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Marine Gastropods: Systematics and Diversity

Part 1: Ecology Part 2: Systematics Part 3: Morphology and Development Part 4: Ecology and Protection
  How Did Snails Come to Live on Land?    

 
YouTube Video: "Mollusks: More than a Shell | Changing Seas". Video: South Florida PBS.
Contents

Introduction: What Does Systematics Mean?

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Systematics is a branch of zoology that deals with the naming, description, and classification of organisms – in our case, of snails and other molluscs. Its aim is not only to catalogue the enormous diversity of species, but also to organise them into a framework of evolutionary (phylogenetic) relationships.

The zoological discipline that focuses specifically on molluscs is called malacology, from the Greek μαλακός (malakos), meaning "soft". It ranges from the description of new species, to the study of their behaviour and ecology, to anatomical and morphological investigations, and even genetic analyses to uncover relationships between species.

In this context, researchers distinguish between the phenotype ("What does a snail look like?") and the genotype ("What does its DNA look like?").

From the Systema Naturae to Cladistics

 

Monophyletic und Polyphyletic - What is That?

In phylogenetic systematics, the focus is not always on the outward characteristics of species, but rather on their genetic relationships and shared ancestry.



The diagram illustrates two monophyletic groups (red and blue), each of which includes all descendants of a common ancestor. In contrast, the green group is paraphyletic – it shares a common ancestor, but some of its descendants are excluded. If the red and blue groups were combined based on external or ecological traits, this would result in a polyphyletic group, which does not reflect true evolutionary relationships.
As early as 1758, Carl Linnaeus (1707–1778) attempted to bring order to the diversity of nature in his Systema Naturae. He introduced the binomial system of naming species still valid today: a Latin genus name followed by a species epithet – for example, Conus marmoreus Linnaeus, 1758, the scientific name of the well-known marbled cone snail.

Systematics: Order in the Snail Realm.

However, Linnaeus' classification was still based purely on phenotypical and anatomical characteristics. He was, as Howard Stark would put it, "limited by the technology of his time."

Today, molecular genetic data play a decisive role: DNA analyses often reveal that species which look similar on the outside are not necessarily closely related – and vice versa. As a result, the traditional classification of gastropods has had to be revised several times.

A key principle of modern systematics (also called cladistics) is that systematic groups must be monophyletic, which means they include all the descendants of a common ancestor, and do not exclude or mix in unrelated groups (See the diagram on the right for clarification):

Wikipedia: Cladistics.
AX, P. (1984): "Das phylogenetische System. Systematisierung der lebenden Natur aufgrund ihrer Phylogenese". Fischer, Stuttgart.
HENNIG, W. (1950): "Grundzüge einer Theorie der phylogenetischen Systematik". Deutscher Zentralverlag, Berlin.

Prosobranchs, Opithobranchs and Pulmonates

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From left to right: a: Prosobranch; b: Opisthobranch; c: Intermediate; d: Pulmonate gastropod
as a consquence of Torsion. Source: Urania Tierleben, Wirbellose 1 (1996). 		 
In classical systematics, THIELE (1929) grouped marine gastropods according to the position of their gills, which is influenced by the typical torsion of the gastropod body and consequently the shell. This led to three main categories: Prosobranchia ("forward-gilled snails"): the gills lie in front of the heart, Opisthobranchia ("rear-gilled snails"): the gills lie behind the heart, and Pulmonata: groups that have lost their gills and converted the mantle cavity into a lung.

  THIELE, J. (1929-1935): Handbuch der systematischen Weichtierkunde. 2 Bände. 1-1154.

However, molecular studies by HASZPRUNAR (1988) and later by PONDER & LINDBERG (1997) demonstrated that the traditional group Prosobranchia is paraphyletic. Some taxa classified as prosobranchs are more closely related to opisthobranchs or pulmonates than to other prosobranchs. Therefore, the term Prosobranchia is no longer used in modern phylogenetic classification. On the present homepage, the term still will be used for orientation purposes, as, albeit obsolete, it is still commonly used in many online sources.

HASZPRUNAR, G. (1988): On the origin and evolution of major gastropods group, with special reference to the streptoneura. In: J. Moll. Stud. 54:367–441, (Abstract).
PONDER, W., LINDBERG, D. (1997): Towards a phylogeny of gastropod molluscs; an analysis using morphological characters. Zoological Journal of the Linnean Society. 119: 83–265 (Abstract).

According to the modern classification proposed by BOUCHET & ROCROI (2005), the various groups of gastropods have been reorganised into new clades that reflect evolutionary relationships more accurately.

Klasse Gastropoda G. CUVIER, 1797
Patellogastropoda D.R. LINDBERG, 1986
Vetigastropoda L. SALVINI-PLAWEN, 1987
Cocculiniformia G. HASZPRUNAR, 1987
Neritimorpha KOKEN, 1896
Caenogastropoda COX, 1960
Heterobranchia J.E. GRAY, 1840
Opisthobranchia H. MILNE EDWARDS, 1848
Pulmonata G. CUVIER in H.M.D. DE BLAINVILLE, 1814

For example, the so-called Hot Vent Taxa – newly discovered gastropods found at hydrothermal vents in the early 2000s – are now included within the Vetigastropoda. The Heterobranchia clade, on the other hand, comprises both opisthobranchs and pulmonates.

BOUCHET, P., ROCROI, J.-P., FRÝDA, J., HAUSDORF, B., PONDER, W., VALDÉS, Á., WARÉN, A. (2005): "Classification and nomenclator of gastropod families". Malacologia 47 (1–2), pp. 1–397. (Archive.org).
Wikipedia: Taxonomy of the Gastropoda (BOUCHET & ROCROI, 2005).

"Prosobranchia"

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Copper limpet (Cellana ornata, Lottoidea).
Picture: Jacqui Geux (iNaturalist): Waitakere Ranges, New Zealand.
Patellogastropoda – True Limpets and Their Relatives

True limpets and their relatives in the clade Patellogastropoda are considered among the most primitive extant gastropods. Most patellogastropods are small, with a shell diameter of less than 8 cm (2.5 in.). They share a common morphology: a flattened, conical shell that gives them the typical limpet-like appearance. As inhabitants of rocky shores and intertidal zones, they have developed various strategies to cope with tidal fluctuations, desiccation, and numerous predators, including humans.


Ventral view of a common limpet (Patella vulgata).
Picture: Erling Svensen (erlingsvensen.no).
The mouse pointer describes the visible body parts!		  
Their strong muscular foot allows them to cling tightly to rocks, making it nearly impossible to dislodge them. Although patellogastropods have two tentacles with tiny eyes at their base, they likely can only distinguish light from shadow (see illustration on the left).

I. F. Smith: Head of a Limpet (Patella ulyssiponensis) with description.

Most patellogastropods are grazers, feeding by scraping algae off rock surfaces using a robust and rigid radula, known as docoglossan (bar-shaped). Their radular teeth are among the toughest known biomaterials in the natural world. Studies show they are composed of layered chitin reinforced with filaments of goethite, an iron-based mineral named after Johann Wolfgang von Goethe. These composite teeth outperform even spider silk and Kevlar in mechanical strength.

RUMNEY, R. et al (2022): "Biomimetic generation of the strongest known biomaterial found in limpet tooth". Nature Communications. 13. (Link).

While the majority of patellogastropods live in the littoral zone of tropical and temperate coasts — often as early colonisers of hard substrates like rocks — some species graze algae directly off seaweed. An example is the blue-rayed limpet (Helcion pellucidus), which lives on kelp fronds.

Following NAKANO & OZAWA (2007), the Patellogastropoda are divided into two major extant superfamilies: the Lottioidea, containing families such as Lottiidae (true limpets or turtle limpets) and Nacellidae (copper limpets), as well as the Patelloidea, containing the family Patellidae (true limpets in the narrower sense).

 
A limpet's radula (Patella rustica).
EM Picture: Salzburg University, with
friendly courtesy.
Patellogastropoda
Lottioidea
Lottiidae (e.g. Lottia)
Nacellidae (e.g. Cellana)
Patelloidea
Patellidae (e.g. Patella)

Limpets (Patellidae, Patelloidea) on molluscs.at.

WoRMS: MolluscaBase eds. (2025): Patellogastropoda LINDBERG, 1986.
NAKANO, T., OZAWA, T. (2007). "Worldwide phylogeography of limpets of the order Patellogastropoda: Molecular, morphological and palaeontological evidence". Journal of Molluscan Studies. 73 (1): 79–99. (Abstract).

Although adult limpets appear uncoiled, torsion is still evident in this group. One clue is the streptoneury of their nervous system: the two main nerve cords (connectives) from the cerebral ganglia cross over each other — a result of the characteristic 180° rotation of the visceral mass (torsion) during larval development.

Vetigastropoda – Ancient, But Multi-Faceted

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Green Ormer (Haliotis tuberculata): Lanzarote, Canary Islands.
Picture: Oscar Sampedro (iNaturalist).		  
The Vetigastropoda represent a group of primitive marine gastropods that nevertheless exhibit many derived features. According to Geiger (2008), around 3,700 extant species are currently known.

Many vetigastropods have shells with slits or holes that aid in respiration. This feature is especially characteristic of families like the keyhole limpets (Fissurellidae) and the abalones (Haliotidae). The ancestral shell microstructure of vetigastropods is thought to have been cross-lamellar, a structure also found in many caenogastropods (HEDEGAARD, 1997).

 
Keyhole Limpet (Fissurella volcano).
Picture: Alex Heyman (iNaturalist), California.		  
"Pāua" (Haliotis iris) are desired art objects within the Māori people.
Picture: Peter de Lange (iiNaturalist), Kapiti Island, New Zealand.
A more derived (apomorphic) trait seen in many vetigastropods, especially in the more recently evolved families, is the presence of a nacreous layer (mother-of-pearl) on the interiuor side of the shell, as found in abalones. Shell morphology is highly variable: from limpet-like shells in the keyhole-limpets (Fissurellidae), to flattened oval shells in ormers and abalones (Haliotidae), and turreted spiral shells in turban shells (Turbinidae) and top shells (Trochidae). While many vetigastropods have dull, earthy shell colours, others, such as those in the superfamily Trochoidea (especially Trochidae and Turbinidae) and the more ancient Pleurotomarioidea, can possess brightly coloured, glossy shells that are highly prized by collectors.

Most vetigastropods have an operculum, a calcareous or horny lid that closes the shell aperture. The head typically bears two tentacles and a well-developed snout, and on the body sides there are often lateral tentacular extensions ( Image).

Sexual reproduction is the norm in most species, although hermaphroditic forms do exist among deep-sea lineages. Fertilisation usually occurs externally, and development proceeds through planktonic veliger larvae.

Feeding strategies among vetigastropods are diverse. Many species feed on bryozoans, tunicates, or sponges. Others, such as abalones (Haliotidae) and top shells (Trochidae), are herbivores, grazing on algae or even seagrass. In the deep sea, some vetigastropods feed on organic detritus in the sediment, while a few, like the remarkable scaly-foot snail (Chrysomallon squamiferum), rely on symbiotic chemosynthetic bacteria for nutrition.

Vetigastropods are found worldwide, from the intertidal zone to hydrothermal vents in the deep sea, and in all climate zones: from tropical reefs to temperate waters and polar seas.

GEIGER, D.L., NÜTZEL, A., SASAKI, T. (2008). "Vetigastropoda". In PONDER, W.F., LINDBERG, D.R. (eds.). Phylogeny and evolution of the Mollusca. University of California Press. pp. 297–330.
HEDEGAARD, C. (1997). "Shell structures of the recent Vetigastropoda". Journal of Molluscan Studies. 63 (3): 369–377. (Abstract).
ROBERTSON, R. (2003). "The edible West Indian "whelk" Cittarium pica (Gastropoda: Trochidae): Natural history with new observations". Proceedings of the Academy of Natural Sciences of Philadelphia. 153 (1): 27–47. (Abstract).
SALVINI-PLAWEN, L.; HASZPRUNAR, G. (1987). "The Vetigastropoda and the Systematics of Streptoneurous Gastropoda (Mollusca)". Journal of Zoology. 211 (4): 747–770. (Abstract).
HASZPRUNAR, G. (1993). "Sententia: The Archaeogastropoda: A Clade, a Grade, or What Else?". American Malacological Union Bulletin. 10: 165–177.

Wikipedia: Vetigastropoda.
WoRMS: MolluscaBase eds. (2025): Vetigastropoda SALVINI-PLAWEN, 1980.

Keyhole Limpets (Fissurellidae)


Greek Keyhole Limpet (Diodora graeca).
Picture: Lou Wagstaffe (iNaturalist): Jersey (Kanalinseln).		  
Shell of Fissurella volcano, Dorsal view.
Picture: H. Zell. 		  
 
Keyhole Limpet (Amblychilepas nigrita): Victoria, Australia.
Picture: John Eichler (iNaturalist).
   
   
Keyhole Limpet (Emarginella incisura): Marshall Islands.
Picture: Scott und Jeanette Johnson (iNaturalist).
At first glance, the shells of keyhole limpets (Fissurellidae) with their flattened, conical shape may resemble those of true limpets (Patellidae). In fact, many species are still referred to as "keyhole limpets" in English or even "slit limpets", owing to this superficial similarity. However, closer inspection reveals key differences.

The defining feature of keyhole limpets is a slit- or hole-like opening at the apex of the shell, which varies in size between species (typically 3 mm to 13.2 cm, .1 in. to ~5 in.). Some species also have a slit-like notch at the edge of the shell instead. This feature indicates a closer relationship to abalones (Haliotidae), which have a series of holes along the shell margin rather than at the top (see image above).

The apical opening serves a specific purpose: water enters the mantle cavity near the head, flows past the paired gills, and exits through the hole. The presence of paired gills and osphradia (chemosensory organs) in the mantle cavity is considered a primitive trait among gastropods.

Like true limpets, keyhole limpets use their broad, flat foot to cling tightly to rocky surfaces, allowing them to withstand wave action in intertidal and subtidal zones. In some species, the mantle extends over the shell margin, and in the case of the largest species, Megathura crenulata (found off California), it completely covers the shell. The epipodium, a lateral ridge between the foot and the mantle, bears numerous tentacle-like extensions, similar to those of abalones. The head, tentacles, and snout are well developed.

Feeding habits vary: most keyhole limpets are herbivores, grazing on algae using their radula, especially in the lower intertidal and deeper rocky zones. However, some species, such as those in the genera Diodora and Emarginella, are carnivorous, preying primarily on sponges (Porifera). Others, like Puncturella species, feed on diatoms, or in the case of Puncturella aethiopica, even on Foraminifera. Keyhole limpets themselves are preyed upon by a range of predators including shorebirds and starfish.

Three keyhole limpet species are found in the North Sea:

Wikipedia: Keyhole Limpet.
WIESE, V., JANKE, K. (2021): "Die Meeresschnecken und -muscheln Deutschlands", S. 52 ff. Quelle & Meyer Verlag Wiebelsheim.
WoRMS: MolluscaBase eds. (2025): Fissurellidae J. FLEMING, 1822.

Further well-known members of the Vetigastropoda Group:

Neritimorpha – Little Tanks With a Broad Foot

Top of Chapter.

  Hot Spring Snails in Lower Austria!

Neritid snails (Neritidae) are an ecologically diverse group that has adapted to a wide range of habitats. They are found in the sea, freshwater, and in some cases even on land, although in the latter case only in amphibious environments.


Hot sping nerite (heodoxus prevostianus).
Picture: © Alexander Mrkvicka, Vienna.

 One of the rarest species in Austria is the thermal spring nerite (Theodoxus prevostianus), a relic from one of the interglacial periods, when the climate in present-day Vienna was significantly warmer than it is today. Since then, this species has survived only in a few thermal springs in Bad Vöslau and Bad Fischau, where the water temperature remains constant at around 24 °C throughout the year.

These Lower Austrian hot springs are also home to two other remarkable snail species: the thermal spring river snail (Esperiana daudebartii daudebartii), also known as the black spring snail, and the tiny spring snail Bythinella pareyssii, a microsnail that lives in the very source areas of the springs.
Today, around 2,000 extant species of Neritomorpha are known. However, this group is far older: fossils have been found dating back to the Early Ordovician. Although relatively small in terms of species numbers compared to other gastropod clades, the Neritimorpha have achieved remarkable ecological and morphological diversity. At least three groups have independently made the transition to land: the extinct Dawsonellidae, and the extant Helicinidae and Hydrocenidae. There is even a genus of shell-less terrestrial slugs within this clade: Titiscania.

BANDEL, K., FRÝA, J. (1999). "Notes on the evolution and higher classification of the subclass Neritimorpha (Gastropoda) with the description of some new taxa". Geologica et Palaeontologica. 33: pp. 219–235. (PDF).
KANO, Y., CHIBA, S., KASE, T. (2002). "Major adaptive radiation in neritopsine gastropods estimated from 28S rRNA sequences and fossil records". Proc. Roy. Soc. Biol. Sci.. 269 (1508): 2457–2465.


Nerite (Smaragdia souverbiana), Romblon, Philippines.
Picture: Kati Eschweiler (iNaturalist).		  
   

A nerite (Nerita tesselata) as a second homr for a hermit crab (Clibana-
rius tricolor): Florida Keys, USA. Picture: Jeffrey Gammon (iNaturalist).		  
Neritomorph gastropods are found in marine, brackish, freshwater, and even terrestrial habitats. Their shells typically exhibit a cap-like (neritiform) shape, and the inner embryonic shell whorls (protoconch) are reduced or resorbed in the adult shell. Most species possess a calcified operculum, used to tightly seal the shell when needed – an important adaptation to environments with fluctuating water levels or salinity. The exception is the aforementioned Titiscania, which has no shell and thus no operculum at all.

Most neritimorphs feed by grazing algae from stones, aquatic vegetation, mangrove roots, or man-made structures like piers. Their tight-fitting operculum helps them survive dry periods or strong currents by sealing moisture in and predators out.

Systematically, modern Neritimorpha are grouped into several subgroups, but only one – the Cycloneritimorpha – still exists today.

Neritimorpha
Paleozoic Neritimorpha (Status unclear, extinct)
Cyrtoneritimorpha (extinct)
Cycloneritimorpha
Helicinoidea: z.B. Helicinidae (terrestrial)
Hydrocenoidea: z.B. Hydrocenidae (terrestrial)
Neritoidea
Neritidae (z.B. Nerita: marine; Clithon: limnic; Theodoxus: limnic)
Neritopsoidea: z.B.
Neritopsidae (marine, often declared as living fossils)
Titiscaniidae (marine, the only slugs inside the Neritomorpha).

Explanatory Note: Marine: Living in the sea; Limnic: Living in freshwater; Terrestrial: Living on dry land.

WoRMS: MolluscaBase eds. (2025): Neritimorpha, KOKEN 1896.

Nerites (Neritidae)

The Neritidae (nerites) are an ancient family, with some 175 extant species and fossil representatives dating back to the Carboniferous (358.9 - 298.9 mio. years ago). Characteristic features include a cap-shaped shell with a missing or reduced columella and resorbed embryonic whorls. The aperture is wide and flat, and can be closed with a calcified operculum that often has two internal processes, which are helpful for species identification.

Despite their often small size, nerite shells are unusually thick-walled, making them well suited to turbulent waters. When moving, the soft body is mostly withdrawn beneath the shell, offering excellent protection. The head bears a broad snout and pointed tentacles, with the eyes sitting on short stalks at the tentacle bases.

Nerites are dioecious (separate sexes) and fertilisation is internal. They are mostly found in the intertidal zone of marine environments or in fast-flowing rivers. Some species are amphibious, occasionally venturing onto land. Others tolerate brackish water, such as the European river nerite (Theodoxus fluviatilis), which can move from river mouths into estuarine zones like the Baltic Sea, tolerating salinity levels up to 10 PSU (10 g salt/litre). A few species are even found in thermal springs, such as the thermal spring nerite (Theodoxus prevostianus), which still survives in small numbers in Lower Austria, Slovenia, and Hungary.

Nerites are mainly algae grazers, although some species can also feed on sponges or even diatoms using their strong radula. Certain species, such as the "racing snails" Neritina turrita or the zebra nerite (Vittina natalensis), are popular aquarium snails, prized for their attractive shells and algae-eating abilities.

Freshwater nerites.

Caenogastropoda – The Versatile Ones

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Name Species Number Percent
Patellogastropoda ca. 1.000 - 1.500 ~ 2%
Vetigastropoda 7.000 - 9.000 ~ 10%
Neritimorpha ca. 2.000 ~ 3%
Caenogastopoda 25.000 - 30.000 ~ 35%
Heterobranchia* 6.000 - 8.000 ~ 9%
Pulmonata 28.000 - 33.000 ~ 39%
Other Groups ca. 2.000 - 3.000 ~ 3%
Gastropoda 71.000 - 86.500  
 
Enlarge Diagram!
  Species numbers of Gastropoda, distributed in subgroups, in Percent. *: Except Pulmonata.
Sources: BOUCHET & ROCROI et al. (2005); WoRMS: MolluscaBase eds. (2025): Gastropoda CUVIER, 1795.
The Caenogastropoda are a large and highly diverse group within the Gastropoda, including sea-living (marine), freshwater (limnic), and even land-living (terrestrial) species. They are among the most ecologically successful and morphologically varied clades of gastropods. It is estimated that around 23,000 to 30,000 extant species belong to the Caenogastropoda, making them the clearly most species-rich group of marine gastropods.

For comparison, the Pulmonata (lung snails), which dominate terrestrial environments, comprise a slightly higher number of species: between 28,000 and 33,000.

BOUCHET, P., ROCROI, J.-P., FRÝDA, J., HAUSDORF, B., PONDER, W., VALDÉS, Á., WARÉN, A. (2005): "Classification and nomenclator of gastropod families". Malacologia 47 (1–2), pp. 1–397. (Archive.org).
WoRMS: MolluscaBase eds. (2025): Gastropoda CUVIER, 1795.

According to the English Wikipedia, Caenogastropoda account for about 60% of extant gastropod species. However, this figure is misleading – more recent and reliable estimates suggest that the actual share is closer to 35%.

Wikipedia: Caenogastropoda.

Due to the torsion typical of gastropods, Caenogastropoda were traditionally placed among the Streptoneura (i.e. "crossed-nerve" snails), in which the two major nerve cords (connectives) cross each other. Based on the position of their gills – which lie in front of the heart – they were also classified as Prosobranchia; however, that systematic group according to medern systematics is obsolete. Older authors also referred to the Caenogastropoda as Monotocardia, since they have a heart with only one atrium, or Pectinibranchia ("comb-gilled snails"), based on the structure of their single gill pair.

Systematically, Caenogastropoda were first defined as a distinct group by L.R. Cox (1960). In terms of older classifications, they roughly correspond to a combination of Mesogastropoda and Stenoglossa as described by THIELE (1929), and are comparable to the Monotocardia of MÖRCH (1865).

 
Tiger cowry shell (Cypraea tigris), Queensland, Australia.
Picture: Nathan Cook (iNaturalist).		  
Common periwinkle (Littorina littorea): Maine, USA.
Picture: Jeff Goddard (iNaturalist).
      
 
Hebrew volute shell (Voluta ebraea), Brazil.
Picture: Thelma Lúcia Pereira Dias (Source).		  
Common whelk (Buccinum undatum) off the Scottish coast.
Picture: Jim Greenfield (iNaturalist).
The most current systematics today is to be found in the World Register of Marine Species:

System of Caenogastropoda: (Strongly simplified version; Complete overview on WoRMS).

WoRMS: MolluscaBase eds. (2025): Caenogastropoda L.R. COX, 1960.

Caenogastropoda

Littorinimorpha:
Calyptraeoidea: Slipper Limpets (Calyptraeidae)
Cypraeoidea: Cowry Shells (Cypraeidae)
Littorinoidea: Periwinkles (Littorinidae)
Naticoidea: Moon Shells (Naticidae)
Stromboidea: Fighting Conches (Strombidae)
Tonnoidea: Tun Shells (Tonnidae)
Truncatelloidea
Vermetoidea: Worm Snails (Vermetidae)

Neogastropoda:
 Buccinoidea: True Whelks (Buccinidae)
Conoidea: Cone Shells (Conidae)
Mitroidea: Mitra Shells (Mitridae)
Muricoidea: Murex snails (Muricidae)
Olivoidea: Olive Snails (Olividae)
Turbinelloidea
Volutoidea: Volute Shells (Volutidae)

Caenogastropoda incertae sedis:
Campaniloidea:
Cerithioidea: Needle Snails (Cerithiidae)
Epitonioidea: Wentletrap Snails (Epitoniidae)

Heterobranchia

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The Heterobranchia ("various gilled snails") represent a highly diverse and heterogeneous group of gastropods. Their classification has undergone frequent revisions in recent years. Broadly speaking, Heterobranchia includes those gastropods in which, during their evolutionary history, torsion was partially or completely reversed. As a result of this process, known as detorsion, the main nerve cords (connectives) became un-crossed, now running parallel again.

Because of this nervous system arrangement, both the Opisthobranchia (sea slugs and their allies) and the Pulmonata (lung snails and slugs) were traditionally grouped together under the term Euthyneura ("straight-nerved" snails). Today, however, this term refers to only a subset of the Heterobranchia: The classical group of Opisthobranchia has largely been dissolved, and its members have been reclassified into new clades based on genetic and phylogenetic data. Meanwhile, the Pulmonata are still mostly recognized as a valid group under the name Eupulmonata, although former subgroups like the Basommatophora (e.g., pond snails and bubble snails) are now treated as part of the Hygrophila, which is sometimes ranked as an equivalent lineage.

Further molecular phylogenetic studies in recent decades have shown that Euthyneura is just one part of Heterobranchia. Several other groups that were historically placed elsewhere are now recognized as basal Heterobranchs, often collectively referred to as “Lower Heterobranchia”. These include, for example, the Valvata snails (Valvatidae, small freshwater snails with feather-like gills) and the sundial shells (Architectonicidae).

The systematics of these basal groups is still far from settled, and future revisions based on genetic and developmental data are likely. The overall picture is one of ongoing change and growing complexity in our understanding of heterobranch evolution.

 
Common Valvata (Valvata piscinalis), Tver', Russia.
Picture: Vladimir Semaschko (iNaturalist).		  
Sundial shell (Architectonica modesta): Bali, Indonesia.
Picture: Scott und Jeanette Johnson (iNaturalist).
      
 
Common sea hare (Aplysia punctata), Killeany Bay, Ireland.
Picture: David Cothran (iNaturalist).		  
Dwarf horn snail (Carychium minimum): Arcata, CA, USA.
Picture: Alex Bairstow (iNaturalist).
System of Heterobranchia: (Strongly simplified version; Complete overview on WoRMS).

Extended display in new window.
WoRMS: MolluscaBase eds. (2025): Heterobranchia BURMEISTER, 1837.
Wikipedia: Heterobranchia.

Heterobranchia

Lower Heterobranchia
Valvatoidea: Valvata snails (Valvatidae)
Architectonicoidea: Sundial shells (Architectonicidae)

Euthyneura
Pteropoda: Sea butterflies (Thecosomata) and sea angels (Gymnosomata).

Ringipleura
Nudipleura: Nudibranchs (Nudibranchia).

Tectipleura
Acochlidiimorpha
Aplysiida: e.g. Sea hares (Aplysiidae).
Cephalaspidea: e.g. Bubble snails (Bullidae).
Sacoglossa: e.g. Plakobranchidae: Green Elysia slug (Elysia viridis); or double-valve snails (Juliidae: Berthelinia).
Eupulmonata
Ellobioidea: e.g. Coast snails (Ellobiidae) and Dwarf horn snails (Carychiidae).
Systellommatophora
Stylommatophora: Landlungenschnecken: e.g. Garden snails (Helicidae) etc.
Hygrophila: Pond snails (Lymnaeidae) and Ramshorn snails (Planorbidae) etc.

"Opisthobranchia"

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According to classical taxonomy (THIELE 1931), the Opisthobranchia ("posterior gill snails") were a group of marine gastropods in which torsion had been secondarily reversed during evolution, causing the gills to be located behind the heart, in contrast to the Prosobranchia ("anterior gill snails"), where the gills lie in front of it (see above). Many opisthobranchs have either strongly reduced or completely absent shells, which makes their fossil record relatively poor. However, molecular clock analyses suggest that opisthobranchs have existed since at least the Carboniferous period (358.9 - 298.9 mio. years ago).


Sea butterflies (Thecosomata, here: Limacina helicina) belong to the
Pteropoda, but they still retain a thin translucent shell.
Source: CHICHVARKHIN, A. (2016).		  
MEDINA, M. et al. (2011). "Crawling through time: Transition of snails to slugs dating back to the Paleozoic, based on mitochondrial phylogenomics". Marine Genomics. 4 (1): 51–59. (Abstract).

 
Bubble snail (Bulla quoyii): California, USA.
Picture: L. Colmer (iNaturalist).
   
 
Bubble snail (Bulla gouldiana): California, USA.
Picture: Alexis T. (iNaturalist).
Following the modern classification (BOUCHET, ROCROI et al., 2017), the majority of the former Opisthobranchia are now included in the clade Tectipleura, which also contains the Pulmonata (see below). One notable exception is the group of nudibranchs (Nudibranchia), which are no longer included, as they form a distinct clade. Molecular studies have shown that most opisthobranchs are more closely related to lung breathing snails (Pulmonata) than previously thought.

What many opisthobranch gastropods have in common is the reduction of their shell, which can be seen in different grades in different groups: Head shield snails (Cephalaspidea), such as the bladder snails (Bullidae) still possess a thin-walled bladder-formed shell. The Anaspidea, among those the sea hares (Aplysiidae) possess a strongly reduced, internal shell. And finally the Pteropoda ("wing-foot snails") are remarkable pelagic gastropods, which are separated into two groups: The sea butterflies (Thecosomata) with a thin translucent shell, and the sea angels (Gymnosomata), without any shell.

Head Shield Snails (Cephalaspidea)

The name Head shield snails (Cephalaspidea) refers to the broad headshield that covers the front of the animal and helps it burrow through the substrate while protecting the mantle cavity from sand intrusion. Cephalaspideans feed on other heterobranchs, marine worms (polychaetes), and foraminifera. Many species have well-developed eyes and sensory structures such as the Hancock's organ, a chemosensory structure near the right rhinophore. Some also have ciliary bundles around the mouth that help track slime trails of prey.

Among them are the bubble snails (Bullidae), which still retain a fragile, inflated shell (see pictures on the right). Others, like members of the family Aglajidae (e.g., Philinopsis cyanea), lack an external shell entirely and are highly efficient predators of other snails. These slugs can swallow their prey whole, shell and all, using an eversible pharynx, and later regurgitate the indigestible parts. This limits them to prey smaller than themselves. Navanax inermis, for example, which inhabits the Pacific coast from California to Mexico, can grow up to 22 cm in length and is the largest species in the family.

Head shield snails (Cephalaspidea).

Sea Hares and Their Relatives (Aplysiida)

In contrast to the tiny pteropods (see below), sea hares (Aplysiidae) can reach impressive sizes. The black sea hare (Aplysia vaccaria), for example, found off the coast of California and Baja California, can grow up to 75 cm and weigh up to 14 kg. Their name derives from their large, ear-like rhinophores, reminiscent of rabbit ears. These bulky slugs have a thin internal shell, which is completely covered by the mantle.

Sea hares became famous not only for their size but also for their exceptionally large nerve cells, which made them model organisms in neurobiological research.

Sea Hares (Aplysiida)

Sap Sucking Slugs (Sacoglossa)


"Bivalve snail" (Berthelinia babai): Victoria, Australia.
Picture: Rebecca Lloyd (iNaturalist).		  
   

"Bivalve snail" (Berthelinia babai): Victoria. Australia.
Picture: Kade (iNaturalist).		  
The Sacoglossa are small marine (and in rare cases, freshwater) slugs and snails typically measuring 1–3 cm in length. Their shells are often reduced or thin-walled, and some species can no longer retract into them. In others, the shell is hidden under the parapodia (lateral mantle flaps), as in sea hares, and may even be used for swimming.

The group is named after their pharyngeal sac, in which the radula sits and old teeth accumulate over time. This structure increases in size as the animal ages. Sacoglossans may have one or two pairs of tentacles, though some have none at all.

 
Green velvet slug (Elysia viridis):  Schouwen-Duiveland, Netherlands.
Picture: Lynn Biscop (iNaturalist).
These seemingly modest animals appear to challenge two core ideas of molluscan biology:

First: Animals do not photosynthesise. Well, not directly, but some sap-sucking slugs have developed a remarkable workaround known as kleptoplasty: They steal chloroplasts from the algae they consume and store them in their tissues, allowing photosynthesis inside their own bodies. This gives species like the green velvet slug (Elysia viridis), which feeds on algae (Codium fragile), their distinctive green colour and earns them the nickname "solar-powered slugs".

Solar Powered Slugs.
WoRMS: MolluscaBase eds. (2025): Plakobranchidae J.E. GRAY, 1840.

Second: Gastropods have unipartite shells; bivalves have two valves. Again, generally true, unless you're looking at the bizarre bivalved sacoglossans of the family Juliidae. However, only the left shell half is homologous to that of other gastropods; the right half is secondarily developed later by the mantle.

These snails were often misidentified as bivalves in the fossil record. Remarkably, it was not until 1959 that living representatives of Juliidae were confirmed to exist — clearly identifiable as gastropods.

Sap Sucking Slugs (Sacoglossa).
WoRMS: MolluscaBase eds. (2025): Juliidae E.A.SMITH, 1885.

Lung Breathing Snails ("Pulmonata")

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Acute bladder snail (Physella acuta): Alsace, France.
Picture: Nataël Adam (iNaturalist).
Pulmonates are arguably the most successful group of gastropods. Instead of gills, they breathe through a vascularised lung, which evolved from the wall of the mantle cavity. Like most marine opisthobranchs, pulmonates never have an operculum (the calcareous "trapdoor" found in many marine snails). It is now widely accepted that pulmonates and opisthobranchs share a common evolutionary origin.

System of the "Pulmonata": (Strongly simplified version; Complete overview on WoRMS).

WoRMS: MolluscaBase eds. (2025): Tectipleura.
BOUCHET, P., ROCROI, J.P. et al. (2017): "Revised classification, nomenclator and typification of gastropod and monoplacophoran families". Malacologia. 61(1-2): 1-526.


Florida leatherleaf (Leidyula floridiana, Veronicellidae).
Picture: Turner Brockman (iNaturalist): Florida Keys, USA.		  
The simplified system used here (based on WoRMS – the World Register of Marine Species) illustrates the modern classification of what was formerly known as the group "Pulmonata":

Eupulmonata
Ellobioidea: e.g. Coast snails (Ellobiidae) und Dwarf horn snails (Carychiidae).
Systellommatophora: e.g. Leather leaf slugs (Veronicellidae).
Stylommatophora: Landlungenschnecken: e.g. Garden snails (Helicidae) etc.
Hygrophila: Pond snails (Lymnaeidae) and Ramshorn snails (Planorbidae) etc.

The Eupulmonata ("true lung snails") include the terrestrial pulmonates (Stylommatophora), which possess retractable tentacles with eyes at the tips (a defining feature of this group), as well as two smaller groups: the Ellobioidea ("shore snails"), which inhabit coastal and sometimes brackish habitats, and the Systellommatophora, a group of tropical slug-like snails with reduced or absent shells.

Opposite the Eupulmonata, there are the Hygrophila, which include most freshwater pulmonates (formerly grouped as Basommatophora). They typically have non-retractable tentacles with eyes at the base and include families such as: Pond snails (Lymnaeidae), Ramshorn snails (Planorbidae), River limpets (Acroloxidae) and Bladder snails (Physidae).

Freshwater Snails.

Among the Systellommatophora there is the Veronicellidae family, also known as leatherleaf slugs, which are found in tropical and subtropical regions of the Americas, Africa, and Asia. These slug-like gastropods live in lowland forests, both primary and secondary, where they feed on detritus and plant matter. Some species have been introduced beyond their native range, for example, to Pacific islands or parts of the United States, where they are considered agricultural pests. In several U.S. states, introduced Veronicellidae from the Caribbean have been classified as invasive threats to agriculture and gardens.

Wikipedia: Veronicellidae.

 

Latest Change: 27.06.2025 (Robert Nordsieck).