Species composition and vertical distribution of chitons (Mollusca: Polyplacophora) in a rocky intertidal zone of the Pacific coast of Costa Rica

Abstract

The present study describes the species composition and vertical distribution of chitons in tide pools and on exposed rock areas of the intertidal zone of Samara, Guanacaste, Pacific Costa Rica. Nine different species of chitons were recorded, and their densities and sizes were determined using quadrate sampling. Physical (period of emersion, temperature and salinity range) and biotic (presence and diversity of food resources) factors were examined to assess their influence on distribution and abundance of each species.Ischnochiton disparwas the predominant species with densities of up to 200 ind m−2, representing 85.4% of all sampled individuals (N = 4193). The size ofI. disparwas positively correlated with the size of the inhabited boulders. Two species (I. disparandStenoplax limaciformis) showed segregation in size:I. disparbeing negatively andS. limaciformispositively correlated to the distance from the shore. Two of the nine species (Chiton stokesiiandAcanthochitona hirundiniformis) occurred mainly on rocky areas outside the tide pools with variable amounts of exposure to air during each tidal cycle. In both species the tide pool specimens were significantly smaller than those collected in the rocky areas. Distribution of all other species was restricted to the tide pools and species had their peak abundance in the zones with more moderate conditions and a higher diversity of algae, resulting in the highest chiton diversity in these areas.