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The commercially fished striped venus clams Chamelea gallina and C. striatula (Bivalvia: Veneridae) are difficult to distinguish by inexperienced observers and the taxonomy of these species is still an issue of discussion. The differences in shape between C. gallina and C. striatula from Portuguese coastal waters were studied through conventional linear and geometric morphometric analysis, using both contour (elliptic Fourier analysis) and landmark-based methods. The relationships shell length vs. height, width, and total weight were significantly different between species. However, because there was a considerable overlap in the data sets, the species could not be distinguished using any combination of those linear measurements. Geometric morphometric methods provided shape variables that led to 0-6% misclassification rates between species; linear morphometric measures led to 16.8% error. Contour analysis revealed differences primarily in the shell umbo and lunular area. The umbo was more "sharp" and the lunula less pronounced in C. striatula than in C. gallina. Generalized procrustes superimposition (landmark analysis) showed that the main differences between species reside in the length of the pallial sinus. Thus, an index was developed (PI: Pallial Index = pallial sinus length/shell length), which was successfully used to separate the species (with 100% correct classification), i.e., specimens with PI lower than 0.119 belonged to C. gallina, whereas greater PI values were attributed to C. striatula. The use of these geometric morphometric methods allowed the detection of differences in shape between these two species and to develop an easy-to-use identification index. We encourage the development of analogous indices that apply the methods of geometric morphometrics to distinguish between other species whose identification is complicated.

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suMMary: ecological aspects of 75 megabenthic bivalve species found on the south coast of portugal (from Vila real to sagres) were studied based on the information provided by seasonal bivalve surveys from 2000 to 2007 (8600 hauls, simultaneously using a razor shell dredge and a clam dredge). of the 75 species identified, 36 occurred less than 30 times, and thus were considered rare in the area. the remaining 39 bivalves were analysed according to their occurrence, bathymetric distribution, geographic preferences, seasonal preferences, burying-depth, beach exposure and river mouth proximity. all species belonging to the pharidae and solenidae families (razor shells) and most tellinidae were significantly more frequent in the razor shell dredge (indicating deeper burying in the sediment), whilst the families cardiidae and donacidae were significantly more frequent in the clam dredge (indicating that these species are probably closer to the sediment surface). as for the season, most bivalve species occurred similarly in both seasons (19 sp; 49%), but a large proportion were more abundant during the autumn surveys (17 sp., 44%), whereas only three species were commoner during the spring surveys. Most species belonging to the families cardiidae and Mactridae were commoner in the autumn surveys. the spatial distribution differed between species and cluster analysis identified four communities with greater geographic affinity. species belonging to the family cardiidae were preferably found in the western part (WB) and the eastern part (s) of the study area, whereas the families donacidae, Mactridae and tellinidae occurred mainly in the central area (eB) and the eastern (s) coastal sectors. overall, shallower species (modal depth at 3-6 m) showed greater occurrences and abundances than the deeper ones, and the depth pattern observed did not change between seasons. donacidae and Mactridae (except Mactra glauca) were represented essentially by shallow species, whereas cardiidae (except Acanthocardia tuberculata and Cerastoderma edule) included mostly species occurring in deeper waters. Bathymetric sequence of species distribution within some families was observed.Keywords: bivalves, geographic distribution, bathymetric distribution, beach morphology, river proximity. resuMen: Ecología de comunidades de bivalvos megabentónicos en playas de arena de la costa meridional de Portugal. -se estudiaron algunos aspectos ecológicos de 75 especies de bivalvos megabentónicos en la costa meridional de portugal (desde Vila real a sagres), basados en la información obtenida a lo largo de varios muestreos estacionales realizados entre el año 2000 y 2007 (8600 muestras). las muestras se obtuvieron utilizando simultáneamente un rastro para capturar navajas y un rastro para capturar almejas). de los 75 taxones identificados, 36 aparecieron menos de 30 veces, considerándose raros en el área. de los 39 taxones de bivalvos restantes se analizaron los patrones de presencia, distribución batimétrica, preferencias geográficas y estacionalidad y ...

This study reports the morphometric relationships and relative growth of 20 uncommon bivalve species collected along the Algarve coast (southern Portugal). Morphometric relationships were established between three linear variables (shell length, height and width) and one ponderal variable (total weight) and the relative growth between variables (isometry vs allometry) was analysed in order to assess their variation throughout ontogeny. In addition, morphometric indices (elongation, compactness, convexity and density) based on ratios of those linear and ponderal variables were calculated in order to further characterize morphologically the bivalve species. A total of 2512 individuals belonging to nine bivalve families were analysed, comprising specimens with broad ranges in both shell length (11.8–109.0 mm SL) and total weight (0.2–354.6 g TW). All morphometric relationships were highly significant (P < 0.001) and displayed invariably high correlation coefficients (r = 0.727–0.998). Among a total of 60 morphometric relationships, 27 isometries, 25 positive allometries and 8 negative allometries were registered. The morphometric indices displayed a remarkable variation among taxa, reflecting the high morphological diversity of these miscellaneous bivalve species. Discriminant analysis provided a spatial visualization of the species morphometric variables that further evidenced their main shape features, the distinctness between some species and families (e.g. Pharidae and Cardiidae) and the morphological resemblance among some species belonging to other families (e.g. Veneridae and Tellinidae). Overall, this information is useful and has practical application in diverse research fields, including studies on systematics and taxonomy, physiology, biology, ecology, fisheries assessment and management.