Biodiversity census of Lake St Lucia, iSimangaliso Wetland Park (South Africa): Gastropod molluscs

Abstract

The recent dry phase experienced by the St Lucia estuarine system has led to unprecedented desiccation and hypersaline conditions through most of its surface area. This has changed only recently, at the end of 2011, with the onset of a new wet phase that has already caused a major shift to oligo- and mesohaline conditions. The estuary mouth, however, remains closed to the ocean, making the weak connection recently established between the St Lucia and the Mfolozi estuaries the only conveyance for marine recruitment. As a result, only 10 indigenous and two alien aquatic gastropod species are currently found living in the St Lucia estuarine lake. This is out of a total of 37 species recorded within the system since the earliest survey undertaken in 1924, half of which have not been reported in the literature before. The tick shell, Nassarius kraussianus, which was consistently found in large abundance prior to the recent dry phase, appears to have temporarily disappeared from the system, probably as a result of the extinction of Zostera marine grasses inside the lake. Population explosions of the bubble shell Haminoea natalensis, with its distinct egg masses, were recorded seasonally until 2009, but the species has subsequently not been observed again. A molecular DNA analysis of the various populations previously reported as belonging to the same assimineid species, variably referred to as Assiminea capensis, A. ovata, or A. bifasciata, has revealed that the St Lucia assemblage actually comprises two very distinct taxa, A. cf. capensis and a species provisionally referred to here as “A.” aff. capensis or simply Assimineidae sp. In the mangroves, the climbing whelk Cerithidea decollata is still found in numbers, while ellobiids such as Cassidula labrella, Melampus semiaratus and M. parvulus are present in low abundances and all previously recorded littorinids have disappeared. A number of alien freshwater species have colonized areas of the system that have remained under low salinity. These include the invasive thiarid Tarebia granifera, which can be found in concentrations exceeding 5000 ind.m-2, the lymnaeid Pseudosuccinea columella and the physid Aplexa marmorata.

First record of the sea slug Stylocheilus striatus (Quoy & Gaimard, 1825) (Anaspidea, Aplysiidae) and swarming behavior for Bazaruto Archipelago, Mozambique with the first record of Pleurobranchus forskalii Rüppel & Leuckart, 1828 (Nudipleura, Pleurobranchidae) for Bazaruto Island (Gastropoda, Heterobranchia)

“…The observed mass-swarming phenomenon is not uncommon for S. striatus within its circumtropical range (Apte 2009, Bidgrain 2005, de Vargas Ribeiro et al 2017, and has been reported for other tropical heterobranch species as well (Rudman 2001, Perissinotto et al 2014. It is known to be locally common in sea grass beds and shallow water, particularly during mating season (Yonow 2012, Bidgrain 2005.…”

“…1): Stylocheilus striatus (Quoy & Gaimard, 1825) (Bebbington 1974, Yonow 2012. Its South African distribution extends from Mngazana in the Eastern Cape to southern Mozambique (Gosliner 1987, Perissinotto et al 2014. All these sites are further than 575 km from Bazaruto Island, with the closest sighting reported from Inhaca Island, Mozambique (Macnae and Kalk 1962).…”

Alien Freshwater Fauna in South Africa

“…Research on the trophic niche of these snails has, for example, found minimal evidence for direct food resource competition with native benthic macroinvertebrates (Miranda and Perissinotto 2014;Hill et al 2015). However, the exceptionally high densities reported from invaded environments may indirectly limit energy transfers within a food web (Hill et al 2015), and can result in decreased benthic macroinvertebrate biodiversity (Facon and David 2006;Perissinotto et al 2014). Native predators of gastropods may also be impacted through the replacement of native snail species as they may be unable to feed on the invader as they lack the ability to break the harder shell of T. granifera (Miranda et al 2016).…”

Description of a new species of Potamonautes MacLeay, 1838, from the iSimangaliso Wetland Park, South Africa

“…The iSimangaliso Wetland Park forms the southernmost region of the Maputaland centre of endemism and constitutes the focus of biodiversity conservation in the region ( Smith et al 2008 ). Recent ecological and biodiversity surveys of Lake St Lucia have been conducted in an attempt to update local taxonomic records, identify undescribed species, highlight the change in diversity over time, and provide illustrated and annotated checklists to use as identification tools ( Nel et al 2012 ; Peer et al 2014 ; Perissinotto et al 2014 ). Surveys have revealed the existence of an undescribed species of Potamonautes along the western shores of False Bay (Fig.…”