus trail that its intended victim has left behind as it travelled.  When it attacks it begins by biting the prey’s exposed soft parts before finally inserting its head into the prey’s shell and consuming the soft body within.  However, when it feeds on small species of snail it usually simply swallows the prey whole

As an active and aggressive carnivore it  and was intentionally introduced the Hawaiian Islands.  The idea was for it to act as a biological control agent against the giant African Land Snail, Achatina fulica which had become an invasive pest there.  Sadly however, as with too many of mankind’s eager attempts to make use of incomplete knowledge, it failed to control A. fulica, and instead seriously depleted the populations of the numerous charming endemic species of tree snails.

Whelks – Gastropod Tigers of the Seas

The family Buccinidae are a large (more than 1500 species) and diverse group of relatively large sea snails, they are commonly called as whelks or true whelks.  Whelks are among the most successful and rapacious of the gastropod carnivores, although some are omnivores and will eat anything.  A number of species have become pests of human bivalve farming, e.g. The Veined Rapa Whelk (Rapana venosa), originally from Japanese waters is now a serious pest in European oyster farms.  However many species are also regularly eaten by human beings.

Whelks are active hunters and often drill a hole through the shell of a mollusc prey item.  Although the word drill is commonly used to describe the wearing away of a hole in the shell of their prey the action is backwards and forwards, not round and around.  Therefore it is probably more correct to say they scrape a hole in the prey’s shell.  This hole making is done with the radula, and the central rachidian tooth does most of the work.  Thus these teeth are often quite worn down at the distal end of the radula.

Cone Shells – Poison Dart Hunters

Another highly successful group of marine carnivores are the cone shells.   The term cone shells is applied to gastropods from the family Conidae (taxonomy under revision).  There are about 600 species in the family and they are all carnivores, many of them are also quite beautiful.  However because their poisons are, in some cases, capable of killing human beings you should never try to pick one up.

Cone shells hunt and eat other animals such as marine worms, small fish, molluscs, and even other cone snails.  Individual species usually specialize on one group of organisms to hunt, thus some eat only fish, others only worms.   Cone snails, like all snails, are relatively slow-moving, therefore to facilitate feeding on faster moving prey they have evolved a venomous harpoon, a modified tooth, called a toxoglossan radula.

They usually only have on or two of these teeth fully grown at any one moment in time.  The structure of the tooth varies according to prey type.  The tooth is hollow and barbed, and is attached to the tip of the radula and held ready in the radular sac. When the snail senses some suitable prey it slowly extends a long flexible tube called a proboscis towards the prey.  This proboscis carries the radular out from the snails body.  The radula tooth is filled with venom from the venom bulb and, still attached to the radula, is fired from the proboscis into the prey by a powerful muscular contraction.

Cone shell venoms are powerful mixture of neurotoxins and the prey is usually paralyzed almost instantly. Species that feed on fish may also release chemicals into the water that cause the prey to be more relaxed and less wary.

True Parasites

Quite a few species of gastropods have evolved to become parasites.  The members of Odostomia are ectoparasites on other molluscs (usually bivalves), polychaetes and maybe even crustaceans. They feed by piercing the body wall of their prey with a buccal stylet.  Once the body wall is penetrated they suck up blood with their buccal pump. They are a pest of commercial bivalves such as oysters, mussels and scallops. Most Odostomia species are host-specific, (meaning they only attack on species).  Odostomia scalaris is an exception to this rule.  It parasitizes a wide range of hosts and has even been found on the European lobster Homarus gammarus.

The Epitoniidae

The family Epitoniidae is a widespread group of parasites. Individuals from a single species have been found from places as far apart as South Africa and Taiwan, i.e. Limiscala maraisi,  however individual specimens are often rare.  They are a diverse group of ectoparasites specializing in using sea anemones and corals as their hosts.  There are hundreds of known species, and probably many more waiting to be discovered.  Because of their small size and scarcity very little is know about them.

The Pyramidellidae

Another family of parasitic gastropods are the Pyramidellidae. All known pyramidellids are ectoparasites on other marine invertebrates.  Their choice of hosts includes other molluscs, polychaetes, tunicates and hydroids.  Some species are host-specific while others parasitize several different species.

They feed by hanging onto their host and extruding a long proboscis into the host body and sucking out body fluids and tissues.  They have a muscular pharyngeal pump to assist in this endeavour. Pyramidellids are parasites of cultured bivalves: oysters, mus­sels and clams, and can become a nuisance due to their rapid reproduction.  For this reason their biology is fairly well studied.

Endoparasites

Perhaps one of the strangest of all parasitic gastropods are the four endoparasitic species of the genus Enteroxenos.  Enteroxenos parastichopoli, the best studied member of the genus, parasitizes the sea cucumber Parastichopus californicus.

These small to medium animals look more like worms than snails. They live entirely within the body of their host and feed by adsorbing nutrients directly from their hosts through their body wall.  They have no mouth or digestive system. Furthermore the male become a parasite of the female.  Adult males end up living embedded in the females body.  Their organs, except for the gonads, atrophy and they become dependent on the female for nutrition.  This makes them effectively a commensal organism within a parasitic organism.

What Do Snails Eat? Final Thoughts

Well, as you can see, the answer is complex and diverse – like everything else in nature! But I hope this page has nevertheless helped answer that ever nagging question of “what do snails eat?”

Why not check out our page on Gastropod Predators.

Image Credits:-  Cover Image by T.R.K.Photography, Powelliphanta annectans by Jess Reedy, – License CC BY-SA 4.0; Shells of Turritella terebra by Andrew Butko, Allied Cowry Nhobgood, Cone Snail by Richard Ling- License CC BY-SA 3.0, Flabellina iodinea by w:en:Aquaimages – License CC BY-SA 2.5