Environmental drivers of the composition and distribution of larval fish assemblages off the south coast of South Africa
- Authors: Trassierra, Jaqueline Anne
- Date: 2019
- Subjects: Fishes -- Larvae -- South Africa -- Eastern Cape , Fishes -- Larvae -- Migration -- South Africa -- Eastern Cape , Fishes -- Larvae -- Dispersal -- South Africa -- Eastern Cape
- Language: English
- Type: text , Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/68154 , vital:29207
- Description: The species composition, distribution and patterns of vertical migration of larval fish assemblages were investigated in March and in September 2013 within two adjacent log spiral bays, Algoa Bay and St Francis Bay, on the south coast of South Africa. Fish larvae were collected by means of a boat towed bongo net (57 cm diameter; mesh aperture 500 μm). An onshore (2 km) and an offshore (3 km) station were each sampled twice during the daytime (06:00 – 18:00) with two horizontal tows: near the surface (0.5 m) and close to the bottom (12 m). Tows were repeated at night (18:00 – 23:00) for onshore sites. Larval catches included 16 fish families and 40 species. A multivariate analysis indicated that the species composition was significantly different between Algoa Bay and St Francis Bay, with Engraulidae, Blenniidae, Sparidae, Soleidae and Cynoglossidae making important contributions to the larval fish catch in Algoa Bay, while Blenniidae, Engraulidae, Tripterygiidae, Sparidae and Gobiesocidae contributed significantly in St Francis Bay. Differences in assemblage composition were noted between the Spring (September–October) and Autumn (March-April) months. The species composition of larval fish assemblages was related to wind speed, wave height, cloud cover, sea water temperature, depth, average current speed and direction. Wind speed, wave height, temperature and depth significantly contributed to the variation in larval fish densities. Abundances of larval fishes were greater offshore than onshore, larvae from pelagic eggs dominated catches offshore, while larvae from demersal eggs dominated onshore catches. Habitat structure strongly influenced the composition of larval fishes between the bays and abundances were significantly greater at night than during the day. Most larval fishes displayed a reverse diel vertical migration pattern and were most influenced by predators, wind speed and cloud cover. This study shows that larval fish assemblages are highly complex and patchy. Spawning mode, individual species behaviour, diel vertical migration, current structure, depth, temperature, wind speed, cloud cover and type of habitat substratum all influence larval fish composition and distribution in the nearshore waters of South Africa.
- Full Text:
- Date Issued: 2019
Mesoscale alongshore and cross-shore transport and settlement of invertebrate larvae on the south east coast of South Africa
- Authors: Dyantyi, Siphelele Buntu
- Date: 2019
- Subjects: Marine invertebrates -- Larvae -- Dispersal -- South Africa -- Eastern Cape , Benthic animals -- Larvae -- Dispersal -- South Africa -- Eastern Cape , Mexilhao mussel -- Larvae -- Dispersal -- South Africa -- Eastern Cape , Mytilus galloprovincialis -- Larvae -- Dispersal -- South Africa -- Eastern Cape , Oysters -- Larvae -- Dispersal -- South Africa -- Eastern Cape
- Language: English
- Type: text , Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/97007 , vital:31386
- Description: Pelagic larval stages of most marine benthic species are important in maintaining coastal populations of adults. Several physio-chemical factors such as currents, winds, larval behaviour and time have an influence on the dispersal and transport of larvae to the adult habitat but their role is however still poorly understood. The aim of this study was to investigate the alongshore and cross-shore transport and temporal delivery of invertebrate larvae at four sites (Kenton on Sea, Cannon Rocks, Schoenmakerskop and Cape St Francis) along the south east coast of Eastern Cape, South Africa. Larval distribution of several taxa was determined during two sampling periods, by collecting water samples at nearshore line transects (3 distances: 900m, 1500m and 2400m – perpendicular to the shore) and at three depths (surface, thermocline/middle, bottom), which ranged from 15m (inshore) to 50m (offshore). Physical properties (current speed and direction, dissolved oxygen, fluorescence, turbidity, temperature, salinity, pH, pressure, density and conductivity) were measured and coupled in order to further understand larval distribution. The larvae were analysed as both total abundance and separately as the abundances of a variety of taxa which were: Perna perna, Mytilus galloprovincialis, oysters, early and late nauplii and cyprids. Larval settlement and recruitment on the rocky shores were measured by monthly deployment and collection of 20 (10 each for barnacles and mussels) artificial collectors at each site, which were preserved in ethanol or frozen for further processing. Multiple Permutational Multivariate Analysis of Variance (PERMANOVA) analyses were used to test the effects of site, depth and distance from the shore for the nearshore larvae (taxa analysed separately). In addition, a distance based linear model (distLM) was performed to analyse the relationship between the total larval abundance and the above mentioned physical variables. Multiple two-way analyses of variance (ANOVA) were performed to test the effects of months and sites on the settlement and recruitment of the larvae (P. perna, M. galloprovincialis, other bivalves, cyprids and juvenile barnacles) arriving on the shore. For the nearshore larval distribution, results from the PERMANOVAs revealed that most taxa showed a significant site and depth interactions with the exception of ‘early nauplii’ taxon. Also nearly all taxa were found within the thermocline, besides ‘oyster’ and ‘cyprids’ which were located at thermocline or bottom. Larvae were also located at variable distances from the shore, with most occurring at the offshore stations. Furthermore, there was a geographical separation of larval abundance according to sites, with most larvae located at Cannon Rocks and Kenton on Sea and least at Schoenmakerskop and Cape St Francis. For the settlement and recruitment, most taxa showed a seasonal trend, with the highest abundance of settlers and recruits expectedly appearing during the summer months of the sampling period. Additionally there was a site effect for most taxa (P. perna, M. galloprovincialis, other bivalves and juvenile barnacles), where settlers and recruits were mostly found at Cannon Rocks. Significant differences in abundance of settlers and recruits amongst the four sites indicate spatial and temporal variability for the targeted 180km stretch of coast. Overall for this study, taxon and ontogenetic stage of larvae were important in the distribution and abundance of larvae. Throughout the time frame of nearshore and intertidal sampling, Cannon Rocks consistently resulted as a ‘hot spot’ for larval abundance, settlement and recruitment, while a broad west to east separation was also observed. These results hence highlight that within this stretch of c.180km coast, time, taxon, ontogeny and post-settlement factors influence early dynamics of benthic populations.
- Full Text:
- Date Issued: 2019
Assessing the impact of climate change on mangrove crabs: the role of ontogenetic macrophysiology and settlement in the persistence of central and marginal populations
- Authors: Mostert, Bruce Petrus
- Date: 2016
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/826 , vital:19994
- Description: After a brief respite in the mid to late 20th century, macro physiology has come to the fore in elucidating large scale ecological patterns and processes as physiological assumptions often form the backbone of many predictive theories associated with species distributions. Critically, macro physiological patterns are valuable in explaining physiological variation across multiple scales and provide insights into the effects of climate change on populations spanning a wide range of latitudes. This can assist in predicting possible distribution expansions, contractions or shifts in light of current climate change scenarios. From this perspective, investigating intra- and inter-specific physiological responses to environmental stress may contribute to better understanding and predicting the effects of climate change on geographical ranges. Further, investigating the physiological effects to environmental stresses across ontogenetic stages allows for the identification of weak links within the lifecycle of a species. Additionally, determining settlement characteristics along a latitudinal cline provides integrated indications of the sustainability of populations, highlighting vulnerable regions in terms of repopulation of viable habitats. In this context, the present study aimed at establishing how temperature, in a physiological context, may affect reproductive biology of two species of mangrove crab, Perisesarma guttatum and Uca urvillei at the centre (Kenya) and edge (South Africa) of their distributional range along the east coast of Africa and highlight possible consequences for range distributions. A third species, Neosarmatium africanum, only in South Africa, was included to provide additional interspecies comparisons. Furthermore, settlement characteristics of brachyuran populations at the centre and edge of their distributional range were considered in order to determine how settlement may contribute to population persistence. Physiological investigations at the centre and edge of distributional range and across ontogenetic stages (larvae, stage 2 and 4 embryos, non-gravid and gravid females) under the concept of oxygen and capacity limitation of thermal tolerance (OCLTT), revealed that, for both species, populations at the centre of their distribution (Kenya) were generally more robust to increasing temperatures and generally displayed greater physiological stability with increasing temperatures compared to their conspecifics in South Africa. Variability in physiological robustness between regions, did however, differ among ontogenetic stages and species but, overall, were evident throughout. Within and between regions, adaptation to oxygen extraction in both milieus (air or water) was displayed for present temperature conditions but aerial respiration largely alleviated increased thermal stress due to overcoming the limitations of reduced oxygen availability and diffusiveness in water for all bimodal ontogenetic stages. Brooding eggs proved to be a physiologically critical process with either heighted oxygen consumption for gravid females or collapse of physiological processes demonstrated by supressed oxygen consumption. The physiological cost of brooding eggs, referred to as maternal costs, was reflected in in both Perisesarma guttatum and Uca urvillei where, in most cases, maternal costs were negative. Again, aerial respiration was able to alleviated increased thermal stress, as shown by positive maternal costs indicating sustained maternal care, but this mechanism was species and regionally specific. Settlement patterns differed between the edge and centre of distribution of the species studied. This difference was predominantly driven by zonal preference within the mangal and/or effects of new and full moon (lunar phase). Overall, settlement dynamics were more widely variable in South Africa, both spatially and temporally, than in Kenya. Finally, empirical physiological data from ontogenetic stages present during the reproductive process (early and late stage embryos) and from non-gravid and gravid females were used in conjunction with data mined from the existing literature to parameterise an individual based model designed to simulate reproductive output at the centre and edge of distribution of Perisesarma guttatum. Physiological data indicate that, in terms of reproductive output across increasing temperatures, populations based at the centre of their distribution presently outperform their counterparts at the edge of the species’ distribution, but reproductive output stagnated as temperature rose. Edge of distribution populations consistently increased reproductive output with increasing temperatures to eventually outperform centre of distribution populations at higher simulated temperatures. Overall, results of the physiological and settlement studies suggest that with increased climate change there may be a contraction of distributional range of the study species from high latitudes to low latitudes, contrary to general poleward shifts/migrations seen in most species, with possible contractions of the entire ecosystem mirroring the disappearance of keystone mangrove macrofauna.
- Full Text:
- Date Issued: 2016
Mesoscale mechanisms of larval transport and settlement in relation to physical factors off the south coast of South Africa: a topographic approach
- Authors: Mian, Shana Iva
- Date: 2016
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5947 , http://hdl.handle.net/10962/d1020844
- Description: The majority of benthic marine invertebrate life cycles include a meroplanktonic phase. Most marine biologists recognise that this stage could play an important role in dispersal, especially for sedentary or sessile organisms. Recent studies on larval dispersal have found that dispersal does not occur to the extent that was previously believed. Some larvae instead remain close to their natal populations through larval nearshore retention mechanisms, both active and passive. This study attempted to a) describe nearshore distribution patterns of several intertidal larvae in relation to nearshore hydrodynamics, b) infer whether larvae are active or passive in the water column and whether this differs among taxa and c) compare pelagic mussel larval abundance with settlement rates, at 2 bay and 2 open coast sites on the south coast of South Africa to note any topographical effects. At each of the four sites, pelagic samples were collected along 2 transects. Larvae and current speed data were collected at various depths (surface, mid-depth and bottom) at distances of 200, 400, 900 and 2400m offshore of the coast. Settlement samples were collected at sites that were onshore of the pelagic transects using plastic souring pads following previous studies. Pelagic larval distribution showed no effect of topography. Instead larval abundance was highly variable among sample periods and among different taxa. Slow swimming D-stage mussel larvae were found to be positively correlated with both upwelling and turbulence indices suggesting that they behave as passive particles in the water column. The abundances of larval taxa with faster swimming speeds than the D-larvae were negatively correlated with upwelling and did not correlate with turbulence. This suggests that these larvae are more active within the water column than the D-larvae which would theoretically allow them to maintain their position in the water column. Further analysis revealed that mean larval depth correlated with mean depth of shoreward moving water masses regardless of the state of upwelling. These results suggest that larvae were actively selecting the depth at which they were found presumably in order to associate themselves with shoreward moving currents. This interpretation was supported by ontogenetic patterns in the behaviour of barnacle larvae, the nauplii of which were found further offshore as they aged, and closer onshore again once they reach the faster swimming cyprid stage. Although there was no detectable effect of topography on the abundances of planktonic larvae, the settlement section of this study revealed an effect of topography, with higher settlement and recruitment rates at bay sites than at the open coast sites. Settlement rates were seen to be negatively correlated with turbulence at bay sites but not open coast sites. Recruitment rates for each sample event were positively correlated with upwelling at each site. This study found pelagic larvae to be generally active within the water column which allowed them to remain close to the shore regardless of topography and upwelling/turbulence regimes. Although meaningful correlation analyses between pelagic larval supply and settlement rates could not be conducted due to loss of settlement collectors, settlement rates were affected by local topography with higher settlement and recruitment in bay sites.
- Full Text:
- Date Issued: 2016
Inter-individual variability and phenotypic plasticity : the effect of the environment on the biogeography, population structure, ecophysiology and reproduction of the sandhoppers Talorchestia capensis and Africorchestia quadrispinosa
- Authors: Baldanzi, Simone
- Date: 2014
- Subjects: Phenotypic plasticity -- Research -- Africa, Southern Talitridae -- Research -- Africa, Southern Amphipoda -- Research -- Africa, Southern Climatic changes -- Environmental aspects -- Africa, Southern Marine biology -- Africa, Southern Adaptation (Biology) -- Africa, Southern
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5846 , http://hdl.handle.net/10962/d1011447
- Description: Climatic envelope models focus on the climatic variables affecting species or species assemblages, and are important tools to investigate the effect of climate change on their geographical ranges. These models have largely been proposed in order to make successful predictions on species‘ persistence, determining which variables are likely to induce range expansion, contraction, or shifting. More recent models, including the ability and the cost for individuals to respond promptly to an environmental stimulus, have revealed that species may express phenotypic plasticity able to induce adaptation to the new environment. Consequently, understanding how species evolve to a changing climate is fundamental. From this perspective, investigating intraspecific responses to an environmental variable may contribute to better understanding and prediction of the effect of climate change on the geographical range and evolution of species, particularly in the case of widespread species. In this context, the present study aimed at establishing how environmental variables (focussing mainly on temperature) may have contributed to shape the spatial distribution, physiology, reproductive biology and connectivity of two species of Southern African sandhoppers (Talorchestia capensis and Africorchestia quadrispinosa, Amphipoda, Talitridae). Most of the work was carried out on T. capensis, due to its widespread spatial distribution. A first investigation of the biogeography of T. capensis and A. quadrispinosa, revealed that, for both species, spatial patterns of abundance, size and sex ratio were not explained by the Abundant Centre Hypothesis (greater abundance at the core of a spatial range), but rather guided by bio-physical forces. Precisely, the abundance of sandhoppers was driven by the morphodynamic state of the beach, salinity and temperatures, with strong differentiation among sites that reflected local environmental conditions. In support of these findings, strong population structure in the genetics of T. capensis was found (three main groups) when investigating its phylogeography and genetic connectivity. Although such defined structure may suggests cryptic speciation, the concomitant within-population variation in the COX1 region of mtDNA, also highlighted the importance of individual genetic variability. High individual variability was also found in the response of T. capensis to temperature, both in its physiology (thermal plasticity) and its reproductive biology (maternal effects). Since temperature is one of the main variables affecting the coastal marine systems of southern Africa and the metabolism of animals in general, its effect on the physiology and reproduction of T. capensis was therefore investigated. Thermal responses to increasing/decreasing temperatures were assessed for separated populations of T. capensis. Individual variability was reported in the oxygen consumption of T. capensis in response to temperature (high variation around the means, especially for increasing temperatures). Among population differences in thermal sensitivity were significantly correlated with air temperature variability experienced over the past 23 years, highlighting the importance of historical temperature fluctuations to the current thermal physiology of these sandhoppers. Temperature also had an important effect on the reproductive plasticity of T. capensis. Different temperatures induced mothers to adjust the size of their offspring (i.e. egg size), with larger eggs produced at lower temperatures. Interestingly, females showed strongly significant among individual variation in the size of the eggs. Given the importance of understanding rapid responses of organisms to climate change and considering the fundamental role played by phenotypic plasticity in evolution, the overall study revealed the significance of individual plasticity and variability in response to the environment and highlighted its importance. Particularly, studying the thermal physiology of separated populations and understanding within population reproductive plasticity in response to temperature, helped to clarify how differences among individual responses have important consequences at the population level, possibly explaining the widespread distribution of T. capensis.
- Full Text:
- Date Issued: 2014
The epibiotic relationship between mussels and barnacles
- Authors: Bell, Caroline Margaret
- Date: 2014
- Subjects: Epibiosis , Mussels -- Ecology -- South Africa , Mussels -- Behavior -- South Africa , Mussels -- Habitat -- South Africa , Barnacles -- Ecology -- South Africa , Barnacles -- Behavior -- South Africa , Barnacles -- Habitat -- South Africa , Perna -- Behavior -- South Africa , Mytilus galloprovincialis -- Behavior -- South Africa , Marine biodiversity -- South Africa
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5847 , http://hdl.handle.net/10962/d1011460 , Epibiosis , Mussels -- Ecology -- South Africa , Mussels -- Behavior -- South Africa , Mussels -- Habitat -- South Africa , Barnacles -- Ecology -- South Africa , Barnacles -- Behavior -- South Africa , Barnacles -- Habitat -- South Africa , Perna -- Behavior -- South Africa , Mytilus galloprovincialis -- Behavior -- South Africa , Marine biodiversity -- South Africa
- Description: Epibiosis is an ecological relationship that has been described as one of the closest possible associations in marine ecosystems. In the space limited rocky intertidal, mussel beds provide important secondary space for barnacles. The epibiotic relationship between mussels and barnacles on the south-east coast of South Africa was considered at different scales, from large-scale, natural patterns of epibiosis on the rocky shore, to fine-scale settlement choices of barnacles and the effects on the condition and growth rates of individual mussels. Mussel and barnacle assemblages were generally stable over a 12-month period. The tracking of individual mussels with and without barnacle epibionts resulted in a significant increase in mortality rate of mussels with epibionts over 12 months (two-way ANOVA, p = 0.028). Barnacles on rocks, as well as on mussels, were also tracked with no significant effect of substratum on mortality of barnacles (two-way ANOVA, p = 0.119). Prevalence and intensity of barnacle infestations was also examined in relation to coastline topography on two co-occurring mussel species, the indigenous Perna perna and invasive Mytilus galloprovincialis. The results were complex, but bay status had significant effects on prevalence and intensity for both mussel species, depending on the time and zone. The effect of bay in relation to time was particularly relevant for M. galloprovincialis (four-way nested ANOVA, Season X Site(Bay): p = 0.0002), where summer prevalence was higher than that of winter in bays, regardless of zone, while in open coast sites, the effect of season was only significant in the mid zone. Patterns of intensity generally showed higher values in summer. Substratum preference by barnacles was investigated by recording settlement, survival and mortality of Chthamalus dentatus barnacles on various treatments. There was a strong preference for the rock-like plastic substratum by primary settlers (pair-wise tests of PERMANOVA: Dead < Rock mimic (p = 0.0001); Replica < Rock mimic (p = 0.019) and Live < Rock mimic (p = 0.0001)). This indicates that barnacles settle on mussel shells only as a secondary choice and that micro-topography is an important variable in barnacle settlement. The effect of barnacle epibiosis on condition index and growth of P. perna and M. galloprovincialis was also examined as a direct indication of the health of mussels subjected to the biological stress of epibiosis. Although not significant (PERMANOVA: P. perna: p(perm) = 0.890; M. galloprovincialis: p(perm) = 0.395), growth for both mussel species was slower for barnacle-infested individuals in summer, which is the main growing season for mussels in the region. Results from condition index calculations, however, showed no negative impacts of epibiotic barnacles (three-way ANCOVA: P. perna: p = 0.372; M. galloprovincialis: p = 0.762). Barnacle epibionts create a new interface between the mussel and its environment and this interaction can affect other members of the community. The possibility of the barnacle epibiont causing increased drag also needs further investigation. Biological processes operating within a wide range of physical stressors drive the interactions on the rocky shore, such as epibiosis. Overall, the results of this study suggest that the epibiotic relationship between mussels and barnacles on the south-east coast of South Africa does not significantly affect the mussel species present and that barnacles only use mussel shells as a secondary choice of substratum.
- Full Text:
- Date Issued: 2014
The effect of mussel bed structure on the associated infauna in South Africa and the interaction between mussel and epibiotic barnacles
- Authors: Jordaan, Tembisa Nomathamsanqa
- Date: 2011
- Subjects: Mytilidae -- South Africa , Mytilus galloprovincialis -- South Africa , Mussel culture -- South Africa , Shellfish culture -- South Africa , Perna -- South Africa , Barnacles -- South Africa , Mussels -- South Africa , Mussels -- Ecology , Barnacles -- Ecology
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5675 , http://hdl.handle.net/10962/d1005360 , Mytilidae -- South Africa , Mytilus galloprovincialis -- South Africa , Mussel culture -- South Africa , Shellfish culture -- South Africa , Perna -- South Africa , Barnacles -- South Africa , Mussels -- South Africa , Mussels -- Ecology , Barnacles -- Ecology
- Description: Mussels are important ecological engineers on intertidal rocks where they create habitat that contributes substantially to overall biodiversity. They provide secondary substratum for other free-living, infaunal or epifaunal organisms, and increase the surface area for settlement by densely packing together into complex multilayered beds. The introduction of the alien invasive mussel Mytilus galloprovincialis has extended the upper limit of mussels on the south coast of South Africa, potentially increasing habitat for associated fauna. The aim of this study was to describe the structure of mussel beds, the general biodiversity associated with multi- and monolayered mussel beds of indigenous Perna perna and alien M. galloprovincialis, and to determine the relationship between mussels and epibiotic barnacles. This was done to determine the community structure of associated macrofauna and the role of mussels as biological facilitators. Samples were collected in Plettenberg Bay, South Africa, where M. galloprovincialis dominates the high mussel zone and P. perna the low zone. Three 15 X 15 cm quadrats were scraped off the rock in the high and low zones, and in the mid zone where the two mussel species co-exist. The samples were collected on 3 occasions. In the laboratory mussel-size was measured and sediment trapped within the samples was separated through 75 μm, 1 mm and 5 mm mesh. The macrofauna was sorted from the 1 mm and 5 mm sieves and identified to species level where possible. The epibiotic relationship between mussels and barnacles was assessed by measuring the prevalence and intensity of barnacle infestation and the condition index of infested mussels. Multivariate analysis was used on the mean abundance data of the species for each treatment (Hierarchical clustering, multidimensional scaling, analysis of similarity and similarity of percentages) and ANOVA was used for most of the statistical analyses. Overall, the results showed that tidal height influences the species composition and abundance of associated fauna. While mussel bed layering influenced the accumulation of sediments; it had no significant effect on the associated fauna. Time of collection also had a strong effect. While there was an overlap of species among samples from January, May and March, the principal species contributing to similarity among the March samples were not found in the other two months. The outcomes of this study showed that low shore mussel beds not only supported a higher abundance and diversity of species, but were also the most structurally complex. Although the condition index of mussels did not correlate to the percentage cover of barnacle epibionts, it was also evident that low shore mussels had the highest prevalence. The levels of barnacle infestation (intensity) for each mussel species were highest where it was common and lowest where it was least abundant. This is viewed as a natural artefact of the distribution patterns of P. perna and M. galloprovincialis across the shore. Mussels are more efficient as facilitators on the low mussel zone than the high mussel zone possibly because they provide habitats that are more effective in protecting the associated macrofauna from the effects of competition and predation, than they are at eliminating the effects of physical stress on the high shore. Although mussels create less stressful habitats and protect organisms from the physical stress of the high shore, there are clear limitations in their ability to provide ideal habitats. The biological associations in an ecosystem can be made weak or strong depending on the external abiotic factors and the adaptability of the affected organisms.
- Full Text:
- Date Issued: 2011
Intertidal patterns and processes : tracking the effects of coastline topography and settlement choice across life stages of the mussels perna perna and mytilus galloprovincialis
- Authors: Von der Meden, Charles Eric Otto
- Date: 2010
- Subjects: Mexilhao mussel -- Behavior -- South Africa Mytilus galloprovincialis -- Behavior -- South Africa Perna -- Behavior -- South Africa Mussels -- Ecology -- South Africa
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5721 , http://hdl.handle.net/10962/d1005407
- Description: Within landscapes, spatial heterogeneity is common and specific landscape features can influence propagule dispersal by wind or water, affecting population connectivity and dynamics. Coastline topographic features, such as bays and headlands, have a variety of biophysical effects on nearshore oceanography, larval transport, retention and supply, and the processes of larval settlement and recruitment. Although this has been demonstrated in several parts of the world, engendering a perception of a general ‘bay effect’, few studies have investigated this generality in a single experiment or region, by replicating at the level of ‘bay’. The Agulhas biogeographic region of the south coast of South Africa is a useful system within which to test for such generality. Using the intertidal mussels Mytilus galloprovincialis and Perna perna as model organisms, patterns of adult distribution were surveyed across four large ‘halfheart’ bays and intervening stretches of open coast, providing replication at the level of ‘bay’ and duplication of ecologically similar species. In support of a general, pervasive influence of bays on intertidal populations, mussel cover was found to be greater in bays than on the open coast for both species, although the effect was strongest for M. galloprovincialis. To explain this adult distribution, settlement, post-settlement mortality and recruitment were examined over 12mo at the same sites, with the prediction that rates of each would favour larger bay populations. Contrary to this, an interaction between month and bay-status was found, with greater settlement and recruitment on the open coast than in bays reflecting extreme settlement and recruitment events at 3 westerly open coast sites during summer. Re-analysis excluding these outliers, revealed the expected effect, of greater settlement and recruitment in bays. While this indicates the broad generality of the bay effect, it highlights exceptions and the need for replication in time and space when examining landscape effects. Measuring post-settlement mortality required testing small-scale settlement behaviour on established and newly deployed settler collectors. It was found that all settlers preferred collectors with biofilm, but that primary settlers avoided conspecific settlers, while secondary settlers were attracted to them. With discrepancies in settler attraction to new and established collectors accounted for, initial (over 2d) and longer-term (over 7d) post-settlement mortality rates were found to be substantial (ca 60 %) for both species. No topographic effect on p-s mortality was evident. Finally, recruit-settler, adult-recruit and interspecies correlations were examined at regional and local scales. Synergistic (or neutral) effects maintained the initial settlement pattern in recruit and adult populations regionally, but not at local scales; striking interspecies correlations suggested the influence of common regional transport processes. Ultimately, the results emphasize the importance of the direction of effects in different life stages and at different spatial scales, and the possibility that antagonistic effects may mask even strong patterns.
- Full Text:
- Date Issued: 2010