Role of Recruitment Processes in Structuring Coralligenous Benthic Assemblages in the Northern Adriatic Continental Shelf

Abstract

Coralligenous biogenic reefs are among the most diverse marine habitats in the Mediterranean Sea. The northern Adriatic mesophotic coralligenous outcrops host very rich and diverse epibenthic assemblages. Several studies quantified the low temporal variability and high spatial heterogeneity of these habitats, while processes driving structuring and differentiation are still poorly understood. To shed light on these processes, temporal and spatial patterns of colonisation were investigated using travertine tiles deployed on three coralligenous outcrops, corresponding to the main typologies of benthic assemblages described in previous studies. Three years after deployment, assemblages colonising travertine tiles resembled the differentiation among sites revealed by the natural assemblages in terms of major ecological groups. Processes structuring and maintaining species diversity have been explored. Pioneer species with high reproduction rate, long distance larval dispersal and fast growth (e.g. the serpulid polychaete Spirobranchus triqueter and the bivalve Anomia ephippium), were the most abundant in the early stages of recruitment on the two outcrops further away from the coast and with lower sedimentation. Their success may vary according to larval availability and environmental conditions (e.g., sedimentation rates). At these sites early-stage lasted 10-12 months, during which even species from natural substrates began colonising tiles by settlement of planktonic propagules (e.g., encrusting calcareous Rhodophyta) and lateral encroachment (e.g., sponges and ascidians). On coastal outcrop, exposed to a higher sedimentation rates, tiles were colonised by fast-growing algal turfs. Resilience of northern Adriatic coralligenous assemblages, and maintenance of their diversity, appeared largely entrusted to asexual reproduction. Exploring the mechanisms that underlie the formation and maintenance of the species diversity is crucial to improve our understanding of ecological processes and to implement appropriate conservation strategies of the Adriatic coralligenous reefs.

Fig 1. Sites location and experimental setup.

(a) sketch map of the study area, showing the three experimental sites and the no take zone (NTZ) limits (for illustrative purposes only); (b) example of plot of travertine tiles and sediment trap, just deployed in P204 in August 2005.

Fig 2. Similarities between assemblages on experimental tiles and on natural substrates three years after tiles deployment.

PCoA ordination plot based on Bray-Curtis similarities of square root-transformed benthic assemblages data: each point was the centroid of the observed similarity among Sites and Substrates.

Fig 3. Abundance of most abundant taxa on experimental tiles and natural substrates three years after tiles deployment.

Mean (± SE) percent cover of (a) turf of algae and hydroids, (b) encrusting calcareous Rhodophyta, c) non-calcareous algae and d) sponges, at each plot and site, both on travertine tiles and natural substrates, at the end of the recruitment experiment (August, 2008).

Fig 4. Temporal trends of similarity between the assemblages on experimental tiles and natural substrates at the end of the experiment.

At each site and date mean similarity (±SE) of assemblages colonising tiles with the respective assemblages on natural substrates in August 2008 was calculated.

Fig 5. Temporal trends of occupation of space on experimental tiles.

Temporal trends of (a) sediment percent cover and (b) proportion of substrate uncovered neither by sediments nor by organisms at each site (mean ± SE).

Fig 6. Differentiation of benthic assemblages on travertine tiles.

PCoA ordination plot based on Bray-Curtis similarities of square root-transformed benthic assemblages data on travertine tiles: each point was the centroid of the observed similarity among Sites and Dates.

Fig 7. Temporal trends of percent cover.

(a) Anomia ephippium, (b) serpulids, (c) encrusting calcareous Rhodophyta, (d) sponges, (e) turf of algae and hydroids and (f) species richness (S) at each site (mean ± SE).

Fig 8. A conceptual model of recruitment processes on experimental tiles deployed on coralligenous outcrops of the northern Adriatic Sea.

Species involved in the early colonisation stage largely depend on larvae availability and environmental conditions in the area. Over time colonisation by local species, living on natural substrates, become more relevant according to their life-cycle and dispersal strategies (see text for details).