Retrotapes exalbidus from southern SouthAmerica: are fossil shells reliable proxy archives for Holocene climate changes?

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

The reproductive cycle of Eurhomalea exalbida (Chemnitz, 1795) (Bivalvia: Veneridae) in Ushuaia Bay (54°50′ S), Beagle Channel (Argentina)

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

The bivalve Tawera gayi, a potential archive of southern South America Holocene climate variability

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

Allozymic variation in the clam genus Eurhomalea (Bivalvia: Veneriidae) along southern South American coast Variación alozímica en el género de almejas Eurhomalea (Bivalvia: Veneriidae) a lo largo de la costa sur de Sudamérica

The correspondence between allozymic variation and specific differentiation was studied in four populations corresponding to the three nominal, allopatric species of clam genus Eurhomalea (E. rufa, E. lenticularis, E. exalbida) described for southern South America. Allozyme variation scored in 12 loci was high as indicated by heterozygosity levels (15.8-20.7 %) and by the presence of only three monomorphic loci (Hk-2, Icd-2, and Xdh-1). These high estimates of allelic variability were influenced by the low levels of interspecific genetic similarity (I = 0.64) and for the high conspecific values of genetic identity observed. The high estimates of substructuring found at the species level (F ST = 0.39) contrasted with the low differentiation (F ST = 0.027) and high migration rate (Nm = 9) existing among conspecific samples. Diagnostic allele fixation coinciding with specific recognition was recorded at locus Hk-1 whereas nearly-fixed differences at loci (Adh, alpha-Gpd, Icd-1, Ldh, Odh, Pgm-3) differed sharply in frequency among species. The Wagner procedure and the neighbor-joining algorithm produced a similar tree topology highly related to the geographic distance and to their taxonomic recognition. The lack of coincidence between patterns of allozymic variation and the two distinctive shell morphs (flat and globoid) occurring in E. exalbida from Ushuaia bay do not support their taxonomic recognition.<br>Se estudió la correspondencia entre la variación bioquímica y la diferenciación específica en cuatro poblaciones correspondientes a las tres especies nominales y alopátricas en las almejas del género Eurhomalea (E. rufa, E. lenticularis, E. exalbida) descritas para la zona sur de Sudamérica. La variación alozímica registrada en 12 loci fue alta como lo indican los altos niveles de heterocigosidad (15,8-20,7 %) y por la presencia de solo tres loci monomórficos (Hk-2, Icd-2 y Xdh-1). Esta alta estimación de variabilidad alélica influyó en los bajos niveles de similitud genética interespecífica (I = 0,64) y en los altos valores de identidad genética conespecífica observados. El alto grado de subestructuración poblacional interespecífica (F ST = 0,39) contrasta con la poca diferenciación (F ST = 0,027) y la alta tasa de migración (Nm = 9) existente a nivel intraespecífica. Coincidiendo con la situación específica de estas tres especies, se observó fijación alternativa de alelos en el locus Hk-1 mientras que los loci (Adh, alfa-Gpd, Idh-1, Ldh, Odh, Pgm-3) están fuertemente diferenciados en sus frecuencias. El procedimiento de Wagner y el algoritmo de Unión al Vecino produjeron topologías similares, altamente relacionadas con la distancia geográfica y con la situación taxonómica. La falta de coincidencia entre los patrones de variación y los dos morfos (planas y globosas) presentes en E. exalbida de la bahía Ushuaia no apoyan su reconocimiento taxonómico como entidades genéticas discretas