Federico Márquez

Shell shape as indicator of pollution in marine gastropods affected by imposex

Tributyltin (TBT) and other pollutants are present in Patagonia, and are associated with maritime traffic and human activities. Cause–effect relationship between TBT and imposex development in females of marine gastropods is well documented. We tested whether the imposex incidence associated with TBT pollution is related to detectable shell-shape variations in the edible marine snail (Buccinanops globulosus). We compared shell shape of B. globulosus in six contiguous sites on a gradient of imposex and maritime traffic, by using geometric morphometric analysis. Our results indicated that the registered differences in shell shape are associated to imposex incidence in the harbour zone where previous works have detected TBT pollution. Gastropods from areas of high maritime traffic presented a rounded shell with a shorter spire, and a smaller relative size of the shell aperture, whereas the opposite shape (fusiform shape, elongated-spired shell and bigger relative size of the shell aperture) occurred in gastropod shells from areas of low maritime traffic. Shell variation registered here could be useful to detect TBT pollution in populations of B. globulosus and another neogastropod species.

Use of shell-shape to discriminate between Brachidontes rodriguezii and Brachidontes purpuratus species (Mytilidae) in the transition zone of their distributions (south-western Atlantic)

Fil: Van Der Molen, Silvina. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Nacional Patagonico; Argentina

Shell variability in Tawera gayi (Veneridae) from southern South America: a morphometric approach based on contour analysis

This study analyses the significance of shell morphological variations in the venerid clam Tawera gayi, a typical element of shallow marine soft bottoms in southern South America and the most common species recovered from Late Quaternary marine deposits along the Beagle Channel, Tierra del Fuego. Fossil and modern Tawera shells from different localities around the southern tip of South America were analysed using the Elliptic Fourier Analysis (EFA) method. Taking into account the palaeontological history of this genus in the southern hemisphere, EFA was also performed on shells of Tawera congeners from South Africa (T. philomela) and New Zealand (T. spissa). The use of EFA permitted the distinction between the three Tawera species and geographical differentiation in the T. gayi groups. These morphological variations of T. gayi appear best related to ecophenotypic plasticity as a response to different environmental conditions, although the palaeobiogeographical history of Tawera in South America cannot be ruled out.

Phenotypic variation of south-western Atlantic clam Mactra isabelleana (Bivalvia: Mactridae)

The phenotypic shell shape variation of Mactra isabelleana was tested using the geometric morphometric method. Four localities were sampled along the Rio de la Plata estuary and the coast of Buenos Aires province. Principal component analysis and canonical variates analysis of the first principal components were performed to reveal the shell variation and differences among localities, respectively. The specimens from different microhabitats mostly overlapped, although differences in shape were observed in the development of the umbo, the enlargement of the dorsoventral axes and the elongation of the posterior end. The ecological and physical parameters that could influence shell shape variation are discussed.

Benthic diversity and assemblage structure of a north Patagonian rocky shore: a monitoring legacy of the NaGISA project

Fil: Rechimont, Maria Emilia. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Nacional Patagonico; Argentina

Shell damage in the Tehuelche scallop Aequipecten tehuelchus caused by Polydora rickettsi (Polychaeta: Spionidae) infestation.

The different types of shell damage caused to the commercially valuable Tehuelche scallop (Aequipecten tehuelchus) by the polychaete Polydora rickettsi are described. X-rays, computerized tomography, shell sections, scanning electron microscopy, Energy Dispersive X-ray analysis (EDAX), mineralogical analyses and geometric morphometrics were applied to that end. Scallop shells presented three types of damage: (1) spots, (2) calcareous alterations, and (3) mud blisters. Microstructural alterations consisted of a simple conchiolin membranous layer in the case of spots, a series of interleaved layers of different degree of calcifications in calcareous alterations, and two different surface morphologies (muddy and mucous layers) in mud blisters. Damage was localized mainly along concentric growth rings, coincidentally with the location of most burrows, as shown by X-ray. Mineralogical analysis showed that in all cases (including non-infested shells) calcite was the calcium carbonate polymorph present. Geometric morphometrics showed that only 5% of shape variation was explained by infestation with P. rickettsi, irrespective of the type of damage. Number of worms per infested shell varied significantly among four beds. Left shells (upward-oriented) were significantly more affected than right shells, which are in closer contact with the bottom.

Shell shape as a biomarker of marine pollution historic increase

Buccinanops globulosus is a TBT sensitive marine gastropod, classified as a good indicator of imposex incidence and used as a model to study adverse contamination effects. Population and maritime industries has incremented pollution in Nuevo gulf harbor since 1970s, promoting morphological changes in B. globulosus shell shape. We study the shell shape of the species comparing present days specimens from the harbor zone with those collected in the same zone before the increasing of maritime activity and pre-Hispanic archaeological Middens. We demonstrated that harbor pollution produces globular shell shape in B. globulosus, an effect that probably allows gastropods to isolate themselves from the external adverse environment. On the contrary, shells from pre-Hispanic periods, unpolluted sites and those collected before the expansion of maritime activities, presented an elongated shell shape. Our study confirms that shell shape variation in marine gastropods can be used as a biomarker of harbor pollution.

A histopathological survey of the razor clam Ensis macha (Pharidae) along the Patagonian Argentina coast

This is the first study performed to determine the health status of the razor clam, Ensis macha, including six different populations along Argentina Patagonian coast and one of Chile. The parasites and pathologies affecting E. macha were analyzed and their prevalence and mean intensity values were calculated. To establish which factors affect the presence and intensity of infection, Generalized Linear Models (GLMs) were applied. Basophilic inclusions, ciliates, coccidians protozoans and turbellarians were found. We report an Aporocotylidae digenean and hemocyte infiltrations. None of the parasites is OIE (World Organisation for Animal Health) notifiable, and none seemed to be pathogenic, with the exception of the digenean. The prevalence of the parasites was affected mainly by environmental factors (such as site of sampling and season) instead of intrinsic conditions of the clam (such as size, condition index, sex and gonadal stage). On the other hand, the maximum intensity of parasites was not only related with cold seasons but also with the partially spawned gonadal stage of E. macha. During this stage, the clams would need to store energy for the next gametogenesis cycle, might be more susceptible to infection by the parasites.

Insights on the history of the scorched mussel Brachidontes rodriguezii (Bivalvia: Mytilidae) in the Southwest Atlantic: a geometric morphometrics perspective

Abstract Some authors support the idea that an important part of the Miocene marine mollusc fauna is still represented in the Argentinean Province. The fossil mollusc Brachidontes lepida (Philippi) is considered a subspecies of the extant B. rodriguezii (dˈOrbigny), a taxon currently present in the Argentinean Biogeographic Province. The aim of this study is to investigate the shell shape relationship between B. lepida and B. rodriguezii using geometric morphometrics. Samples of B. rodriguezii (n = 63) from four localities distributed in Uruguay and Argentina, of B. lepida from the Paraná Formation (n = 26) and of two other extant Brachidontinae present in the region were included in this study. Canonical variate analyses showed that extant species differed in shell shape, with the discriminant function properly allocating 93% of the individuals to their respective species. Using the extant discriminant function, approximate 92% of B. lepida individuals were allocated to B. rodriguezii. This result suggests that B. lepida is more similar in shell shape to B. rodriguezii than to the other extant species present in the region. Considering the material from the Pliocene of Cerro Laciar and from the Pleistocene deposits of Buenos Aires and Bustamante, the presence of B. rodriguezii from the Late Miocene in the warm temperate region is discussed.

Allometric differences on the shell shape of two scorched mussel species along the Atlantic South American Coast

Scorched mussels (Bivalvia: Mytilidae) are an important component of rocky shore assemblages around the world. The mussel beds of the mid-intertidal coasts of the south-western Atlantic are composed of two species of scorched mussels, which dominate the physiognomy and structure of the rocky shore communities. The present study investigates the variation in the shell-shape allometric trajectories of both species of mussels throughout their distributional range in the southwestern Atlantic coast, from latitude 34°S to 53°S. Shells of Brachidontes rodriguezii were collected at two Uruguayan localities and four Argentinean localities, while shells of Perumytilus purpuratus were collected at six Argentinean localities. Shell shapes of the specimens were studied by geometric morphometrics using landmark and semi-landmark methods. Different evolutionary histories and thermal regimes characterize these two species. Brachidontes rodriguezii seems to have a long history in the region and is present in a restricted area, the warm temperate region of the south-western Atlantic, while P. purpuratus seems to have a recent history and is present in a wide thermal range, involving the cold-temperate regions of the south-western Atlantic and south-eastern Pacific and the warm-temperate region of the south-eastern Pacific. Based on these considerations, we prompt the hypothesis that allometric trajectories of P. purpuratus shell shape is more variable and adapted to a specific habitat than B. rodriguezii. The impact of allometry was larger in P. purpuratus than in B. rodriguezii. Graphical evidence was gathered, showing that these two species differ in allometric growth forms and that the allometric shell morphology changes reflect environmental constraints and adaptation. While the intertidal mussel beds of the South American coasts are ecologically similar in appearance, we found that the allometric variation of the scorched mussels occurs in response to diverging evolutionary processes such as phenotypic plasticity in P. purpuratus and canalization in B. rodriguezii.