Between May of 2000 and July of 2001 a study aiming to determine the seasonal variation of syllid local population abundances was carried out. Samples were taken every second month and consisted of two replicates, each one being a quadrat 20 cm wide. After fixing, the samples were washed and the draining water was passed through a 1 mm sieve; then the syllids were identified and counted. For each replicate a number of ecological indexes were calculated, namely population density, number of species and Shannon's and Simpson's diversity indexes, computing mean values for each sample. In order to identify the most representative species of the taxocoenosis the Ci (constancy index, frequency of the species ranging from 0 to 1) and Di (dominance index, relative abundance ranging from 0 to 100 considering all the collected individuals) were computed for each one.

Analysis of the eight samples yielded 6548 individuals and 48 species belonging to the four subfamilies of Syllidae, of which Exogoninae and Syllinae were the most abundant and diverse. During the study period, the syllid taxocoenosis suffered dramatic changes. The abundance, that in May of 2000 was 33,375±775 individuals/m2, diminished to 2162.5±37.5 in September. After this date, the population density slowly increased. The number of present species evolved in a very similar way. Twelve species occurred in all the samples, although just six were abundant enough to yield Ci×Di products exceeding 5, thus being considered as good descriptors of the taxocoenosis. The variation of population density for most of these species more or less paralleled that of the total population of syllids. With regards to relative abundance, the taxocoenosis was always clearly dominated by Exogoninae species, especially by Salvatoria vieitezi, whose dominance scores always fluctuated around 45%. Syllis prolifera was the only important non-Exogoninae species in terms of dominance, although this parameter showed strong variation in this species.