Unusual composition and vertical zonation patterns amongst the biota of pebble and cobble shores

Abstract

Research on the composition and distribution patterns of intertidal biota has focused mostly on either sandy or rocky shores, with much less work on boulder shores and almost none on shores of particle grain sizes between 1 - < 256 mm. Recent studies have shown, however, that pebble (4–64 mm) and cobble (65–256 mm) shores comprise a very distinct habitat type, sheltering a biota less than 10% similar to those of either sandy, or boulder and rocky shores. This paper examines the composition and distribution patterns of the biota of seven pebble and five cobble shores along the South-Western coastline of South Africa, each surveyed by sampling a transect of eight equally-spaced 20 × 20 cm quadrats from low to high shore. Overall, 39 taxa were recorded, but only 14 occurred more than once and are thus considered characteristic of pebble and cobble habitats. The biota comprised many species typical of hard-substrata, some typical of sandy beaches, and a few unique to pebble and cobble habitats. Dominant species were mostly air-breathing Arthropoda, plus one pulmonate Gastropoda. Macro-algae were notably absent. Unlike the burrowing species of sandy shores, or attached species of rocky and boulder shores, pebble and cobble shore species were almost all motile, no doubt to withstand the mobility of rolling grains. Also, unlike most other intertidal shores, the fauna was concentrated around the high shore, where food was most available in the form of drift kelp. Some air-breathing species migrated well into the intertidal at low tide, but presumably retreat above the high-water mark during high tide.