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Marine Gastropods: Ecology and Habitats

Teil 1: Ecology Teil 2: Systematics Teil 3: Morphology and Development Teil 4: Ecology and Protection
  How did snails come to live on land?    

 
YouTube Video: "Secret World of Mollusks". Video: Wild Nature Now.
Contents

Introduction

Top of Page.

Europe is an example of a continent surrounded by various very different seas, which differ not only in temperature, salinity and current systems, but also in their geological history and biodiversity:

These seas differ not only in their physical and chemical characteristics but also in their faunal composition. While the Atlantic and Mediterranean host many tropical to subtropical elements, the fauna of the North Sea and Baltic is dominated by boreal to temperate species, often with special adaptations to tides or low salinity.

In the following sections, we will explore the most important marine habitats for snails across the world’s oceans – from the tidal zones of the North Sea and Atlantic to the coral reefs of tropical oceans, the open waters of the seven seas, and the deepest reaches of the deep sea. The focus will be on both systematic groups and ecological specialities.

Wikipedia: Sea Snails.

Ecology and Habitats

Top of Page.

Marine snails have spread to every part of the ocean – from the surf zone to the deep sea, from the poles to the equator. They can be found in coral reefs, on sponges, buried in sand, clinging to rocks and seaweed, or swimming freely in open water. In doing so, they have developed some of the most unusual ways of life.

The violet snail (Janthina janthina), for example, floats on the ocean surface using a self-made raft of mucus bubbles. It drifts across the sea and feeds on large jellyfish when it encounters them. Even at deep-sea hydrothermal vents – the so-called “black smokers” – snails can be found. The most famous is the scaly-foot snail (Chrysomallon squamiferum), with its iron-rich shell and scale-covered foot.

No less remarkable are the worm snails (Vermetidae), which become permanently attached to coral reefs and feed by casting out a sticky mucus net to capture plankton from the water. Cone snails (Conidae), meanwhile, hunt worms, other snails, and even fish using venom and harpoon-like radular teeth. Even relatively simple species like the limpets (Patellidae) display interesting behaviours, such as grazing within defined territories along the shore.

NORDSIECK, F. (1958): Meeresschnecken - Vom wunderlichen Treiben unbewusster Künstler. Franck'sche Verlagshandlung, Stuttgart.
WIESE, V., JANKE, K. (2021): Die Meeresschnecken und -muscheln Deutschlands. Quelle & Meyer Verlag Wiebelsheim.

Coastal Zones: Different Coasts – Different Habitats

 
Characteristics Tidal Coasts Coasts Without Notable Tides
Examples Atlantic Coast, North Sea, English Channel Mediterranean Sea, Black Sea, Baltic Sea
Tidal Range notable (2–6 m) minimal (cm-Bereich)
Exposed Zones changable dry - flooded permanently flooded, humid
Main Adaptation protection against desiccation, temperature changes exposure to wave action, light and shadow
Typical Gastropod Groups Littorinidae, Patellidae Neritidae, Cerithiidae, Nudibranchs
Habitat Structure Clear zone structure Continuous change
Biodiversity high, zone specific different, often endemic
The coastlines of European seas differ not only in landscape, but also in ecology, particularly in terms of whether they experience significant tidal fluctuations (see table on the right).

Tidal coasts, such as those along the Atlantic, the North Sea, or the English Channel, are shaped by the regular alternation between ebb and flood tides. The tidal range can reach several metres, creating a dynamic environment where organisms must constantly adapt to changing conditions: dryness, strong sunlight, temperature fluctuations, and mechanical stress from waves. Snail families such as the periwinkles (Littorinidae) and limpets (Patellidae) have evolved remarkable strategies to cope with these challenges – for example, tightly closing opercula or strong muscular feet for clinging to rocks.

Non-tidal or low-tide coasts, such as those of the Mediterranean, the Black Sea, or the Baltic, at first glance appear more stable. Although there is little to no tidal range, other factors determine the living conditions here: exposure to wave action, light levels, types of substrate, and water movement. In these regions, other snail groups are found, such as Neritidae in brackish waters, Cerithiidae on muddy flats, or various nudibranchs (Nudibranchia) that often live among seagrass or algae.

While tidal shores often show a clearly zoned structure, from the splash zone down to the sublittoral, non-tidal shores tend to display more gradual transitions between shallow and deeper waters. Species composition also differs: tidal zones host a wide variety of highly specialised organisms in a narrow space, whereas low-tide seas are more likely to harbour endemic species that have adapted to stable environmental conditions over long periods.

The Wadden Sea - A Habitat Between Tides

Top of Chapter.

 
A netted dog whelk(Tritia reticulata) on the search for prey.
Picture: François Roche (iNaturalist).
The Wadden Sea stretches along the North Sea coast from the Netherlands through Germany to Denmark. It is one of the largest tidal flats in the world and has been designated a UNESCO World Natural Heritage Site.

Twice a day, this shallow coastal zone is flooded and exposed by the tides. This creates dynamic habitats with mudflats, sandflats, and mixed flats that are highly productive. The Wadden Sea provides ideal conditions for many marine snails.

Typical species include:

The high nutrient content from incoming organic matter makes the mudflats a rich feeding ground – not only for snails, but also for many migratory birds, fish, and other invertebrates.

BUND: Kleine Wattschnecke, große Wirkung.
Wikipedia: Wattenmeer (Nordsee).

Snails in the Wadden Sea must endure extreme conditions: oxygen deficiency in the mud, large temperature fluctuations, and variations in salinity caused by rainfall or evaporation under strong sunlight. Many species survive these phases by burrowing into the sediment or through special metabolic adaptations.

Periwinkles (Littorinidae), for example, can withstand periods of exposure thanks to their operculum – a shell lid that allows them to seal off their shell's opening (aperture).
Common whelks (Buccinum undatum), on the other hand, occupy different depth zones of the Wadden Sea during the various stages of their life cycle, where they hunt other snails, bivalves, and worms.

Ocean Floor – Life in Sand and Mud

Top of Chapter.

 
Helmet snail (Cassis cornuta) on the ocean floor at Kwajalein Atoll.
Picture: Scott & Jeanette Johnson (iNaturalist, Marshal Islands.
The ocean floor is a highly diverse habitat – from shallow continental shelves to the slopes of the deep sea. For many snail species, the seabed offers shelter, food, and hiding places, whether in the form of soft sediment, gravel, or biological material such as mussel shells.

Examples of common benthic snails:

Many of these snails are well camouflaged or remain buried most of the time, making them easy to overlook while diving or snorkelling – yet they form an important part of the benthic community. Other species, such as the helmet snails (Cassidae), have developed extremely hard, thick-walled, and durable shells to protect themselves from predators.

Fighting Conches (Strombidae) - Jumping Snails With Style


Queen conch (Aliger gigas) looking for food.
Picture: Donald Davesne (iNaturalist), Martinique (Caribbean).		  
Conches (Strombidae) are a remarkable family of marine snails, primarily known from tropical shallow-water habitats. Many species live on sandy or muddy sea bottoms, often near seagrass beds or coral reefs, where they feed on algae and detritus.

The name "conch" refers to their large, thick-walled shells, which are flared at the aperture and almost completely protect the crawling animal. A distinctive feature of the family is the so-called stromboid notch – a U-shaped indentation on the outer lip of the shell. Conchs have long eyestalks, allowing them to peek out from under the wide shell aperture without exposing their head. Often, one eye peers through the stromboid notch, and the other through the siphonal canal, which normally houses the breathing siphon (as described before, this is a long elongated mantle funnel that allows the conch to breathe, but also check the water for olfactory cues on mating partners or enemies).

 
Queen conch (Aliger gigas) watching from beneath its shell rim.
Piucture: Pauline Walsh Jacobson (iNaturalist, Virgin Islands.		  
Queen conch (Aliger gigas). With its siccle-shaped operculum it can de-
fend itself or "jump" across the ocean floor.
Picture: Robin White (iNaturalist, Southern coast of Cuba.
Some members of the Strombidae have names reflecting their unusual shell shapes – such as the spider conches (Lambis), including the common spider conch (Lambis lambis), the scorpion conch (Lambis scorpius), and the chiragra spider conch (Harpago chiragra).

Other genera, such as the West Indian fighting conch (Strombus pugilis) have names that evoke battle (pugilist is an old name for a boxer) because of their curved, sickle-shaped operculum: These snails use the operculum to anchor themselves in the substrate and then catapult themselves forward with a powerful movement of their muscular foot. Apart from this characteristic leap across the seafloor, they also use the operculum effectively for defence – hence the name "fighting conch".

Conchs are often beautifully coloured and patterned, and their attractively shaped shells make them popular among collectors. In the Caribbean, they are also eaten raw or grilled. Indigenous peoples have used the large, curved shells of conchs as horns and other musical instruments.

Due to overharvesting, the Caribbean queen conch (Aliger gigas, formerly Strombus gigas), whose shell can reach up to 30 cm (12 in.), is now considered a threatened species. Lambis truncata grows even larger, up to 35 cm (14 in.), but the largest conch of them all is Titanostrombus goliath, found off the Atlantic coast of Brazil, which can reach a shell length of up to 40 cm (16 in.).

The Ocean Floor (on molluscs.at).
Wikipedia: True Conches (Strombidae).
WoRMS: MolluscaBase eds. (2025): Strombidae, RAFINESQUE 1815.

Seagrass Meadows - Green Oases Under Water

Top of Chapter.

Green lungs beneath the waves: Why seagrass meadows matter!

Seagrass meadows are far more than just habitats for snails and fish – they are among the most productive ecosystems on Earth.


Seagrass meadow (Posidonia oceanica) in the Mediterranean. Bouches du Rhône, France. Picture: Frédéric Ducarme (Wikipedia).

Although they cover only a small portion of the ocean floor, their contribution is immense:

  • Oxygen production: Seagrasses and algae produce more than half of the world’s available oxygen.
  • Carbon storage: They trap carbon in the seabed and help combat climate change – even more efficiently than many forests on land.
  • Biodiversity: They provide shelter, food and breeding grounds for countless marine species – including many types of sea snails.
  • Coastal protection: Their roots stabilise the seabed and protect shorelines from wave erosion.

A beautiful thought: If every person planted a tree or adopted a patch of seagrass, it would already make a big difference for the planet.

  
 
Needle snail (Bittium reticulatum) at the French Mediterranean coast (Hérault). Picture: Sylvain Le Bris (iNaturalist).
   
 
Elysia viridis at the French Mediterranean coast (Hérault).
Picture: Sylvain Le Bris (iNaturalist). 
   
  What Does PSU Mean?

Practical Saline Unit" (PSU), is a unit to measure sealinity in solutions,especially in sea water.

1 PSU is the equivalent of 1g Salz of 1000g (= 1l) water, thus 0,1%, measured on the basis of the saline solution's electrical conductivity.
Seagrass meadows are among the most productive and ecologically important habitats in shallow coastal waters. Unlike algae, seagrasses are true flowering plants that have roots, leaves, and flowers. In the cooler waters of Europe, the most common species is common eelgrass (Zostera marina), while in the Mediterranean, Neptune grass (Posidonia oceanica) also occurs.

These dense underwater meadows not only provide shelter for many juvenile fish and invertebrates, but also offer an ideal habitat for numerous snail species. Snails use the seagrass blades as a food source, as climbing structures, and as places to lay their eggs. In addition, seagrass meadows play a vital role in carbon dioxide uptake and oxygen production, as well as in stabilising coastal sediments (see box on the left).

Typical inhabitants include:

  • Needle snails (Cerithiidae, e.g. Bittium reticulatum), which graze on algae growing on the seagrass leaves
  • Bubble snails (Bullidae), which forage in the sediment
  • Nudibranchs (Nudibranchia)
  • Sacoglossans (e.g. the green velvet slug Elysia viridis, which lives in symbiosis with algae)
  • Mudsnails (Peringia ulvae, see above), often found in transition zones between mudflats and seagrass

Seagrass meadows are sensitive to turbidity, eutrophication, and mechanical disturbance. Their decline affects entire coastal ecosystems – including the specialised snail species that live there – and also has far-reaching consequences for the global carbon cycle.

WWF: Artenlexikon Seegraswiesen (in German).
NABU: Seegras: Kinderstube der Ostsee (in German).
Wikipedia: Seagrass.
YouTube Video: NABU: "Lebensraum Seegraswiese - Einzigartige Unterwasserwelten" (in German).

The Batic Sea: A Young Brackish Sea Changed Through Time

Top of Chapter.

The Baltic Sea is not a typical sea, but a geologically young, almost enclosed inland sea. It differs significantly from the neighbouring North Sea in several key respects:

  • Salinity is very low, especially in the northern and eastern parts – in some areas only 1–3 PSU (practical salinity units), compared to up to 18 PSU in the Kattegat and about 35 PSU in the open Atlantic.
  • Water exchange with the North Sea is limited to the narrow Danish straits and the Øresund, preventing thorough and regular mixing.
  • The Baltic Sea is virtually non-tidal, with barely measurable tidal range.

Many true marine species cannot survive long-term under these conditions, while strictly freshwater species have often developed salt-tolerant forms. This results in a depauperate but highly specialised mollusc fauna, including marine snails adapted to brackish water (e.g. Peringia ulvae) and freshwater snails that have migrated from river systems.

For example, the common river nerite (Theodoxus fluviatilis) is found mainly in estuaries and on hard substrates in the western Baltic, thanks to its high brackish water tolerance (up to ~10 PSU). The pond snail (Ampullaceana balthica) is less tolerant (up to ~3 PSU) and therefore restricted to less saline areas, especially shallow, sheltered bays and brackish lagoons along the Baltic coast.

Unterwasserwelt Ostsee: Gehäuseschnecken (in German).
Haus der Natur Cismar, Ostholstein: Artenliste der Mollusken der Ostsee (in German).
Weichtier des Jahres 2004: Gemeine Kahnschnecke (Theodoxus fluviatilis) (in German).

Geological Development

 
Name Time in Years* Water Type Indicator Organisms
End of Weichselian Glaciation ca. 12,000 dry none
Baltic Ice Lake 12,500 - 10,000 freshwater none
Yoldia Sea 10,000 - 9,250 saline to brackish water Saltwater clam Yoldia arctica**
Ancylus Lake 9,250 - 7,100 freshwater River limpet Ancylus fluviatilis
Littorina Sea 7,100 - 4.000 saline to brackish water Periwinkle Littorina littorea
Lymnaea Sea 4,000 - 1,500 saline to brackish water,
but slightly freshening		 Pond snail Lymnaea ovata***
Mya Sea 1,500 - today saline to brackish water Soft-shell clam Mya arenaria

Developmental History of the Baltic Sea since the End of the Last Glaciation. Source: Wikipedia, revised.

*: BCE.
**: Nowadays Portlandia arctica: WoRMS: MolluscaBase eds. (2025): Portlandia arctica (J. E. GRAY, 1824).
***: Nowadays Ampullaceana balthica: WoRMS: MolluscaBase eds. (2025): Ampullaceana balthica (LINNAEUS, 1758).

Common river nerite (Theodoxus fluviatilis).
Bild: Deistung (iNaturalist), Mecklenburg, DE. 		  
Following the end of the last Ice Age (the Weichselian Ice Age) around 12,000 years ago, the Baltic Sea formed in several stages, primarily shaped by varying inflows of salt water from the North Sea. These marine incursions (transgressions), such as the so-called Littorina Transgression, occurred during periods when the land bridge between present-day Denmark and Sweden was flooded.

At other times, tectonic uplift of the land reduced or blocked the inflow of seawater, leading to increased freshening of the Baltic Sea through the influx of river water.

The different developmental phases of the Baltic Sea are named after the indicator organisms most commonly found as fossils in sediment layers from each respective period.

This gradual transformation of the Baltic Sea not only affected ecological habitats but also had significant consequences for human settlement patterns in the surrounding regions at the time.

Today, regular water exchange occurs between the North Sea and the Baltic via the Kattegat and the Belt Sea. As a result, the western Baltic is saltier than the eastern part, which receives large amounts of freshwater from the rivers flowing into the sea. While tides are present due to the connection with the North Sea, the tidal range in the Baltic is minimal compared to other marine regions.

Wikipedia: Baltic Sea.

Coral Reefs – Endangered Diversity of Colour and Form

Top of Chapter.

Wikipedia: Coral Reef.


Giant tun shell (Tonna galea) on the ocean floor overgown with algae
(Acetabularia acetabulum) in Greece.
Picture: Stergios Vassilis (iNaturalist).		  
Coral reefs are often called the "rainforests of the sea" due to their immense biodiversity. These ecosystems are built by tiny, colony-forming cnidarians (Cnidaria), especially stony corals (Scleractinia), fire corals (Millepora), and the blue coral (Heliopora coerulea). These animals secrete calcium carbonate from their base, forming reef structures over long periods of time – large enough to exert a significant physical and ecological influence on their surroundings. Coral reefs are the largest structures on Earth created by living organisms. The total known global reef area is around 600,000 km². In the Maldives, reefs rise up to 2,200 metres from the seafloor.

Coral reefs are complex marine ecosystems, home to a diverse community of plants and animals, including worms, molluscs, sponges, echinoderms, and crustaceans. For many fish species, coral reefs provide a safe nursery, protected by the stinging inhabitants of the reef.

Selected Snail Species

Coral reefs in the warm, clear, shallow waters of the tropics offer a habitat for countless species of marine snails. Accordingly, their ways of life are just as diverse:

  • Cowry shells (Cypraeidae) are mostly grazers. They feed on the algae growing on corals and may occasionally consume small coral polyps as well.
  • Egg cowries (Ovulidae), close relatives of cowries, are carnivores. They feed mainly on coral polyps. Some species live as ectoparasites, clinging to coral colonies with their foot and consuming all the polyps in reach. Their mantle – which covers the shell – often forms special extensions that mimic algae or coral polyps, allowing the snail to blend into its surroundings.
  • Cone snails (Conidae) are highly specialised predators. They hunt using harpoon-like radular teeth and potent neurotoxins (conotoxins). Their prey includes marine worms, other snails and molluscs, and even fish. The venom of larger species can be so powerful that it poses a serious danger to humans.

 
Blue dragon slug (Pteraeolidia ianthina) in New South Wales, Australia.
Picture: Richard Ling (Flickr).
   
  Blue Dragons

The English expression Blue Dragon might mean several
different species of sea slugs: Aside from the nudibranch
shown above, that could also be the pelagic species Glau-
cus atlanticus and Glaucus marginatus. Those, however,
are also Nudibranchia.
   
 
Worm snail (Thylacodes squamigerus), Monterey, California.
Picture: Cricket Raspet (iNaturalist).
Tun Shells (Tonnidae)

The giant tun snail (Tonna galea) feeds primarily on echinoderms, especially sea cucumbers (Holothuriidae). Its prey is subdued with acidic saliva, which also helps dissolve the calcareous skeleton. The saliva of a tun snail contains 2–4% free sulfuric acid as well as aspartic acid. Tun snails inhabit coral reefs as well as open sea floors.


Marbled cone snail (Conus marmoreus) approaching chocolate banded
cowry (Talparia talpa). Marshall Islands, Western Pacific.
Picture: Scott und Jeanette Johnson (iNaturalist).		  
Nudibranchs (Nudibranchia)

Nudibranchs are among the most well-known sea slugs and are famed for their spectacular colours and forms. Despite lacking a protective shell, many species are not defenceless: they compensate by incorporating the stinging cells (cnidocytes) of their prey, such as coral polyps, into their own tissues. Rather than digesting them, the nudibranch stores these kleptocnidae (Greek for "Stolen nettle cells") in its brightly coloured dorsal appendages, where they serve to defend against predators.

The often elaborate mantle extensions (papillae) seen in harmless cowry shells (Cypraeidae) and egg cowries (Ovulidae) may function as a form of mimicry, imitating the warning appearance of nudibranchs.

Although adult nudibranchs have no shell, their planktonic veliger larvae do possess a small shell, which is lost during the final metamorphosis into the juvenile slug.

Nudibranchs (Nudibranchia)
Wikipedia: Nudibranchs.

Worm Snails (Vermetidae)

Worm snails (Vermetidae) are another highly unusual group of marine snails, often found on coral reefs. Their appearanc makes it difficult to recognise them as gastropod at first glance because they have a very  unique way of life: they build irregular, tube-like shells that resemble the calcareous tubes of sessile marine worms, rather than a snail shell. However, unlike those of worms, their shells possess the typical three-layered structure of gastropod shells: a calcareous middle layer (ostracum), covered on the outside by a thin organic periostracum and lined on the inside with a nacreous layer (hypostracum).

Worm snails cement their shells to hard substrates such as coral, sometimes becoming overgrown by the coral itself or forming dense colonies with other individuals.

These snails are filter feeders, but unlike bivalves, they filter the surrounding water using their gills, and in some species, with a mucus net secreted from the foot to trap plankton.

Like many other marine gastropods, worm snails reproduce via planktonic veliger larvae. Even the juvenile snails they transform into still initially possess spiral-shaped shells. Only once the juvenile worm snail settles in its permanent place, it begins to grow the worm-tube shaped shell characteristic for this group.

Wikipedia: Worm snails.

Threats to Coral Reefs

Many reef-dwelling snail species are highly specialised and just as sensitive to environmental changes as the coral reefs they inhabit. Tropical corals are particularly vulnerable to changes in water temperature and light conditions – both of which can shift rapidly with fluctuations in sea level.

One of the most visible consequences of rising atmospheric carbon dioxide levels is coral bleaching. As ocean temperatures increase and seawater becomes more acidic due to CO₂ and other atmospheric pollutants, the symbiotic algae living within the coral tissues begin to die. Without these algae, the coral loses both its primary food source and its colour – hence the term "bleaching".

Wikipedia: Coral Bleaching.

Other factors also put stress on coral reefs and the snails that live there, including increased sedimentation (often associated with excess algal growth), as well as ocean acidification. Human activity poses a direct threat as well – coral pieces are broken off and sold, and so are the colourful shells of many reef snails, particularly cowries (Cypraeidae) and cone snails (Conidae). These are popular as collectibles and for making jewellery and decorative items.

As a response, several International Years of the Reef have been declared in the past to raise awareness. But whether current protection measures will be enough remains uncertain. What is especially alarming is the fact that the degradation of coral reefs endangers not only the corals themselves, but also the immense biodiversity associated with them – including many species of snails and bivalves.

Open Ocean: Life in the Water Column

Top of Chapter.

Far from coastlines, reefs, and the seafloor, only a few snail species inhabit the open ocean. However, these few are highly specialised: pelagic species that float or swim freely in the water column. Many of them belong to groups that, at first glance, hardly resemble typical snails anymore.

Sea Butterflies and Sea Angels (Pteropoda)

 
Sea butterfly (Limacina helicina), Sea of Japan, NW coast.
Source: CHICHVARKHIN, A. (2016).

Sea angell (Clione limacina), Beaufort Sea, Alaska.
Picture: Kevin Raskoff (Source).		  
Pelagic gastropods of the group Pteropoda are divided into two main types: Sea butterflies (Thecosomata, e.g. Limacina helicina) – which retain a thin, fragile shell, and sea angels (Gymnosomata, e.g. Clione limacina) – which have no shell left at all.

Sea Butterflies and Sea Angels (Pteropoda).

Both groups share several features: they are tiny, transparent, and in some cases even bioluminescent. Their foot has evolved into wing-like lobes used for swimming – which is reflected in the scientific name Pteropoda (from Greek, meaning "wing-footed"), coined by CUVIER in 1804. Sea angels move through the water using a rowing-like motion of their wing-like appendages.

Sea butterflies catch plankton with a mucus net suspended in the water. Sea angels, on the other hand, are predators – their primary prey are sea butterflies. Some species possess additional tentacles, sometimes even with suction cups, to grasp and consume their prey. While some wait to encounter their prey by chance, others actively hunt.

Sea butterflies are highly sensitive to ocean acidification, which can severely weaken or dissolve their delicate shells.

LISCHKA, S., RIEBESELL, U. (2017): Metabolic response of Arctic pteropods to ocean acidification and warming. Polar Biology 40, pp. 1211–1227. (Abstract).

To protect themselves from predators, the Antarctic sea angel Clione antarctica produces a defensive substance called pteroenone, a molecule only recently discovered. Amphipods (Hyperiella dilatata) exploit this: they capture a sea angel from the dense swarms and carry it around for protection against their own predators.

In some areas, Antarctic sea angels can occur at densities of up to 300 individuals per cubic metre of seawater, forming an important food source for plankton-feeding marine animals.

Wikipedia: Pteropoda.
Wikipedia: Clione_limacina.
Wikipedia: Limacina_helicina.
CHICHVARKHIN, A. (2016): "Shallow water sea slugs (Gastropoda: Heterobranchia) from the northwestern coast of the Sea of Japan, north of Peter the Great Bay, Russia". (Link).

Other groups of pelagic snails include the Pterotracheoidea (formerly Heteropoda), such as Atlanta and Carinaria. These snails are known for their transparent bodies, large eyes, tentacles, and reduced or internal shells.

Violet Snails (Janthina)

 
A violet snail (Jantina janthina) unter ihrem Schleimfloß an der
Wasseroberfläche. New South Wales, Australien.
Bild: Norm Farmer (iNaturalist).
   
 
The shell of a violet snail (Jantina janthina).
Picture: H. Zell. 
Violet snails (e.g. Janthina janthina) are holopelagic gastropods – extraordinary open-ocean dwellers that live permanently in the water column. Unlike freely swimming marine nudibranchs, violet snails have retained their shell, which is thin-walled and coloured in shades of bluish violet to purple, giving the group its name.

However, violet snails do not actively swim. Instead, they float at the sea surface by hanging from a raft of mucus bubbles, which they produce using a gland on the front part of their foot. These snails drift on the surface upside-down, and their shell pigmentation reflects this lifestyle: the underside (facing upward in life) is darker than the upper side – a form of counter-shading that helps them remain camouflaged from both above and below. In contrast to their delicate, violet-coloured shells, the soft body of Janthina is usually much darker, often nearly black.

Despite their "flowery" name, violet snails are carnivorous, feeding on jellyfish. Their preferred prey includes the by-the-wind sailor (Velella velella) and the Portuguese man o’ war (Physalia physalis). To safely consume these stinging cnidarians, violet snails have evolved a long, cylindrical proboscis with the mouth at its tip. This allows them to feed without being harmed by the stinging cells.

The tentacles of Janthina are long and forked, about half the length of the foot. Their eyes are tiny and located at the base of the tentacles.

Violet snails are distributed across the warm waters of tropical and temperate seas. Living specimens are rarely seen near the shore, but their shells – sometimes still attached to remnants of their bubble rafts – are often washed ashore after storms. Occasionally, Janthina may appear near coral reefs to feed on jellyfish stranded by currents or wave action.

Wikipedia: Janthina janthina.
WoRMS: MolluscaBase eds. (2025): Janthina janthina (LINNAEUS, 1758).
Sea Snails of the Open Sea (on molluscs.at).

Wentletrap Snails (Epitoniidae)


Wentletrap snail (Epitonium scalare), Western Australia.
Picture: Glen Whisson (iNaturalist).		  
From a systematic perspective, violet snails belong to the family Epitoniidae, also known as wentletraps. However, unlike the holopelagic Janthina species, true wentletraps are benthic snails that live on sandy substrates, typically in close association with sea anemones or corals, which form the main part of their diet.

The name wentletrap refers to the distinctive axial ribs (costae) that are especially prominent in the genus Epitonium, giving the shell the appearance of a spiral staircase – which is exactly what wentletrap means in Dutch (wenteltrap = winding stair).

Epitonium scalare, also known as the precious wentletrap, In historical times, was also highly prized by shell collectors. For example, Cosimo III de' Medici is said to have owned one in his collection. The Holy Roman Emperor Franz Stephan of Lorraine, husband of Maria Theresia of Austria, once paid 5,000 guilders to acquire a wentletrap specimen for his cabinet of curiosities.

From Franz Stephan’s natural history collection eventually emerged the Natural History Museum in Vienna (Naturhistorisches Museum Wien), whose mollusc department still features a wentletrap on its homepage today.

Wikipedia: Epitonium scalare .
Jacksonville Shell Club: Epitonium scalare - The Precious Wentletrap.
WoRMS: MolluscaBase eds. (2025): Epitoniidae S. S. BERRY, 1910 (1812).
Naturhistorisches Museum Wien: Die alte Naturaliensammlung 1748-1796.
Naturhistorisches Museum Wien: Mollusken-Sammlung des Naturhistorischen Museums.

Red Whelk (Neptunea antiqua)


Angular whelk (Neptunea despecta): Tromsø, Norway.
Picture: Don-Jean Leandri-Breton (iNaturalist).		  
The red whelk (Neptunea antiqua) is a close relative of the common whelk and belongs to the true whelks (Buccinidae). Its distribution ranges from the Baltic Sea and the North Sea to the Bay of Biscay, and northwards to the Arctic Ocean. Red whelks inhabit subtidal zones at depths between 15 and 1,200 metres, usually on soft substrates, where they actively hunt their prey. Their diet consists mainly of bivalves, but also includes polychaete worms and crustaceans.

Thanks to their long, extendable proboscis, they are likely capable of reaching into the tubes of burrowing worms – much like the giant Australian elephant snail (Syrinx aruana).

The angular whelk (Neptunea despecta) is found from the North Atlantic to the Arctic and inhabits similar depth ranges. It tolerates water temperatures as low as 5 °C, and specimens of Neptunea have even been found at depths of up to 2,000 metres.

Despite these deep-water records, Neptunea species are still considered snails of the continental shelf and upper slope, only occasionally venturing into greater depths. Other gastropods, by contrast, live permanently in much deeper and more extreme marine environments.

TAYLOR, J.D. (1978): The diet of Buccinum undatum and Neptunea antiqua (Gastropoda: Buccinidae). Journal of Conchology 29, S. 309.
Wikipedia: The Red Whelk.

Deep Sea: Gastropods Under Extreme Conditions

Top of Chapter.

  Chemosynthesis – Life Without Light

While plants and algae rely on sunlight for photosynthesis, life at hydrothermal vents is based on an entirely different principle: chemosynthesis.

Instead of using sunlight, microorganisms harness chemical energy – for example from hydrogen sulfide (H₂S), methane (CH₄), or iron compounds – to build organic matter from CO₂.

A simplified formula:
CO₂ + H₂S → sulfuric acid (H₂SO₄) + organic material

These chemosynthetic bacteria live:
  • freely in the water,
  • attached to surfaces (e.g. mussel shells, snail feet),
  • or in symbiosis with animals, such as Bathymodiolus mussels.
The animals do not feed directly on their environment, but on the products of their symbiotic bacteria – a way of life that requires no sunlight at all.

This form of life challenges our understanding of where life can exist – not only in the deep sea, but perhaps also on distant moons like Jupiter’s Europa or Saturn’s Enceladus.
The deep sea refers is the part of the ocean that begins beyond the continental shelf, usually from depths of 200–800 metres downward. Areas deeper than 1,000 metres make up roughly 62% of the Earth’s surface. Conditions here are extreme: perpetual darkness, near-freezing temperatures, and immense pressure.

Sunlight only penetrates the upper layers of the ocean. Below 200 metres, photosynthesis is no longer possible, and from around 600 metres depth, there is complete darkness. Water temperatures drop steadily with depth: between 500 and 1,000 metres they range from 1.5 to 5 °C, and at 6,000 metres they approach freezing. Pressure increases dramatically with depth – at 1,000 metres it reaches about 100 bar (1,000 kg/cm²), and at 4,000 metres about 400 bar. By comparison: normal atmospheric pressure at sea level is about 1 bar (1,013 hPa).

These conditions are extremely hostile to life. With no photosynthesis, there are no plants to form the base of the food chain. And yet, the deep sea is home to a surprising number of highly specialised species that have adapted to these challenges. Some feed on marine snow – dead organic matter that sinks from above – while others are predators, or live in symbiosis with chemosynthetic bacteria, which generate energy from sulfur or methane compounds. This form of chemosynthesis is unique on our planet.

Due to the harsh environment, the growth and reproduction rates of deep-sea animals are often extremely slow, and many species can live for decades or even centuries. Some have evolved remarkable adaptations such as bioluminescence, which they use to lure prey or attract mates.

Two Worlds of the Deep Sea

Biologists distinguish between two major zones of the deep sea:

The deepest point on Earth is the Mariana Trench, reaching depths of up to 11,000 metres at the Challenger Deep, southwest of the island of Guam. This makes it far deeper than Mount Everest is high (8,849 metres).

Wikipedia: Deep Sea.
WWF: Faszinierendes Leben in der Tiefsee (in German).
Wikipedia: Mariana Trench.
Wikipedia: Challenger Deep.

Seamounts

Seamounts are usually of volcanic origin, rising from the deep sea and in some cases reaching into the sunlit (euphotic) zone. Where light penetrates, plant growth becomes possible – which in turn supports a particularly high level of biodiversity. On many seamounts, unique biological communities have evolved, consisting of cold-water corals and deep-sea sponges, which in turn are home to crustaceans, starfish, deep-sea mussels and snails. Thanks to the abundance of prey, fish populations around these seamounts are often particularly rich.

Geomar.de: Seamounts - Die Oasen der Ozeane (in German).

Hydrothermal Vents and Black Smokers

 
Deep sea gastropods (Lepetodrilus) at a black smoker in Nova Scotia,
Canada. Source: ROGERS A.D. et al. (2012). (Link).
Hydrothermal fields are found, for example, along mid-ocean ridges, where tectonic plates diverge, or at so-called hot spots – weak zones in the Earth's crust where magma lies close beneath the seafloor. Seawater seeps into these zones, becomes superheated by underlying magma, dissolves minerals from the rock, and is then expelled at temperatures of several hundred degrees Celsius through chimney-like openings and cracks in the seabed. These features are known as black smokers.

To date, over 350 hydrothermal vent fields with black smokers have been discovered. In some of these locations, up to 300 different species have been recorded – many of them endemic, meaning they are found nowhere else on Earth.


Habitat of Bathymodiolus in the Deep Sea.
Picture: NOAA.		  
When this hot, acidic, mineral-rich water, containing high concentrations of hydrogen sulfide, methane, and dissolved metals, meets the near-freezing (ca. 2°C) ambient seawater at depths of 2,000 to 3,000 metres, metal and mineral compounds precipitate, including metal sulfides.

Bacteria use the dissolved sulfur compounds as an energy source via chemosynthesis (see above). Just like plants at the surface, these bacteria form the base of a unique food web – supporting life forms such as tube worms, specialised snails and bivalves, various crustaceans, and even fish.

Many of these animals do not feed on the bacteria directly, but live in symbiosis with them. A prominent example is the deep-sea mussel Bathymodiolus, which lives in dense beds around black smokers and hosts a diverse community of other organisms. Bathymodiolus gains energy through symbiotic bacteria housed in specialised gill cells known as bacteriocytes.

Wikipedia: Bathymodiolus.

The numerous snail species discovered in the early 2000s near black smokers – many with remarkable adaptations to their extreme environment – were originally grouped as "hot vent taxa". Today, following the classification of BOUCHET and ROCROI (2005), they are assigned to the Vetigastropoda.

Scal-Foot Snail (Chrysomallon squamiferum): "No Animal has Teeth  Made From Iron!"

In the 2001 French movie "Le Pacte des Loups" (The Brotherhood of the Wolves), the naturalist Grégoire de Fronsac, sent by the famous Buffon to the Gévaudan, declares: “No animal has teeth made from iron!”
And indeed, this seems highly unlikely.

 
Scaly-foot gastropod (Chrysomallon squamiferum) seen from the right.
Source: NAKAMURA, K. et al. (2012): Link.
But in the deep sea, on mussel beds formed by Bathymodiolus mussels, lives one of the most extraordinary snails on the planet: the scaly-foot snail (Chrysomallon squamiferum).
It does not have iron teeth – but perhaps even more astonishingly, it possesses a shell and foot scales reinforced with iron minerals.

The species was first discovered in 2001, the same year the film was released. Specimens came from a hydrothermal vent field in the Indian Ocean, more than 2,800 metres deep, where superheated water (over 300 °C) rich in sulphur and iron escapes from the seafloor. Like the deep-sea mussel Bathymodiolus, Chrysomallon hosts symbiotic bacteria, but in its case, they reside in the oesophagus. The snail does not feed in the usual sense – it nourishes itself through the metabolic products of these symbionts. As a result, its digestive tract is greatly reduced, and although Chrysomallon does have a radula, it does not use it, as it does not feed directly.


Vertical layers of a normal gastropod shell and a Chrylomallon squami-
ferum shell in comparison. Picture: Robert Nordsieck.		  
The snail does have tentacles, but no discernible eyes – unsurprising, given the total darkness of its habitat. The shell, which can reach up to 4.5 cm in size, resembles a limpet or a nerite at first glance. But the similarity ends there. The shell wall consists of three layers – as in other snails (hypostracum, ostracum, periostracum) – but in Chrysomallon, this design is modified:

The innermost layer is a thin sheet of calcium carbonate (which forms the main bulk of most other snail shells). This is covered by a thick organic layer, which is in turn coated with a hard outer layer made not of calcium, but reinforced with iron sulphide minerals, such as pyrite (FeS₂). This unique structure makes the scaly-foot snail the only known multicellular organism with iron sulphide in its shell (contrary to some sources like Wikipedia, the shell is not a skeletal element).

The shell has a similar function like the Chobham armour of modern battle tanks: it is hard, but thanks to the thick organic middle layer, flexible enough to absorb impacts. Instead of cracking, it may deform under pressure – distributing the energy and preventing breakage. The genus name Chrysomallon is derived from Greek and means “golden-haired” – a reference to the pyrite (“fool’s gold”) embedded in the shell’s outer layer.

The name scaly-foot snail) and the Latin species name squamiferum (“scale-bearing”) refer to the remarkable sclerites covering the snail’s foot. These scale-like structures are composed of conchiolin – the same organic material found in the periostracum of more typical snail shells – and are further reinforced with pyrite and greigite minerals.

It remains unclear whether these sclerites primarily serve as protection, or are instead a detoxification mechanism for harmful sulphur compounds – a mystery still under investigation.

Wikipedia: Scaly-Foot Gastropod.
NAKAMURA, K. et al. (2012). "Discovery of New Hydrothermal Activity and Chemosynthetic Fauna on the Central Indian Ridge at 18°–20°S". PLoS ONE 7(3): e32965, (Link).
ROGERS, A.D., TYLER, P.A. et al. (2012) The Discovery of New Deep-Sea Hydrothermal Vent Communities in the Southern Ocean and Implications for Biogeography. PLoS Biol 10(1): e1001234, (Link).
SUZUKI, Y., et al. (2006): Sclerite formation in the hydrothermal-vent "scaly-foot" gastropod. In: Earth and Planetary Science Letters, Vol. 242, (1–2), pp. 39-50., (Abstract).

Abyssal Plains

Even in the vast, featureless abyssal plains of the deep sea, life is not necessarily sparse. Manganese nodules, found at depths greater than 4,000 metres, have proven to be centres of diverse benthic communities – including sponges, sea cucumbers, cephalopods, and a variety of bottom-dwelling invertebrates.

These nodules serve as "rock substitutes", offering the only hard substrate in an otherwise soft, muddy seabed. As such, they play a key role in structuring the habitat and supporting life.

 

Latest Change: 16.08.2025 (Robert Nordsieck).