Description: 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 than around 5000 years or 500 years BP. These findings correlate well with an episode of cooling at ca. 5000 BP, followed by a period of ameliorization (the Hypsithermal at ca. 4000 BP), and towards the end of the Holocene, at ca. 500 years BP, a new cooling event was recorded. This sclerochronological study of the growth patterns and the oxygen isotope ratios in fossil R. exalbidus shells demonstrated that this species clearly exhibited annual cycles showing seasonality patterns through the mid-to-late Holocene, providing an opportunity to analyze intra-seasonal time scales in the fossil record. This study was supported jointly by the CONICET and the DAAD and was made as part of the Scientific Visit of SG to the AWI and the international cooperation from MINCyT and BMBF to BL.

Description: The venerid Tawera gayi could be a suitable Holocene bioarchive for Southern South-America given that it is found in a wide distribution range from the Beagle Channel (54º 50´ S) to 33º S along the Pacific coast, and to the North Patagonia (36°S) in the South Atlantic. In the Beagle Channel, both extant T. gayi populations and shell beds of mid-Holocene origin can be found. On the other hand there is reliable life history information in modern populations (Lomovasky et al. 2005 J. Appl. Ichthyol. 21, 64-69), i.e. shell growth patterns and the confirmation of the annual periodicity of the translucent bands. Finally, the shells provide geochemical proxies, e.g., d18O for temperature reconstruction. In order to investigate climate variability in the Beagle Channel, the individual age, growth increments and isotopes analyses of modern and fossil shells of T. gayi were used. The shell cuts of T. gayi showed a pattern of alternating broad opaque and narrow translucent bands, which were confirmed by acetate peels too. In general, the translucent bands showed a pink to purple colour, similar to internal part of the shell. Both modern and fossil populations showed a maximum age of 13 years old. Radiocarbon dating revealed ages ranging between ca 3800 to 4400 years b.p. in the fossil shells corresponding to the Holocene Climate Optimum. The δ18O values obtained in fossil shells ranged from 1.316‰ to –0.064‰ We correlated the most positive δ18O values with winter forming the translucent bands and the most negative δ18O with summer. The comparison of the von Bertalanffy growth curve showed no difference in the H∞ between modern (32.50 mm; Confidence Interval (CI)=31.07, 33.94) and fossil (33.23 mm; CI=31.94, 34.51) populations, but higher values (p 〈 0.05) were observed in the growth rate k and t0 in the modern (0.37 (0.31, 0.42) and 1.12 (0.98, 1.25) respectively) than fossil shells (0.24 (0.21, 0.27) and 0.57 (0.44, 0.69) respectively). This study demonstrated that this species clearly exhibited annual cycles showing seasonality patterns from the mid-Holocene to the present with translucent bands corresponding to slow or halted growth formed in fall/winter; the growth rate was lower during the past warm epochs than the present possible related to a different productivity in the Channel and/or a lower metabolic rate of the clams exposed to a higher temperature.

Description: 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.

Description: 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.

Description: 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.

Description: 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.