ALBERT

Description: Population structure, growth and production of the surf clam Donax serra (Bivalvia, Donacidae) inhabiting highly exposed sandy beaches of Namibia, were investigated between November 1997 and December 1999. From length-frequency distribution and tagging-recapture data a von Bertalanffy growth function with an asymptotic length (L*) of 82 mm and a growth constant (K) of 0.274 y-1 was established. Regarding growth performance of Donacidae, D. serra fits in a group of species inhabiting cold-temperate and upwelling regions. The intertidal biomass of the studied population ranged between 141 g ash free dry mass (AFDM) m-2 y-1 and 546 g AFDM m-2 y-1. Individual production was maximal at 56.5 mm shell length (0.83 g AFDM ind.1 y1) and annual production ranged between 167 g AFDM m-2 y-1 and 637 g AFDM m-2 y-1, resulting in productivity values (P/B) between 1.167 y-1 and 1.589 y-1. This data underlines the importance of D. serra for the beach/surf ecosystem. Further the findings of this study are crucial to support future aquaculture or exploitation activities and management.

Description: The surf clam Donax serra (Bivalvia, Donacidae) dominates sandy beach communities of two southern African biogeographical regions, a cold (Benguela current) and warm province (Agulhas current). Morphometric and behavioural differences led to a controversial discussion of whether or not populations from the two provinces belong to the same species. Shell size measurements confirmed morphological differences: clams from the cold province were significantly rounder, flatter and less wedge-shaped than clams from the warm province. In this study a genetic approach was used to relate phenotypic differences to genetic variability of four populations of D. serra separated by up to 2 500 km of shoreline. Genetic analysis of twenty-two protein-coding loci was carried out by starch-gel electrophoresis. Populations studied are conspecific (genetic distances range from 0.003 to 0.044) and possess genetic variation (alleles per locus: 1.73 - 1.91; mean heterozygosity: 18 - 22%; percentage polymorphism: 45.5 - 59.1%) in the range of most other marine bivalves, which allows for potential adaptation to environmental changes. Wrights fixation indices show little to moderate genetic divergence among the subpopulations relative to the limiting amount under complete fixation (FST = 0.016 - 0.089), moderate divergence of individuals relative to the total population (FIS = 0.265 - 0.452), and comparably high divergence of individuals relative to the compound population (FIT = 0.300 - 0.473). The effective number of individuals exchanged between populations in each generation is high enough (1.44 - 8.65) to counteract genetic drift. We propose that the observed differences represent phenotypic plasticity enabling this species to inhabit different biogeographic regions. Gene flow, balanced selective pressure and evolutionary inertia are discussed as explanations for similarities of the two outlying populations. The substantial subdivision of the two Namibian populations indicates a potential biotic barrier and requires separate studies of the population dynamics.

Description: The surf clam Donax serra (Röding, 1798) dominates macrobenthic communities of extended and exposed sandy beaches of those southern African biogeographical re-gions where phytoplankton production is high: the cold Benguela upwelling system and areas of the warm Agulhas current system, which experience occasional upwelling and estuarine input. It feeds on phytoplankton and detritus, serves as food item for marine and terrestrial predators and is exploited by recreational anglers. The overall aim of this study was to investigate the population dynamics and ecology of Namibian D. serra in order to contribute essential knowledge for the understanding of its role in the Benguela upwelling ecosystem as well as for its potential use in aquaculture and exploitation activities. In particular two populations of the central Namibian coast were studied during November 1997 and December 1999.Histological examinations of D. serra gonads and the condition index indicated a sea-sonal reproductive cycle, coupled to the annual mean sea surface temperature cycle. Spawning started in August/September and lasted until February. Recruits, however, were present for only three months in the intertidal zone. The period when these juve-niles are abundant is decoupled from the spawning period and therefore cannot be predicted. Starvation, hydrodynamic processes, chemical parameters and different re-lease times during the spawning period are likely to cause spatial and temporal differ-ences in settlement time and recruitment strength.Individual growth is best described by a Von Bertalanffy growth function with an asymp-totic length (L∞) of 82 mm and a growth constant (K) of 0.274 y -1 in both intertidal populations. Growth performance of D. serra agrees with values of cold-temperate and upwelling donacids while temperate Donax-species have intermittent and tropi-cal/subtropical congeners show lowest values. The intertidal biomass of the studied populations ranged between 141 g ash free dry mass (AFDM) m-2 and 546 g AFDM m-2, which is much higher that in D. serra populations at warm temperate beaches and dis-tinctly surmounts the range for several non-upwelling Donax-species. In line, annual production exceeds values calculated from populations of habitats without permanent upwelling as well as those of non-upwelling donacids. The present values ranged be-tween 167 g AFDM m-2 y-1 and 637 g AFDM m-2 y-1, resulting in productivity values be-tween 1.167 y-1 and 1.589 y-1. Individual production was maximal at 56.5 mm shell length (0.83 g AFDM ind.-1 y-1).To investigate whether toxic hydrogen sulphide affects survivorship of juvenile D. serra and thus is a potential community structuring factor, the reaction of these clams to low oxygen concentrations and sulphide presence was examined. In vitro exposure ex-periments were conducted using an innovative gas-tight continuous flow system. Hy-drogen sulphide was adjusted to a concentration (0.1 mmol l-1) as regularly found during native "sulphide eruptions", which occur sporadically in the highly productive inshore regions of the central Namibian Benguela. During the first 2 h of hypoxic and hypoxic-sulphidic exposure test clams emerged to the sediment surface, which might support the drift to locations with more favourable conditions. Juvenile clams possess a high sulphide detoxification capacity and are adapted to sulphur events by their ability to oxidise the penetrating hydrogen sulphide to non-toxic thiosulphate. In addition, they are able to switch to anaerobic energy production, indicated by a significant accumulation of succinate and alanine. However, tested clams were not able to withstand long periods of exposure, the median survival time (LT50) under hypoxic sulphide incubation was 80 h. Spatial and temporal extended sulphur events are assumed to be a potential community-structuring factor, owing to their negative impact on recruitment.Shell size measurements confirmed that clams from the cold Benguela were significantly rounder, flatter and less wedge-shaped than clams from the warm Agulhas. A genetic comparison of four D. serra populations inhabiting both regimes aimed to clarify if populations are separate (sub-) species reflected in allelic variation. Genetic analysis of twenty-two protein-coding loci was carried out by starch-gel electrophoresis. Popula-tions studied are conspecific and possess genetic variation in the range of most other marine bivalves, which allows for potential adaptation to environmental changes. Little to moderate genetic differentiation among sub-populations relative to the maximal dif-ferentiation under complete fixation (FST = 0.016 - 0.089), moderate differentiation of individuals relative to their sub-population (FIS = 0.265 - 0.452), and comparably high differentiation of individuals relative to the compound population (FIT = 0.300 - 0.473) were found. The effective number of individuals exchanged between populations in each generation is high enough (1.44 - 8.65) to counteract genetic drift. Therefore it is proposed that observed morphological differences represent phenotypic plasticity ena-bling this species to inhabit different biogeographic regions. Gene flow, balanced se-lective pressure and evolutionary inertia are proposed as explanations for similarities of the geographically most distant populations. The substantial differentiation of the two Namibian populations indicates a potential biotic barrier and requires separate studies of the population dynamics.The results of this investigation, especially the high growth and production rates as well as the ability to inhabit substrates in high abundances are encouraging for future aquacultural use of this species. Work perspectives are identified for further support of culturing activities of D. serra, which will moreover contribute to a broader understanding of sandy beach ecology.

Description: Reproduction and recruitment of the surf clam Donax serra on two Namibian beaches were studied over a period of two years. Histological examination of the gonads indi-cated a discontinous annual reproductive cycle, related to monthly mean sea surface temperatures. The spawning season lasted from August/September until February but juveniles (2-6 mm anterior-posterior shell length) were only present for three months in the intertidal zone. The condition index indicates that the species spawns during autumn and summer, but histological validation is needed. The period when juveniles are abun-dant is decoupled from the spawning period and therefore cannot be predicted clearly, even if the spawning time is known. Starvation, hydrodynamic processes, chemical parameters and different release times during the spawning period are thought to cause the differences in settlement time and in recruitment strength between locations.

Description: The goal of this presentation is to give a brief overview over the German activity in cold water scientific diving. First, we will outline the requirements for scientific diving under German legislation. Secondly, two locations where much of the cold water diving is performed will be described, Kongsfjord in Spitsbergen (Arctic) and Potter Cove at King George Island (Antarctic). Some examples of scientific projects will be presented.

Description: Toxic "sulphide eruptions" sporadically occur in the highly productive inshore regions of the central Namibian Benguela upwelling system. The surf clam Donax serra (Röding, 1798) dominates the intertidal and upper subtidal of large exposed sandy beaches of southern Africa and its recruitment seems to be affected by sulphide events. The reaction of juvenile surf clams to low oxygen concentrations and sulphide occurrence (0.1 mmol l-1) was examined by in vitro exposure experiments in a gas-tight continuous flow system. After 2 h of hypoxic- and hypoxic-sulphidic conditions clams moved to the sediment surface, aiding their passive transport to areas with more favourable conditions. The clams showed a high sulphide detoxification capacity by oxidising the penetrating hydrogen sulphide to non-toxic thiosulphate. Moreover, juvenile D. serra switched to anaerobic energy production, indicated by the significant accumulation of succinate and, to some extent, alanine. Test animals were not able to reduce their energy requirements enough to withstand long periods of exposure, leading to a median survival time (LT50) of 80 h under hypoxic sulphide incubation. In conclusion, natural "sulphide eruptions", especially those with a large spatial and temporal extension, have to be considered as an important factor for D. serra recruitment failures. Hydrogen sulphide is assumed to be a potential community structuring factor.

Description: The surf clam Donax serra (Röding, 1798) dominates macrobenthic communities of extended and exposed sandy beaches of those southern African biogeographical regions where phytoplankton production is high: the cold Benguela upwelling system and areas of the warm Agulhas current system, which experience occasional upwelling and estuarine input. It feeds on phytoplankton and detritus, serves as food item for marine and terrestrial predators and is exploited by recreational anglers. The overall aim of this study was to investigate the population dynamics and ecology of Namibian D. serra in order to contribute essential knowledge for the understanding of its role in the Benguela upwelling ecosystem as well as for its potential use in aquaculture and exploitation activities. In particular two populations of the central Namibian coast were studied during November 1997 and December 1999.Histological examinations of D. serra gonads and the condition index indicated a seasonal reproductive cycle, coupled to the annual mean sea surface temperature cycle. Spawning started in August/September and lasted until February. Recruits, however, were present for only three months in the intertidal zone. The period when these juveniles are abundant is decoupled from the spawning period and therefore cannot be predicted. Starvation, hydrodynamic processes, chemical parameters and different release times during the spawning period are likely to cause spatial and temporal differ-ences in settlement time and recruitment strength.Individual growth is best described by a Von Bertalanffy growth function with an asymptotic length (L*) of 82 mm and a growth constant (K) of 0.274 y-1 in both intertidal populations. Growth performance of D. serra agrees with values of cold-temperate and upwelling donacids while temperate Donax-species have intermittent and tropical/subtropical congeners show lowest values. The intertidal biomass of the studied populations ranged between 141 g ash free dry mass (AFDM) m-2 and 546 g AFDM m-2, which is much higher that in D. serra populations at warm temperate beaches and distinctly surmounts the range for several non-upwelling Donax-species. In line, annual production exceeds values calculated from populations of habitats without permanent upwelling as well as those of non-upwelling donacids. The present values ranged between 167 g AFDM m-2 y-1 and 637 g AFDM m-2 y-1, resulting in productivity values between 1.167 y-1 and 1.589 y-1. Individual production was maximal at 56.5 mm shell length (0.83 g AFDM ind.-1 y-1).To investigate whether toxic hydrogen sulphide affects survivorship of juvenile D. serra and thus is a potential community structuring factor, the reaction of these clams to low oxygen concentrations and sulphide presence was examined. In vitro exposure experiments were conducted using an innovative gas-tight continuous flow system. Hydrogen sulphide was adjusted to a concentration (0.1 mmol l-1) as regularly found during native "sulphide eruptions", which occur sporadically in the highly productive inshore regions of the central Namibian Benguela. During the first 2 h of hypoxic and hypoxic-sulphidic exposure test clams emerged to the sediment surface, which might support the drift to locations with more favourable conditions. Juvenile clams possess a high sulphide detoxification capacity and are adapted to sulphur events by their ability to oxidise the penetrating hydrogen sulphide to non-toxic thiosulphate. In addition, they are able to switch to anaerobic energy production, indicated by a significant accumulation of succinate and alanine. However, tested clams were not able to withstand long periods of exposure, the median survival time (LT50) under hypoxic sulphide incubation was 80 h. Spatial and temporal extended sulphur events are assumed to be a potential community-structuring factor, owing to their negative impact on recruitment.Shell size measurements confirmed that clams from the cold Benguela were significantly rounder, flatter and less wedge-shaped than clams from the warm Agulhas. A genetic comparison of four D. serra populations inhabiting both regimes aimed to clarify if populations are separate (sub-) species reflected in allelic variation. Genetic analysis of twenty-two protein-coding loci was carried out by starch-gel electrophoresis. Populations studied are conspecific and possess genetic variation in the range of most other marine bivalves, which allows for potential adaptation to environmental changes. Little to moderate genetic divergence among sub-populations relative to the maximal divergence under complete fixation (FST = 0.016 - 0.089), moderate divergence of individuals relative to their sub-population (FIS = 0.265 - 0.452), and comparably high divergence of individuals relative to the compound population (FIT = 0.300 - 0.473) were found. The effective number of individuals exchanged between populations in each generation is high enough (1.44 - 8.65) to counteract genetic drift. Therefore it is proposed that observed morphological differences represent phenotypic plasticity enabling this species to inhabit different biogeographic regions. Gene flow, balanced selective pressure and evolutionary inertia are proposed as explanations for similarities of the geographically most distant populations. The substantial differentiation of the two Namibian populations indicates a potential biotic barrier and requires separate studies of the population dynamics.The results of this investigation, especially the high growth and production rates as well as the ability to inhabit substrates in high abundances are encouraging for future aquacultural use of this species. Work perspectives are identified for further support of culturing activities of D. serra, which will moreover contribute to a broader understanding of sandy beach ecology.

Description: El estudio se realizó en Kongfsjord, un fiordo ártico , ubicado en la costa O de Spitsbergen a 79 Nº 12ºE. Este es influenciado por masas de agua árticas y atlánticas. La producción primaria es reducida debido al aporte de sedimentos provenientes de corrientes de deshielo de los glaciares circundantes. A esta latitud sería de esperar un fiordo típicamente Ártico, pero debido a la influencia de la Corriente del Golfo, este presenta características subárticas a tal punto que se han registrado temperaturas de hasta 7º C. Este estudio fue realizado durante el verano boreal, en julio del 2001 y julio del 2002. Para analizar las comunidades macrobentónicas y estudiar los patrones de distribución , diversidad y abundancia se tomaron muestras fotográficas a 15, 20, 25, y 30 metros de profundidad en tres estaciones distintas caracterizadas por ser de sustrato duro, blando y de transición. Análisis multivariados (ordenamiento) de la composición de especies, mostraron un claro patrón de zonación en función de la profundidad con un incremento de la diversidad en las tres estaciones. En la estación de fondos duros, la que además presentó una mayor diversidad, a los 15m la comunidad está dominada por macroalgas, especialmente feófitas, observándose también poríferos, Balanus y erizos. Con el incremento de la profundidad comienza a desarrollarse una comunidad animal particularmente dominada por suspensívoros y predadores como anémonas, Balanus, ascidias y erizos. En el área de sustratos blandos la diversidad es menor que en sustratos duros pero mayor en el área de sustratos mixtos. Siendo dominante a los 15 y 20 m macroalgas feófitas, a los 25 m la cobertura de las algas se ve reducida a pequeños parches y dominan aquí y a 30 m bivalvos, poliquetos y ofiuros. La comunidad menos diversa resultó ser la de sustrato mixto siendo dominantes a los 15 m las féeofitas y rodófitas en menor medida, esto ocurre también a los 20 m y a los 25 m dominan las rodófitas, a los 30 m dominan poliquetos, Balanus y briozoos.La comunidad sustratos duros sería la mas diversa no solamente por su ubicación en la entrada del fiordo, lo cual favorece en particular a los suspensívoros, debido a la entrada de aguas ricas en materia orgánica sino también a que su superficie horizontal le otorga una ventaja con respecto a las otras dos estaciones porque el impacto del hielo en esta comunidad seria mucho menor.