Introduction to gastropods
The mollusc class Gastropoda is composed of snails with a shell and slugs without a shell, or with a reduced, ±internal shell. Traditionally gastropods were subdivided into 3 sub-classes, Prosobranchia, Opisthobranchia and Pulmonata, but phylogenetic analyses, both based on morphological characters and molecular data have instigated a reorganization of the classification of higher taxa (sub-classes, orders, and in some cases even families and superfamilies). Unfortunately this means that there is currently a lot of name-changes, and both old and new names are in use by different scientists. The most up-to-date introduction to gastropod classification available on the internet is Newworldencyclopedia. Newer changes can be found in Bouchet and Rocroi (2005).
The key will help you to identify the following gastropods:
Bouchet, P. and Rocroi, J.-P. 2005. Classification and nomenclator of gastropod families. Malacologia 47(1-2). ConchBooks, Hackenheim, Germany, 397pp.
Most of the snails with a limpet-like (=patelliform) shell are, in fact, true limpets (Patellogastropoda and some Vetigastropoda, both formerly in the Prosobranchia, Archaeogastropoda). For some British and North Sea species descriptions are available on-line. Native species in Nordic waters are:
Subclass: Patellogastropoda
Family: Lottiidae (=Acmaeidae)
Erginus rubellus (O. Fabricius, 1780) (=Acmaea r.), only in northern Norway, Iceland and Greenland (WORMS)
Testudinalia testudinalis (O.F. Müller, 1776) (=Acmaea t.; =Tectura t.) – Tortoiseshell limpet, see: http://www.conchsoc.org/encyclopedia/speciesInfo.php?taxon_version_key=NBNSYS0000178291 and http://species-identification.org/species.php?species_group=mollusca&id=964
Tectura virginea (O.F. Müller, 1776) (=Acmaea v.) – White tortoiseshell limpet, see: http://www.conchsoc.org/encyclopedia/NBNSYS0000178292 and http://species-identification.org/species.php?species_group=mollusca&id=964
Family: Patellidae
Ansates pellucida (=Helcion p.; Patina p.) (Linnaeus, 1758) – Blue-rayed limpet, see: http://www.conchsoc.org/encyclopedia/NBNSYS0000175715 and http://species-identification.org/species.php?species_group=mollusca&id=549
Patella ulyssiponensis Gmelin, 1791(=Patella aspera Lamarck, 1819) – China limpet, see: http://www.conchsoc.org/encyclopedia/NBNSYS0000177033 and http://species-identification.org/species.php?species_group=mollusca&id=888
Patella vulgata Linnaeus, 1758 – Common limpet, see: http://www.conchsoc.org/encyclopedia/NBNSYS0000177034and http://species-identification.org/species.php?species_group=mollusca&id=889
Subclass: Vetigastropoda
Family: Fissurellidae
Emarginula crassa (J. Sowerby, 1813) – Large slit-limpet, see: http://species-identification.org/species.php?species_group=mollusca&id=668
Emarginula fissura (Linnaeus, 1758) – Common slit-limpet, see: http://species-identification.org/species.php?species_group=mollusca&id=669
Fissurisepta granulosa Jeffreys, 1883 (=F. papillosa auct., non Seguenza, 1863), S and W Norway
Puncturella noachina (Linnaeus, 1771) – Punctured limpet, see: http://species-identification.org/species.php?species_group=mollusca&id=917
Family: Lepetidae
Iothia fulva (=Lepeta fulva) (O.F. Müller, 1776) – Fulvous tortoiseshell limpet, see: http://www.conchsoc.org/encyclopedia/NBNSYS0000175884 and http://species-identification.org/species.php?species_group=mollusca&id=737
Lepeta caeca (O.F. Müller, 1776) – Blind limpet, see: http://species-identification.org/species.php?species_group=mollusca&id=737
Propilidium exiguum (Thompson, 1844) – Curled limpet, see: http://www.conchsoc.org/encyclopedia/NBNSYS0000186101 and http://species-identification.org/species.php?species_group=mollusca&id=914
Family: Lepetellidae
Lepetella laterocompressa (de Rayneval & Ponzi, 1854) (=L. tubicola Jeffreys, 1882), NW Sweden, Norway
Capulus ungaricus (Linnaeus, 1758) (Family: Capulidae), see: http://species-identification.org/species.php?species_group=mollusca&id=592
Some species in the family Velutinidae, e.g., Lamellaria perspicua (Linnaeus, 1758) and Velutina plicatilis (O.F. Müller, 1776), may also superficially seem limpet-like. However, a flat spire is usually visible on the shell. See: http://species-identification.org/species.php?species_group=mollusca&id=753 and http://species-identification.org/species.php?species_group=mollusca&id=1014
In freshwater a couple of limpet-like snails occur, Acroloxus lacustris (Linnaeus, 1758) and Ancylus fluviatilis O.F. Müller, 1774. These are pulmonate snails. However, there is little chance of confusing these with the marine introduced species.
A few other members of the Calyptraeidae (same family as the slipper limpet, Crepidula fornicata occur to the south of Nordic waters, and can be expected to expand northwards if temperatures increase. Crepipatella dilatata (Lamarck, 1822), a native to South America, has just been found in Spain (Collin et al., 2009), and Calyptraea chinensis (Linnaeus, 1758) occurs naturally from NW Africa and the Mediterranean to southern England and western Scotland, has been introduced to Ireland in 1962 (Minchin, 2007), and has recently extended its distribution to Belgium (Kerckhof et al., 2007).
Calyptraea chinensis see: http://species-identification.org/species.php?species_group=mollusca&id=591
Collin, R., Farrell, P. and Cragg, S. 2009. Confirmation of the identification and establishment of the South American slipper limpet Crepipatella dilatata (Lamarck 1822) (Caenogastropoda: Calyptraeidae) in Northern Spain. Aquatic Invasions 4(2): 377-380.
Kerckhof, F., Haelters, J. and Golasch, S. 2007. Alien species in the marine and brackisch ecosystem: the situation in Belgian waters. Aquatic Invasions 2(3): 243-257.
Minchin, D. 2007. A checklist of alien and cryptogenic aquatic species in Ireland. Aquatic Invasions 2(4): 341-366.
Neogastropods are highly derived snails, characterized by a distinct siphonal canal at the anterior shell margin. Presently they are considered a sub-order of Caenogastropoda; previously they were one of 3 orders of Prosobranchia. The snails have a long siphon, which can be waved about to detect the smell of food, enemies or partners. This siphon extends through the siphonal canal. They have specialized radular teeth and often have special diets as well as feeding methods, e.g. the harpoon-like teeth of the venomous cone-shells (Conus spp.). Many species are predatory and some are known to drill holes in the shells of other molluscs, such as oysters, and hence can be pests to oyster culture. Their association with cultured oysters or other bivalves also means that they are often accidentally introduced with oysters to new areas. Neogastropods are highly susceptible to anti-fouling agents, such as TBT. They develop imposex, which eventually sterilizes or even kills the snails. The ban of TBT may enhance the chances of successful transfer of these snails with aquaculture organisms (Faasse & Ligthart, 2007).
No introduced neogastropods are known from Nordic waters at the present time. However, a few are moving steadily closer, and one species, Rapana venosa (Valenciennes, 1846), has now reached the southern North Sea (Kerckhof et al., 2006). With the occurrence of dense beds of the invasive oyster, Crassostrea gigas, it is likely that R. venosa will be able to spread further north. Two other alien neogastropods, both known as oyster drills, the American Urosalpinx cinerea and the Japanese Ocinebrellus inornatus have recently been found in the Netherlands (Faasse & Ligthart, 2009). Also, Ocenebra erinaceus (Linnaeus, 1758), which is a native of the UK, seems to be extending its range northwards. Thus, it now occurs and breeds in the Limfjord, Denmark (Jensen & Hoffmann, 2007). It probably has migrated through its own means.
For more information on Ocenebra erinaceus see the Marine Species Identification Portal.
For more information on Ocinebrellus inornatus (Récluz, 1851)(synonyms: Ceratostoma inornatum; Ocenebra japonica (Dunker, 1860); Pteropurpura (Ocinebrellus) inornatus) see USGS and the Global Invasive Species Database.
Faasse, M. and Ligthart, M. 2007. The American oyster drill, Urosalpinx cinerea (Say, 1822), introduced to the Netherlands – increased risks after ban on TBT? Aquatic Invasions 2(4): 402-406.
Faasse, M. and Ligthart, M. 2009. American (Urosalpinx cinerea) and Japanese oyster drill (Ocinebrellus inornatus) (Gastropoda: Muricidae) flourish near shellfish culture plots in the Netherlands. Aquatic Invasions 4(2): 321-326.
Jensen, K.R. and Hoffmann, E. 2007. Ny rovsnegl i Limfjorden. Dyr i Natur og Museum 2007, nr. 1: 7-9. (in Danish) [New predatory gastropod in the Limfjord]
Kerckhof, F., Vink, R.J., Nieweg, D.C. and Post, J.N.J. 2006. The veined whelk Rapana venosa has reached the North Sea. Aquatic Invasions 1: 35-37.