The Eurasian water shrew (Neomys fodiens), known in the United Kingdom as the water shrew, is a relatively large shrew, up to long, with a tail up to three-quarters as long again. It has short, dark fur, often with a few white tufts, a white belly, and a few stiff hairs around the feet and tail. It lives close to fresh water, hunting aquatic prey in the water and invertebrates nearby.
Like many shrews, the water shrew has venomous saliva, making it one of the few venomous mammals. It lives a solitary life and is found throughout the northern part of Europe and Asia, from Britain to Korea.
Evolution
With genetic as well as morphological evidences, the speciation of N. fodiens from other species in genus Neomys seem to be earlier than the divergence between N. teres and N. anomalus, despite a large portion of geographic range of N. anomalus overlaps with N. fodiens. Another phylogenetic study also supports such claim with phylogenetic tree and associated split times between lineages. Subsequently, several hypotheses of the evolutionary history of the genus Neomys have been made. One of the hypotheses claims that the speciation between species N. fodiens, N. anomalus, and N. teres was heavily affected by climatic oscillations in Pleistoscene, in which N. fodiens expanded southwards during cold periods, but moved back northwards in the subsequent warm periods. As a result of geographic barriers, the isolated populations in Pontic Mountains speciated into N. teres while the population in the Balkan region become N. anomalus. The female has five pairs of nipples. The legs are short and the hind feet are powerful, with a fringe of short, stiff hairs on the outer edge, both of which features assist while it is driving its body through the water. The tail is slender and has a keel of short white hairs on the underside. This shrew often utters shrill cries as it scurries about. When wet their colouration becomes less distinct, with their white undersides less noticeable. Like other shrews, N. fodiens has large and hooked first upper incisors. The species can by identified by its four unicuspid teeth and blade-like first lower incisors. It rarely strays far from water and is found in and around ditches, streams, ponds, watercress beds, fish ponds, damp meadows and rough bushy ground adjoining water. On the whole they are solitary animals that seem to mutually avoid each other and there is no social hierarchy.
The venom produced by the Eurasian water shrew has been shown to be potent against the field vole (Microtus agrestis), and lethal at a minimum dose of fifteen milligrams per kilogram body weight. The venom consists of a paralytic peptide which has been patented for use in neuromuscular therapy.
Overall, the Eurasian water shrew venom has stronger paralytic properties and lower cardioinhibitory activity. Thus acting as an immobilizing agent towards their prey or to organisms that may threaten them.
Hantaviruses have also been detected in shrews in Hungary and Poland, raising public health concerns despite epidemiological mechanisms largely unknown.
Conservation
The International Union for Conservation of Nature lists the Eurasian water shrew as being of "Least concern" in its Red List of Threatened Species. This is because it has a large population distributed across a wide range and its population seems fairly stable. In some areas habitat degradation is occurring and wetlands are being drained but not to such an extent as to increase the status to "Vulnerable". Other possible threats include agricultural products and sewage which may pollute waterways and reduce the availability of food. In western Spain, a separate subspecies (N. f. niethammeri) has a very limited range and may be declining in numbers. Climate change may pose threats on Eurasian water shrews such as a range shrift towards northern Europe or a severe habitat loss in continental Europe, depending on climate scenarios.