LITTORINIDAE, Children, 1834

Mead, A., Carlton, J. T., Griffiths, C. L. & Rius, M., 2011, Introduced and cryptogenic marine and estuarine species of South Africa, Journal of Natural History 45 (39 - 40), pp. 2463-2524 : 2491-2492

Family LITTORINIDAE

Littorina saxatilis (Olivi, 1792) Introduced (= Littorina punctata of authors; = Littorina rudis Maton, 1797 )

The history of this well-studied, North Atlantic, intertidal snail in South Africa remains to be fully explored. Kilburn (1972) was the first to properly recognize this species in the modern-day marine fauna of South Africa. He noted that it had previously been identified as Littorina punctata in Langebaan (west coast) by Barnard (1963) and from the Berg River Estuary (west coast) and Knysna Lagoon (southeast coast) by Day (1969). In addition, a fourth African population is known from Luderitz, in Namibia ( Reid 1996). Kilburn (1972) described the morphology and colour of the Langebaan Lagoon snails and noted that the population “was found to be a very large and well-established one. In habitat it occurred chiefly on firm, slightly muddy sandflats in the upper midtidal region, especially on Zostera beds.” Kilburn remarked that as it had been previously known only from the North Atlantic “it at first appeared probable that it had been introduced.” However, he then noted that “Subsequently I have material collected from Pleistocene beds on the Cape Flats and adjacent areas, which indicates that the population is an indigenous one.” Reid (1996) noted that Littorina saxatilis has not been found alive in the immediate Cape Town area. Reid (personal communication, 2008) reported that he had examined “possibly subfossil samples from raised beaches” in the Cape Town area; this may be the same material to which Kilburn refers (Kilburn notes that he was in communication with a “Mr S. Fenwick,” and the samples examined by Reid were collected by S. Fenwick). The shells are at the Natal Museum and are described as being from the “bed of the Diep River, Table Bay,” and from the “shores of Zandvlei, Muizenberg.” The shells have not been radiocarbon dated, nor are there any further data on the actual age of the strata from which the shells were recovered.

One year later, Schalke (1973) reported that Littorina saxatilis (the number of specimens is not mentioned) were found in boreholes at Rietvlei, immediately north of Table Bay. The snails were said to be found in two horizons in a borehole, with one level antedating 45,000 years before present, and the other with an age range of 40,500 to 36,500 years before present. Strata level ages were determined from radiocarbon datings and pollen analyses, but the Littorina saxatilis shells themselves were not aged.

Hughes (1979; as Littorina rudis ) then reported on further details of morphology, colour and reproduction of the Langebaan and Knysna populations, and on searches for Littorina saxatilis at other sites (none was found). Knight et al. (1987) undertook genetic analyses of the South African populations, comparing them with both North American and European material; South African Littorina saxatilis “showed a severely reduced heterozygosity compared with Atlantic populations.” Both Hughes (1979) and Knight et al. (1987) suggested ship-mediated introduction.

Reid (1996) provided a detailed review of the history of the occurrence of Littorina saxatilis in South Africa, noting the reported fossil material, the known living populations, and previous hypotheses (relictual but natural distribution, or human-mediated introduction) that had sought to explain the presence of this species in the South Atlantic Ocean. Reid offered a third hypothesis, that migrating birds may have carried Littorina saxatilis from Europe to Africa. Reid also noted the existence of a fourth southern Africa population, in Luderitz, in Namibia, based upon Natal Museum material.

We consider that the most probable origin of the modern-day populations of Littorina saxatilis in Namibia and in South Africa is human-mediated introduction, possibly in the days of wooden sailing ships transporting shore ballast from Europe. Genetic analyses are required to match the South African populations with North Atlantic populations, not only to determine possible origin, but also to determine whether unique haplotypes exist in the former, and if so how many, to determine (by molecular clock estimations) how long this snail has been in the southern hemisphere. Although it is not likely that snails would survive on birds on the wing from Europe, its presence in locations such as the Berg River Estuary could well be accounted for by post-introduction dispersal by birds within South Africa ( Kalejta and Hockey 2008).

The ostensible fossil material from the Cape Town area is not dated, and could represent Holocene occurrences; if so, these could represent specimens transported out of a region such as Saldanha Bay (Langebaan Lagoon), or introduced populations from Europe that failed to survive. Of more interest certainly are the Pleistocene Rietvlei specimens: these would bear re-examination and verification as Littorina saxatilis , and it would be of no small interest to perform radiocarbon testing on the shells. Even if these prove to be Littorina saxatilis with good stratigraphic control, we suspect that there is no link between these fossils and modern-day populations in South Africa. Had Littorina saxatilis become established tens of thousands of years ago in South Africa, it would have long since become very widespread, despite its lack of planktonic larvae (given time, Littorina saxatilis are transported by floating materials, for example, or simply expand their range by moving along coastlines for aeons); instead, it remains highly restricted to a few locations, suggestive of relatively recently established populations.

We have as yet no first date of record of living Littorina saxatilis populations in South Africa. The dates of collection of Barnard’s (1963) specimens (identified as Littorina punctata ) from Langebaan, of other material, and of the Namibia population, remain to be determined (we note that Museum material should be searched for under both the name Littorina punctata and other names as well).