Distribution and habitats of Pisidium viridarium Kuiper, 1956 (Bivalvia: Sphaeriidae) as reflected by the records of the National Freshwater Snail Collection of South Africa

摘要

The genus Pisidium includes some of the smallest bivalves in the family Sphaeriidae. The distribution of this group is cosmopolitan and they can utilize virtually any freshwater habitat,including peat bogs. Without doubt their centre of evolution lies in the Holarctic Region and theorigin of Pisidium is considered to be Mesozoic. This article focuses on the geographical distribution and habitats of P. viridarium, the most well represented species of this genus in the database of the National Freshwater Snail Collection (NFSC) of South Africa. Details pertaining to the habitats of the 639 samples of P. viridarium as recorded at the time of collection were extracted from the database of the NFSC. The number of loci (1/16 th square degrees) in which the 639 collection sites were located, was distributed in pre-selected intervalsof mean annual air temperature and rainfall, as well as intervals of mean altitude, to illustratethe frequency of occurrence within specific intervals. A temperature index was calculated for allmollusc species in the database from their frequencies of occurrence within the selected intervals and the results used to rank them in order of their association with low to high climatic temperatures. To evaluate the significance of the difference between frequencies of occurrence in, on, or at the various options for each parameter investigated, chi-square values were calculated. Furthermore, an effect size value was calculated to determine the contribution of each parameter towards establishing the geographical distribution of this species as reflected by the data in the database. Additionally, a multivariate analysis in the form of a decision tree was constructed from the data which enabled the selection and ranking of those variables that maximally discriminated between the frequency of occurrence of P. viridarium in, on, or at the various options for each parameter as compared to all other mollusc species in the database.