Evaluación del mejillín Brachidontes rodriguezii como modificador del hábitat en ambientes intermareales de sustrato móvil y efectos sobre la estructura comunitaria asociada

Abstract

Uno de los principales objetivos de la ecología bentónica ha sido entender los mecanismos que regulan la relación entre organismos y sedimentos. Un habitante bentónico de los intermareales en Argentina, con un gran impacto sobre la biodiversidad, el funcionamiento y la estabilidad de los ecosistemas, es Brachidontes rodriguezii. Los estudios de esta especie se han desarrollado casi exclusivamente en intermareales rocosos; el objetivo general de esta tesis es evaluar el rol del mejillín como estructurador de ambientes intermareales de sustrato móvil de la Bahía San Antonio (Río Negro) y los consecuentes efectos de esta modificación de hábitat sobre la estructura comunitaria asociada. El estudio se llevó a cabo en dos sitios, en los horizontes inferior (HI) y medio (HM) del mediolitoral. Para caracterizar los sitios, se evaluó la granulometría y la velocidad de flujo, y para evaluar las características poblacionales y biológicas de B. rodriguezii, se analizó la estructura de tallas, la densidad, el índice de condición y el ciclo reproductivo. Además, se analizó la fuerza bisal y el número y grosor de hilos bisales y se realizó un diseño experimental para evaluar la variación de la densidad de mejillines en el peso y tamaño de grano al que se adhieren, el sedimento y porcentaje de agua retenido en la matriz bisos-sustrato y la fauna asociada. Ambos sitios presentaron un amplio rango de tamaño de grano y velocidad de flujo, propios de cada horizonte. Independientemente de las estaciones, B. rodriguezii se adhiere preferencialmente a los granos de mayor tamaño. La fuerza bisal varió estacionalmente entre horizontes y sitios, y fue mayor en mejillines adheridos a una gran cantidad de sedimento y/o a sedimento de mayor tamaño. El número de hilos bisales fue mayor en aquellos horizontes con menor densidad de mejillines y el grosor de éstos fue variable entre horizontes y sitios. En ambos sexos se observó maduración y desove durante todo el año, con picos durante el verano. El índice de condición varió solamente entre meses y sitios. Las densidades fueron mayores en HI de ambos sitios. En todas las estaciones, se observaron diferencias significativas en la estructura de tallas entre horizontes en cada sitio. La fauna asociada al mejillinar presentó diferencias entre estaciones, sitios y horizontes, respondiendo este arreglo faunístico principalmente a la acción conjunta del peso del sedimento adherido a los bisos, al porcentaje de agua retenido en el sedimento y a la velocidad de flujo. Al manipular la densidad de mejillines se observó que a mayor densidad, aumentaron la abundancia de organismos asociados y el peso y tamaño de grano del sedimento adherido a los bisos y disminuyó el peso retenido por la matriz mejillines-sustrato. Estos resultados fueron variables a lo largo del año, por lo que se discute si la densidad en sí misma tuvo efecto en las variables analizadas o si las densidades elegidas en el diseño experimental fueron suficientemente contrastantes como para detectar los efectos mencionados.

One of the main objectives of the benthic ecology has been to understand the mechanisms that regulate the relationship between organisms and sediments. Brachidontes rodriguezii is a benthic species inhabitant of the intertidal in Argentina, with a great impact on biodiversity, functioning and stability of ecosystems. Studies focused on this species have been developed almost exclusively in rocky intertidal. The general objective of this thesis is to evaluate the role of the mussel as a structuring species in the intertidal environments of mobile substrates of San Antonio Bay (Río Negro) and the consequent effects of this habitat modification on the associated community structure. The study was carried out in two sites (Punta Delgado: PD and Punta Verde: PV), in the lower and middle levels of the intertidal. Granulometrics and flow velocity were evaluated to characterize the sites, and the size structure, density, condition index, shell morphology, shell thickness, and the reproductive cycle were studied in detail to evaluate population and biological characteristics of B. rodriguezii. The byssal strength and the number and thickness of byssal threads were also analyzed. In addition, an experimental design was carried out to assess the effect of the mussel´s density on the weight and grain size to which they adhere, the sediment and percentage of water retained in the byssus-substrate matrix and the associated fauna. Both sites presented a wide range of grain size and flow velocity, typical at each level. Regardless of seasonality, B. rodriguezii is mostly attached to larger grains. The byssal strength varied seasonally between vertical levels and sites, and was higher in mussels adhered to a large amount of sediment and / or larger sediments. The number of byssal threads was higher in those levels with lower density of mussels and the thread thickness was variable between levels and sites. Female spawning was observed mainly in summer while males released gametes with peaks throughout the year. The condition index varied only between seasons. Along with the highest densities, the lower level of both sites presented the longest mussels, with the lowest morphometric index Width / Length (ET/LT) as well as the lowest shell thickness, while the middle level of both sites presented the lowest densities, the highest morphometric indexes ET/LT and higher shell thickness. On the other hand, for the morphometric index Width / Length of the mussels in PD, variations were observed between months and not between levels, whereas in PV this index showed only variations between levels. In all the seasons, significant differences were observed in the size structure between levels in each site. The fauna associated to mussels presented a large number of species and abundance of organisms. The structure of the communities associated to mussels, although it differed between levels and seasons, mainly varied between levels, responding mainly to the joint action of the weight of the sediment adhered to the byssus, to the percentage of water retained in the sediment and to the speed of flow. The mussels retained greater sediment weight in the middle level at both sites, while the grain size of the sediment retained only varied between PV levels, being higher in the medium level. The water content present in the sediment was higher in the lower level for PD and PV, while in PV these percentages were even higher in winter. When manipulating the density of mussels it was observed that at higher density, the abundance of associated organisms, the weight and grain size of the sediment adhered to the byssus and the water contained in the musselssubstrate matrix, increased. However, the weight of the sediment retained by the musselsubstrate matrix decreased. These results were variable throughout the year, so it is discussed whether the density itself had an effect on the variables analyzed or if the densities chosen in the experimental design were sufficiently contrasting to detect the aforementioned effects.