Distribution of a population of annelid polychaetes in the "trottoir" of the midlittoral zone on the coast of North-East Spain, Western Mediterranean

10 pages, 10 figures, 1 tableThe calcareous cornices of the algae Lithophyllum tortuosum Foslie, the so-called "trottoirs", are one of the most characteristic formations of the littoral in the Western Mediterranean. The marked inconstant insolation and wave action are the environmental factors that most affect the fauna colonizing the cornice, and the distribution of animal populations should thus be affected by these factors. To test this hypothesis, the distribution of a population of polychaetes located on the coast of North-East Spain was studied from May 1982 to May 1983, taking three types of variables into consideration: (1) Spatial-at three levels (upper, medium and lower levels of the cornice), with two fractions in each level (inner and outer), from three different sampling locations; (2) seasonal-during six different months of the year; (3) biological-number of species, number of individuals, sizes and feeding strategies. In the population studied, the Syllidae dominated (37% of total identified species);Fabricia sabella was the most common species throughout the cornice, its average abundance being 38% of the total number of individuals in the population. No significant differences were observed in individual sizes of the polychaete population or between sampling stations. The number of species, the number of individuals and diversity increased from the upper level to the lower level of the cornice. The discretely motile filter-feeding species were dominant in the upper level, and motile carnivores, motile herbivores and sessile filter-feeders in the lower level. These three latter groups also dominanted the outer fraction of the cornice, whereas discretely motile filter-feeders dominanted the inner fraction. Differences in biological variables between levels were more marked outside than inside the cornice. Seasonal differences arose from higher insolation and a low humidity in summer and, in general, were most marked in the inner fraction and the lower level of the cornice, i.e., the most sheltered zones. In these zones, biological factors play a predominant role, although environmental factors do govern the whole polychaete population of the "trottoir" to varing degrees. © 1988 Springer-Verla