Seasonality and trophic diversity in molluscan assemblages from the Bay of Tunis (southern Mediterranean Sea)

Soft bottom molluscan assemblages from the Bay of Tunis have been studied in order to analyse their seasonality and trophic diversity in relation to environmental variables. A total of 147 species of molluscs was identified, with gastropods displaying the highest species richness and bivalves the highest abundances, and including five non-indigenous species such as the dominant bryozoan grazer Polycerella emertoni. Carnivorous and scavenger gastropods were among the most frequent species, reflecting a diverse benthic community. Seasonal changes were significant, being more acute at 3-4 m than at 10-15 m depth, and were correlated mainly with seawater temperature and percentage of organic matter in the sediment. The high affluence of tourists in summer was coincident with high decreases in species richness and abundance of molluscs, together with a strong siltation of the sediment. Nevertheless, most trophic groups persisted and the trophic diversity was relatively high. Significant relationships were found between the index of trophic diversity and Shannon-Wiener diversity and evenness indices, suggesting that the identity of the species with its particular trophic trait, together with the balanced distribution of the individuals among the species would be the drivers for the maintenance of the molluscan food web under environmental stress. The abundance of P. emertoni altered the trophic structure of the molluscan assemblage, bringing the ectoparasite trophic group to an unusual peak of dominance. Soft bottom molluscan assemblages of the Bay of Tunis should be taken into account in monitoring programs for anthropogenic impacts and for non-indigenous species trends throughout the Mediterranean basin.Versión del editor0,568

Molluscs as bioindicators of the regression of a Zostera marina bed in southern Spain

Seagrass beds are experiencing a sharp decline worldwide and, unfortunately, a continued decrease is expected, mainly due to anthropogenic influence, with negative repercussions for coastal biodiversity. One of these declining seagrasses is Zostera marina L. which is the most widespread in the northern hemisphere (North America, Europe and Asia). One of the most recent observed cases of eelgrass decline occurred in the Special Area of Conservation “Acantilados de Maro - Cerro Gordo” (southern Spain) between 2005 and 2007. These eelgrass beds were probably the deepest ones (5–18 m depth) in Europe and supported a highly diverse associated fauna (Rueda et al., 2009). Different methods have been designed for evaluating ecological integrity or condition status of coastal waters. In general, there is an increasing interest in developing assessment tools for different physicochemical or biological elements of the ecosystems. Taking this into account, what might be the characteristics of a good ecological indicator? In Europe, and within the Water Framework Directive (Directive 2008/56/EC), biological elements are limited to composition, abundance (and biomass) of phytoplankton, other aquatic flora, benthic invertebrate fauna, and fishes. The data used to quantify these biological elements describe distributions/gradients, ratios, biodiversity indices and classification schemes. Environmental science must complement the structural ecosystem properties through an approach towards the ecosystem function and dynamics. In this way, trophic webs constitute probably an appropriate type of measurement of the ecosystem function. Molluscs constitute ca. 25% of the benthos and their dynamics have been shown to reflect the dynamics of the whole community. Therefore, we analyzed the changes in the molluscan taxocoenosis associated with the regression of a Zostera marina bed from the SAC of Acantilados de Maro - Cerro Gordo from 2005 to 2009. We analyzed changes in biotic variables: species richness, abundance, diversity index, biotic indexes and trophic diversity of the molluscan assemblage, and abiotic variables: grain size and percentage of organic matter in sediment. The aim was to check which of these indicators expressed better the environmental changes caused by the regression of the eelgrass bed. Species richness was the best indicator for showing loss of species and the Trophic Index (Danovaro et al., 2004) was the indicator that better expressed the drastic change of the assemblage, with a strong reduction of the trophic groups. Regarding biotic indexes, AMBI, MAMBI and MEDOCC, the absence of eutrophication makes these indicators less accurate to reflect the environmental change caused by the loss of the eelgrass bed. In fact, the best descriptor for this event would be the integrity of seafloor, because the regression of the eelgrass bed changes completely the bottom landscape. The loss of seagrasses and its effects are going to be one of the important topics in future scientific studies due to this continuous declining trend, so tools allowing us to anticipate this threat are becoming essential. In southern Spain, protection of seagrasses is urgently needed for preserving not only the beds but also the diverse communities associated with them

First record of the starfish Luidia atlantidea Madsen, 1950 in the Mediterranean Sea, with evidence of persistent populations

The starfish Luidia atlantidea, an echinoderm known hitherto from infralittoral and circalittoral bottoms of the northwestern African coasts, is reported for the first time in the European margin and in the Mediterranean Sea. A total of 31 specimens of different sizes (disc diameters from 0.8 to 2.9 cm) were collected from the mechanized dredges fleet targeting four commercial bivalves (Acanthocardia tuberculata, Callista chione, Chamelea gallina, Donax trunculus) between February and July 2013 in the northern Alboran Sea (southern Spain). Most individuals had broken arms due to the collecting gear. The specimens were generally collected in low numbers, at shallower depths (1-11 m) than the bathymetric range reported for this species along the northwestern African coasts (10-80 m). The data provided here, and supported by old specimens collected between 1980-1990, suggests that local populations of L. atlantidea occur in shallow infralittoral soft bottoms of the northern Alboran Sea. This constitutes a new extended northeastern limit for the distribution of L. atlantidea and increases the known number of Luidia species in the Mediterranean Sea.Postprint0,708

Invertebrados de afinidad subtropical en descartes de la flota marisquera del mar de Alborán

En el mar de Alborán confluyen masas de agua y organismos de diferentes regiones biogeográficas (Lusitánica, Mauritánica y Mediterránea), lo cual le confiere un enorme interés ecológico y biológico. Además, esta cuenca alberga una importante flota artesanal multiespecífica, cuyo principal recurso son los moluscos bivalvos. El estudio de descartes de estas pesquerías puede resultar una herramienta útil, no sólo para caracterizar la fauna asociada, sino para evaluar el impacto ambiental asociado a esta actividad o aumentar el conocimiento sobre especies que son raras en el ámbito Europeo. Entre febrero y julio del 2013, se analizaron más de un centenar de muestras de descartes de la flota artesanal de rastros o dragas mecanizadas con base en los puertos de Caleta de Vélez, Fuengirola y La Línea de la Concepción. Dicha flota explota poblaciones de coquina –Donax trunculus Linnaeus, 1758-, chirla -Chamelea gallina (Linnaeus, 1758)-, concha fina -Callista chione (Linnaeus, 1758)- y corruco –Acanthocardia tuberculata (Linnaeus, 1758)-, los cuales representan importantes recursos marisqueros de fondos blandos infralitorales del mar de Alborán. En cada muestra de descarte, se identificaron, cuantificaron y pesaron todas las especies presentes, entre las cuales se encontraban algunas especies de afinidad subtropical africana. El equinodermo Luidia atlantidea Madsen, 1950, asteroideo común en fondos blandos infralitoralescircalitorales del noroeste africano, ha estado presente en el 17% de los descartes analizados, con un total de 31 ejemplares recolectados mayoritariamente a baja abundancia (1-2 individuos lance-1) y en fondos de Arenas Finas Bien Calibradas (3,5-5,5 m de profundidad) de la caleta de Vélez y Fuengirola. La mayoría de los ejemplares se hallaron en descartes de lances dirigidos a la chirla (87%) y mostraron daños severos (pérdida de varios brazos) tras su captura (90%). Otras dos especies del género Luidia habitan los fondos blandos del mar de Alborán, como son Luidia ciliaris (Philippi, 1837) (generalmente en el infralitoral y mostrando 7-8 brazos) y Luidia sarsi Düben&Koren, in Düben, 1845 (generalmente en el circalitoral-batial y con 5 brazos como L. atlantidea). La disposición, número y forma de las placas superomarginales y el patrón de coloración de brazos y espinas marginales representan caracteres que sirven para distinguir L. sarsi y L. atlantidea. El presente estudio constituye el primer registro de L. atlantidea para las costas europeas y mediterráneas. En los descartes se han hallado otros invertebrados de afinidad subtropical africana que son poco comunes en las costas europeas, como es el gasterópodo Sinumbi fasciatum (Récluz, 1851), cuyas únicas poblaciones europeas y Mediterráneas se encuentran en el mar de Alborán o el decápodo Albunea carabus (Linnaeus, 1758), considerado como “raro” por diferentes estudios científicos y que está presente en las costas occidentales africanas y en determinadas zonas del Mediterráneo. De este decápodo se capturaron 23 ejemplares, con longitudes de cefalotórax entre 11 y 22 mm, fundamentalmente en descartes de chirla (69,6%) y de coquina (30,4%), tanto en zonas frente a desembocaduras de ríos (en concordancia con estudios previos) como frente a zonas sin aportes fluviales. La mayoría de ejemplares eran machos (78,3%), debido a que provienen de profundidades menores (1,5-5,3 m) a las indicadas para las hembras (10-40 m), y no presentaban daños o, si los presentaban, eran de tipo leve (pérdida de algunos apéndices tras su captura) (65,2%). La presencia de estas especies de afinidad subtropical africana podría estar ligada a diversos factores como (1) el continuo flujo de larvas, a través del estrecho de Gibraltar, hacia el mar de Alborán por la corriente atlántica más superficial, (2) el calentamiento global que favorece el asentamiento y desarrollo de especies de latitudes inferiores en esta zona y/o (3) los descartes de barcos pesqueros que faenaron en el pasado las costas africanas y que tenían como base los puertos del sur de la península ibérica.Versión del edito