Spatial structure of recruitment in the mussel Perna perna at local scales: Effects of adults, algae and recruit size

Abstract

To test the assumption that there is no spatial structure in small-scale recruitment variability of rocky shore mussels, we examined spatial dependence in the distribution of density of recruits (late plantigrades: 0.5 to 3.5 mm; larger recruits: 3.5 to 10 mm) and adults of the brown mussel Perna perna within local scales (30 lags ranging between 0.35 and 10.5 m) in mid- and upper mussel beds. Spatial heterogeneity was estimated by analyzing scaling properties of semivariograms using a fractal approach. Relationships between density of mussel recruits and adults and biomass of the red alga Gelidium pristoides at the different scales were examined by cross-semivariograms. We found that the distribution of adults showed spatial dependence at all transects, often with higher spatial heterogeneity (higher fractal dimension, D) at smaller scales (1st scaling region). The distribution of larger recruits exhibited spatial dependence at all transects, revealing a spatial structure, which was related to that of adults. In contrast, the distribution of late plantigrades showed mainly spatial independence (random pattern; 1.97 < D ≤ 2). Densities of both size classes of recruits were positively related to those of adults at all transects and scales, but the relationship was stronger for larger recruits than late plantigrades, explaining why there was clearer spatial structure of larger recruits. The relationship with algae was mainly negative for larger recruits, while it tended to be positive at many scales for late plantigrades. Thus, both adult mussels and G. pristoides are suitable habitats for plantigrades, while mussels are the main habitat for larger recruits. This may mean that recruits on algae either die or migrate to mussel clumps at a certain size. This study highlights the importance of recruit size when analyzing recruitment patchiness of mussels, and has implications for sustainable management of P. perna.