Destructive standard squares or low-impact visually driven collection? A comparison of methods for quantitative samplings of benthic hydrozoans

The patchy distribution pattern of benthic hydrozoans reflects high sensitivity of colonies to local and micro-scale environmental factors, which may affect the outcome of biodiversity inventories based on different sampling methodologies. We compared three quantitative sampling methods for benthic hydrozoans, differing for strategy (scraping standardized surfaces or picking visually located colonies) and tactics (extension of examined surface). Each of the three methods consisted of eight replicate samplings to compare relative and absolute method efficiencies, also in terms of minimum sampling area representative of the hydroid diversity in the study site. A single horizontal belt transect on a vertical rocky cliff was chosen at 15 m depth near Otranto (South Adriatic). A total of 37 species were identified by the three methods and their relative abundances were evaluated. The hydroid assemblage was dominated by generalist species with high polyp numbers able to colonize different substrata. We compared the outcomes of each of the three sampling methods, and by a cost-benefit analysis we identified the optimal sampling methods to be adopted according to the different targets of the investigations. On data standardized to the same total sampled areas, the three sampling methods did not originate significant differences in the number of polyps of each recorded species (PDA, Polyp Detection Ability), whereas the visually-oriented collection (VIS) and the scrapings over 20 × 20 cm standard surfaces (S20) were more effective in the detected number of species (SDA, Species Detection Ability) than over 10 × 10 cm squares (S10). For ecological purposes, samplings based on five S20 replicates can be considered an appropriate experimental design to get a statistically robust representation of the studied hydroid assemblage at Punta Palascia. However, variation in depth, exposure and geographical locations may require different sampling approaches, and pre-surveys should be always carried out before addressing taxonomical surveys of sessile organisms.