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: Body size, geographical distribution and biomass make Adamussium colbecki (Smith 1902) one of the most important bivalve species in the Antarctic. Based on samples collected in austral summer 1999/2000 in Terra Nova Bay, the annual formation of shell growth bands visible on x-ray photographs was verified by stable isotope analyzis. A general von Bertalanffy growth function was fitted to size-at-age data of 25 individuals (H*= 108.86 mm, K= 0.114 y-1, t0= -0.367, D= 1.284). Maximum age of A. colbecki is likely to be more than 18 years. Somatic production calculated from mass-specific growth rates was 235 kJ m-2 y-1. Gonadal productivity estimated by an average value for reproductive output of pectinids amounted to 59 kJ m-2 y-1. Annual somatic and gonad production-to-biomass ratios (P/B) were 0.199 and 0.043, respectively. According to its consumption and production A. colbecki is likely to play a significant role in the trophic web of Terra Nova Bay.

Description: Standard metabolic rates of the endemic Antarctic scallop, Adamussium colbecki (Smith, 1902), were measured in austral summer and under simulated winter conditions. Average mass-specific metabolic rates were significantly different between summer (151.17 ± 45.06 µl O2 g-1 h-1) and winter (106.52 ± 39.65 µl O2 g-1 h-1) animals. The overall metabolic rates of A. colbecki are comparable to those of other Antarctic bivalve species, but well below those of temperate scallop species. Data for 24 scallop populations (13 species) from different latitudes give no evidence for elevated metabolic rates in A. colbecki as suggested by the concept of metabolic cold adaptation. A world-wide comparison of metabolic rate and overall growth performance of scallops indicates that in the Antarctic scallop the energetic advantage of low basal metabolism does not counterbalance the disadvantage of the prolonged seasonal period of food shortage.

Description: Regular sea urchins of the families Cidaridae and Echinidae are widespread and sympatrically occurring epibenthic species in Antarctic waters. Food preference and water depth distribution of the five most abundant species (Ctenocidaris gigantea, C. spinosa, Notocidaris mortenseni, Sterechinus antarcticus, S. neumayeri) were analysed based on trawl and photograph samples. Both diet and water depth contribute to niche separation among these species. All sea urchins consume bryozoans and sediment, but echinids feed predominantly on diatoms in the fluff, when available. Cidarids do not consume diatoms, most likely owing to morphological constraints; their typical food consists of sponges and hydroids. C. spinosa and S. neumayeri prefer shallow water depths, whereas N. mortenseni and S. antarcticus prefer deeper regions. C. gigantea is the most variable species regarding food composition and living depth.