An assemblage of cirratulids in a confined marine basin (Mediterranean Sea) was investigated at small spatio-temporal scales in an experiment in which artificial granules of expanded fire-clay were used as a bare substratum for colonization. Analysis of core samples of 3.5 l undisturbed natural sediments (source area) and equal volumes of artificial substratum (new settlement area), plunged into the sea bottom were carried out from May to July 2008 after 0, 15, 30 and 60 days. Caulleriella bioculata showed the highest total mean density of 10.1 ± 5.8 N l−1 in the natural sediment, and the lowest at 0.13 ± 0.08 N l−1in the artificial substratum, with a marked temporal decrease. Individuals were not strongly aggregated and were found in a deeper sediment layer. The density of large Cirriformia tentaculata was 0.5 ± 0.3 N l−1, with individuals more aggregated and confined to shallower sediments, versus 0.10 ± 0.06 N l−1 but found deeper in the artificial substratum; temporal trends were not straightforward for this species. The adult stage of sedentary cirratulids appeared to actively move into the new available substratum from the neighbouring sediments. An early cirratulid juvenile stage was observed in both microhabitats at a comparable density of 1.8 ± 2.8 N l−1 with highly aggregated individuals (cohorts). The peculiar morphology of branchiae, the threadlike body shape and the multivariate morphological differences between two sub-populations settled in the different substrata are reported for these problematic specimens.

Assessing and monitoring the environmental impact off Mar del Plata (38°S 57°W) led to the need to identify the cirratulid polychaetes. Cirratulids are indicators of moderately impacted areas or early indicators of organic contamination. Some subtidal sand patches affected by sewage discharge have high abundances of a new, rare and difficult to place species, such as Caulleriella trispina sp. nov. This species is characterized by having characters of three bitentaculate cirratulid genera, including three kinds of spines: unidentate, sub-bidentate and bidentate. The genera and their taxonomic characters are discussed as well as the need for monitoring and environmental assessments for correct identification. A key to the known species from the warm-temperate region of Argentine Cirratulidae is given.