Abstract
This paper focuses on the geographical distribution and habitats of Gyraulus costulatus as reflected by the 736 localities currently on record in the database of the National Freshwater Snail Collection (NFSC) of South Africa. Although a wide variety of water body types was reported for this species, the largest number of samples were yielded by rivers, streams and dams which were described as perennial with predominantly fresh and clear water. A mainly stony substrate and the presence of aquatic vegetation were recorded for the majority of localities at the time of survey. An integrated decision tree constructed from the data indicated that altitude, temperature and water bodies were the most important of the factors investigated that determined the documented geographical distribution of this species in South Africa. This was supported by the effect size values calculated separately for each factor. Since the early eighties of the previous century, surveys to monitor and update the geographical distribution of freshwater snails and snail-borne parasitic diseases have been relegated to a low priority issue and this is a matter of concern. It is recommended that the exact role of G. costulatus in the epidemiology of economically important helminth parasites in South Africa should be investigated.
Highlights
Hierdie artikel handel oor die geografiese verspreiding en habitats van Gyraulus costulatus soos gereflekteer deur data wat tans in die databasis van die Nasionale Varswaterslakversameling (NVWSV) op rekord is
Reënval, temperatuur- en hoogte bo seevlakdata is goedgunstiglik deur die Computing Centre for Water Research (CCWR), Universiteit van Natal, beskikbaar gestel. ’n Temperatuurindeks is vir elke spesie in die databasis aan die hand van die voorkomsfrekwensie in die geselekteerde temperatuurintervalle opgestel en die resultate is gebruik om elke spesie in rangorde te plaas op grond van sy assosiasie met lae tot hoë klimatologiese temperature
Figuur 2: Besluitnemingsboom van die voorkomsfrekwensie van Gyraulus costulatus by bepaalde veranderlikes teenoor die voorkomsfrekwensie van alle ander spesies wat in die databasis van die Nasionale Varswaterslakversameling opgeneem is. 0 = persentasies en getalle van alle ander spesies, 1 = persentasies en getalle van Gyraulus costulatus
Summary
Inligting met betrekking tot die habitats en geografiese verspreiding van G. costulatus wat vanaf 1956 tot die hede dateer, is uit die databasis van die NVWSV onttrek. Die voorkomsfrekwensie van G. costulatus in spruite, fonteine en panne het in elk van die gevalle meer as 2.0% uitgemaak van die totale getal monsters van alle Mollusca-spesies wat in elkeen van hierdie drie waterliggaamtipes versamel is (tabel 1). Die grootste getal monsters is in habitats versamel met ’n gemiddelde jaarlikse reënval van 601 – 900 mm (tabel 4) en die frekwensievoorkoms by hierdie interval het statisties betekenisvol verskil van die ander vier intervalle (p < 0.05). Die effekgroottes wat vir die temperatuurindekse bereken is, dui daarop dat twaalf van die 53 Mollusca-spesies in die databasis in hierdie opsig nie betekenisvol (w < 0.5) van G. costulatus verskil het nie (tabel 5). TABEL 2: Watertoestande in die habitats van Gyraulus costulatus soos tydens die versameling opgeteken
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