Ancylus fluviatilis

Flow cap screw ( Ancylus fluviatilis)

The flow cap screw ( Ancylus fluviatilis ), also called river limpet, is the best known and only one in Central Europe species of the genus Ancylus. It is now placed in the family of the plate screw ( Planorbidae ); earlier it was assigned to the now defunct family " Ancylidae ". She comes out in rivers and other watercourses on rocky or stony ground, some lakes before.

Features

Morphology: The flow cap screw with a maximum of 11 mm shell length ( mostly adult only 5-8 mm) than adult animal a small to medium sized, but some places frequent and widespread water lung worm ( Basommatophora ). The larger forms (more than 8 mm ) occur in Central Europe only in relatively calcareous streams with optimal nutritional and temperature conditions (eg, near Lake Constance ) ago. Characteristic is their cup-shaped shell with twisted slightly to the right - rear tip; the apex of the shell is coarsely sculptured Radiärrippen. At hatching, the animals measure less than 1 mm shell length.

Karyology: Compared to the majority of the other representatives of the cup-shaped Planorbidae the species is characterized by an increased number of chromosomes. During the haploid set of chromosomes in the majority of these species and genera, so far as they have been studied (especially Rhodacmea cahawbensis, Laevapex fuscus, not at Ferrissia species) at n = 15-17 is what within the basic equipment of all water snails (n = 15 - 18) corresponds, it is for Ancylus fluviatilis at n = 30 ( tetraploid, some were also n = 60 ( oktoploid ) reported ) ..

Similar Species

The flow cap screw resembles superficially the Teichnapfschnecke. However, this is rechtsgewunden anatomically. The apex of the housing is therefore in the flow cap screw to the back right, at the Teichnapfschnecke to the rear left.

Another possibility of confusion, especially of juvenile river cap screw, is the perhaps, introduced from North America, but perhaps also from other regions of Europe and traditionally referred to as Ferrissia wautieri type ( the exact assignment and naming is still unclear ). This species has, like the flow cap screw, a left-handed shell, but is only 3-4 mm long, tends to rather länglicherer shell and sometimes forms after dry periods a septum at the bottom of the sled out.

Geographical distribution and species status

The type is large parts of Western, Central, Eastern and Northern Europe (up to southern Sweden, norway, finland - ) distributed. For large parts of the Mediterranean area ( Iberian Peninsula, Italy, etc. ) occurring and nominally been also designated Ancylus fluviatilis forms are, to the molecular genetic findings to independent forms, holding not less than the relationship status of its own species, but so far not nomenclaturally independently are designated. In addition, on the occurrence of this form of group in northeastern Africa ( coastal areas from Morocco to Tunisia, Hoggar Mountains of Algeria ) and is reported in the highlands of Ethiopia to 2240 m above sea level. For details, see also under genus Ancylus.

Ecological distribution

The snails are found in oxygen-rich stationary and flowing water and also in karst springs. They needed (not too little and not too abundant ) growth of algae, which is why they are in Central Europe, particularly in rivers often, but are also found in lakes ( even common in northern and western Europe) definitely hard substrate with a suitable. In contrast to many other freshwater snails, the animals tolerated base -poor acidic environment. You are in Germany a pointer type for the water quality class II

Southern European, North African and Near Eastern representative of the group of species can probably survive to a certain degree of desiccation of water by forming a protective layer on the bottom of the shell; at the Central European forms, this is only limited and short-term possible by strong water-efficient hard stitching on the stone surface.

Way of life

Nutrition: The flow cap screw feeds on plant nursery ( periphyton ) and detritus, with diatoms, small green algae, locally also water lichens and other food components are absorbed and digested. The crushing and digesting the diatoms support the numerous stored in the gizzard grains of sand that are recorded and stored on the eating process. The animals consume in the active feeding stages 1-5 % of their own body mass, but always put back feeding breaks ( see next section ).

Activity: The animals move over rocks, boulders and bedrock, where it ( for no apparent day -night rhythm ) rest or show at irregular intervals activity phases. They spend many many hours of continuous rest periods rather at the ( more protected against predators influence) bottom of the rocks, as they travel around the active grazing of food in a largely undirected random pattern over the rocks. Where food in appropriate composition and thickness that is abraspelbar with the radula is present, they remain and grazed by alternating movements of the head at the same time slow forward creep underground more or less completely. During the winter months, the animals often remain long at rest, which is also a state of reduced metabolic activity.

Spread Biology: The irregular activity phases with periods of persistent clinging immovable and intense grazing on nutritionally favorable points are also used to spread in the water and finding the often patchily distributed favorable micro-habitats. This enters the individual animal over a year away to 1 m or more from the Schlüpfort. The orientation and propagation takes apparently random; only a slight component of a flow upward direction can be measured. The spatial distribution corresponds to the propagation principle of diffusion. However, " Road transport " River of cap screw has become known, especially by the random solid stitching on the feet of water birds or even the body of larger airworthy water beetles. The most famous observation over a fixed stapling beetles is that of Charles Darwin in his book " The Origin of Species " (p. 386 of the original English edition). This form of propagation is in the long term (eg in the recolonisation formerly glaciated areas after the ice ages ), the more significant compared to the diffusive Nahausbreitung that it is the gradual colonization of a larger portion waters following a local first settlement.

Reproductive biology

The flow cap screw is like all water snails hermaphroditic, wherein, in the copulation of a partner than males, which may act as other females. Also superimposed Kopulationsketten of four to five animals in which the supreme all intervening act only as males, the lowest only as females, and both males and females as have been described.

However, further investigations showed that in many populations there is a marked tendency for self-fertilization. By measuring polymorphic Allozymloci was concluded that only 13 to 15 % of the young snails emerged from cross-pollination and that therefore must be expected of superficial observation of copulations often with the use of own sperm itself. The ability to self-fertilization is basically a survival advantage, since even individual animals can build up a population again.

The eggs are deposited ( by 5-7 eggs on average ) at a distance of a few days in the form of several gelatinous egg capsules, each 1-10 eggs. The onset of copulation and oviposition usually occurs from a shell length of 4 to 5 mm. After the onset of oviposition body growth compared to the previous growth is around 1 /5 of the growth rate is reduced, but is still going on. The number of eggs per capsule depends on the size of the adult animal and the nutritional status. The total number of eggs laid per adult animal is highly variable and is at most about 100 eggs. Oviposition begin in the spring from a temperature of 7 ° to 10 ° C.

Paleontology

As an index fossil in the area of ​​today's Baltic Sea led to the designation of the Ancylus Lake.