Intertidal barnacle distribution:a case study using multiple working hypotheses

Abstract

The roles of larval supply, selection of habitat, availability of space, larval physiological condition, and early post-settlement mortality in determining vertical distribution patterns of the barnacle Semibalanus balanoides (L.) were simultaneously examined in Passamaquoddy Bay, New Brunswick, Canada. Pump samples and settlement surveys showed that about 60% of planktonic cyprids were collected from 1.5 to 4.0 m depth, while over 90% of newly settled cyprids colonized low intertidal levels. Selection of habitat by cyprids and patterns of settlement were studied using precolonized planks that had been previously placed to obtain various periphyton gradients. These planks precolonized at the low, mid-, and high intertidal levels and placed in different orientations, induced variations in the distribution of newly settled cyprids. For example, vertical planks turned upside down had fewer settlers at low intertidal levels compared to surfaces maintained in the same orientation at the same level. When placed horizontally at the same level, sections precolonized at the low intertidal level collected more larvae than sections initially placed at the high and mid-intertidal levels. We used the triacylglycerol/cholesterol ratio (TAG/CHOL) as a measure of physiological condition in planktonic and newly settled cyprids, and newly metamorphosed spat. The TAG/CHOL ratio in planktonic cyprids decreased over the sampling period, but there was no variability with position in the water column. For newly settled cyprids and newly metamorphosed spat, the TAG/CHOL ratio increased with decreasing intertidal level. Quadrats freed of conspecific adults were colonized by more cyprids than were uncleared controls. Early post-settlement mortality was greater in uncleared than in cleared quadrats, but this effect varied with intertidal level. Early post-settlement mortality increased with increasing intertidal level in cleared quadrats, while it did not significantly vary in uncleared controls. The present study shows that the vertical distribution of S, balanoides on the shore results from a complex relationship involving larval supply, selection of habitat, availability of space, and early post-settlement mortality. We also present the first evidence of correlations between larval physiological condition and early vertical distribution in S. balanoides.