Macrofauna associated with intertidal mussel beds in the Knysna estuarine embayment, South Africa
- Authors: Hodgson, Alan N , Smith, Frances , Smith, Peter , Claasens, Louw
- Date: 2021
- Subjects: To be catalogued
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/443554 , vital:74131 , https://hdl.handle.net/10520/ejc-afzoo_v56_i1_a44
- Description: The alien invasive mussel, Mytilus galloprovincialis, forms biogenic habitats in the embayment of Knysna Estuary. Invertebrate macrofauna inhabiting mussel beds were sampled from six sites in the embayment. In addition, mussel cover, mussel bed depth, density and size of mussels were recorded. With the exception of two sites (The Heads and Featherbed) where some Perna perna was present, we confirmed that the mussel beds were composed of the invasive species M. galloprovincialis. At The Heads, mussel beds were single-layered and in patches, whereas at other sites they were multi-layered with 100% cover. Bed depth, mussel density and shell size varied among sites. A total of 65 taxa (mainly Crustacea and Mollusca) were identified across all sites with species richness and diversity being significantly different among sites. Forty-three percent of taxa were new records for the estuary. Macrofaunal richness and diversity were highest at the two sites (The Heads and Featherbed) closest to the mouth of the embayment. There were negative relationships between species diversity and mussel density, and diversity and macroinvertebrate abundance. At most sites macrofaunal abundance was dominated by one or two species. Multivariate analysis revealed a significant difference in species composition among sites. Where wave action was present (The Heads, Featherbed and Leisure Isle) only a few deposit feeders were present in the mussel beds. At sites where wave action was minimal (Thesen Wharf, Marina and Railway Bridge) fine silt was present in the mussel matrix and there was an increase in the number of deposit feeding species.
- Full Text:
- Date Issued: 2021
An endangered seahorse selectively chooses an artificial structure
- Authors: Claassens, Louw , Booth, Anthony J , Hodgson, Alan N
- Date: 2018
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/123718 , vital:35486 , https://doi.10.1007/s10641-018-0732-4
- Description: The development of a residential marina estate within the Knysna estuary, South Africa, introduced Reno mattresses (horizontal wire cages filled with rocks) as a novel habitat for the endangered Knysna seahorse Hippocampus capensis. Consistently high seahorse densities on these artificial structures, despite the availability of seagrass habitat, begged the question of whether this habitat was chosen by the seahorse in preference to natural vegetation. An in situ habitat choice experiment was conducted which focused on the choice made by adult H. capensis between natural vegetation (Zostera capensis) and artificial (Reno mattress) habitat within a choice chamber. Seahorses were significantly more likely to move away from Z. capensis onto a Reno mattress structure or remain on this structure. This study concludes that higher H. capensis densities on Reno mattresses within Thesen Islands Marina are owing to some positive feature of this habitat and the underlying processes responsible for the choice made by this species (additional food, holdfasts, protection) can now be investigated.
- Full Text:
- Date Issued: 2018
Disentangling the identities and distribution patterns of the introduced beachfleas Orchestia gammarellus and Platorchestia platensis (Crustacea: Amphipoda Talitridae) in South Africa
- Authors: Diemer, Natalie , Griffiths, Charles L , Hodgson, Alan N
- Date: 2016
- Subjects: To be catalogued
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/443163 , vital:74084 , https://doi.org/10.1080/15627020.2016.1268930
- Description: Two introduced beachfleas, Orchestia gammarellus and Platorchestia platensis, have been recorded from South Africa, but have been misidentified by some earlier researchers and, prior to this study, each was also known from just a few, scattered observations, such that their true distributions remained obscure. We illustrate both species to clarify the features that identify and distinguish them and determine their true distributions by re-examining historical samples and by collecting new material from estuaries and lagoons along the west and south coasts of South Africa. Adult males of O. gammarellus have slender antennae and characteristic expanded, flattened, oar-like distal segments on pereopod 7. Adult males of P. platensis have the peduncle of antenna 2 strongly swollen and have bulbous, but not flattened, distal segments on pereopod 7. Both species occur under wrack and other debris along the banks of estuaries and sheltered lagoons. Orchestia gammarellus is found only along the south-west coast, in the Berg River Estuary, Langebaan Lagoon and the Diep River Estuary. Previous records from Knysna were misidentifications of P. platensis. Platorchestia platensis is found at one west coast site, Langebaan Lagoon, but is widely distributed from Cape Point eastwards to at least Algoa Bay. Both species share the same habitats as several other native beachfleas and competitive interactions between native and introduced species merit investigation.
- Full Text:
- Date Issued: 2016
Sperm ultrastructure and spermatodesm morphology of the spittle bug Locris transversa (Thunberg 1822)(Hemiptera: Cercopidae)
- Authors: Hodgson, Alan N , Ridgeway, Jaryd A , Villet, Martin H
- Date: 2016
- Subjects: To be catalogued
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/442333 , vital:73976 , https://doi.org/10.1080/07924259.2016.1157104
- Description: The structure of the spermatozoon and spermatodesm of the spittlebug Locris transversa (Thunberg 1822) was investigated by light and transmission electron microscopy. Males produced only one size class of sperm, which was 93–106 μm long. During spermatogenesis groups of spermatozoa are arranged around, and attached by their acrosomes to, a small central extracellular matrix to form a ball-shaped spermatodesm. Spermatodesmata were found in the testis, vas deferens and seminal vesicle of males and spermatheca of females. The sperm are filiform, each consisting of a ~15-μm-long head containing an anteriorly positioned conical ~2-μm-long acrosome and ~13-μm-long nucleus, and a midpiece and tail with a 9+9+2 axoneme. The acrosome, which has two posterior extensions that lie along one side of the anterior region of the nucleus, contains longitudinally orientated microfilaments. The nucleus has two unequal anterior extensions, whereas posteriorly it is flattened laterally to accommodate a putative centriolar adjunct and anterior ends of the two mitochondrial derivatives. The basal body lies posterior to the nucleus. The mitochondrial derivatives are elongated and extend for almost the entire length of the tail. The tail has glycogen within its centre, and towards its terminal end large deposits of glycogen surround the mitochondrial derivatives and axoneme. Compared to other members of the Cicadomorpha, the spermatozoa and spermatodesms of cercopids so far studied have some structural features in common with cicadellids and others in common with cicadas; therefore, if sperm features are to be useful in phylogenetic studies of cicadomorphs, attention must be given to examining key taxa to establish which character states are plesiomorphic or apomorphic.
- Full Text:
- Date Issued: 2016
Evolutionary retention of defensive lateral pedal glands in the smallest siphonariid limpet (Gastropoda: Pulmonata)
- Authors: Pinchuck, Shirley C , Allanson, Brian R , Hodgson, Alan N
- Date: 2015
- Subjects: To be catalogued
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/443312 , vital:74107 , https://hdl.handle.net/10520/EJC183115
- Description: Despite its cryptic habitat and habits, light and transmission electron microscopy has revealed that like many other siphonariids Siphonaria compressa, the smallest species of this genus, possesses lateral pedal glands. The pear-shaped glands (about 120 µm long Ã? 70 µm maximum diameter) open via a pore, and are multicellular with three types of secretory cell that surround a central lumen. The glands of this minute pulmonate limpet are similar in structure to its larger relatives, suggesting that they are defensive in function.
- Full Text:
- Date Issued: 2015
Some life-history parameters of the non-native amphipod Platorchestia platensis (Talitridae) in a warm temperate South African estuary
- Authors: Hodgson, Alan N , Booth, Anthony J , David-Engelbrecht, Viginia , Henninger, Tony O
- Date: 2014
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/126976 , vital:35940 , https://doi.10.1080/0035919X.2014.941961
- Description: Density, population structure, growth, mortality and aspects of reproduction of the non-native amphipod Platorchestia platensis were studied for 20 consecutive months (October 2008 to May 2010) in the supralittoral wrack of the Knysna Estuary, South Africa. Amphipod density varied over the sampling period with the lowest numbers recorded in summer (January and February). Ovigerous females with embryos, and juveniles were found in most months with peaks in recruitment found in both April and October, suggesting that in Knysna this species is a biannual breeder. Average female size was significantly greater in winter, with larger females tending to brood more embryos. The largest adults (13.5 mm total length) were always male although the monthly sex ratio was nearly always female biased. Growth rate estimated from a cohort analysis was about 1 mm per month and monthly survival about 69%. The ability to reproduce all year round may be one reason why this introduced species has become established within this warm temperate estuary.
- Full Text:
- Date Issued: 2014
Temporal and spatial variability in the abundance, biomass and distribution of Palaemon peringueyi (Decapoda: Crustacea) are influenced by biological but not physico-chemical factors in the permanently open Kariega Estuary, South Africa
- Authors: Nyalungu, Nonhlanhla P , Hodgson, Alan N , Froneman, P William , Dopolo, M , Masubelele, M
- Date: 2014
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/68106 , vital:29200 , https://doi.org/10.2989/16085914.2014.960792
- Description: Publisher version , Biological and physico-chemical factors affecting abundance and biomass of shrimp Palaemon peringueyi were investigated in 2010–2011 in the lower, middle and upper reaches of the freshwater-deprived permanently open Kariega Estuary. Shrimp abundance and biomass ranged from 0 to 88 ind. m−2 and 0 to 4.4 g wwt m−2, respectively. Shrimps were most abundant in the lower reach, and highest abundances and biomasses were recorded in the lower reach in June when salinities were low. The shrimps were almost always absent in the upper reach. Juveniles were found mostly in the lower and middle reaches, whereas adults were distributed in all three reaches. Regression analyses showed no statistically significant relationship for either abundance or biomass of the shrimps with temperature, salinity and dissolved oxygen (p > 0.05). The relationship between shrimp abundance and biomass and vegetation cover was positive and statistically significant (R2 = 0.109 and 0.185, respectively; p < 0.05). Vegetation cover plays an important role in determining the presence, distribution and abundance or biomass of P. peringueyi in this estuary.
- Full Text: false
- Date Issued: 2014
Growth and longevity of Exosphaeroma hylocoetes (Isopoda) under varying conditions of salinity and temperature
- Authors: Henninger, Tony O , Froneman, P William , Booth, Anthony J , Hodgson, Alan N
- Date: 2010
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/124571 , vital:35630 , https://doi.org/10.3377/004.045.0118
- Description: Numerous studies have documented the importance of both temperature and salinity in influencing aquatic crustacean metabolic processes such as respiration and growth. For example, increased water temperatures have been shown to increase respiration rates in various species of shrimp (Chen & Nan 1993; Spanonopoulos-Hernándeza et al. 2005; Allan et al. 2006), and copepods (Isla & Perissinotto 2004). The response of invertebrates to changes in salinity is more complex, largely reflecting their evolutionary origins (Kinne 1966). For example, juvenile blue swimming crabs, Portunus pelagicus, displayed significantly faster growth and higher survival in response to increasingsalinity (Romano & Zeng 2006). Additional factors that may influence the growth rates of crustaceans include photoperiod (Gambardella et al. 1997), food availability (Shuster & Guthrie 1999) and sex (Newman et al. 2007).
- Full Text:
- Date Issued: 2010
Reproductive seasonality of southern African inshore and estuarine invertebrates–a biogeographic review
- Authors: Hodgson, Alan N
- Date: 2010
- Subjects: To be catalogued
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/447415 , vital:74623 , https://doi.org/10.1080/15627020.2010.11657249
- Description: A literature review reveals that reproductive seasonality of less than 1% of South Africa’s inshore marine invertebrates has been studied. Approximately 95% of studies have been on large, long-lived, gonochoristic, iteroparous species, and nearly 50% on molluscs. There is only a weak relationship between biogeographic region and timing of reproduction. Nevertheless, a relatively large proportion of west coast species studied breed in winter, whereas south coast species tend to have breeding peaks in spring/summer, although many nearly always have mature gametes and probably reproduce year-round. East coast species are mainly continuous or summer breeders. The factors that control gametogenesis and trigger spawning are largely unresolved, although temperature and food are thought to be important in many species.
- Full Text:
- Date Issued: 2010
The role of macrophytes as a refuge and food source for the estuarine isopod Exosphaeroma hylocoetes ()
- Authors: Henninger, Tony O , Froneman, P William , Richoux, Nicole B , Hodgson, Alan N
- Date: 2009
- Subjects: To be catalogued
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/458360 , vital:75735 , xlink:href="https://doi.org/10.1016/j.ecss.2009.01.01"
- Description: The role of submerged macrophytes as refugia from fish predation and as possible food sources for the estuarine isopod Exosphaeroma hylocoetes (Barnard, K.H., 1940) was investigated. Laboratory experiments tested the effectiveness of artificial vegetation, replicating submerged vegetation, in enabling isopods to elude selected fish predators Rhabdosargus holubi, Glossogobius callidus, Monodactylus falciformis and Clinus cottoides. Isopods preferentially hid in the vegetation (>90%), even in absence of fish. The predatory fish had varying success in finding isopods within the vegetation. Isopod mortality ranged from 2% (R. holubi) to a maximum of 87% (C. cottoides) within vegetation, depending on the fish predator present. Stable isotope and fatty acid analyses ruled out the submerged macrophyte Ruppia maritima and inundated fringing grasses as direct food sources, but highlighted the epiphytic biota (mainly diatoms) found on the submerged vegetation and sediments as more likely food sources. These findings are consistent with gut content analyses. The results suggest that the close association of E. hylocoetes with R. maritima is the result of the vegetation providing the isopod with a refuge against fish predation as well as areas of increased food availability.
- Full Text:
- Date Issued: 2009
The population dynamics of the estuarine isopod Exosphaeroma hylocoetes (Barnard, 1940) within three temporarily open/closed southern African estuaries
- Authors: Henninger, Tony O , Froneman, P William , Hodgson, Alan N
- Date: 2008
- Subjects: To be catalogued
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/447627 , vital:74662 , https://doi.org/10.1080/15627020.2008.11657237
- Description: The population dynamics of the estuarine isopod, Exosphaeroma hylocoetes, was investigated monthly from February 2006 to August 2007 in three temporarily open/closed Eastern Cape estuaries, the East and West Kleinemonde and Kasouga Estuaries. Mean isopod abundances and biomasses ranged between 0 and 4791 ind/m2 and 0 and 9.65 mg/dwt/m2 in the West Kleinemonde Estuary and between 0 and 108 ind/m2 and 0 and 0.318 mg/dwt/m2 in the nearby East Kleinemonde Estuary. In the Kasouga Estuary, the values ranged between 0 and 3650 ind/m2 and 0 and 5.105 mg/dwt/m2. Temporal changes in the abundance and biomass of E. hylocoetes within the three systems was primarily linked to mouth phase, with populations crashing when the mouth was open and to a lesser extent, seasonality. In all three estuaries maximum abundance and biomass was recorded in the middle reaches, which could be ascribed to the presence of submerged macrophytes. E. hylocoetes demonstrates strong sexual dimorphism, with males being significantly larger than females (P 0.05). Sex ratios were skewed in favour of females in all three estuaries.
- Full Text:
- Date Issued: 2008
Spermiogenesis in three species of cicadas (Hemiptera: Cicadidae)
- Authors: Chawanji, A S , Hodgson, Alan N , Villet, Martin H , Sanborn, Allen F , Phillips, Polly K F
- Date: 2007
- Language: English
- Type: Article
- Identifier: vital:6865 , http://hdl.handle.net/10962/d1011207
- Description: Spermiogenesis in three species of cicadas representing one cicadettine (Monomatapa matoposa Boulard) and two cicadines (Diceroprocta biconica [Walker] and Kongota punctigera [Walker]) was investigated by light and electron microscopy. Although spermiogenesis was occurring in the testis of adult males of all species, earlier spermiogenic stages were observed in D. biconica only. While spermiogenesis was similar to that described for other insects, some differences were noted. For example granular material did not assemble around the centriole to form a centriolar adjunct but did accumulate in the cytoplasm of early spermatids adjacent to a region of the nuclear membrane where nuclear pores were aggregated. In late spermatids this material accumulated anterior to the mitochondrial derivatives in a developing postero-lateral nuclear groove. While this material has been named the ‘centriolar adjunct’ by previous authors, its formation away from the centriole raises questions about its true identity. Second, during acrosome maturation an ante-acrosomal region of cytoplasm develops. Although present in later spermatids, this region is lost in spermatozoa. Interspecific variations in chromatin condensation patterns and the number of microtubule layers encircling the spermatid nucleus during spermiogenesis were noted.
- Full Text:
- Date Issued: 2007
The effect of temperature and salinity on oxygen consumption in the brachyuran crab Cyclograpsus punctatus (Crustacea: Decapoda: Grapsidae)
- Authors: Winch, James W W , Hodgson, Alan N
- Date: 2007
- Subjects: To be catalogued
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/447426 , vital:74624 , https://doi.org/10.1080/15627020.2007.11407384
- Description: The aquatic oxygen consumption of the estuarine brachyuran crab, Cyclograpsus punctatus, was investigated after a 24-hour acclimation period at different temperature (12.5, 20, 30°C) and salinity (9, 17.5, 35, and 44°) combinations . Salinity had no significant effect on oxygen consumption at 12.5 and 20°C in both large and small crabs. At 30°C and 44°, however, the rate of oxygen consumption declined in large crabs (from 0.233 to 0.176 mg O2 g wwt/h) and increased in small crabs (0.300 to 0.469 mg O2 g wwt/h). There was also a significant increase in the oxygen consumption (0.300 to 0.536 mg O2 g wwt/h) of small crabs at the high temperature/low salinity combination. Temperature had a significant effect on the rate of oxygen consumption at all salinities, with Q10 values ranging from 1.16 to 2.85. In all cases the mass-specific oxygen consumption rate of small crabs was higher than that of large crabs. The results suggest that the confinement of this species to the lower reaches of estuaries may, in part, be due to physiological constraints on juveniles and small individuals.
- Full Text:
- Date Issued: 2007
Effects of temperature and salinity on the standard metabolic rate (SMR) of the caridean shrimp Palaemon peringueyi
- Authors: Allan, E Louise , Froneman, P William , Hodgson, Alan N
- Date: 2006
- Language: English
- Type: Article
- Identifier: vital:6831 , http://hdl.handle.net/10962/d1007567
- Description: The standard metabolic rate (SMR) of the caridean shrimp Palaemon peringueyi to changes in temperature (15–30 °C), salinity (0–45‰) and a combination thereof was investigated. The rate of oxygen consumption of the shrimp was determined using a YSI oxygen meter. At a constant salinity of 35‰ the respiration rate of P. peringueyi increased with an increase in temperature and ranged between 0.260 and 0.982 μl O[subscript 2] mg wwt[superscript −1] h[superscript −1]. The Q[subscript 10] value over the temperature range 15–25 °C was estimated at 3.13. At a constant temperature of 15 °C the respiration rate of P. peringueyi also increased with an increase in salinity and ranged between 0.231 and 0.860 μl O[subscript 2] mg wwt[superscript −1] h[superscript −1]. For combination experiments the absence of any significant difference in the respiration rate of P. peringueyi at the four temperatures over the salinity range 15–35‰ suggests that the shrimp is well adapted to inhabiting environments characterised by variations in salinity and temperature such as those encountered within the middle and lower reaches of permanently open estuaries with substantial freshwater inflow. On the other hand, the total mortality of the shrimp recorded at salinities < 5‰ at all four temperatures suggests that the upper distribution of the shrimp may reflect physiological constraints. Similarly, the increase in the respiration rate of the shrimp at the four temperatures at salinities > 35‰ suggests that the shrimp may experience osmotic stress in freshwater deprived permanently open and intermittently open estuaries where hypersaline conditions may develop.
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- Date Issued: 2006
Professor Emeritus Denis William Ewer MBE FRSSAf 1913 - 2009: biographical memoir
- Authors: Hodgson, Alan N
- Date: 2006
- Language: English
- Type: text , Article
- Identifier: vital:6884 , http://hdl.handle.net/10962/d1011643
- Description: Denis Ewer went to University College School in London. In 1931 he entered Trinity College, Cambridge (his father had studied mathematics at Trinity) where he read for the Natural Science Tripos, obtaining a B.A. in 1934. In his second year at Cambridge he gained the nickname of 'Jakes' via a literary route (his father's nickname also had literary origins). The word 'ewer' can mean a jug, chamber pot, or jerry, and hence the progress to 'jakes', Elizabethan English for an outside lavatory. It was at Cambridge that he also met the future communist spy, Kim Philby. After graduating, Jakes Ewer moved to the University of Birmingham where he undertook his doctoral studies (graduating PhD in 1940) under H. Munro Fox. World War II, however, temporarily halted any further academic career, and during the war years he was an Experimental Officer for the Army Operational Research Group (Ministry of Supply) achieving the rank of Major. For his work, which included being a Scientific Advisor to the Chief, Air Defence Division, Supreme Headquarters Allied Expeditionary Force, he was awarded the M.B.E. (Member of the British Empire).
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- Date Issued: 2006
Sperm morphology in five species of cicadettine cicadas (Hemiptera: Cicadomorpha: Cicadidae)
- Authors: Chawanji, A S , Hodgson, Alan N , Villet, Martin H
- Date: 2006
- Language: English
- Type: Article
- Identifier: vital:6930 , http://hdl.handle.net/10962/d1011948
- Description: Mature spermatozoa from five species of cicadas of the subfamily Cicadettinae (Quintilia wealei, Melampsalta leucoptera, Stagira simplex, Xosopsaltria thunbergi and Monomatapa matoposa) were examined by light and electron microscopy. In each species sperm are elongate, aggregated into organized bundles with their heads embedded in a homogenous matrix to form spermatodesmata, and exhibit polymegaly. The head of the sperm consist of an anteriorly positioned conical acrosome that has a tubular substructure and a deep, posterior invagination that forms the subacrosomal space (eccentrically positioned anteriorly). The acrosome is flattened anteriorly; posteriorly it extends along either side of the nucleus as two tubular processes that gradually decrease in diameter. The filiform nucleus tapers anteriorly and intrudes into the subscrosomal space. Posteriorly the nucleus has a lateral invagination that houses material of the so-called centriolar adjunct. Posterior to the centriolar adjuct and the nucleus are two crystalline mitochondrial derivatives and a centriole, respectively, the latter giving rise to the axoneme, which has a 9 + 9 + 2 arrangement of microtubules. In these respects the sperm are similar to those of platypleurine cicadas. However, some features seem unique to cicadettines, including the structural organization of an enlarged centriolar adjunct and the dimensions of the tails. The enlarged centriolar adjunct has a lamella-like substructure and can be considered a synapomorphic character in the Cicadettinae. It is, therefore, potentially useful in the separation of this subfamily from the Cicadinae. In addition, the great length of the sperm nucleus of long-headed sperm in M. matoposa could be a synapomorphy of this genus and related taphurine and cicadettine species.
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- Date Issued: 2006
A century of Zoology and Entomology at Rhodes University, 1905 to 2005
- Authors: Hodgson, Alan N , Craig, Adrian J F K
- Date: 2005
- Language: English
- Type: Article
- Identifier: vital:6922 , http://hdl.handle.net/10962/d1011905
- Description: The history of the Department of Zoology & Entomology at Rhodes University is traced from its origin with a single staff member, through the era of permanent heads of department to the present day staff. Significant achievements and developments in teaching and research are highlighted, and some anecdotal material is included.
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- Date Issued: 2005
Sperm morphology in four species of African platypleurine cicadas (Hemiptera: Cicadomorpha: Cicadidae)
- Authors: Chawanji, A S , Hodgson, Alan N , Villet, Martin H
- Date: 2005
- Language: English
- Type: Article
- Identifier: vital:6917 , http://hdl.handle.net/10962/d1011880
- Description: Mature spermatozoa from four species of platypleurine cicadas (Albanycada albigera, Azanicada zuluensis, Platypleura capensis and P. hirtipennis) were examined by light and electron microscopy. The filiform sperm have a similar ultrastructure in all species but notable variations were found in sperm dimensions. All species produce more than one discrete length of nucleated, motile sperm, a form of polymorphism termed polymegaly. Polymegaly is expressed in two ways: sperm have bi- or trimodal head and tail lengths. The anterior parts of sperm heads are embedded in an elongate homogenous matrix forming a spermatodesm. The conical acrosome is deeply invaginated posteriorly, and sits on top of the nucleus. The acrosomal contents are differentiated internally with an electron-lucent central medulla and a denser cortex. The homogenously electron-dense nucleus is pointed anteriorly and is generally cylindrical, although posteriorly there is a lateral invagination that extends part-way along the nucleus. This invagination houses fine granular material of the centriolar adjunct. Vesicle-like elements that are associated with both the posterior nucleus and the centriolar adjunct are also found within the invagination. Immediately posterior of and adjoining the centriolar adjunct is a pair of mitochondrial derivatives that are elongated and extend for almost the entire length of the tail. The absence of accessory bodies in cicada sperm suggests that within the Cicadomorpha, the families Cicadidae and Cercopidae are closely affiliated.
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- Date Issued: 2005
An estimation of the standing stock and population structure of Upogebia africana (Crustacea: Thalassinidae) in the Knysna Estuary
- Authors: Hodgson, Alan N , Allanson, Brian R , Cretchley, Robyn
- Date: 2000
- Language: English
- Type: Article
- Identifier: vital:6923 , http://hdl.handle.net/10962/d1011907
- Description: Within the Knysna Estuary Upogebia africana has an extensive distribution from north of Leisure Isle to the Red Bridge, occupying 62% of the available intertidal zone. Six sites were surveyed within the above distributional range. U. africana was found from the high-water level (Spartina zone) to the shallow subtidal zone (to about 1.5 m depth). The density, biomass and population structure of the mud prawns were found to be highly variable within the estuary. Only small animals (carapace length –× = 10.9 mm) were found in the upper estuary at the Red Bridge. Although large animals were present, the Invertebrate Reserve had a low density and biomass of U. africana (– × = 11.7 m–2; 3.9 g.m–2 dry weight). By contrast a relatively inaccessible centre mud bank (Oyster Bank) in the middle reaches of the estuary had much larger populations (–× = 176.5 m–2; 65.3 g.m–2 dry weight). This site is, therefore, a natural mud prawn reserve. Within the intertidal zone, mud prawn density (74–76 m–2) and biomass (26–27 g.m–2 dry weight) was usually greatest in the Spartina and lower Zostera zones. Sex ratios throughout the estuary did not deviate significantly from 1:1.
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- Date Issued: 2000
Intertidal population structure of the edible mollusc Turbo sarmaticus (Vetigastropoda) at an unexploited and exploited sites along the coast of the Eastern Cape Province, South Africa
- Authors: Foster, Gregory G , Hodgson, Alan N
- Date: 2000
- Subjects: To be catalogued
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/447403 , vital:74622 , https://doi.org/10.1080/15627020.2000.11657088
- Description: The intertidal population structure of the large edible gastropod Turbo sarmaticus was examined at four sites along the Eastern Cape Province of South Africa. One site was free from exploitation, two were subjected to low levels of exploitationand one was heavily exploited. Within the intertidal zone at all sites there was a size-related distribution, with smalleranimals being found towards the upper mid-shore and larger animals occurring in a downshore direction. The mean shore densities (1.2–animals/m2) and biomass (4.4–9.4 g/m2) of T. sarmaticus at the sites where there was no, or minimal, exploitation of animals were not significantly different (P0.001). However, the largest animals (up to 110mmshell length) were found at the unexploited site, an offshore island. The lowest density (0.2 animals/m2) and biomass (0.4 g/m2) of T. sarmaticus was recorded at the site where the exploitation of this gastropod was intense. In addition, no animals of a legally exploitable size (>70 mm shell length) were found. It is suggested that intense over-exploitation is threatening the population at this site.
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- Date Issued: 2000