Distribution of hydroids along fronds of the kelp Ecklonia radiata

Abundances of organisms generally increase with structural heterogeneity and complexity of habitat. Patterns of distribution of hydroids on the kelp Ecklonia radiata were studied in Australia to examine the generality of this pattern. It was predicted that the cover of hydroids on E. radiata would be greater in the most heterogeneous (e.g., peripheral, crinkled blades) and complex (e.g., middle and bushy) parts of the alga. When different zones were independently compared along laminae of E. radiata, two clear patterns were found. Covers of hydroids in basal, middle and distal zones were greater in the periphery than in the centre of kelp. Second, covers in the periphery and in the centre of kelp were greater in middle than in distal zones and greater in distal than in basal zones. Both patterns conform to those predicted. At a finer spatial resolution, hydroids were significantly more abundant in troughs than on ridges of the peripheral blades of E. radiata. The primary substrata on which kelp was growing (pier-pilings or shark-nets) did not affect the distribution of hydroids. Biological and abiotic factors associated with the kelp itself most probably influence the distribution of hydroids along fronds, but manipulative experiments are needed to establish what models explain these patterns.