Bythinella bicarinata
Encyclopedia
Bythinella bicarinata is a species
of very small freshwater snail
, an aquatic
gastropod mollusk in the family Amnicolidae
.
of this species measures about 2.2-2.6 mm long and has a pupoidal shape (shaped like a fly's puparium). In the population first described, there are two ribs running along the length of the shell; hence the scientific name bicarinata ("two-keel
ed").
(A1ce) by the IUCN, as its habitat
– essentially the Fontaine de la Vierge and nearby spring
s in the Dordogne
– was being affected by pollution
and the local snail populations had declined more than 80% in the late 20th century.
and nDNA ITS1
sequence
data showed that the "typical" two-keeled Bythinella bicarinata are actually part of a clade
widespread in central to northeastern France and nearby regions. Consequently, Bythinella dunkeri, Bythinella lalindei, Bythinella moulinsii and Bythinella poujolensis are now provisionally treated as junior synonyms of Bythinella bicarinata. These all lack the shell keels, though Bythinella lalindei and Bythinella poujolensis are otherwise essentially identical. The other two differ a bit more, though those described as Bythinella moulinsii are quite variable in size and shape. As the study proposing the synonymy applied the phylogenetic species concept which does not recognize subspecies
, the question whether the other taxa are valid subspecies or simply local morphs remains unresolved.
Considering the lack of the two-keeled phenotype
outside the extreme southeastern end of the range of Bythinella bicarinata and the fact that the region where it occurs is at the very limits of the range, it is probably advisable to recognize several subspecies. In any case, Bythinella bicarinata in the loose sense is not globally threatened. The populations from Belgium
and western Germany described as Bythinella dunkeri were classified as Vulnerable
(B1+2c), as they were known from less than ten freshwater springs which are affected by pollution. Given that they occur at the other end of the species' range and are consistently most distinct morphologically
from the "keeled" specimens, they might also warrant recognition as subspecies.
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
of very small freshwater snail
Freshwater snail
A freshwater snail is one kind of freshwater mollusc, the other kind being freshwater clams and mussels, i.e. freshwater bivalves. Specifically a freshwater snail is a gastropod that lives in a watery non-marine habitat. The majority of freshwater gastropods have a shell, with very few exceptions....
, an aquatic
Aquatic animal
An aquatic animal is an animal, either vertebrate or invertebrate, which lives in water for most or all of its life. It may breathe air or extract its oxygen from that dissolved in water through specialised organs called gills, or directly through its skin. Natural environments and the animals that...
gastropod mollusk in the family Amnicolidae
Amnicolidae
Amnicolidae is a family of small freshwater snails with a gill and an operculum, aquatic gastropod mollusks.This family is in the superfamily Rissooidea and in the clade Littorinimorpha...
.
Shell description
The shellGastropod shell
The gastropod shell is a shell which is part of the body of a gastropod or snail, one kind of mollusc. The gastropod shell is an external skeleton or exoskeleton, which serves not only for muscle attachment, but also for protection from predators and from mechanical damage...
of this species measures about 2.2-2.6 mm long and has a pupoidal shape (shaped like a fly's puparium). In the population first described, there are two ribs running along the length of the shell; hence the scientific name bicarinata ("two-keel
Keel
In boats and ships, keel can refer to either of two parts: a structural element, or a hydrodynamic element. These parts overlap. As the laying down of the keel is the initial step in construction of a ship, in British and American shipbuilding traditions the construction is dated from this event...
ed").
Distribution
This species was believed to be endemic to France and severely threatened with extinction. In 1996 it was classified as Critically EndangeredCritically Endangered
Critically Endangered is the highest risk category assigned by the IUCN Red List for wild species. Critically Endangered means that a species' numbers have decreased, or will decrease, by 80% within three generations....
(A1ce) by the IUCN, as its habitat
Habitat
* Habitat , a place where a species lives and grows*Human habitat, a place where humans live, work or play** Space habitat, a space station intended as a permanent settlement...
– essentially the Fontaine de la Vierge and nearby spring
Spring (hydrosphere)
A spring—also known as a rising or resurgence—is a component of the hydrosphere. Specifically, it is any natural situation where water flows to the surface of the earth from underground...
s in the Dordogne
Dordogne
Dordogne is a départment in south-west France. The départment is located in the region of Aquitaine, between the Loire valley and the High Pyrénées named after the great river Dordogne that runs through it...
– was being affected by pollution
Pollution
Pollution is the introduction of contaminants into a natural environment that causes instability, disorder, harm or discomfort to the ecosystem i.e. physical systems or living organisms. Pollution can take the form of chemical substances or energy, such as noise, heat or light...
and the local snail populations had declined more than 80% in the late 20th century.
Genetics
However, analysis of mtDNA COICytochrome c oxidase
The enzyme cytochrome c oxidase or Complex IV is a large transmembrane protein complex found in bacteria and the mitochondrion.It is the last enzyme in the respiratory electron transport chain of mitochondria located in the mitochondrial membrane...
and nDNA ITS1
Internal transcribed spacer
ITS refers to a piece of non-functional RNA situated between structural ribosomal RNAs on a common precursor transcript. Read from 5' to 3', this polycistronic rRNA precursor transcript contains the 5' external transcribed sequence , 18S rRNA, ITS1, 5.8S rRNA, ITS2, 28S rRNA and finally the 3'ETS...
sequence
DNA sequence
The sequence or primary structure of a nucleic acid is the composition of atoms that make up the nucleic acid and the chemical bonds that bond those atoms. Because nucleic acids, such as DNA and RNA, are unbranched polymers, this specification is equivalent to specifying the sequence of...
data showed that the "typical" two-keeled Bythinella bicarinata are actually part of a clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
widespread in central to northeastern France and nearby regions. Consequently, Bythinella dunkeri, Bythinella lalindei, Bythinella moulinsii and Bythinella poujolensis are now provisionally treated as junior synonyms of Bythinella bicarinata. These all lack the shell keels, though Bythinella lalindei and Bythinella poujolensis are otherwise essentially identical. The other two differ a bit more, though those described as Bythinella moulinsii are quite variable in size and shape. As the study proposing the synonymy applied the phylogenetic species concept which does not recognize subspecies
Subspecies
Subspecies in biological classification, is either a taxonomic rank subordinate to species, ora taxonomic unit in that rank . A subspecies cannot be recognized in isolation: a species will either be recognized as having no subspecies at all or two or more, never just one...
, the question whether the other taxa are valid subspecies or simply local morphs remains unresolved.
Considering the lack of the two-keeled phenotype
Phenotype
A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...
outside the extreme southeastern end of the range of Bythinella bicarinata and the fact that the region where it occurs is at the very limits of the range, it is probably advisable to recognize several subspecies. In any case, Bythinella bicarinata in the loose sense is not globally threatened. The populations from Belgium
Belgium
Belgium , officially the Kingdom of Belgium, is a federal state in Western Europe. It is a founding member of the European Union and hosts the EU's headquarters, and those of several other major international organisations such as NATO.Belgium is also a member of, or affiliated to, many...
and western Germany described as Bythinella dunkeri were classified as Vulnerable
Vulnerable species
On 30 January 2010, the IUCN Red List of Threatened Species identified 9694 Vulnerable species, subspecies and varieties, stocks and sub-populations.-References:...
(B1+2c), as they were known from less than ten freshwater springs which are affected by pollution. Given that they occur at the other end of the species' range and are consistently most distinct morphologically
Morphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
from the "keeled" specimens, they might also warrant recognition as subspecies.