Population dynamics and ecology of the surf clam Donax serra (Bivalvia, Donacidae) inhabiting beaches of the Benguela upwelling system

jlaudien [ at ] awi-bremerhaven.de


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.

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Laudien, J. (2002): Population dynamics and ecology of the surf clam Donax serra (Bivalvia, Donacidae) inhabiting beaches of the Benguela upwelling system , Berichte zur Polar- und Meeresforschung, 99 p. .

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