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Estuarine abiotic characteristics vary with tidal variations and fresh water input, both driven by climatic conditions and often cyclical climatic events. Estuaries are very productive and bivalves often represent a substantial proportion of the biomass with important ecological role. Salinity fluctuation is often a key environmental factor affecting bivalves shell growth and individual condition index (CI), parameters that in turn can be used to show the quality of the environment for one population. The stout razor clam Tagelus plebeius is an euryhaline filterfeeder species that inhabits sandy-silt tidal flats within a wide salinity range. The objective of this study was to evaluate, by survey sampling and in situ transplant experiments among sites with different salinity, whether shell growth and CI of T. plebeius vary with the salinity regime. We hypothesized a higher growth and CI related to the intermediate salinity. Clams and abiotic parameters were sampled in three estuaries of the northern Argentinian coast with different salinity regimes (Bahía Samborombón (36°19′S), Mar Chiquita coastal lagoon (37°32′S) and Bahía Blanca (38° 47′S)) and in three sites along the salinity gradient of the Mar Chiquita coastal lagoon where the manipulative experiments were also conducted. Shells were more elongated in sites with lower salinity; but the growth rate (k) and the CI were higher in sites with intermediate salinity. Furthermore, clams transplanted from intermediate salinity sites to those with higher or lower salinity showed a disadvantage in shell growth. Considering the scenario of climatic-driven salinity changes in these estuaries and given that T. plebeius is the only native bivalve species in those intertidal zones, having a relevant ecological role and conspicuous fossil presence in Holocene outcrops, our results highlight the relevance of this clam as a valuable target for future studies on conservation biology and paleobiology.

In southern South America little is known about the biotic response of marine species to large scale climate variability during the Holocene. Fossil shells of the aragonitic bivalve Retrotapes exalbidus (previously called Eurhomalea) offer the possibility to investigate climate variability in the Beagle Channel and past seasonal dynamics of sea water temperature during the mid-to-late-Holocene. This selection is based on two reasons: extant R. exalbidus preserve annual increments in the outer shell layer (Lomovasky et al., 2002 J. Sea Research 48, 209-216); and, although not very common as other venerids, this species is well preserved in different Holocene marine outcrops along the channel. Holocene fossil shells of R. exalbidus were sectioned, polished, photographed and measured, and after examination three of them were selected for chemical sampling. In each case, one-half of the shell was used to resolve the annual growth bands and the other half was used for stable isotopes sampling. In addition, a fragment of each shell was used to C-14 dating performed in the Poznań Radiocarbon Laboratory with the accelerator mass spectrometry (AMS) technique. Ontogenetic ages were measured by counting the annual growth increments under a stereo microscope. Our results show differences between the three specimens. In the ontogenetic oldest individual (14 years), which gave a calibrated mean value age of 3839 BP, the δ18O values ranged from 1.53‰ to –1.16‰. The two other specimens (8 years), with calibrated mean ages of 5190 BP and 431 BP, gave δ18O values from 1.55‰ to 0.44‰ in the oldest specimen, and from 1.29‰ to 0.72‰, in the youngest one. Besides, we found variations in annual growth increment widths at different radiocarbon ages, probably correlated with environmental changes over the mid-to-late Holocene. We correlated the most positive δ18O values with winter and the most negative δ18O with summer. In addition, the summer values around 3800 yr BP are more negative tha

The Patagonian scallop Zygochlamys patagonica occurs on the SW Atlantic shelf of South America between 36�S to 55�S in beds at depths around 100 m. Stable oxygen and carbon isotope ratio analysis in scallops from four large beds (Uruguay, 36�17#S, Reclutas, 39�20#S, Tango B, 42�30#S, and Beagle, 55�10#S) in combination with condition indexes and oceanography date strongly suggest that shell growth increments in this species are formed annually. Most translucent growth bands coincide with low values of both d13C and d18O(i.e., they are formed at times of high remineralization activity and of higher water temperature). This pattern is consistent throughout the distributional range. Given the specific Argentinean shelf oceanography, higher temperatures at this depth occur during austral autumn—beginning winter in the region of Reclutas and Tango B but during summer–autumn at the northern and southern limit of the distributional range. Year-round investigations at Reclutas beds indicate that translucent shell growth bands coincide with a pause in somatic growth during austral winter, when energy investment is shifted from somatic growth to gonad development.

Growth, productivity and potential for exploitation of the clam Tawera gayi from shallow waters (3 to 5 m) of Ushuaia Bay, Beagle Channel were investigated. Mean abundance and biomass in the study area were 1091 ± 737 ind m^-2 and 901.83 g SFWM m^-2 (shell free wet mass) respectively. Individual growth was described best by the von Bertalanffy growth model with the parameter values H? = 28.03 mm, K = 0.288 y^1, t0 = -0.34 (r^2 = 0.83). Annual production of the population was estimated to be 120.45 g SFWM m^-2 y^-1, corresponding to a P/B ratio of 0.134 y^-1. The single negative exponential mortality model does not fit the population mortality pattern, but predation by gastropods (Xymenopsis muriciformis, Trophon geversianus, Natica sp.) appears to be the major cause of mortality. These highly mobile predators together with the comparatively slow growth and low turnover of T. gayi in Ushuaia Bay limit its potential for sustainable commercial exploitation.

The reproductive cycle of Eurhomalea exalbida (Chemnitz, 1795) in Ushuaia Bay, Beagle Channel, was studied from October 1998 to December 1999. Clams were collected monthly by SCUBA diving at 3 to 5 m water depth. The degree of sexual maturity was determined histologically (n=318). The sex ratio was 1:1 (Chi-squared p > 0.05). First sexual maturity occurred at 39 mm shell heigth in males and 40 mm in females (i.e. 4 years of age).Five distinct histological stages of sexual maturity could be established in males: (1) Early Active, (2) Late Active, (3) Ripe, (4) Partially Spawned and (5) Spent. Females had oocytes ranging from small and immature to large and developed in their ovaries throughout the year, hence we could not classify reproductive stages based on a qualitative histological scale of sexual maturity. Therefore, quantitative measures using an image analyzer were used. In males a high percentage of ripe individuals were present between January and August. The percentage of early active and partially spawned individuals was highest in November 1998 (70%) and 1999 (60%). In females, the mean number of developed oocytes per gonadal area, the mean diameter (minor axis) of oocytes, and the percentage of gonadal area occupied by oocytes were lowest in November, indicating a spawning peak. These results indicate synchronous gonadal development and spawning in males and females. The more intensive spawning activity in November coincides with the higher biomass and production of phytoplankton in spring.

In the hard clam Eurhomalea exalbida autofluorescent granules were detected in high concentrations in the connective tissue around the intestine and in other tissues. Autofluorescence combined with Sudan Black B and PAS positive reactions suggested that these granules were lipofuscin-like. The concentration of this material in the connective tissue (CT) around the intestine was quantified by image analysis and expressed as total area fraction occupied by lipofuscin granules. Lipofuscin concentration was distinctly better related with individual age as determined from stable isotope validated shell growth bands, than with any morphometric parameter. This relationship was described best by a Gompertz model: Lipofuscint = 24.79 * e -e 0.029 * (Age t 58.578) (N = 38; r2nl = 0.882). Age was predicted from lipofuscinCT concentration by a von Bertalanffy model: Age t = 68.00 * (1 - e -0.146 * (Lipofuscint + 0.028))0.664 (N = 38; r2nl = 0.933). Our findings suggest that lipofuscinCT concentration in E. exalbida is a function of individual age. If this holds true for bivalves in general, lipofuscin may be a suitable proxy for age in species with less clear shell growth band patterns.

Growth, mortality and productivity of the hard clam E. exalbida from Ushuaia Bay, Beagle Channel were investigated. The parameters of the von Bertalanffy growth function were estimated to be H = 73.98 mm, K = 0.180 y 1, t0 = 0.147 y. Maximum individual production amounted to 2.742 g shell free wet mass (SFWM) at 49.5 mm shell height. Animals between 40 mm and 70 mm shell height contribute most to overall population somatic production P of 133.89 g SFWM m-2 y-1. Mean annual biomass B amounted to 1122.69 g SFWM m-2 y-1. Annual P/B ratio and mortality rate Z were estimated to 0.119 y-1 and 0.141 y-1, respectively. Slow growth and low turnover makes this population less suitable for sustainable commercial exploitation.