Eurasian water shrew

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.^[4] Another phylogenetic study also supports such claim with phylogenetic tree and associated split times between lineages.^[5] 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.^[4]

Its karyotype has 2n = 52 and FN = 98.^[1]

The Eurasian water shrew grows to a length of about 10 cm (4 in) long with a tail length of 8 cm (3 in) and weight of 15 to 19 grams (1⁄2 to 5⁄8 oz). The dense short fur on the head, back and sides is greyish-black. The underparts are dirty white and are sharply demarcated from the dorsal surface. Sometimes they are tinged with rusty brown or occasionally are entirely dark grey. There is a white spot just behind the eye and often another near the small, rounded ear which is nearly hidden in the fur. The nose is black and the snout long and tapering.^[6] 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.^[6][7] When wet their colouration becomes less distinct, with their white undersides less noticeable.^[7]

The Eurasian water shrew is found throughout Europe with the exception of Iceland, Ireland, certain Mediterranean islands and the Balkans.^[6] In Asia, its range extends from western Siberia and Asia Minor to North Korea and the Pacific coast of Siberia.^[10] 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.^[6]

The Eurasian water shrew is most active near dawn and primarily nocturnal. It is thoroughly at home in the water and able to dive up to two meters underwater. This is despite its high buoyancy that requires it to leap to get below the surface.^[7] Its short fur holds air, and the skin does not get wet when it swims. When it emerges from the water it enters one of its many burrows and any moisture adhering to the fur is absorbed by the earth walls.^[10] It generally remains within a home range of about 500 square meters although this range changes periodically throughout the year. When foraging on land the Eurasian water shrew is known to travel up to 3 kilometers.^[7]

The Eurasian water shrew mostly feeds on aquatic organisms which are caught while it is swimming. It can remain underwater for twenty seconds before it has to surface to breathe.^[10] When attacking prey, it will salivate profusely, producing its neurotoxic saliva which it attempts to deliver at the base of the skull. Even prey larger than the shrew can be subdued by the toxic secretions allowing it to fulfill its high metabolic needs.^[9] They feed on aquatic crustaceans, freshwater snails, other mollusks, small fish, aquatic larvae, aquatic insects, spiders, amphibians, especially frogs and newts, and small rodents are also eaten.^[7][10] It also feeds on land on invertebrates such as insect (dipteran) larvae, beetles, worms, and millipedes.^[7][10] According to a study, N. fodiens showed a higher consumption frequency of small invertebrates (larvae) compared to medium-sized earthworms and beetles. Additionally, medium-sized prey were consumed more gradually than small prey items (earthworms versus larvae and earthworms versus beetles).^[12]

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.^[13] The venom consists of a paralytic peptide which has been patented for use in neuromuscular therapy.^[14] 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.^[15]

Hantaviruses have also been detected in shrews in Hungary and Poland, raising public health concerns despite epidemiological mechanisms largely unknown.^[16][17]

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.^[2]

Eurasian water shrews in alpine lakes are facing pressure from introduced fishes as they reduce the availablility of aquatic invertebrates--the major prey of the shrews.^[18] 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.^[19]