Nelson A. F. Miranda

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.

Environmental influences on living marine stromatolites: insights from benthic microalgal communities

Extant marine stromatolites act as partial analogues of their Achaean counterparts, but are rare due to depleted ocean calcium carbonate levels and suppression by eukaryotic organisms. Unique, peritidal tufa stromatolites at the interface between marine and freshwater inputs were discovered in South Africa in the past decade. Our aim was to investigate the benthic microalgal community (green algae, diatoms and cyanobacteria) of these stromatolites to assess succession and dominance patterns using real-time, in situ measurements of algal concentrations and composition. These biological measurements were modelled using generalized linear modelling (GLM) multivariate statistics against water physical and chemical parameters measured at regular monthly intervals, from January to December 2014. Salinity peaked and temperature dipped in winter, with both correlated to microalgal community change (GLM: P < 0.01). Diatoms and cyanobacteria, which construct the stromatolites, were consistently the dominant groups within the algal community, with minimal green algae present throughout the year. Importantly, this demonstrates a unique, relatively stable microalgal stromatolite community as opposed to those of other marine stromatolites, which likely require seasonal and stochastic disturbance to persist. This has implications in terms of interpreting community succession and differential layering in modern and fossilized stromatolites respectively.

Non-reliance of metazoans on stromatolite-forming microbial mats as a food resource

Grazing and burrowing organisms usually homogenise microalgal mats that form on benthic sediments of many aquatic ecosystems. In the absence of this disruption, microalgal mats can accrete laminated deposits (stromatolites). Stromatolites are rare in modern coastal ecosystems, but persist at locations where metazoans are largely excluded. This study aimed to assess the trophic structure at stromatolite locations where metazoans co-occur, to determine the grazing influence exerted by the metazoans on the stromatolite-forming microalgae (cyanobacteria and diatoms). Stable isotope signatures (δ13C and δ15N) were used as food-web tracers and dietary composition of consumers was calculated using source mixing models. Results clearly demonstrate that the dominant macrofaunal grazers do not utilise stromatolite material as a food resource, but rather subsist on autochthonous macroalgae. For instance, the mean (±SD) dietary composition of two of the most abundant grazers, Melita zeylanica (Amphipoda) and Composetia cf. keiskama (Polychaeta), consisted of 80 ± 11% and 91 ± 7% macroalgae, respectively. This suggests that the stromatolite-forming benthic microalgae are not disrupted significantly by grazing pressures, allowing for the layered mineralisation process to perpetuate. Additionally, grazers likely have a restrictive influence on pool macroalgae, maintaining the competitive balance between micro- and macroalgal groups.

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

Abstract A new species of freshwater crab, Potamonautes isimangaliso sp. n., is described from the western shores of False Bay, Hluhluwe, within the iSimangaliso Wetland Park, South Africa. While bearing a superficial resemblance to Potamonautes lividus, the new species has been found to be genetically distinct, diverging from the former by 7.4–7.8% in mtDNA. Potamonautes isimangaliso most closely resembles Potamonautes lividus, but is distinguished by a unique suite of carapace characters, colouration, and size. The new species also lives in close association with oxygen-poor, fresh ephemeral pans, while the habitat of Potamonautes lividus is well above the surface water line of the closest water body. An updated identification key for the Potamonautes species of South Africa is provided.

Genetics and shell morphometrics of assimineids (Mollusca, Caenogastropoda, Truncatelloidea) in the St Lucia Estuary, South Africa

Abstract The Assimineidae are a family of amphibious microgastropods that can be mostly found in estuaries and mangroves in South Africa. These snails often occur in great numbers and are ecologically important to the St Lucia Estuary, which forms a crucial part of the iSimangaliso Wetland Park, a UNESCO World Heritage Site. Genetic and shell morphometric analyses were conducted on individuals collected from nine localities distributed from the northern lake regions to the southern lake and the mouth of the St Lucia estuarine lake. Mitochondrial (COI) and nuclear (28S) DNA was used to construct Bayesian Inference, Neighbour-joining, Maximum Parsimony and Maximum Likelihood trees. Principal Component Analysis and Cluster Analysis were performed on standard shell parameter data. Results indicate that two different taxa are present in St Lucia. The taxon comprising individuals from the South Lake and St Lucia Estuary Mouth is identified as Assiminea cf. capensis Bartsch, in accordance with the latest taxonomic consensus. The taxon comprising assimineid individuals from False Bay, North Lake and South Lake, is here tentatively named “Assiminea” aff. capensis (Sowerby). These two taxa exhibit patterns of spatial overlap that appear to vary depending on environmental parameters, particularly salinity. The need to resolve the complex taxonomy of assimineids is highlighted.

Phytoplankton community dynamics within peritidal pools associated with living stromatolites at the freshwater–marine interface

Recently-discovered peritidal stromatolite ecosystems in South Africa form at the interface of freshwater seeps and the ocean intertidal zone, sharing several similarities with both tidal pool and estuarine ecosystems. While the overall ecology of tidal rock pools has been well studied, the dynamics of the phytoplankton assemblage have been comparatively neglected. In addition, there are no studies to date which describe the dynamics of phytoplankton within a habitat associated with stromatolites. The aim of this study was to investigate the coarse-scale phytoplankton community composition of a series of peritidal pools associated with living stromatolites, using a spectral fluorescence analysis tool, in relation to source-specific drivers related to both freshwater and marine forces. Three sites were sampled monthly from January to December 2014. Physico-chemical, biotic and meteorological parameters were recorded to assess some of the factors which might influence the phytoplankton size-fractionation and community composition using a generalised linear modelling approach. Results indicate that fresh or marine pool state, temporal differences associated with season, macronutrients (N and P), and benthic microalgal biomass are important drivers of the phytoplankton assemblages. Specifically, a transition from fresh to marine pool conditions resulted in an increased abundance of smaller phytoplankton size fractions and a shift from Chlorophyta and Cyanophyta to Bacillariophyta and Cryptophyta. Overall, the community was dominated by Chlorophyta and Bacillariophyta. There was consistency between the drivers and composition of the phytoplankton community compared to those from the few other comparable published studies. Furthermore, this study demonstrates a system which is dominated by benthic rather than pelagic microalgae in terms of biomass, thereby supporting the persistence of actively accreting stromatolites.

A review of fiddler crabs (genus Uca Leach, 1814) in South Africa

Fiddler crabs occur globally across tropical and subtropical coastal habitats including mangrove swamps, which are in decline worldwide. The genus has been well-studied in the Americas, Australia and Asia, whereas information on African species is scarce. This review summarises the existing literature on fiddler crabs and highlights gaps in research for species at their southernmost distribution ranges in South Africa. Biological and ecological aspects including life cycle and reproduction, feeding ecology, biotic interactions and tolerance to environmental fluctuations and pollution are discussed. The systematics and distribution of the five fiddler crab species that occur in South Africa is provided. The St Lucia estuarine lake is presented as a case study for the effects of environmental change on Uca populations. Future research should focus on less-studied Uca populations, such as those found in the Indo-Pacific region. The effects of climate change and habitat modification on Uca populations should also be more widely investigated.

Benthic assemblages of wetlands invaded by Tarebia granifera (Lamarck, 1822) (Caenogastropoda: Thiaridae) in the iSimangaliso Wetland Park, South Africa.

Tarebia granifera is a freshwater/estuarine gastropod invading many tropical and sub-tropical areas around the world. This snail is native to southeast Asia and was accidentally introduced into South Africa during the last decade. The current study investigated shallow-water benthic assemblages of different invaded and uninvaded localities across locations spanning a large range of environmental conditions in the iSimangaliso Wetland Park. Using a correlation-based approach, we found that native benthic assemblages were more closely associated with environmental conditions than with densities of T. granifera. However, there were significant negative correlations between T. granifera abundance and Shannon Diversity at two of the invaded locations. This alien species has successfully invaded, and become dominant in, different types of water bodies with different assemblage compositions and physico-chemical characteristics, ranging from freshwater ponds to saline estuaries and lakes. The current data set is presented as an essential baseline for future studies. It is recommended that future work focuses on specific localities, in order to determine if changes in diversity are driven by non-native species or by other disturbances (e.g., climate change).

Feeding dynamics of the fiddler crab (Uca annulipes) in a non-tidal mangrove forest

To investigate the lack of tidal influence on the feeding dynamics of fiddler crabs, we used an in situ gut fluorescence technique to measure gut pigment content of Uca annulipes in the non-tidal mangrove habitat of the St Lucia Estuary. Measurements were taken over a 24-h cycle and in the two extreme seasons, austral summer and winter, to examine any diel and seasonal shifts in feeding. Three hour gut evacuation experiments were conducted to determine the gut evacuation rate and potential sexual differences in feeding. It was found that under lack of tidal fluctuations, U. annulipes feeding is influenced by diel rhythms. In summer, males displayed a bimodal pattern of feeding, becoming more active in the morning and late afternoon with a gut evacuation rate of 0.795h–1, whereas females remained generally inactive and displayed short bouts of feeding during the day with a gut evacuation rate of 0.322h–1. The summer grazing impact of U. annulipes on microphytobenthos was higher compared with winter. In winter both sexes were fairly inactive, but displayed a greater consumption efficiency (65% compared with 45% in summer). U. annulipes feeding dynamics in a non-tidal habitat are shown to vary seasonally, daily and among sexes.

Chemical cues released by heterospecific competitors: behavioural responses of native and alien invasive aquatic gastropods

Chemical cues released by organisms have the potential to impact the behaviour of species, particularly in aquatic environments. These interactions affect the dynamics of resource utilisation at a small scale. Chemical cues may also be significantly involved in heterospecific competition and in reducing biotic resistance thus facilitating invasion success. This occurs if one species causes another to vacate optimal niches. This study aimed to assess (1) the behavioural responses of native gastropods to chemical cues released by other native gastropod species with which they did not co-occur, and (2) whether native gastropods from different localities exhibit similar behavioural responses to chemical cues released by the alien invasive Tarebia granifera, irrespective of their prior exposure to the alien. Behavioural responses were measured using an experimental approach both in situ and in the laboratory. Principal Components Analyses were used to visualize the variation in the behavioural responses of gastropods which were measured in terms of displacement, number of steps and turning angles. Gastropod behaviour was similar for populations from different sites, irrespective of their prior exposure to T. granifera. The behaviour of native gastropods was only different when exposed to chemical cues naturally released by T. granifera. Additionally, native gastropods did not behave differently when exposed to the chemical cues of an alien physid, Aplexa marmorata. This suggests that a change in behaviour when exposed to chemical cues of T. granifera may not be the result of having different evolutionary histories, but rather that T. granifera has evolved a unique metabolite which acts a deterrent to potential competitive snails. The role of chemical cues in mediating interactions between native and invasive species needs further research, as they may be important factors influencing biotic resistance and facilitating invasion success.