Population structure and spatial distribution of the gastropod molluscs Osilinus atrata and Osilinus sauciatus in the rocky intertidal zone of the Canary Islands (Central East Atlantic)

Abstract

A hierarchical sampling design was used to study the population structure and the horizontal (differences among islands, locations within islands and sites within locations) and vertical (differences among intertidal bands) distribution patterns of the gastropod molluscs Osilinus atrata and O. sauciatus along the rocky intertidal zone of the Canary Islands. Differences in the spatial patterns of abundance, size and distribution were recorded for both species. Osilinus atrata was more abundant than O. sauciatus throughout the archipelago. Its abundance varied among islands and it presented a heterogeneous distribution in the bands from one island to another, though it was more frequent in the middle and high bands. Osilinus sauciatus was only found at the eastern islands and did not exhibit significant differences between the bands, even though 91.35% of the individuals appeared in the high band and the rest in the middle band. On the other hand, O. sauciatus had a larger mean size than O. atrata, but both species presented the largest mean sizes in the high band.