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Shells and crustaceans

 

Thick-Shelled Freshwater Mussel (Unio crassus), Vipava Valley  

Description: The thick-shelled freshwater mussel belongs to the family of freshwater mussels which live in running and standing waters of the Primorsko region. Since it feeds on suspended phytoplankton by means of filtration, the species is very sensitive to chemical pollution because the substances, dissolved in water accumulate more in such organisms than in herbivores or carnivores. With increased pollution, this species of mussel has completely disappeared from almost all watercourses in the Vipavsko region. But recently, the system of waste-water treatment plants has been improved and the thick-shelled freshwater mussel has slowly started to repopulate the brooks. Despite the regulatory protection of this species in Slovenia, the main reason for its survival has been the improvement of the environmental conditions. The most serious competition is represented by the lately introduced swan-mussel (Anodonta cygnea) which originates from the Danubian hydrographical basin. After the larvae of the thick-shelled freshwater mussel hatch, they live attached to fishes, where they also feed, sucking nutritious substances. In this way the species, which cannot swim against the current, move up the watercourses as the period of hatching coincides with the migration of fishes during the spawning season.

 

 

Freshwater White-clawed Crayfish (Austropotamobius pallipes), Vipava Valley

Description: In the middle of the 20th century, the freshwater white-clawed crayfish was a frequent species, found in every brook in the Vipavsko region, if only the brook was not intermittent. Since the Canadian waterweed (Elodea canadensis) spread across the Atlantic, the number of freshwater white-clawed crayfish populations has decreased substantially. Only isolated populations have survived, especially those in brooks with waterfalls which were a natural barrier for other specimens and disease-germs to spread up the watercourse. Only over the past few decades, the excessive spreading of the Canadian waterweed has decreased. But due to increased organic and chemical pollution, the freshwater white-clawed crayfish is still much more rare than it would otherwise be.

 

 

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