Helgol Mar Res (2007) 61:17–29 DOI 10.1007/s10152-006-0050-7 ORIGINAL ARTICLE Environmental factors structuring polychaete communities in shallow rocky habitats: role of physical stress versus habitat complexity Alberto Serrano · Izaskun Preciado Received: 2 January 2006 / Revised: 4 September 2006 / Accepted: 20 September 2006 / Published online: 7 November 2006 © Springer-Verlag and AWI 2006 Abstract Polychaetes inhabiting 12 diVerent hard Keywords Polychaetes · Rocky environments · bottom habitats were studied. A total of 157 species Physical disturbance · Habitat complexity · belonging to 32 families were identiWed. DiVerences Cantabrian Sea among habitats in polychaete density, species rich- ness, and diversity were analysed, as well as the rela- tionships between these ecological indices and depth Introduction range, slope and in-bay/out-bay gradient. A high fau- nal homogeneity was found: all biotopes were domi- Shallow rocky ecosystems provide a great variety of nated by a low number of eurytopic species. Intertidal habitats suitable for polychaetes, which are often one habitats and subtidal ones with scarce algal cover of the dominant taxa there (e.g., Bianchi and Morri were typiWed by vagile polychaetes (syllids, nereids), 1985; Giangrande 1988). In the littoral rocky bottoms while sessile polychaetes (serpulids, sabellids) of the Santander Bay (Fig. 1), hydrodynamic and topo- appeared typically among subtidal large macrophytes, graphic variables produce a patchy distribution of ben- habitats with a calcareous substrate and shaded habi- thic macrohabitats (García-Castrillo et al. 2000). Tidal tats. Multivariate analyses showed that habitat com- rhythms generate harsh conditions, with barnacles and plexity, determined by physical disturbance, is the turf algae being the only organisms able to establish main structuring factor for polychaete populations. three-dimensional biotopes in the intertidal (Puente Biotopes with the highest structural complexity dis- 2000). In the subtidal, sedimentation is the key factor played a high number of companion species increas- conditioning the diVerent habitats, as it simpliWes the ing ecological indices and denoting a well-structured macrophytic coverage by decreasing the vertical strati- habitat. On the other hand, communities such as those Wcation and replacing canopy algae by crustose and in the upper intertidal, mainly controlled by physical turf types (Gorostiaga and Díez 1996). Because of this, environmental variables, showed a poorer polychaete turf-forming algae monopolize subtidal localities of fauna, dominated by ubiquitous species and a few high luminosity and siltation. Polychaete responses to well-adapted specialists. environmental changes are diverse. Polychaetes are soft-bodied organisms with low resistance to desicca- tion and sand abrasion (Serrano 2002). Mobile poly- chaetes can behaviourally avoid environmental stress. However, since most vagile polychaetes are medium- Communicated by H.-D. Franke. or small-sized organisms with low motility at macro- habitat scale, they rely on the existence of microhabi- A. Serrano (&) · I. Preciado tats or “refuges” in harsh areas or during periods of Instituto Español de Oceanografía (IEO), Promontorio de San Martín s/n, P. O. Box 240, environmental stress to survive (Bailey-Brock et al. 39080 Santander, Spain 1980). Conversely, the distribution of sessile polychaetes e-mail: [email protected] is much more dependent on environmental conditions, 123 18 Helgol Mar Res (2007) 61:17–29 Island (HI, n = 34), located outside, at the entrance, and inside of the Bay of Santander (Atlantic coast of northern Spain; Fig. 1), respectively. Rocky bottoms reach 20 m of depth at MI, 10 m at MP, and 5 m at HI; below these depths the bottom is completely covered by sand deposits. The underwater topography at MI consists mostly of large boulders with deep vertical walls forming an intricate system of narrow channels. At MP the seascape is a large rocky platform with a moderate slope heading oVshore. Finally, at HI the bottom is characterized by horizontal surfaces, with vertical walls extending from the sides of the island. MI and MP are highly exposed to wave action, while HI is sheltered from direct wave action but aVected by strong tidal currents (Castillejo et al. 1984). At MI the presence of silt is conWned to the bottom of the chan- nels, whereas at MP and HI sedimentation is intense all Fig. 1 Map of Santander Bay (Cantabrian Sea, North-Atlantic over the bottom surface. coast of Spain) showing the three sampling sites: HI Horadada Is- land, MP Magdalena Peninsula, and MI Mouro Island The benthic assemblages found in the study area were deWned in previous studies (García-Castrillo and morphological as well as physiological adaptations et al. 2000; Puente 2000) relating to tidal level, algal are necessary to avoid stress situations. coverage, and especially, to the presence of a basal Well-structured macrophytic habitats are located far encrusting layer of Mesophyllum lichenoides. We from environmental extremes, i.e., in the study area, on grouped these communities into 12 diVerent “habi- horizontal surfaces with low sedimentation rates. Cal- tats” following previous general studies on the poly- careous encrusting algae are relatively abundant in chaete fauna of the area (Serrano 2002). Hence, we environments of low light intensity and sedimentation considered three intertidal habitats: barnacles domi- rates, and are hence common underneath macroalgae nated by Chthamalus stellatus (BAR, n = 5), the alga (Connell 2003). Finally, invertebrate assemblages dom- Corallina elongata (COR, n = 37) and the lower inter- inate shaded environments on higher slope surfaces tidal (LIA, n = 16) grouping Bifurcaria bifurcata and (Preciado and Maldonado 2005), where competition Codium tomentosum assemblages. Another four habi- with macroalgae and siltation are limited (Moore tats were characterized as subtidal without Mesophyl- 1977). lum lichenoides substratum: two animal-based habitats Patchiness in macrohabitat distribution sometimes without algal cover, Anemonia viridis beds (ANE, generates parallel distributions of associated popula- n = 3) and Sabellaria spinulosa “reefs” (SAB, n =2); a tions. On the other hand, taxa may perceive environ- seasonal small-sized algae habitat (SSA, n =22) mental variability at diVerent scales, thus exhibiting grouping communities dominated by several species patterns of distribution, which do not match that of (Aglaothamnion sp., Asparagopsis armata, Falkenber- macrohabitats. The aim of the present work was to elu- gia rufolanosa, Dictyopteris polypodioides, Dictyota cidate if polychaete spatial distribution matches the dichotoma); and a macroalgae habitat dominated by patchy distribution of benthic habitats, and to deter- Cystoseira baccata (CYS, n = 20). These four habitats mine the main factors structuring polychaete popula- showed a high siltation-resilience, and were located in tions in shallow rocky habitats. We examined patterns rock-sand ecotones. Additionally, there were four sub- of polychaete spatial distribution along depth, slope, tidal habitats with Mesophyllum substrate: Laminaria and in-bay/out-bay gradients in littoral rocky habitats ochroleuca (LAM, n = 48), Gelidium sesquipedale of the Atlantic coast of northern Spain. (GEL, n = 39), a “small red algae” habitat (SRA, n = 17) co-dominated by Calliblepharis ciliata and Pterosiphonia complanata, and a Mesophyllum Methods lichenoides community without macroalgal cover (MES, n = 7). Finally, shaded walls, overhangs, and The study was carried out at three diVerent sites: caves were grouped in a sciophilous habitat category Mouro Island (MI, number of quadrants (n) = 294), (SCI, n = 141), dominated by macrofauna, mostly Magdalena Peninsula (MP, n = 29), and Horadada sponges and cnidarians. 123 Helgol Mar Res (2007) 61:17–29 19 Sampling was conducted by Scuba diving. A total of inclination, site, and habitat. Sponge and algal abun- 357 random quadrats were scraped, collecting all fauna dance (both calculated as wet weight per quadrat) were and Xora within them. We used 625 cm2 sampling quad- also included, as indicators of a darkness/light aYnity rats, except in the LAM community, where quadrats of gradient and also as a habitat complexity measure (see 2,500 cm2 were used. For further statistical analyses, above). Density data were log-transformed to diminish abundance values were calculated as ind m¡2. the eVect of uneven density distributions and rare taxa. DiVerences in polychaete density (average number The Monte-Carlo test was used to test the statistical of individuals m¡2), species richness (average species signiWcance of the Wrst and all canonical axes together number per quadrat) and Shannon–Wiener diversity using 999 permutations under the reduced model. were examined in relation to depth level, substrate RDA results were represented graphically in two bi- inclination, site, and habitats, using a Kruskal–Wallis dimensional ordinations generated by bi-plot scaling, one way ANOVA on ranks. When signiWcant diVer- focusing on inter-species distances, and representing ences were detected, pairwise “a posteriori” Dunn’s species and samples by points and environmental vari- tests were run to identify the groups responsible for ables by vectors. such diVerences. We considered Wve depth ranges: ¡5 to 0 m, 0 to 5 m, 5 to 10 m, 10 to 15 m, 15 to 20 m; and four substrate inclination semiquantitative ranges: hor- Results izontal to subhorizontal surfaces