Ostracoda of the Italian ricefields thirty years on: new synthesis and hypothesis

We compare data from a survey of ostracode species carried out during 1994-1998 from Northern Italian ricefields with data from the same area collected in the ’60s. Twenty-five species were recorded during a survey of 19 ricefields in 1994-98 as against 46 species found in 16 ricefields over thirty years ago. Three of these species (Ilyocypris biplicata, Chlamydotheca incisa and Chrissia sp.), as well as six among the 27 species found in the '60s but not recorded during 1994-1998, were found in Italy only in the ricefield habitat. Three species were recorded for the first time in Italy: Hemicypris dentatomarginata, Ilyocypris monstrifica and Chrissia sp. Eight taxa (Chlamydotheca incisa, Chrissia sp., Cypretta turgida, Dolerocypris sinensis, H. dentatomarginata, Isocypris beauchampi, Strandesia spinulosa and Tanycypris pellucida) were considered endemic to South America, Africa or Asia and are thought to have been introduced to Italy with useful plants, notably rice varieties. The recording of these species indicates once again the importance of man as an agent for passive dispersal of ostracode and the role of ricefields as a suitable habitat for new exotic colonising species. Heterocypris incongruens is the most widespread species and several other species (D. sinensis, Cypridopsis vidua, Ilyocypris gibba and Isocypris beauchampi cicatricosa) were found in more than 50% of the ricefields sampled: 40% of the species occur in just one or two ricefields. In Northern Italy today, species similarity between ricefields is inversely related to their geographic distance but is not spatially structured. This may be due to the fact that, in ostracodes, the production of passive dispersal resting eggs constrains regional differentiation. Species richness is significantly related to the amount of cultivated area. The decrease in species richness observed over the last thirty years or so may be related to the widespread use of pesticides in local current agricultural practices.