Medidas morfológicas de moluscos de agua dulce

Name: Medidas morfológicas de moluscos de agua dulce
License: https://creativecommons.org/licenses/by-nc-sa/2.5/ar/
Keywords: Shell strength

Datos de investigación

Medidas morfológicas de moluscos de agua dulce

Autores: Cristini, Paula Andrea Icon

; Frontini, Patricia Maria Icon

; Ballarre, Josefina Icon

Publicador: Consejo Nacional de Investigaciones Científicas y Técnicas

Fecha de depósito: 27/08/2024

Fecha de creación: 2021

Clasificación temática:

Paleontología

Resumen

Las conchas de los moluscos están compuestas por dos fases: una fase inorgánica de carbonato de calcio en forma de aragonita, calcita o una combinación de ambas, y una fase orgánica en gran parte proteica, la matriz. Los fragmentos de valvas son un componente importante tanto en los ensambles de muerte como ensambles fósiles y pueden proporcionar información valiosa sobre ambientes modernos y pasados. La fragmentación puede tener diferentes orígenes: ecológicos (por ejemplo, depredación, bioturbación) y postmortem (por ejemplo, bioestratinómica, diagenética y tectónica). La resistencia a la fragmentación depende de la resistencia de la mecánica de la valva, que está relacionada con varias propiedades intrínsecas como la mineralogía, la cantidad de magnesio y proporción de materia orgánica así como el tamaño y espesor de la concha. En general, la resistencia de la valva se evalúa mediante experimentos de compresión y se define como la fuerza máxima requerida para romper un caparazón catastróficamente. Dado que las valvas de los moluscos se comportan como materiales frágiles (ausencia de procesos dúctiles) la fractura está controlada por la presencia de defectos (discontinuidades, perforaciones, cambios en la microestructura o fracturas no visibles al ojo). En este trabajo definimos la resistencia mecánica como la fuerza máxima compresiva por unidad de área requerida para romper la valva catastróficamente, o lo que se conoce como tensión mecánica o stress (fracture stress). La probabilidad de encontrar una falla o de defecto a que pueda inducir la rotura de la valva está relacionada con el volumen del espécimen. Así, en este estudio se aplicará la distribución de probabilidad de Weibull, que es un método estadístico comúnmente utilizado para determinar la resistencia de materiales frágiles que incluye la influencia del volumen y la distribución del tamaño de la falla o defecto. La distribución de Weibull permite predecir la probabilidad de fallo a cualquier nivel de tensión proporcionando información sobre la fiabilidad de un material. Se presentan los datos morfológicos ancho (mm), largo (mm), alto (mm), espesor (mm), biovolumen (mm3), peso (g), fuerza (N), volumen (cm3), Área proyectada (m2), Fracture stress (MPa) y globosidad (largo/alto) de 7 especies de moluscos de agua dulce con el que se realizó el estudio.

Métodos

Sites and shell sampling The study was carried out with the most representative mollusk species living in freshwater shallow lakes (or associated to them) of the Pampa region: the gastropods Heleobia parchappii (d´Orbigny 1835), Biomphalaria peregrina (d´Orbigny 1835), Uncancylus concentricus (d´Orbigny 1835), Physa acuta (Draparnaud 1805), Succinea meridionalis (d´Orbigny 1834–1847) and Pomacea canaliculata (Lamarck 1822) and the bivalve Musculium argentinum (d’Orbigny 1835) (for details see Tietze and De Francesco 2010; Tietze et al. 2011). The gastropods Heleobia australis (d´orbigny 1835) and Chilina parchappii (d´Orbigny 1835) were also included in the analyses (Figure 1). In spite of inhabiting estuarine environments today, H. australis has been recorded in many fossil lacustrine deposits from the region (De Francesco et al. 2013), and C. parchappii is the only species in the Pampa region that inhabits exclusively streams and has no record in shallow lakes (Tietze and De Francesco 2010). Molluscs were sampled from Nahuel Ruca (NR) lake (37°37’21’’S; 57°25’42’’W), except Heleobia australis and Chilina parchappii that were sampled in Mar Chiquita (MC) coastal lagoon and Vivorata stream (VS), respectively (Figure 2). The region is a vast grassy plain that covers central Argentina, which is characterised by a quite uniform relief, except for the presence of two mountain ranges (Tandilia and Ventania) located south of Buenos Aires province (Diovisalvi et al. 2015). The climate is temperate humid or sub-humid with a mean annual temperature of 15°C and a mean annual precipitation of 1100 mm (Feijoo and Lombardo 2007). Nahuel Ruca, like most lakes of the region, is a shallow (2 m) lake without thermal stratification except for short periods of time (Quiros and Drago 1999; Fernandez Cirelli and Miretzky 2004). Living molluscs were collected among the submerged vegetation, under stones and on the substratum. Fifty molluscs of each species were collected from life assemblages both manually (picking up by hand) and with a 0.5 mm mesh size sieve (Tietze and De Francesco 2010). Molluscs were treated with menthol to extract the soft parts and retain only the shells (Gunkel and Lewbart 2008; Gianelli et al. 2015). Thirty shells of each species were used for the morphological and mechanical characterisation (see explanation below in morphological and mechanical characterisation sections, respectively). The rest of the shells were used for the mineralogical characterisation (see explanation below in mineralogical characterisation section). Allshells presented a good taphonomic condition without signs of physical or biological destruction, a grade 0 (until 15% of damage) in the semi-quantitative scale of Cristini and De Francesco (2017). Intrinsic properties of shells Mineralogical characterization To carry out the mineralogical characterisation and estimate the percentage of organic matter in the matrix, shells of each species were ground using an agate mortar until a grain size smaller than 2 micron was achieved. First, molluscs were characterised by x-ray diffraction (XRD, Equipment PANalytical X´Pert PRO, 40kV, 40 mA, running from 5 to 90 (2 theta) of Bragg angle, steps of 0.02 degrees, Cu Kα radiation). The percentage of organic matter of the matrix was estimated by a thermogravimetric analysis (TGA, Thermo Gravimetric Analyser, Shimadzu TGA 50 H, in a platinum cell with 19 mg of sample, in synthetic air, at room temperature until 1100 °C, at 10oC per minute). Morphological characterisation Thirty shells of each species were used for morphological measurements, biovolume and globosity estimation, and mechanical experiments. Length, width, height and shell thickness were measured following Kosnik et al. (2006) (Figure 3) using a digital caliper (Wembley CD-150 resolution 0.01 mm/0.0005’). Shells were also weighed with digital balance (accuracy 0.0001 g). Biovolume and globosity were estimated, as indicators of the size and the shape, respectively. Biovolume was calculated based on morphological measurements following Powell and Stanton (1985) (Figure 3). For gastropods, operational biovolume was the equation for the volume of a cone [Bv = 1/3(π (length/2))2 x height], except for the limpet U. concentricus and the planorbid B. peregrina, in which cases a formula for a general prismatoid [Bv = 1/6(B1 + 4 M + B2) x height] and a cylinder [(Bv = π (length/2)2 x height] were used, respectively. For bivalves, the operational biovolume was calculated as the cube of the maximum anterior-posterior length (Bv = length3) (See Figure 3) (Powell and Stanton 1985). Since true biovolume is time-consuming to measure, the authors used an operational measurement of biovolume based on the conversion of easily measured shell parameters (length, width, etc.) into a cubic form. Regressions of true biovolume, obtained by filling shells with paraffin under vacuum, dissolving the shell in 10% HCl and weighing the paraffin, with operational biovolume for three mollusk species, showed correlation coefficients above 0.95, proving the efficacy of this method (Powell and Stanton 1985 and references therein). Globosity was calculated as the ratio between the height and the length for gastropods and the ratio between the height and the width for the bivalve (Lowell et al. 1994). To analyse the crystal arrangement of shells, Scanning Electron Microscopy (SEM) images were taken with a JEOL JSM-6460LV, Japan microscope. Mechanical characterisation After the morphological characterisation, the same 30 shells of each species were subjected to load measurements. All of the shells had been previously dried out at room temperature to avoid any procedure such as drying at heater temperature, which might influence in the mechanical structure of the shell, before the mechanical test. The mechanical experiments were performed using an Instron 4467 Universal Testing Instrument. The system is made up of a load frame, in which a specimen of the tested material is mounted, and that applies compression load to the specimen, and a control console that provides the calibration, test setup, load–displacement measurements and test operating controls. The instrument was equipped with a 500 N load cell, with an accuracy of 1/1000 of the load, at a crosshead rate of 1 mm/min. Thirty shells of each species were placed in the position of maximum stability (maximum supported area), with the shell aperture face down (West and Cohen 1996), between plates and loaded until breaking (Figure 4(a)). Load vs. displacement data for each shell of each species were plotted and the maximum load (Pmax) was measured in Newton (N, unit of force) (Figure 4(b)). Shell strength of 30 shells of each species was calculated as the force measured in Newton (maximum load applied) divided by the resistant area of the shell (m2), which was defined in this study as fracture stress and expressed in Pascal (Figure 5). The resistant area of the shell was estimated for each species as the projected area from the cubic form of the species (explained in Morphological characterisation section) and the position of the shells in the mechanical experiments (explained in the mechanical characterisation section). For gastropods, the projected area was calculated from the triangle of a cone (A = length x height/2), except for the limpet U. concentricus and the planorbid B. peregrina in which cases the equation of an ellipse (A = length/2 x width/2 x π) was used. For the bivalve, the projected area was also calculated from an ellipse (A = length/2 x height/2 x π).

Palabras clave: Shell strength, Weibull, Molluscs, Taphonomy, Freshwater

Alcance geográfico

.

Identificador del recurso

URI: http://hdl.handle.net/11336/243139

Colecciones

Datos de Investigación(IDEAN)
Datos de Investigación de INSTITUTO DE ESTUDIOS ANDINOS "DON PABLO GROEBER"

Datos de Investigación(INTEMA)
Datos de Investigación de INST.DE INV.EN CIENCIA Y TECNOL.MATERIALES (I)

Citación

Cristini, Paula Andrea; Frontini, Patricia Maria; Ballarre, Josefina; (2024): Medidas morfológicas de moluscos de agua dulce. Consejo Nacional de Investigaciones Científicas y Técnicas. (dataset). http://hdl.handle.net/11336/243139

Condiciones de uso

Las buenas prácticas científicas esperan que se otorgue el crédito adecuado mediante una citación. Utilice un formato de citación y aplique estas normas de reutilización.

Excepto donde se diga explícitamente, este item se publica bajo la siguiente descripción: Creative Commons Attribution-NonCommercial-ShareAlike 2.5 Unported (CC BY-NC-SA 2.5)