A Rocha Portugal Observatory Report 2009-2010

A Rocha Portugal Observatory Report 2009-2010 17
were observed around the sample areas (roughly 30 specimens of each species within a 0.5m radius of the
sample area).
Figure 6 Number of live specimens found at the Alvor sampling site (6b).
All the sample sites in this study were located within the subtidal area and had either no, or very limited,
submerged vegetation present. Aquatic vegetation within the Alvor estuary is in general very poor in
diversity and extent.
Discussion and Conclusion
The species found alive in this study of the Alvor Estuary were also found by Almeida et al (2008) in the Ria
Formosa which is located in the eastern Algarve, Portugal (Bivalvia: C. edule, O. edulis, S. plana, P. aurea, T.
decussatus, and Gastropoda: C. chinensis, C. vulgatum, H. ulvae, N. pfeifferi, N. reticulatus, G. umbilicalis, J.
striatus, M. lineata).
Some of the species found during this study are included on lists of sensitive, tolerant and opportunistic
species used by Simboura and Zenetos (2002) in their study. Most of them, like Tellina tenuis, Tapes
decussatus, Spisula subtruncata, Cerithium vulgatum, Cyclope neritea, Paphia rhomboides are on the list of
sensitive species; being described as “sensitive to disturbance in general”. Some species like Hydrobia acuta
and Bittium reticulatum are also on the list of sensitive species but they are described as being tolerant
species. A very large number of Bittium reticulatum shells were found in this survey, however no living
specimens were found. Although Bittium reticulatum is an estuarine species it is mostly found on seagrass
vegetation (Almeida et al, 2008; Fernandez et al, 1988), therefore, it is highly possible that most of the
shells found in the samples were moved in by the tide. The Upper Estuary site (5a), where the abundance
of Bittium reticulatum was almost always (with the exception of the March sample) greater than that of any
other species, may be an exception due to the proximity of the sample site to a large vegetated area. Other
than in this area, the aquatic vegetation within the Alvor Estuary is poor and this may explain the lack of
several species of gastropods which are commonly associated with seaweed and other aquatic plants
(Sousa, 2007).