JOURNAL OF EXPERIMENTAL MARINE BIOLOGY AND ECOLOGY
Abstract
Scaling up from local, short-term experiments to larger-area and longer-term ones is crucial to address the role of scale in
ecology. Few studies, however, examined large-scale spatial variability in the distribution and abundance of marine organisms, with
rare attempts to directly compare spatial variation at local (centimetres–metres) vs. regional (1000's of kilometres) scale. Here, we
used a hierarchical design to describe the spatial distribution of the hydroids epiphitic of the brown alga Cystoseira amentacea, a
habitat-forming species that provides a continuous, extensive settling substrate at regional scale along the rocky coasts in the
Mediterranean Sea. This continuity provides the potential to deal with scale-related variability, increasing area of investigation
without adding differences deriving from habitat heterogeneity or changes in topographic complexity. Hydroids were selected for
their abundance and for their life cycle features (rapid growth, small body size, early sexual or asexual reproduction and short life
span), allowing rapid responses to changes in environmental conditions. The aim of this study was to analyse whether the structure
of hydroid assemblages living on C. amentacea had a consistent pattern of variation among three portions of the algal thallus (i.e.,
basal, middle, and distal) across a spectrum of scales and whether having or not a pelagic stage could exert a significant influence
on the distribution patterns of the species. A total of 32 species were identified. Multivariate analyses showed that hydroid
colonization of Cystoseira occurs differently along each thallus, with patterns of variation in the structure of assemblages differing
at an even smaller spatial scale than that of single plants. However, such differences varied from patch to patch. Among the 14
species identified as “important” to define the hydroid assemblage inhabiting Cystoseira, only one (Clytia hemisphaerica) has free
medusae, the other species reproducing by fixed gonophores or by short-lived medusoids. Univariate analysis showed significant
differences among portions of thalli in terms of spatial variability at the various scales investigated, thus suggesting that patterns of
multivariate variation along the three portions of thalli might vary across scale. Overall, our results suggest that patterns of
distribution of hydroids along C. amentacea thalli significantly vary across spatial scales but that the observed differences can be
hardly interpreted on the basis of life-cycle patterns.
