Boletín de la RSEHN. Sección Biológica

Se aporta el inventario de los invertebrados bentónicos encontrados en los fondos intermareales y submareales de la Ensenada de San Simón, situada en el interior de la Ría de Vigo (Galicia). Los fondos blandos de dicha ensenada se muestrearon durante 1999. El cultivo del mejillón sobre batea es una práctica muy arraigada en la zona, de naturaleza fangosa y sometida a significativas fluctuaciones de salinidad debido a la desembocadura de varios canales de agua dulce. Además, praderas de las fanerógamas marinas Zostera marina L. y Z. noltii Hornem. caracterizan el interior de la ensenada. Se han contabilizado un total de 379 taxones, pertenecientes a 12 filos. La mayor riqueza de especies la muestran los anélidos (134 taxones), artrópodos (133) y moluscos (73).

Species appearing in the macrobenthic study of the Ensenada de San Simón (located in the inner Ría de Vigo, north-western Spain) are cited. The intertidal and subtidal soft-bottoms of this inlet were sampled during 1999 (Fig. 1). Culture of mussels on rafts is a common practice in large areas of the mouth of the inlet. Soft-bottoms of this inlet are mainly muddy with high organic matter content. Sandy sediments are present in tidal channels in the inner inlet where low content of total organic matter was also found. Intertidal and shallow subtidal areas have Zostera noltii Hornem. 1832 and Zostera marina L. meadows. Large freshwater input occurs in the innermost part of the inlet which translates into fluctuations of salinity on both a tidal and seasonal basis. Areas around the outer part of the inlet have muddy sands. Samples were taken at each site, by means of a van Veen grab. Samples were sieved through 0.5 mm mesh and the retained material was fixed in 10% buffered formalin. Fauna was sorted from the sediment and preserved in 70% ethanol. Temperature and pH were measured in situ from water and sedimentary samples taken from each site. An additional sedimentary sample was taken at each site for grain-size analyses and to determine content of calcium carbonate and total organic matter (Table 1). Sampling yielded 379 taxa, of which 7 were Cnidarians, 3 Platyhelminthes, 4 Nemertins, 1 Nematod, 134 Annelids (5 oligochaetes and 129 polychaetes), 3 Sipunculids, 133 Arthropods, (11 ostracods, 5 copepods, 109 malacostraceans, 2 insects, 3 aracnids, 3 picnogonids), 73 Molluscs (3 chitons, 1 scaphopod, 37 gastropods, 32 bivalves), 1 Phoronid, 11 Echinoideans, 3 Chaetognaths and 6 Chordates (3 ascideans and 3 osteichthyes). A complete check list is presented with the species name with author and date, and the number of the site where it appeared. The polychaetes dominated in 20 sites according to the number of species. Sites situated in the deeper area showed the largest number of polychaete species. The intertidal zone was colonized by large numbers of crustacean species. The polychaete families Syllidae, Spionidae and Phyllodocidae showed the largest number of species, Syllidae appearing mostly in outer areas. Some species of Spionidae, such as Polydora ciliata or Streblospio shrubsolii, showed higher densities in the sandy inner bottoms, while others, as Prionospio pulchra, showed preference by the external part. Pseudopolydora paucibranchiata is an ubiquitous species, appearing along the inlet. Among molluscs, the piramidellid gastropods showed the higher species richness (6 species). Amphipod is the crustacean order with higher number of species (60), appearing mostly in outer muddy bottoms, with the exception of some cases (Melita palmata, Leptocheirus pilosus, Corophium acherusicum, Microdeutopus gryllotalpa or Carcinus maenas). In Table I, we can see that the number of species increase from the intertidal areas towards the outer part of the inlet, with more than 10 m depth. The inner sediments are likely to be subjected to changes in salinity, temperature and desiccation levels due to their intertidal character and the freshwater input from several rivers (VILAS et al., 1995). Seagrass meadows are known to provide a complex habitat that may be colonized by many species (ORTH, 1977; PETERSON et al., 1984; SOMERSFIELD et al., 2002). In Ensenada de San Simón, however, these meadows are located in areas subjected to changes in salinity which is a major limiting factor for many species (KIKUCHI, 1987; PLANAS & MORA, 1987; JUNOY, 1996). In fact, PLANAS & MORA (1987) indicated the dominance of eurihaline species in estuarine communities, which are submitted to great changes in physical conditions. The highest values of number of species were found in the mouth of the inlet. These subtidal sediments show more stable conditions in terms of salinity and hydrodynamics (NOMBELA et al., 1987). The sites with the smallest number of species were muddy bottoms close to the mouth of freshwater channels and the main harbour in the inlet. Thus, it may be suspected that fluctuations in salinity coupled with effects of human activities, such as organic enrichment and disposal sewage, may be responsible for the scarce macrobenthic fauna.