Distribution and habitats of Pisidium viridarium Kuiper, 1956 (Bivalvia: Sphaeriidae) as reflected by the records of the National Freshwater Snail Collection of South Africa
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. The 132 different loci from which the samples were collected, display a relatively continuous distribution in the south-eastern part of the North West Province, the northern part of Gauteng, the central part of Mpumalanga and Lesotho. P. viridarium is sporadically distributed in the Eastern Cape and Free State, poorly represented in the Northern and Western Cape and completely absent from Limpopo. Its absence in certain areas of South Africa should be attributed to unfavourable environmental conditions rather than to a lack of opportunities to disperse: there are many agents and frequent opportunities for passive dispersal reported in literature for Sphaeriidae and the presence of P. viridarium in this country had already been recorded in 1950.This species was reported from 13 of the 14 habitat types represented in the database. However, the majority of samples were collected in marshes and in habitats of which the water conditions were described as clear, fresh and standing. The decision tree analysis indicated that temperature, altitude, type of water-body and substratum were the most important factors, of those investigated, that influenced the geographical distribution of this species in South Africa. It is known from reports in literature that bivalves can accumulate heavy metals from superficial sediments and the fact that P. viridarium is a filter feeder, a bottom dweller and can utilize a relatively large variety of habitats under a wide range of climatic conditions, could make it a useful candidate for monitoring heavy metal levels in freshwater habitats. It is recommended that the feasibility of such a possibility be investigated. In view of the reports from elsewhere in the world that Pisidium species can serve as intermediate hosts for helminth parasites that can affect the well-being of humans and animals, it is suggested that the potential of P. viridarium toact as intermediate host for harmful helminth parasites be evaluated. Efforts should also be made to update the geographical distribution of molluscs especially those that could play an important role in the life cycle of economically important helminth parasites.