Distribution and habitats of Ceratophallus natalensis (Mollusca: Planorbidae) in South Africa

Abstract

Species of the genus Ceratophallus are known only from some islands in the western Indian Ocean and Africa, where they occur in Eritrea and the area extending southwards and westwards into the Western Cape of South Africa. The most recent classification recognises nine species of this genus of which only two, namely Ceratophallus natalensis (Krauss) and Ceratophallus gibbonsi (Nelson) occur in South Africa. Although partially sympatric, C. gibbonsi is largely associated with a tropical climate and perennial water-bodies while C. natalensis is fairly common in cooler areas and are often found in ephemeral rain pools. This article focuses on the geographical distribution and habitats of C. natalensis, the most widespread species of the genus, as reflected by the data on record in the National Freshwater Snail Collection (NFSC). Details pertaining to the habitats of 1 797 samples of C. natalensis as recorded at the time of collection were extracted from the database of the NFSC. In view of the finding of Brown1 that C. natalensis and C. gibbonsi are partly sympatric, but that the latter is associated with fully tropical climatic areas, samples of Ceratophallus collected in loci ( 1 / 16 th square degrees) that fall within the altitude interval ranging from 0-500 m, were selected for closer investigation. A number of 153 samples from 64 loci falling within this altitude interval was subsequently identified as C. natalensis. The number of loci in which the 1 797 collection sites were located, was distributed in intervals of mean annual air temperature and rainfall, as well as intervals of mean altitude, to illustrate the frequency of occurrence within specific intervals. A temperature index was calculated for all mollusc species in the database from their frequencies of occurrence within selected temperature intervals and the results were used to rank them in order of their association with low to high climatic temperatures. Chi-square values were calculated to evaluate the significance of the difference between the frequency of occurrence in, on, or at the different options for each variable. Additionally, an effect size value was calculated to determine the significance of the effect of all the different variables discussed in this paper on the geographical distribution of C. natalensis in South Africa. A multivariate analysis in the form of a decision tree was also constructed. This is a statistical model that enables the selection and ranking of those variables that can maximally discriminate between the frequency of occurrence of a given species under specific conditions as compared to all other mollusc species in the database.

The 1 797 samples of C. natalensis of which the collection sites could be located on a 1:250 000 topo-cadastral map series of South Africa were spread over 334 loci. This species was recovered from all types of water-bodies represented in the database but the largest percentage was reported from dams (31.2%), streams (22.2%) and rivers (15.7%). The majority of samples came from habitats with perennial (60.0%), standing (62.9%), clear (63.8%) and fresh (76.7%) water, while 42.1% of the samples were recovered from habitats with a muddy substratum. More than 95% of the samples were reported from sites that fell within the temperature interval ranging from 16-20°C and the majority of samples (64.8%) came from sites falling within the rainfall interval ranging from 601–900 mm. More than 50% of the samples came from sites falling within the altitude interval ranging from 1 001–1 500 m and this differed significantly from the frequency of occurrence within all the alternative altitude intervals. The temperature index calculated for C. natalensis ranked it in the eighth position for all species in the database due to its association with low climatological temperatures. However, the effect size values calculated for this index for 18 of the other 53 species in the database did not differ significantly from the value calculated for C. natalensis. A moderate to large effect size value was calculated for temperature and altitude suggesting that these two variables played an important role in the geographical distribution of C. natalensis, a finding also substantiated by the results of the decision tree analysis. Harrison8 divided the riverine and stream invertebrates of southern Africa into two main groups – a so-called old element with Gondwanaland affinities with relatives in other southern continents and a Pan-Ethiopian (Sub-Saharan) element. Also included in this last element are some species associated with temperate climate and sometimes with mountains, that obviously related to the African fauna. According to this author this group includes the following four sub- groups: (1) widespread species found in tropical and temperate climates, (2) warm stenothermal, tropical species, (3) Highveld-temperate climate species, (4) montane, cold stenothermal species and (5) temporary mountain stream species. According to Brown9 C. natalensis could be placed in the more or less eurythermal sub-group (1). This is supported by the geographical distribution of the collection sites of the samples of C. natalensis depicted in figure 1. Presently this species is the only member of the subfamily Planorbinae reported from Lesotho and it is currently on record from nine loci in that country in our database. It is therefore not surprising that the temperature index calculated for this species ranked it under the eight species in the database most closely associated with cooler climatic conditions. It is reported in literature that this species can utilise a wide variety of habitats including ephemeral water-bodies like marshes, slow-flowing streams, natural depressions and ditches only briefly filled with water. This is supported by the results of the present investigation which indicated the presence of this species in all types of water-body on record in the database and that 26% of the samples came from temporary habitats. It is not known whether C. natalensis can serve as intermediate host for any human helminth parasite. However, Loker et al. 12 reported the shedding of 11 different trematode cercariae from naturally infected snails in Tanzania and Frandsen and Christensen13 mention at least 10 different cercariae which could be shed from species of Ceratophallus under natural conditions. To our knowledge no efforts have been made to establish the possible role of C. natalensis as intermediate host of economically important helminth parasites in South Africa. In view of its relatively wide distribution in this country and the reports that it can act as intermediate host of a variety of helminth parasites elsewhere in Africa, it is recommended that its role as potential intermediate host in South Africa should be investigated.