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Anisus vorticulus (Troschel, 1834)
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Species name:
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Anisus vorticulus (Troschel, 1834)
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Taxon name:
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Planorbis vorticulus Troschel, 1834
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Originally described in:
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Troschel, F. H. 1834. De limnaeaceis seu de gasteropodis pulmonatis quae nostris in aquis vivunt. - pp. [1-5], 1-65, [1]. Berolini. (Dissertatio Zoologica).
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Distribution:
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SE England, Belgium to N Italy, Denmark, Estonia, Greece, Ukraine and Caspian Sea (not Norway)
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Diagnosis:
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Shell horny brown, upper side slightly concave, lower side almost flat, 5-5.5 convex whorls with distinct suture, last whorl with blunt edge in the centre of the periphery, aperture elliptical and usually oblique.
Like Anisus vortex, usually smaller, keel blunt, whorls rounded at both sides.
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Size:
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0.6-1.2 x 2.5-7 mm
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Biology:
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Standing or slowly floating clean waters usually rich in vegetation (Lemna trisulca, Hydrocharis, Ceratophyllum), oxygen and calcium contents, no visible sediment preference. Often floating on the surface among Lemna. Feeds on diatoms or algae which need sunlight (cutting the bankside vegetation improves the conditions for A. vorticulus). Not sensitive against careful weed removal and against measures to remove 1 m thick anaerobe mud layers. Tolerates frozen waters in winter and dried-out waters in sumer. Does not tolerate hypertrophic conditions, sensitive against too fast water currents and against destruction of water banks by cattle. In Switzerland up to 500 m. Seems to be dispersed by birds and can increase popula-tion densities rapidly under appropriate conditions.
Snails in NW Poland begin copulating at 3 mm shell diameter. Together up to 500 eggs (0.55 x 0.45 mm) per individual are laid between May and August in cocoons containing 1-6 eggs, juveniles hatch after 10 days (24°C), diameter reaches 4 mm after 3 months, no growth between November and March, maximum size is reached in June. Life span slightly more than 1 year, snails usually die in the second winter.
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Threatened:
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Scattered and rare, threatened by continuous habitat destructions, drainage, overfrequent dredging, fertilization in agricultural zones and introduction of nutrients to water bodies. All surviving populations in England are threatened by these causes. Populations in Austria have dramatically declined since the 1990s, including in Donauauen National Park, where the species became extinct between 1991 and 2006 due to serious mismanagement of the National Park administration. The last surviving populations in Austria (near Bad Deutsch-Altenburg in an old river arm with low water current) are seriously threatened by sceduled changes in river regulation (2006/2009) which is (Reischütz 2009) expected to cause another dramatic decline and possibly extinction.
Critically endangered in Germany, Czech Republic, S Switzerland and Austria, endangered in Italy and all other parts of Switzerland, vulnerable in England and Poland.
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Family:
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Planorbidae
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Higher group:
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Gastropoda
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Comments:
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References: Meier-Brook 1983: 92, Grossu 1987: 138, Falkner 1990: 132, Vogt et al. 1994: 79, Manganelli et al. 1995: 11, Turner et al. 1998: 121, Kerney 1999: 62, Glöer 2002: 262, Cioboiu 2006 (W Romania, Danube and delta), Glöer & Groh 2007 (measures for conservation), Myzyk 2008 (life cycle), Reischütz 2009, Welter-Schultes 2012: 60 (range map Europe).
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