Temporal variation of the free-living Isopoda (Crustacea: Peracarida) in two subtidal muddy bottoms of the Ría de Aldán (Galicia, NW Iberian Peninsula)
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1
Universidade de Vigo, Departamento de Ecoloxía e Bioloxía Animal, Spain
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2
University of Vigo, Estación de Ciencias Mariñas de Toralla (ECIMAT), Spain
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3
Universidad Autónoma de Madrid, Departamento de Biología (Zoología), Spain
Introduction
Isopods are a group of crustaceans characterized by an extremely high morphological and functional diversity, and can be found in all kinds of substrata. Many of them are parasites on either fishes or other crustaceans, but here we focus on free-living benthic isopods. More specifically, we address the temporal variation of free-living Isopoda assemblages in shallow fine sedimentary bottoms, an issue that still remains poorly studied. The Ría de Aldán is a small estuarine-like embayment; it is about 7-km long by its main axis and has a maximum width of 3.5 km by its opening, with depths of up to 45 m. It is located at the SW of the Ría de Pontevedra and is part of the Galician Rías Baixas, a group of tectonically-formed bays located in the NW of the Iberian Peninsula. The influence of the eastern boundary upwelling system of the North Atlantic subjects the Rías to both seasonal and event-scale variability.
Material and methods
Two areas were studied for this work: an 18m-depth muddy sand bottom in the westerly part of the ría (Site 1), and a 17m-depth organically enriched muddy bottom in its innermost part (Site 2). Five replicate samples were taken monthly at each site from May 1998 to May 1999 by means of a Van-Veen grab with an operating surface of 0.056m2. Samples were sieved through a 0.5-mm mesh, and then fixed in 10% buffered formalin. An additional sample was taken to determine sediment characteristics (granulometric composition, carbonate content and total organic matter content). Isopods were sorted, identified and counted in the laboratory using a stereo microscope. Non-metric multidimensional scaling (nMDS) analysis was used to obtain a visual representation of the similarities among samples. The correlation between sediment features and species abundances and assemblage univariate descriptors was evaluated by means of the Spearman’s correlation coefficient.
Results
Along the study, 176 individuals belonging to 14 different taxa were collected. Both abundance and taxa richness were considerably higher at Site 1 overall. However, both values were very variable at both sites throughout the study period. Maximum values for both of them were found at Site 1 in June and November 1998, coincident with remarkable changes in the characteristics of the sediment. No isopods were found at Site 2 in 5 out of 13 sampling dates. From the 14 taxa identified, 9 were exclusive of Site 1, 2 were only present at Site 2 and the remaining 3 taxa were found at both sites. Jaera (Jaera) nordmanni (Rathke, 1837), Uromunna sp. and Lekanesphaera rugicauda (Leach, 1814) were the most abundant taxa both at Site 1 and overall. Ordination of the samples through the nMDS analysis showed a poor separation between sites and also a very weak seasonal structure, reflecting the high variability of the results. Spearman’s correlation coefficients at Site 1 showed that both total abundance and taxa richness, as well as the abundance of most of the species, were significantly (P < 0.05) positively correlated to coarser and more heterogeneous sediment with lower total organic matter content and higher carbonate content. At Site 2, only the abundance of one of the species showed significant correlations with sediment characteristics, showing the same pattern as the significant correlations found for Site 1.
Discussion
Our results reveal that isopods are rather scarce in the two studied bottoms. This agrees with the findings of different studies on the spatial distribution of benthic assemblages carried out in shallow sedimentary bottoms of both Ría de Aldán and nearby areas, as well as in other coastal areas around the world. Such studies found that both isopods and other peracarid crustaceans were numerically dominant in sandy bottoms, while they presented much lower abundances in finer sediments. Also in agreement with this, most of the isopods found in this study were collected at Site 1, the one with a coarser sediment. Furthermore, total abundance and taxa richness at this site presented significant correlations with indicators of a coarser and less organically enriched sediment. However, these correlations must be carefully considered, a thorough observation of the data reveals that they are highly influenced by just two of the sampling dates: June and November 1998. Sediment characteristics on these dates were utterly different from those found during the rest of the studied period; sediment was coarser, more heterogeneous, with a higher carbonate content and a lower total organic matter content, so it was much closer to the kind of sediment on which peracarids are usually numerically dominant. The abundances of several species showed the same correlations as total abundance and taxa richness, but those species were more or less abundant on June and/or November and either absent or very scarce during the rest of the study period. Uromunna sp. was the only taxa present on most of the sampling dates at Site 1 and did not show any significant correlation with sediment characteristics. Similarly, the only species that showed significant correlations with sediment characteristics at Site 2 was only present on two sampling dates, one of which was July 1998, characterized by a remarkable change on sediment characteristics similar to those observed on June and November at Site 1. These alterations on the characteristics of the sediment, for which no satisfactory explanation has been found, seem to shape the main features of the temporal variation of isopod assemblages on Site 1. They are also responsible for most of the abundance and diversity of isopods at this bottom, as the presence of this group on sampling dates with ordinary sediment features for this site (i.e. muddy sand) is merely anecdotic, only slightly higher than in the case of the muddy sediment present at Site 2.
Acknowledgements
F. Aneiros was funded by FPU program (Spanish Education Ministry, AP2010-2010). Partial funding for this study was obtained from FEDER GRC2013-004 project (Xunta de Galicia).
Keywords:
temporal variation,
Peracarida,
Isopoda,
Benthic macrofauna,
muddy sediments,
Galician Rías
Conference:
XIX Iberian Symposium on Marine Biology Studies, Porto, Portugal, 5 Sep - 9 Sep, 2016.
Presentation Type:
Poster Presentation
Topic:
1. ECOLOGY, BIODIVERSITY AND VULNERABLE ECOSYSTEMS
Citation:
Aneiros
F,
Moreira
J and
Troncoso
JS
(2016). Temporal variation of the free-living Isopoda (Crustacea: Peracarida) in two subtidal muddy bottoms of the Ría de Aldán (Galicia, NW Iberian Peninsula).
Front. Mar. Sci.
Conference Abstract:
XIX Iberian Symposium on Marine Biology Studies.
doi: 10.3389/conf.FMARS.2016.05.00121
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Received:
15 Apr 2016;
Published Online:
03 Sep 2016.
*
Correspondence:
Mr. Fernando Aneiros, Universidade de Vigo, Departamento de Ecoloxía e Bioloxía Animal, Vigo, Pontevedra, 36310, Spain, f.aneiros@uvigo.es