Folia Historico-Naturalia Musei Matraensis - A Mátra Múzeum Természetrajzi Közleményei 10. (1985)
De Wilde, J. J. — Van Goethem, J. L. — Marquet, R.: The distribution of the species of the genus Deroceras Rafinesque, 1820 in Belgium (Gastropoda, Pulmonata, Agriolimacidae)
Fol. Hist.-nat. Mus. Matr. 10. 1985. The distribution of the species of the genus (yCjJ Deroceras RAFINESQUE, 1820 in Ве1 ё тпУ шъз (Gastropoda, Pulmonata, Agriolimacidae) J. J. DE WILDE, J. L. VAN GOETHÉM, R. MARQUET Brüssel, Antwerpen ABSTRACT: Four species belonging to the genus Deroceras are known from Belgium: D. reticulatum /MÜLLER, 1774/, D. laeve /MÜLLER, 1774/, D. caruanae /POLLONERA, 1891/ and D. agreste /LINNAEUS, 1758/. The distribution of D. reticulatum , P. laeve and D. caruanae as known at the end of 1971 is compared with a map showing all records of landsnail species in Belgium till then. A similar comparison is given for the year 1982. From the period 1972-1982 year to year maps- showing all records of each of these three Deroceras species are compared with a corresponding map compiling all landsnail records of the same year. At present P.reticulatum , D.laeve and D. caruanae are very widespread and common. D. caruanae was found for the first time in Belgium in 1968. As shown by means of a series of cumulative maps, D. caruanae became very rapidly widespread /1973 onwards/ presenting an explosive dispersal . Undoubtedly ,man assisted in its introduction and dispersal. P. agreste seems to be very rare in Belgium with a limited number of local populations in the four southern provinces. INTROPUCTION The last survey of the Belgian non-marine molluscs was given by W. АРАМ /1947; 1960/. As part of his contribution to the E.I.S., one of us /J. VAN GOETHÉM/ assisted by members of the Recent Invertebrates Section of the K.B.I. N., started a new malacological survey in Belgium. From 1970 numerous explorations were made each year, according to a program for covering the whole Belgian territory within the next 10 or 15 years /e.g. figs 13,17,21,... give an idea of the sampled U.T.M. squares for the respective years/. After some years we hit upon the idea to compile not only locality data but also to record information on collecting-sites and habitats. This would further our knowledge on the autecology of the species and their occurrence in time. The object became now to study a changing fauna and to detect the dynamic processes involved as for instance introductions, decreasing populations, changes in densities, migrations, etc. For this new, comprehensive project : "An extensive faunistic and ecological survey on landsnails in Belgium /1977-1982/", a grant was obtained from the Belgian Fund for Joint Basic Research. This survey was made by the "Laboratorium voor Algemene Dierkunde" of the "Rijksuniversitair Centrum Antwerpen", directed by Prof. Dr. W. N. VERHEYEN, and by the Recent Invertebrates Section of the "Koninklijk Belgisch Instituut voor Natuurwetenschappen" , directed by Dr. J. VAN GOETHÉM [see J. VAN GOETHÉM /in press/, R. MARQUET /in press/]. The aim of this paper is to describe the occurrence and distribution of each species of Deroceras in Belgium and to investigate the dynamics of their distribution patterns. 103
