Extracted from Summary, p. 172. The semi-terrestrial grapsoid crabs Cyclograpsus punctatus and Sesarma catenata have been studied in two ways. Part I of this thesis gives an account of their distribution in the Kowie River estuary in relation to certain environmental factors. Part II deals with further aspects of their biology as well as the ecological relationship between the two species.

Clibanarius virescens (Krauss) is a common and abundant hermit crab along the East coast of South Africa. Despite this, its biology is poorly known, both in South Africa and throughout the rest of its range in the West Indo-Pacific region. This study focuses on the descriptive analysis of the shell resource used by C. virescens and of the effects of this resource on crab populations. The underlying hypothesis of the study is that biogeographically imposed gradients in the morphology of intertidal gastropod shells used by hermit crabs affect hermit crab population parameters. The gradient of potential change in the shell resource was captured by sampling at 12 localities, encompassing the range of C. virescens in South Africa. Seasonal changes in shell use, population structure and reproduction at a single locality (Cape Recife) were recorded over a period of 13 months. The breeding season of C. virescens at the southern extreme of its range extended from December to June. The population structure shows some seasonal change, but no clear trends emerge. The shell resource changed substantially in nature over the region studied. Based on shell use, localities clustered into separate southern and northern groups with a break occurring between Dwesa and Coffee Bay. Southern localities were characterised by use of Burnupena cincta, B. lagenaria and B. pubescens. Northern localities were characterised by the use of Morula granulata, M. nodosa and Peristernia forskalii. Intertidal shells used by C. virescens show fewer adaptations to predation in southern localities than shells from northern localities. Southern shells ii are relatively large, light and have wider apertures than those from northern localities which are generally smaller, heavier and have decreased aperture widths. Shell parameters affect population size-distributions as southern crabs were larger and heavier than northern crabs. C. virescens show sexual size dimorphism in which male crabs uniformly dominate the larger size classes at all localities. Differences in the sex ratio between males and females show more variable patterns. Most southern localities show no difference in the number of male and female crabs, but most northern localities show a skewed sex ratio in favour of females. Shell use and population size distributions of females affect reproductive output. Southern females produced significantly larger clutches than northern females. Reproductive output was related to crab mass and shell volume. Shell use patterns conform to the biogeographical regions in the range of the study and have a clear effect on both the population size distributions and reproductive output of Clibanarius virescens.

Crabs are conspicuous inhabitants of temperate salt marshes and tropical mangroves and interact with their environment through several processes. However, detailed information on crab community processes is absent for most South African estuaries and nearshore coastal regions. This study evaluated the primary producers supporting crab species in the salt marsh dominated Swartkops estuary and the mangrove Mngazana estuary. Various methods estimating crab abundances were also assessed in different microhabitats and the larval distribution of crabs in the coastal zone was also investigated. Various methods for estimating crab abundance have been employed in the past, each with its inherent biases. The microhabitat of a mangrove forest in Australia was structurally altered by the manipulation of the litter, pneumatophores and the associated algae. These alterations did not affect the behavioural activity or the numbers of crabs recorded in any of the experimental treatments by either visual counts or pitfall traps. However, the number of crabs caught in the pitfall traps differed between the sites. Species-specific behaviour which was not investigated in this study may bias crab abundance estimates when using pitfall traps and therefore requires further investigation. Benthic consumers inhabiting shallow coastal environments may ultimately have the origin of their nutrition in a number of possible sources. Isotopic and gut content analysis of Thalamita crenata and juvenile Scylla serrata in the Mngazana estuary in South Africa revealed that these two portunids are able to share a habitat by resource partitioning. Differences were noted for species-specific utilization of primary producers not only between seasons within a site, but also between sites. This highlighted the use of locally produced primary producers sustaining food webs in estuaries. Mangrove production in the Mngazana estuary is very important and contributes to most of the carbon in the underlying sediments in the mangrove forest. However, the relatively large number of species and biomass encountered in this estuary may also be attributed to the fact that the different species are able to exploit of a number of different resources. The variation in stable isotope analysis of the different crab species throughout the estuary indicated that these crabs able to occupy the same habitat by feeding on a number of different resources and may preferentially select for a specific primary producer. A stable isotope of crabs in the salt marsh Swartkops estuary indicated that the dominant primary producer sustaining crab communities may even take place on a relatively smallscale. Sesarma catenata found at the inner marsh site recorded more depleted carbon signatures than those encountered in the other sites approximately 100 m away, and reflected signatures similar to the locally-encountered inner marsh plants. The relatively enriched nitrogen signatures of the anthropogenically-impacted Swartkops estuary is an indication of extensive inputs due to urbanization and industrialization, in contrast to the relatively pristine Mngazana estuary which exhibited low nitrogen signatures. Emphasis has been placed on the abiotic component of the exchange of nutrients and energy, although living organisms may also be transported, both actively and passively, between ecosystems. Little variation in either species composition or abundance was found between seasons for the larval distribution of brachyuran crabs on the east coast of South Africa. Due to the lack of published larval descriptions, larvae could not be identified to species level and it was therefore not possible to identify whether the larvae were hatched or spawned in an estuary or in a marine environment, or whether the larvae originated in the northern tropical regions. Frequent wind-reversals which are common in this region may retain larvae close inshore and supply the southern temperate locations with larvae from the northern locations. In conclusion, this study has shown that in highly productive systems with a number of potential primary producers, the crabs that inhabit the estuary show a marked diversity in resource utilization which could potentially allow a number of closely related species to occupy different trophic levels. This study also highlights the importance of locally produced sources in an estuary, which may occur on very small scales and this needs to be factored in with the design of any future stable isotope studies of this nature.

Thesis (PhD)--University of Stellenbosch, 2002. / ENGLISH ABSTRACT: In the present study, the systematic status, aspects of the evolutionary biology and phylogenetic relationships among species of the African freshwater crab genus Potamonautes in South Africa are examined. Systematic research between allopatric populations of P. brincki, using allozyme and morphometric data has revealed the existence of a new undescribed freshwater crab species occurring in mountain streams of the Cape Peninsula. This species is described in the present study. In addition, the diagnostic value of carapace dentition patterns in the taxonomy of freshwater crabs is explored among two toothed river crab species from South Africa. The latter study utilized P. warreni and P. unispinus where considerable variation in the dentition pattern of the former species has been recorded. Fixed differences in allozyme loci demonstrated that these two taxa should indeed be considered separate, yet closely related species, while the morphometric data failed to reveal this taxonomic separation. The holotype of P. warreni is re-described. The relationship of hybrid taxa between P. depressus and P. clarus populations are investigated. Evident from this study is that populations that occur equidistant from the two parental taxa have undergone extensive introgressive hybridization. Considering the pronounced sequence divergence and the occurrence of fixed allozyme loci between populations, it is postulated that the hybrid populations should be considered to be on a unique evolutionary trajectory worthy of conservation. The population genetic structure of P. calcaratus is examined as this species is unique in its occupation of water holes. Results demonstrate that the genetic population structure is the result of recent colonization and moderate gene flow among populations. Phylogenetic relationships between the southern African freshwater crab fauna is investigated with the use of sequence data from two mitochondial genes (12 S rRNA and 16 S rRNA), allozymes and morphology in an attempt to firstly, test the usefulness of freshwater crabs as biogeographic indicators, secondly to explore the relationship among hybrid taxa, and thirdly to examine Bott's (1955) subgeneric divisions. Results demonstrated that freshwater crabs can be used as biogeographic indicators, that hybrid taxa are phylogenetic ally closely related and that Bott's subgeneric divisions have no systematic basis. Among the three data sets, the sequence data provided the best resolution, while the phylogenetic inferences derived from the allozyme data and the morphology was limited. / AFRIKAANSE OPSOMMING: In hierdie studie word die sistematiek, aspekte van die evolusionêre biologie en filogenetiese verwantskappe tussen spesies van die varswater krap genus Potamonautes ondersoek in Suid Afrika. Navorsing op geografies geisoleerde populasies van P. brincki word met behulp van allosieme en morfometriese data ondersoek en dui daarop dat daar 'n nuwe onbeskryfde spesie op die berge van die Kaapse Skiereiland voorkom. Die nuwe spesies word beskryf in hierdie studie. Die diagnostiese waarde van tand patrone in varswater krappe word tussen twee spesies P. warreni en P. unispinus ondersoek. Genetiese data dui daarop dat alhoewel P. warreni morfologies baie naverwant is aan P. unispin us, is genoegsame verskille in allosiem lokusse wat daarop dui dat die twee taksa volwaardige spesies is. Die holotipe van P. warreni word herbeskryf. Die verwantkap tussen populasies van twee hibried taksa, P. depressus en P. clarus, word ondersoek. Resultate van hierdie studie dui daarop dat noemenswaardige verskille voorkom tussen die twee hibridie taxa en dat die hibriede populasies geneties verskillend is. 'n Argument word aangevoer vir die beskerming van hibriede taksa. Die populasie genetika van P. calcaratus word ondersoek en vergelyk met vorige studies. Die data dui daarop dat minimale geen vloei tusses populasies is en dat die populasie struktuur waarskynlik die resultaat is van onlangse kolonisasie. Die filogenetiese verwantskap tussen die suider Afrikaanse krappe word ondersoek met DNA, allosieme en morfologiese karakters met drie hoofdoele. Eerstens om die gebruik van varswater krappe as biografiese indikatore te toets. Tweedens om die verwantskap van hibriede taksa te ondersoek, en derdens om die status van Bott (1955) se subgenera te bepaal. Resultate dui daarop aan dat varswater krappe wel gebruik kan word as biografiese indikatore. Hibridieserende taxa naverwant is derdens en dat Bott se subgenera geen taksonomiese basis het nie. Die DNA data het die beste resolusie gebied vir die filogenetiese vrae, terwyl die allosieme en morfologiese data 'n laer resolusie bedied het, en meer beperk was.

The source of a reliable supply of seed is one of the most important factors determining the success of aquaculture. At present mud crab culture is reliant on wild caught juveniles, which limits the expansion of mud crab farming globally. This, coupled with the paucity of knowledge of the spawning and egg characteristics of mud crabs, prompted the investigation into the captive spawning, incubation and egg quality of Scylla serrata in South Africa. A total of 112 mature females were caught from three estuaries in northern KwaZulu Natal and acclimated to captive conditions. Mature females were present in the estuaries throughout the year and ranged from 90 to 200 mm carapace width. Except for 40 crabs all the females spawned in captivity. On average spawning took place 38 ± 23 days after capture but was not dependent on crab size. Fecundity was significantly correlated to crab size (p = 0.026), with larger females producing more eggs. The average fecundity per batch was 5.79 ± 2.07 million eggs. Hatch success rate for all egg batches was high averaging 84 ± 6 %. Egg dropping occurred in all egg batches during incubation. On average 6.5 % of the egg mass was dropped during incubation. At the average temperature of 27⁰C eggs in the incubation system hatched within 292 ± 12 hours after extrusion. In-vitro incubation of eggs proved to be an ineffective means of incubating Scylla serrata eggs with a hatch success rate of only 25 ± 5 %. Egg quality was assessed both directly, using variables such as hatch success rate and hatch synchronicity and retrospectively by using stress tests and comparing survival of larvae from different coloured egg batches. Lipid class composition and fatty acid methyl ester content for 28 batches of eggs and 16 batches of larvae were analyzed. The biochemical composition of the egg was correlated to survival of larvae in the stress tests to ascertain if the lipid content of the eggs determines their quality. Stress tests were used to retrospectively assess egg quality. Four stressors were administered to newly hatched larvae and the LD50 values recorded. An average LD50 of 64 hours was recorded for the starvation tests, 40% for the salinity tests, 37 ppm for the formalin tests and 39.7 mg/l NH3 for the ammonia stress tests was recorded. Egg colour ranged from pale yellow to orange-red. Egg colour was not influenced by female size or the time the females spent in captivity prior to spawning. Egg colour can therefore not be used as an indicator of quality. The lipid profiles of newly extruded eggs had no effect on hatch synchronicity or hatch success rate. However, the stress tests identified female size (p ≤ 0.02), DHA content of the egg (p ≤ 0.02), Σ omega-3 fatty acids of eggs (p ≤ 0.02) and EPA content of eggs (p ≤ 0.007) as possible determinants of egg quality. Larger crabs tended to produce poorer quality eggs. Egg quality also decreased as the amount of DHA, EPA and the Σ omega-3 fatty acids decreased in the eggs. To summarize, the results of this study indicated that the following parameters affect egg quality - crab size, DHA, EPA and Σ omega-3 fatty acid content of eggs.